Scholarly article on topic 'Plant growth promoting rhizobia: challenges and opportunities'

Plant growth promoting rhizobia: challenges and opportunities Academic research paper on "Agricultural biotechnology"

Share paper
Academic journal
3 Biotech
OECD Field of science

Academic research paper on topic "Plant growth promoting rhizobia: challenges and opportunities"

3 Biotech

DOI 10.1007/s13205-014-0241-x


Plant growth promoting rhizobia: challenges and opportunities

Subramaniam Gopalakrishnan • Arumugam Sathya • Rajendran Vijayabharathi • Rajeev Kumar Varshney • C. L. Laxmipathi Gowda • Lakshmanan Krishnamurthy

Received: 8 May 2014/Accepted: 19 July 2014

© The Author(s) 2014. This article is published with open access at

Abstract Modern agriculture faces challenges, such as loss of soil fertility, fluctuating climatic factors and increasing pathogen and pest attacks. Sustainability and environmental safety of agricultural production relies on eco-friendly approaches like biofertilizers, biopesticides and crop residue return. The multiplicity of beneficial effects of microbial inoculants, particularly plant growth promoters (PGP), emphasizes the need for further strengthening the research and their use in modern agriculture. PGP inhabit the rhizosphere for nutrients from plant root exudates. By reaction, they help in (1) increased plant growth through soil nutrient enrichment by nitrogen fixation, phosphate solubilization, siderophore production and phytohormones production (2) increased plant protection by influencing cellulase, protease, lipase and b-1,3 glucanase productions and enhance plant defense by triggering induced systemic resistance through lipopolysac-charides, flagella, homoserine lactones, acetoin and butanediol against pests and pathogens. In addition, the PGP microbes contain useful variation for tolerating abiotic stresses like extremes of temperature, pH, salinity and drought; heavy metal and pesticide pollution. Seeking such tolerant PGP microbes is expected to offer enhanced plant growth and yield even under a combination of stresses. This review summarizes the PGP related research and its benefits, and highlights the benefits of PGP rhizobia belonging to the family Rhizobiaceae, Phyllobacteriaceae and Bradyrhizobiaceae.

S. Gopalakrishnan • A. Sathya • R. Vijayabharathi • R. K. Varshney • C. L. L. Gowda • L. Krishnamurthy (&) International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru 502 324, Andhra Pradesh, India e-mail:

Published online: 03 August 2014

Keywords Rhizobium • PGPR • Biocontrol • Stress • Heavymetal • Co-inoculation


Imbalance in nitrogen (N) cycling, nutritional status, physical and biological properties of soil, incidence of pests and diseases, fluctuating climatic factors and abiotic stresses are the interlinked contributing factors for reduced agricultural productivity. Agricultural sustainability, food security and energy renewability depends on a healthy and fertile soil. However, rapid acceleration of desertification and land degradation by numerous anthropogenic activities leads to an estimated loss of 24 billion tons of fertile soil from the world's crop lands (FAO 2011). The intensity of such degradation can be realized by the extent of highly degraded (25 %) and slightly/moderately degraded (36 %) lands, while only 10 % of land is listed to be improving all though high level use of agricultural chemicals have increased the productivity of available limited lands, high energy and environmental costs associated with their use necessitate the search for alternative methods of soil fertility and pest management. Recent estimations indicate that by 2030, the increasing population growth and changing consumption patterns would increase the demand for food by at least 50 %, energy by 45 % and water by 30 % (IFPRI 2012). These expectations cannot be met sustainably unless the soil fertility and productivity has been restored in the already degraded lands. A reversal of the decline in soil health is a possibility through the use of green and farm yard manures, composts and crop residues and by crop management options, such as natural fallow, intercropping, relay cropping, cover crops, crop rotations and dual purpose legumes. Among these practices, legumes

^j^dlxLc dLLcJI CÚJZLO A\ 0

^GT^ig^ H Springer

are the well-acknowledged builders and restorers of soil fertility, primarily through their association with symbiotic nitrogen fixation.

Use of microbial agents for improving agricultural productions, soil and plant health had been practiced for centuries. By the end of the ninenteenth century, the practice of mixing natural soil with seeds became a recommended method of legume inoculation. Rhizospheric soil, inhabited and influenced by the plant roots, is usually rich in nutrients when compared to the bulk soil, due to the accumulation of numerous amino acids, fatty acids, nucleotides, organic acids, phenols, plant growth regulators/promoters, putrescine, sterols, sugars and vitamins released from the roots by exudation, secretion and deposition. This results in enrichment of microorganisms (10- to 100-folds than the bulk soil) such as bacteria, fungus, algae and protozoa, among which bacteria influence the plant growth in a most significant manner (Uren 2007). Such rhizobacteria were categorized depending on their proximity to the roots as (1) bacteria living in soil near the roots (rhizosphere) (2) bacteria colonizing the root surface (rhi-zoplane) (3) bacteria residing in root tissue (endophytes), inhabiting spaces between cortical cells and (4) bacteria living inside cells in specialized root structures, or nodules, which includes two groups—the legume associated rhizo-bia and the woody plant associated Frankia sp. (Glick 1995). Bacteria that belong to any of these categories and promote plant growth either directly (nitrogen fixation, phosphate solubilization, iron chelation and phytohormone production) or indirectly (suppression of plant pathogenic organisms, induction of resistance in host plants against plant pathogens and abiotic stresses), are referred as plant growth promoting rhizobacteria (PGPR). Vessey (2003) preferred to categorize the bacteria that belong to the above mentioned first three groups as extracellular PGPR (eP-GPR) and the fourth group as intracellular PGPR (iPGPR). This ePGPR includes the genera Bacillus, Pseudomonas, Erwinia, Caulobacter, Serratia, Arthrobacter, Micrococcus, Flavobacterium, Chromobacterium, Agrobacterium, Hyphomycrobium and iPGPR includes the genera Rhizo-bium, Bradyrhizobium, Sinorhizobium, Azorhizobium, Mesorhizobium and Allorhizobium.

Research on exploring the potential of such PGPR has been reviewed periodically by many researchers (Bhatta-charyya and Jha 2012; Gray and Smith 2005; Johri et al. 2003; Lugtenberg and Kamilova 2009). There are many reviews focusing on both ePGPR and iPGPR. However, we intend to provide a detailed review on iPGPR, the rhizobia that belong to the families Rhizobiaceae (excluding the Frankia sp.), Bradirhizobiaceae and Phyllobacteriaceae, having unique association with root nodules of legumes and induce plant growth in many ways and improving sustainability in agriculture. Similar review on the capacity

of rhizobia in inducing the plant growth of nonleguminous plants has been published by Mehboob et al. (2012).

In Rhizobiaceae family, the constituents increased considerably from 8 in the year 1980 to 53 in 2006 (Willems 2006). Dispersion of host plants to new geographical locations might serve as a major source for these new rhizobia species. Still, increasing number of rhizobial species is expected because of following reasons. Only 57 % of 650 genera of leguminous plants have been studied for nodulation. Exploration of large number of legume species can potentially lead to the identification of many more rhizobial species. Recent advancements in the taxonomic research with the aid of specific molecular tools are another reason. So, identification and exploration of such potential rhizobia with plant growth promoting properties will be useful for sustainable agriculture.

Plant growth promoting traits of rhizobia

Plant growth promotion by Rhizobia can be both direct as well as indirect. Both these types of promotions are discussed as follows:

Direct promotions

Nitrogen fixation

Nitrogen (N) is required for synthesis of nucleic acids, enzymes, proteins and chlorophyll and hence it is a vital element for plant growth. Although 78 % of the atmospheric air is N, this gaseous form is unavailable for direct assimilation by plants. Currently a variety of industrial N fertilizers is used for enhancing agricultural productivity. However, economic, environmental and renewable energy concerns dictate the use of biological alternatives. Biological nitrogen fixation (BNF) is a process of converting atmospheric N into plant assimilable N such as ammonia through a cascade of reactions between prokaryotes and plants with the use of complex enzyme systems (Wilson and Burris 1947). BNF accounts for about 65 % of N currently used in agriculture. Legumes are BNF capable and meet their own N needs. Major part of N fixed by legumes is harvested as grains, while the soil and the succeeding crops also get benefitted by N in the form of root and shoot residues. Legume crops substantially reduce the N requirement from external sources (Bhattacharyya and Jha 2012). However, N fixation efficiency of legumes varies, and depends on the host genotype, rhizobial efficiency, soil conditions, and climatic factors. Reported quantum of nitrogen fixation ranged from 126 to 319 kg N ha- in groundnut, 33 to 643 kg N ha- in soybean, 77 to 92 kg N ha- in pigeonpea, 25 to 100 kg N hain cowpea, 71 to 74 kg N ha-1 in green gram and 125 to

143 kg N ha-1 in black gram (Peoples and Craswell 1992). Crops like wheat, rice, sugarcane and woody species have also the capacity to fix atmospheric N using free living or associative diazotrophs like Cyanobacteria, Azospirillum, Azocarus etc. However, the contribution of legume-rhizobia symbiosis (13-360 kg N ha- ) is far greater than the non-symbiotic systems (10-160 kg N ha-1) (Bohlool et al. 1992). The symbiotic N contribution is also reported to benefit the cereal crops, such as maize, rice, wheat and sorghum with a relative yield increase of 11-353 % (Peoples and Cranswell 1992).

Rhizobia can be used as inoculants for enhanced N fixation and studies demonstrated their predominance in nodules for 5-15 years after initial inoculation (Lindstrom et al. 1990), and confirming that they are effective colonizers persisting in soil for many years in the absence of their host (Sanginga et al. 1994). Still, BNF systems can be realized only through analysis and resolution of major constraints to their optimal performance in the field, their adoption and use by the farmers. The constraints include environmental, biological, methodological, production level and sociocultural aspects (Bohlool et al. 1992).

BNF ability, N self sustainability and protein-rich grains of legumes require high energy and productivity tradeoffs (Hall 2004). Hence, improving yield potential of BNF capable legumes, to a level of cereals, is considered difficult. Legumes do not establish a rapid crop ground coverage leading to low intercepted photon (radiation) use efficiency and a low proportion of carbohydrate that is partitioned to the grain. In addition, this area of research had attracted less plant breeding attention till now. The energy costs of biochemical pathways for the production of proteins and lipids are far greater than that of carbohydrates. This explains why the protein-rich legumes lack behind in yield potential compared to cereals. Production of proteins require 2.5 and lipids 3.0 units of photosyn-thates (glucose), while a mere 1.2 units are required for the carbohydrates (Penning de Vries et al. 1983) and such a high energy requirement for protein synthesis and accumulation in seeds increases the amount of photosyn-thates requirement thereby reducing the productivity potential of legumes and oil seeds. The seed biomass production efficiency of legumes is shown to be lower (0.66) per unit of photosythates required as compared to 0.72 of cereals (Sinclair and de Wit 1975). Also, legume seeds (26 mg g-1 seed) need double the requirement of nitrogen compared to cereals (13 mg g-1 seed) which is one more limiting factor for grain yield productivity. In addition, legumes need to be BNF-capable spending large amount of energy in this symbiotic relationship contributing both for the current yield and for enriching the soil by 30-40 kg N for every ton of plant biomass productivity (Peoples et al. 2009). Thus, not only the BNF

requires energy, but also it acts as a limiting factor to yields when conditions are uncongenial.

Nitrogenase, a major enzyme involved in the nitrogen fixation has 2 components: (1) dinitrogenase reductase, the iron protein and (2) dinitrogenase (metal cofactor). The iron protein provides the electrons with a high reducing power to dinitrogenase which in turn reduces N2 to NH3. Depending on the availability of metal cofactor, three types of N fixing systems have been identified (1) Mo-nitroge-nase (2) V-nitrogenase and (3) Fe-nitrogenase. Complexity of nitrogen fixation can be clearly understood by the contribution of several gene clusters for (1) nitrogen fixation (nifHDK—nitrogenase, nifA, fixLJ, fixK—transcriptional regulator, nifBEN—biosynthesis of the Fe-Mo cofactor, fixABCX—electron transport chain to nitrogenase, fix-NOPQ—cytochrome oxidase, fixGHIS—copper uptake and metabolism, fdxN—ferredoxin) (2) nodulation (nodA— acyltransferase, nodB—chitooligosaccharide deacetylase, nodC—N-acetylglucosaminyltransferase, nodD—tran-scriptional regulator of common nod genes, nodIJ—Nod factors transport, nodPQ, nodX, nofEF, NOE—synthesis of Nod factors substituents, nol genes—several functions in synthesis of Nod factors substituents and secretion); and (3) other essential elements (exo—exopolyssacharide production, hup—hydrogen uptake, gln—glutamine synthase, dct—dicarboxylate transport, nfe—nodulation efficiency and competitiveness, ndv—p-1,2 glucan synthesis, pls— lipopolysaccharide production) (Laranjo et al. 2014). Another study reported the coexistence of symbiosis and pathogenicity determining genes in Rhizobium (Velazquez et al. 2005). This coexistence enables the induction of nodules depending on plant species. Although BNF is an energy expensive process, it is the only process through which the atmospheric N is converted to plant usable organic N making the greatest quantitative impact on N cycle. Legume-rhizobia (Rhizobium/Bradyrhizobium/Mes-orhizobium) symbiosis is a cheaper source of N and an effective agronomic practice ensuring adequate supply of N than the application of fertilizer-N. However, various environmental factors limit nitrogen fixation, such as soil moisture deficiency, osmotic stress, extremes of temperature, soil salinity, soil acidity, alkalinity, nutrient deficiency, overdoses of fertilizers and pesticides; since all these soil and environmental factors affect the survival and infectivity rate of rhizobia—an important driver for BNF (Zahran 1999). Recent research is focused to identify rhi-zobial strains with resistance to these environmental stresses and explore their potentiality under field conditions. Details of such rhizobia have been discussed in later parts of this review.

Nitrification is an important process in nitrogen cycle in which ammonia is converted to nitrite and nitrate by nitrifying bacteria such as Nitrosomonas and Nitrobacter.

¿1 Snrinopr

KAcsTiuiUi, ^wj ^ springer

The nitrification products are vulnerable to leaching and denitrification and an estimated 45 % of applied fertilizer is lost by leaching (Jarvis 1996) and 10-30 % by denitrification (Parker 1972). Therefore, a reduced rate or inhibition of nitrification provides enough time to plant for assimilation of fixed N. Plants also produce secondary metabolites such as phenolic acids and flavonoids for inhibiting nitrification. The natural ability of plants to suppress nitrification is not currently recognized or utilized in agricultural production (Subbarao et al. 2006). However, they have no effects on other soil microbial community. For example, it had been demonstrated that nitrification inhibitor produced by B. humidicola as root extracts were seen to inhibit nitrifying bacteria, with no adverse effects on other soil microorganisms such as Azospirillum lipofe-rum, R. leguminosarum and Azotobacter chroococcum (Gopalakrishnan et al. 2009). This work also demonstrated that, this inhibitory effect vary with the soil type. Nitrification and denitrification remain to be the only known biological processes that generate nitrous oxide (N2O), a powerful greenhouse gas contribute to global warming. Biological nitrification inhibition is seen as a major mitigation process towards global warming besides improving N recovery and N use efficiency of agricultural systems (Subbarao et al. 2012).

Phosphate solubilizers

After nitrogen, phosphorus (P) is the most limiting nutrient for plant growth. It exists in both inorganic (bound, fixed, or labile) and organic (bound) forms and the concentration depends on the parental material. The concentration had been shown to range from 140 ppm in carbonate rocks to more than 1,000 ppm in volcanic materials (Gray and Murphy 2002). Although the parent material has a strong control over the soil P status of terrestrial ecosystems (Buol and Eswaran 2000), the availability of P to plants is influenced by pH, compaction, aeration, moisture, temperature, texture and organic matter of soils, crop residues, extent of plant root systems and root exudate secretions and available soil microbes. Soil microbes help in P release to the plants that absorb only the soluble P like monobasic (H2PO4-) and dibasic (H2PO42-) forms (Bhattacharyya and Jha 2012). Although the P fertilizer provides the plants with available form of P, excessive application of them is not only expensive, but also damaging to environment.

Phosphorus accounts for about 0.2-0.8 % of the plant dry weight, but only 0.1 % of this P is available for plants from soil (Zhou et al. 1992). The soil solution remains to be the main source of P supply to plants. The P content of agricultural soil solutions are typically in the range of 0.01-3.0 mg P L- representing a small portion of plant needs. The rest must be obtained from the solid phase

through intervention of biotic and abiotic processes where the phosphate solubilizing activity of the microbes has a role to play (Sharma et al. 2013). Rhizobia, including R. leguminosarum, R. meliloti, M. mediterraneum, Brady-rhizobium sp. and B. japonicum (Afzal and Bano 2008; Egamberdiyeva et al. 2004; Rodrigues et al. 2006; Vessey 2003) are the potential P solubilizers. These bacteria synthesize low molecular organic acids which acts on inorganic phosphorous. For instance, 2-ketogluconic acid with a phosphate-solubilizing ability has been identified in R. leguminosarum (Halder et al. 1990) and R. meliloti (Halder and Chakrabarty 1993). Sometimes mineralization of organic P takes place by several enzymes of microbial origin, such as acid phosphatases (Abd-Alla 1994a, b), phosphohydrolases (Giigi et al. 1991), phytase (Glick 2012; Richardson and Hadobas 1997), phosphonoacetate hydro-lase (McGrath et al. 1998), D-a-glycerophosphatase (Skrary and Cameron 1998) and C-P lyase (Ohtake et al. 1996). Some bacterial strains are found to possess both solubilization and mineralization capacity (Tao et al. 2008). Importance of this P solubilizing capacity in enhancing plant growth by M. mediterraneum has been demonstrated in chickpea and barley plants (Peix et al. 2001).

Siderophore formation

Iron, a typical essential plant micronutrient, is present in soils ranging from 0.2 to 55 % (20,000-550,000 mg/kg) with a significantly different spatial distribution. Iron can occur in either the divalent (ferrous or Fe2?) or trivalent (ferric or Fe3?) states which is determined by the pH and Eh (redox potential) of the soil and the availability of other minerals (e.g., sulphur is required to produce FeS2 or pyrite) (Bodek et al. 1988). Under aerobic environments, iron exists as insoluble hydroxides and oxyhydroxides, which are not accessible to both plants and microbes (Rajkumar et al. 2010). Generally bacteria have the ability to synthesis low molecular weight compounds termed as siderophores capable of sequestering Fe3?. These sidero-phores are known to have high affinity for Fe3?, and thus makes the iron available for plants. The siderophores are water soluble and are of two types viz. extracellular and intracellular. Fe3? ions are reduced to Fe2? and released into the cells by gram positive and negative rhizobacteria. This reduction results in destruction/recycling of sidero-phores (Rajkumar et al. 2010). Siderophores can also form stable complex with heavy metals such as Al, Cd, Cu etc. and with radionucleides including and U and NP (Neubauer et al. 2000). Thus, the siderophore producing bacteria can relieve plants from heavy metal stress and assist in iron uptake. Rhizobial species, such as R. meliloti, R. tropici, R. leguminosarum bv. viciae, R. leguminosarum bv. trifolii,

R. leguminosarum bv. phaseoli, S. meliloti and Brady-rhizobium sp. are known to produce siderophores (Antoun et al. 1998; Arora et al. 2001; Carson et al. 2000; Chabot et al. 1996).

Phytohormone production

Phytohormones are the substances that stimulate plant growth at lower/equal to micromolar concentrations. These include indole-3-acetic (IAA) acid (auxin), cytokinins, gibberellins and abscisic acid.

Indole-3-acetic acid (IAA)—IAA is the foremost phy-tohormone that accelerates plant growth and development by improving root/shoot growth and seedling vigor. IAA is involved in cell division, differentiation and vascular bundle formation and an essential hormone for nodule formation. It has been estimated that 80 % of bacteria isolated from the rhizosphere can produce IAA (Patten and Glick 1996). The salient ones are A. caulinodans, B.ja-ponicum, B. elkanii, M. loti, R. japonicum, R. leguminosarum, R. lupine, R. meliloti, R. phaseoli, R. trifolii and Sinorhizobium spp. (Afzal and Bano 2008; Antoun et al. 1998; Biswas et al. 2000; Boiero et al. 2007; Chi et al. 2010; Chandra et al. 2007; Dazzo et al. 2005; Naidu et al. 2004; Senthilkumar et al. 2009; Yanni et al. 2001; Weyens et al. 2009). IAA production in rhizobium takes place via indole-3-pyruvic acid and indole-3-acetic aldehyde pathway. On inoculation of R. leguminosarum bv. viciae, 60-fold increase in IAA was observed in the nodules of vetch roots (Camerini et al. 2008). One of the highest productions of IAA had been reported with the inoculation with B. japonicum-SB1 with B. thuringiensis—KR1 (Mishra et al. 2009). Co-inoculating Pseudomonas with R. galegae bv. orientalis had shown to produce IAA that had contributed to increases in nodule number, shoot and root growth and nitrogen content. Both environmental stress factors (acidic pH, osmatic and matrix stress and carbon limitation) and genetic factors (auxin biosynthesis genes and the mode of expression) were shown to influence the biosynthesis of IAA (Spaepen et al. 2007; Spaepen and Vanderleyden 2011).

Cytokinins—Cytokinin stimulates plant cell division and in some instances root development and root hair formation (Frankenberger and Arshad 1995). It is documented that 90 % of rhizospheric microorganisms are capable of releasing cytokinins and about 30 growth-promoting compounds of the cytokinin group has been identified from microbial origin (Nieto and Frankenberger 1990, 1991). Rhizobium strains are also reported as the potent producers of cytokinins (Caba et al. 2000; Senthilkumar et al. 2009).

Gibberellins—Gibberellins, the plant hormones responsible for stem elongation and leaf expansion has been denoted as GA1 to GA89 depending on the

approximate order of their discovery. It is also believed that certain types of dwarfness are due to gibberellin deficiency, but it has no effect on roots. Application of gibberellins is known to promote bolting of the plants, parthenocarpy in fruits, increase fruit size and number of buds and break down the tuber dormancy. Gibberellins also help in seed germination as in the case of lettuce and cereals and control flowering and sex expression of flowers. Many PGP microbes are reported to produce gibberellins (Dobbelaere et al. 2003; Frankenberger and Arshad 1995) including Rhizobium, S. meliloti (Boiero et al. 2007).

Abscisic acid—Abscisic acid in plants is synthesized partially in the chloroplasts and the whole biosynthesis primarily occurs in the leaves. The production of abscisic acid is accentuated by stresses such as water deficit and freezing temperatures. It is believed that biosynthesis occurs indirectly through the production of carotenoids. The transport of abscisic acid can occur in both xylem and phloem tissues and can also be translocated through paranchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins (Walton and Li 1995). Abscisic acid was reported to stimulate the stomatal closure, inhibit shoot growth while not affecting or even promoting root growth, induce seeds to store proteins and in dormancy, induce gene transcription for proteinase inhibitors and thereby provide pathogen defense and counteract with gibberellins (Davies 1995; Mauseth 1991). Rhizobium sp. and B. japonicum had been reported to produce abscisic acid (Boiero et al. 2007; Dobbelaere et al. 2003).

1-aminocyclopropane-1-carboxylic acid (ACC) deaminase ACC deaminase is a member of a large group of enzyme that utilizes vitamin B6 and considered to be under tryptophan synthase family. Rhizobia has the ability to uptake ACC and convert it into a-ketobutyrate and NH3. This is used as a source of carbon and nitrogen. Hence, on inoculation of rhizobia producing ACC deaminase, the plant ethylene levels are lowered and result in longer roots providing relief from stresses, such as heavy metals, pathogens, drought, radiation, salinity, etc. Strains, such as R. leguminosarum bv. viciae, R. he-dysari, R. japonicum, R. gallicum, B. japonicum, B. elk-ani, M. loti and S. meliloti had been known to produce ACC deaminase (Duan et al. 2009; Hafeez et al. 2008; Kaneko et al. 2000; Ma et al. 2003a, b, 2004; Madhaiyan et al. 2006; Okazaki et al. 2004; Sullivan et al. 2002; Uchiumi et al. 2004). IAA producing bacteria are reported to produce high levels of ACC and known to inhibit ethylene levels (Glick 2014). Inoculation with these bacteria had shown to promote root elongation, shoot growth, enhanced rhizobial nodulation and minerals uptake (Glick 2012). It had also been shown that the

¿1 Snrinopr

KAcsTiuiUi, ^wj ^ springer

rhizobia producing ACC deaminase are also efficient nitrogen fixers. The structural gene of ACC deaminase (acds) in Mesorhizobium sp. is under the control of nif promoter (Nascimento et al. 2012) which generally controls the nif gene responsible for nitrogen fixation.

Indirect growth promotions

There are many indirect ways through which rhizobia act as plant growth promoters with their biocontrol properties and induction of systemic resistance against phytopathogens and insect pests. PGP organisms have the ability to produce many active principles forbiocontrol of various phytopathogens with antibiosis production. This includes (1) production of antibiotics such as 2,4-diacyetyl phloroglucinol (DAPG), kanos-amine, phenazine-1-carboxylic acid, pyoluteorin, neomcycin A, pyrrolnitrin, pyocyanin and viscosinamide. Among them, DAPG is important since it has a broad spectrum antibacterial, antifungal and antihelminthic activity; (2) secretion of sid-erophores enabling iron uptake depriving the fungal pathogens in the vicinity; (3) production of low molecular weight metabolites such as hydrocyanic acid (HCN) which inhibits electron transport and hence disruption of energy supply to the cells; (4) production of lytic enzymes such as chitinase, p-1,3 glucanase, protease and lipase which lyse the pathogenic fungal and bacterial cell walls; (5) successfully competes for nutrients against phytopathogens and thereby occupies the colonizing site on root surface and other plant parts and (6) induces systemic resistance in plants by any of the metabolites mentioned above or by the inducting the production of phenyl alanine lyase, antioxidant enzymes such as peroxidase, polyphenol oxidase, superoxide dismutase, catalase, lipoxygenase and ascorbate peroxidase and also by phytoalexins and phenolic compounds in plant cells (Reddy 2013).

Biocontrol abilities of rhizobia

Biocontrol is a process through which a living organism limits the growth or propagation of undesired organisms or pathogens. Several rhizobial strains are reported to have the biocontrol properties. Hence, usage of these strains against soil borne pathogens can lead to potential control. The mechanisms of biocontrol by rhizobia include, competition for nutrients (Arora et al. 2001), production of antibiotics (Bardin et al. 2004; Chandra et al. 2007; Deshwal et al. 2003a), production of enzymes to degrade cell walls (Ozkoc and Deliveli 2001) and production of siderophores (Carson et al. 2000; Deshwal et al. 2003b). The production of metabolites such as HCN, phenazines, pyrrolnitrin, vi-scoinamide and tensin by rhizobia are also reported as other mechanisms (Bhattacharyya and Jha 2012). For example, the strains including R. leguminosarum bv. tri-folii, R. leguminosarum bv. viciae, R. meliloti, R. trifolii,

¿1 Snrinopr

KAcsToAoUiq rnqiiU ^ springer

S. meliloti and B. japonicum have been reported to secrete antibiotics and cell-wall degrading enzymes that can inhibit the phytopathogens (Bardin et al. 2004; Chandra et al. 2007; Ozkoc and Deliveli 2001; Shaukat and Siddqui 2003; Siddiqui and Mahmood 2001; Siddiqui et al. 1998, 2000). Rhizobial strains also compete for nutrients by displacing the pathogens. Rhizobia starve the pathogens of available iron by producing high affinity siderophores and thereby limit the growth of the pathogen (Arora et al. 2001). A study on colonization behavior of P. fluorescens and S. meliloti in alfalfa rhizosphere had sufficiently demonstrated the usage of biocontrol agents to suppress pathogens (Villacieros et al. 2003).

Pathogens that infect okra and sunflower, such as Macrophomina phaseolina, Rhizoctonia solani and Fusarium solani were shown to be controlled with the usage of B. japonicum, R. meliloti and R. leguminosarum (Ehteshamul-Haque and Ghaffar 1993; Ozkoc and Deliveli 2001; Shaukat and Siddqui 2003). Some more examples are cyst nematode of potato controlled by R. etli strain G12 (Reitz et al. 2000), Pythium root rot of sugar beet by R. leguminosarum viciae (Bardin et al. 2004) white rot disease in Brassica campestris by M. loti and sheath blight of rice by R. leguminosarum bv. phaseoli strain RRE6 and bv. Trifolii strain ANU843 (Mishra et al. 2006; Chandra et al. 2007). Xanthomonas maltophilia in combination with Mesorhizobium had been shown to enhance plant growth and productivity in chickpea. This was also been shown to enhance nodule number, nodule biomass and nodule occupancy (Pathak et al. 2007). The incidence of collar rot in chickpea was also shown to reduce by Pseudomonas sp. CDB 35 and BWB 21 when co-inoculated with Rhizobium sp. IC59 and IC76 (Sindhu and Dadarwal 2001). Brady-rhizobium sp. had been shown to control the infection of M. phaseolina in peanut, while enhancing seed germination, nodule number and grain yield (Deshwal et al. (2003b). The use of R. leguminosarum RPN5, B. subtilis sBPR7 and Pseudomonas sp. PPR8, isolated from root nodules and rhizosphere of common bean, were shown to be successful against M. phaseolina, F. oxysporum, F. solani, Sclerotinia sclerotiorum, R. solani and Colletotrichum sp. as dual culture or as cell free culture filtrate (Kumar 2012).

Induction of plant resistance

The use of PGP strains are reported to trigger the resistance of plants against pathogens. This phenomenon is referred as induced systemic resistance (ISR). In this process, a signal is generated involving jasmonate or ethylene pathway and thus inducing the host plant's defense response. Various rhizobial species are reported to induce systemic resistance in plants by producing bio-stimulatory agents including R. etli, R. leguminosarum bv. phaseoli and

Table 1 Rhizobia and their ability in plant growth promotion (Modified from Ahmed and Kibert, 2014)

Rhizobia Growth promoting

substances produced



Rhizobium sp.

Siderophores, P-solubilization, IAA, HCN

Growth hormones, IAA, siderophores, HCN, ammonia, exo-polysaccharides

R. phaseoli IAA

R. ciceri Siderophores

R. meliloti Siderophores

R. Cytokinin


Bradyrhizobium Siderophores, IAA,

HCN, P-solubilization

Bradyrhizobium IAA, HCN, ammonia, sp. siderophores, exo-


B. japonicum IAA, Siderophores

Mesorhizobium IAA, Siderophores, sp. HCN, Ammonia, exo-

polysaccharides, antifungal activity,

M. ciceri

IAA, Siderophores

Abd-Alla (1994a, b), Antoun et al. (1998), Deshwal et al. (2003a), Duhan et al. (1998), Khan et al. (2002), Tank and Saraf (2010)

Ahemad and Khan (2009a, 2010a, b, c, 2011a, b, c, 2012a), Joseph et al. (2007), Wani et al.(2007b), Zafar-ul-Hye et al. (2013) Arora et al. (2001), Berraho et al. (1997), Noel et al. (1996), Prabha et al. (2013), Zahir et al. (2010)

Abd-Alla (1994a), Antoun et al. (1998), Deshwal et al. (2003b), Duhan et al. (1998) Ahemad and Khan (2011c, d, e, 2012b), Khan et al. (2002), Wani et al. (2007a) Shaharoona et al. (2006), Wittenberg et al. (1996)

Ahemad and Khan (2009b, 2010d, e, 2012c), Ahmad et al. (2008), Khan et al. (2002), Wani et al. (2008a) Wani et al. (2007c)

R. leguminosarum bv. trifolii (Mishra et al. 2006; Peng et al. 2002; Singh et al. 2006; Yanni et al. 2001). Even individual cellular components of the rhizobium had been shown to induce ISR viz. lipopolysaccharides, flagella, cyclic lipopeptides, homoserine lactones, acetoin and butanediol (Lugtenberg and Kamilova 2009).

Abiotic stress resistance of rhizobia

The best option for developing stress tolerant crops with minimized production costs and environmental hazards can be the use of PGP microbes as stress relievers. Among

them, indigenous and native microbes were more effective and competitive as they are well adapted to the local environments (Mrabet et al. 2005). Rhizobia when used as microbial inoculants have shown many direct and indirect PGP properties including traits for stress are represented in Tables 1 and 2. Rhizobia having some key tolerance mechanism/pathways against certain stress factor such as abiotic stresses, heavy metals and pesticides are required as these are the major constraints for sustainable agriculture. These mechanisms help rhizobia to execute their beneficial PGP traits under stress conditions. The following are some of the resistance mechanisms adopted by rhizobia for their survival and PGP traits for plants under stress conditions.

Abiotic stresses, such as drought, extremes of temperature, soil salinity, acidity, alkalinity and heavy metals causes severe yield loss. The response of legumes to various stresses depends on the host plant reaction, but this reaction can be influenced by the rhizobia and the process of symbiosis (Yang et al. 2009). The role of microorganisms in adaptation of crops to various abiotic stresses is reviewed by Grover et al. (2010). There are comprehensive reviews on tolerance and nodulating capacity of Rhizobium and Bradyrhizobium to soil acidity, salinity, alkalinity, temperature and osmotic stress conditions (Graham 1992; Kulkarni and Nautiyal 2000). A list of related studies that are in Table 2.

Extremes of temperature

Global warming and the resultant climate change are expected to cause land degradation with salinization, increase the drought episodes and desertification (USDA 2012). High temperatures lead to increased drought intensity, due to enhanced transpirational water loss. This can lead to reduction in nodule number, rhizobial growth, rate of colonization and infectious events, and can lead to delay in nodulation or restrict the nodule to the subsurface region. This phenomena was observed in alfalfa plants grown in a desert environment of California (USA) forming fewer nodules in the top 5 cm soil horizon, while extensively nodulating below this depth (Munns et al. 1979).

The optimum temperature for rhizobial growth is 28-31 °C, while many of them are unable to grow beyond 37 °C. Rhizobia isolated from hot and dry environments of the Sahel Savannah are reported to tolerate temperature up to 45 °C, but they were found to lose their infectiveness (Eaglesham and Ayanaba 1984; Hartel and Alexander 1984; Karanja and Wood 1988). Similarly, a heat treatment of 35 and 37 °C to R. phaseoli was found to cause loss of melanin synthesis plasmid DNA and symbiotic properties (Beltra et al. 1988). In contrast, at 35 and 38 °C, R. legu-minosarum bv. phaseoli was found to be infective and

¿1 Snrinopr

KAcsTöojüUig ^laU ^ springer

Table 2 Attributes of rhizobia exerted against abiotic stress on host plants/in vitro


Crop species

Screening medium

Growth condition



Drought stress

R. tropici co-inoculated with Paenibacillus polymyxa

Bradyrhizobium sp.

Kidney bean

-7, -70 and <-85 kPa

PEG 6000 induced


Enhanced plant height, shoot dry weight and nodule number

In vitro and pot Enhanced culture drought

tolerance, and EPS

M. mediterraneum LILM10


Field study

R. elti (engineered for enhanced trehalose-6-phosphate synthase)

Kidney bean

Pot studies

Temperature stress Mesorhizobium spp.

20, 28 and 37 °C

In vitro

Rhizobium sp. DDSS69

Heat shock at 60 °C for 15 min; 46 °C for 3 h

In vitro

In vitro

Figueiredo et al. (2008)

Uma et al. (2013)

production; nodulation, nodule ARA, nodule N

Increased nodule

number, shoot dry weight and grain yield

Water deficient tolerant strains were also NaCl tolerant.

Enhanced nodules, nitrogenase activity and biomass production

Higher tolerance than wild type strains

Overproduction of 60 kDa protein by all the isolates

All the isolates revealed more tolerance to 20 °C than 37 °C.

Variations in the expression of protein profile

Induction of 135 and 119 kDa proteins

Variation in the protein profile of stressed and non-stressed cells

Romdhane et al. (2009)

Suarez et al. (2008)

Rodrigues et al. (2006)

Sardesai and Babu (2001)

Salt/osmotic stress

jjjilliir dllall iüja» c •

KAcsTäLLöUiq rnqjiU ^ springer

Table 2 continued


Crop species

Screening medium

Growth condition



M. ciceri ch-191

Chickpea -salt resistant 50, 75, 100 mM and sensitive cultivars

In vitro

M. ciceri ch-191

100-400 mmol NaCl/L In vitro

Acacia rhizobia. (40 strains)

0.4-1.4 M NaCl

In vitro

M. ciceri, M. mediterraneum and S. medicae


25 mM NaCl


Rhizobia strains


5.5 Ds m

Field study

Decreased plant dry weight, nitrogenase activity in sensitive cultivars Less N2 fixation inhibition, higher root to shoot ratio, normalized nodule weight and shoot K/Na ratio and reduced foliar accumulation of Na + in resistant cultivars Higher tolerance was noticed on 200 mmol/L Altered protein and LPS levels Higher proline accumulation than glutamate Presence of small and large plasmids Intracellular accumulation of free glutamate

Three rhizobia strains has tolerated 1.4 M NaCl M. ciceri enhanced the nodulation and CAT activity

Least decrease in nodule protein and SOD activity Increased plant biomass, nodule number and nodule dry weight

Tejera et al. (2006)

Soussi et al. (2001)

Gal and Choi (2003)

Mhadhbi et al. (2004)

Islam et al. (2013)

1 Snrinapr

KACSTöajüUIg (D^JilJ ^ Springer

Table 2 continued

Rhizobia Crop species Screening medium Growth Remarks References


M. ciceri, M. mediterraneum


Mannitol—50 mM induced

Aerated hydroponic cultures

pH stress

Mesorhizobium spp.

5, 7 and 9

In vitro

Maintenance of Mhadhbi et al.

growth and nitrogen fixing activity

Increased antioxidant enzyme activity in nodules

Large range of isolate variation in growth at pH 7/9 and others at pH 5/7


Rodrigues et al. (2006)

formed nodules in P. vulgaris, but these nodules were found to remain ineffective (Hungria and Franco 1993). Nevertheless, heat-tolerant, actively nodulating and N2 fixing Rhizobium strains have been identified. Adaptation of microorganisms to stress is a complex regulatory process, as it involves the use of proteins and lipopolysac-charide (LPS) with the up-regulation of an array of genes. Upon exposing the wild and heat resistant Rhizobium sp. to 30 and 43 °C, changes in the cell surface including extracellular polymeric substances/exo polysaccharides (EPS), LPS and proteins had been demonstrated (Nandal et al. 2005). Michiels et al. (1994) reported first about the presence of a large set of small heat shock proteins in Rhi-zobium sp. They observed the expression of eight heat shock proteins in heat-sensitive strains, where as it was only two in the heat-resistant strains, which indicates that the heat shock proteins also play key roles in normal cell growth. Munchbach et al. (1999) reported 12 small heat shock proteins in B. japonicum and classified them into Class A—sHsp similar to Escherichia coli IbpA and IbpB, and Class B—sHsps similar to sHsps from other prokary-otes and eukaryotes. Among the sHsp family, 13 genes for small heat shock proteins was detected on B. japonicum (Han et al. 2008). Among these regulatory systems, chap-erones such as DnaK-DnaJ and GroEL-GroES are the key components of heat shock or stress response. These chap-erones help on hydrophobic domains of the target protein to regain their native structure since they get denatured upon stress (Hartl and Hayer-Hartl 2009). Alexandre and Oliveira (2011) reported 53 strains of Mesorhizobium sp. for heat stress and shock protein production by chaperone analysis, which revealed the increased transcripts of dnaK and groESL. They also observed a higher induction of chaperone genes in heat-tolerant isolates than in heat-

¿1 Snrinopr

kacstolloUIq rnqjiU ^ springer

sensitive isolates of the same species, however, such phenomenon was not found to express during cold stress conditions. This indicates that although the chaperones are heat shock proteins, their gene expression is stress specific.

Plants have various mechanisms for drought tolerance including drought-escapism, dehydration postponement and dehydration tolerance (Turner et al. 2000). Plants generally overexpress zeatin for delayed leaf senescence as a drought tolerance mechanism. Alfalfa plants inoculated with engineered strains of S. meliloti with ipt gene showed elevated zeatin concentrations and antioxidant enzymes in their leaves and survived better under severe drought conditions (Xu et al. 2012). Vanderlinde et al. (2010) noticed the production of EPS as another tolerance mechanism. Intra species difference in competitive efficiency was demonstrated by Krasova-Wade et al. (2006) in which Bradyrhizobium ORS 3257 was found to compete their best under favorable water conditions while Bradyrhizobium ORS 3260 was the best under limited water conditions.


Soil salinity is one of the production constraints in the arid and semi-arid tropics world-wide, and about 40 % of the world's land surface is affected by salinity-related problems (Zhan et al. 1991). Salinity decreases the nutrition uptake of plants, particularly P, due to their binding with Ca ions in salt-stressed soils. It is also known that higher concentration of ions (Na?, Cl-, SO4 -) in saline soils gets accumulated in the plant cells and inactivate enzymes that inhibits protein synthesis and photosynthesis (Serraj et al. 1994; Zhu 2001). Salinity affects bacterial infection process (by decreasing the number and the deformation of root hairs), nodule growth and functioning (by limiting the

nutrient supply via photosynthesis products and oxygen consumption) and BNF (by reducing the nodule metabolism, leghemoglobin content and atmospheric nitrogen diffusion).

Rhizobial species are known to vary in their salt sensitivity. Some of them are categorized as salt tolerant, such as R. meliloti (Zhang et al. 1991), R. fredii (Yelton et al. 1983), Rhizobium sp. from Acacia senegal, Prosopis chil-ensis (Zahran et al. 1994) and Vigna unguiculata (Mpe-pereki and Makoneses 1997), chickpea, soybean (El Sheikh and Wood 1990), and pigeonpea (Subbarao et al. 1990) whereas others as salt sensitive such as R. leguminosarum (Chein et al. 1992). The existence of a high degree of phenotypic and genotypic diversity in Sinorhizobium populations sampled from marginal soils of arid and semi-arid regions of Morocco has been demonstrated recently (Thami-Alami et al. 2010). It was also observed that these salt tolerant isolates also turned out to be water stress tolerant. The effect of salt stress on halotolerant rhizobia by their LPS (Lloret et al. 1995; Zahran et al. 1994), protein profiles (Saxena et al. 1996) and exopolysaccharide (Lloret et al. 1998) have been studied. Large variability in the efficiency of host plant and rhizobial strains on BNF under salinity had been reported (Jebara et al. 2001; Aouani et al. 1998).

Salinity generates negative osmotic potential that lowers the soil water potential. Similar to plants, rhizobia also produce many group of metabolites called compatible solutes [trehalose, N-acetylglutaminylglutamine amide (NAGGN) and glutamate], osmoprotectants [betaine, gly-cine-betaine, proline-betaine, glucans, trehalose, sucrose, ectoine, 3-dimethylsulfoniopropionate (3-dimethylpropio-thetin or DMSP), 2-dimethylsulfonioacetate (2-dimethyl-thetin or DMSA)] and pipecolic acid and cations [calcium, potassium] as tolerance mechanism (Chen 2011; Streeter 2003; Sugawara et al. 2010). Salt tolerance mechanisms involve several gene families which have been reported largely in S. meliloti followed by R. etli, R. tropici, Rhi-zobium sp., S. fredii and B. japonicum. The identified gene families includes betaine (betS/betABCI/hutWXV) (Boscari et al. 2002), glycine-betaine (AraC) and proline-betaine (betS/prb) (Boscari et al. 2004; Payakapong et al. 2006; Alloing et al. 2006), glucans (ndvABCD), sucrose (zwf) and trehalose (zwf) (Chen et al. 2002; Jenson et al. 2002; Barra et al. 2003), cation efflux (phaA2/phaD2/phaF2/phaG2) (Jiang et al. 2004) and rpoH2 (Tittabutr et al. 2006), ntrY, ntrX, greA, alaS, dnaJ, nifS, noeJ, kup (Nogales et al. 2002), omp 10, relA, greA and nuoL (Wei et al. 2004). Osmolyte production depends not only on type of stress, but also on degree of stress. It was reported in S. meliloti that at lower level of salt concentration glutamate accumulates; at higher levels, glutamate and NAGGN accumulates, whereas at extremely higher concentrations, all

the three osmolytes, glutamate, NAGGN, and trehalose accumulates (Smith et al. 1994). Osmoprotectants, the compatible solutes/osmolytes, also play a dual role as evidenced in S. meliloti by proline-betaine which serves as both osmoprotectant (under high osmotic stress) and energy source (under low osmotic stress) (Miller-Williams et al. 2006).

Soil acidity

Soil acidity (low pH) is yet another abiotic stress that affects plant growth and cause crop failures which might be due to high concentration of protons and low concentration of calcium and phosphate in acidic soils. Low survival and poor growth of rhizobia and inhibition of initiation and formation of root nodules are the important responses that lead to the failure of rhizobia-legume symbiosis in acid soils (Richardson et al. 1988). The addition of lime on acid soils has been followed as a common practice to raise the soil pH creating a favorable conditions for the growth and survival of root nodule bacteria (Watkin et al. 1997). Graham et al. (1994) proposed some strains of Rhizobium, Azorhizobium and Bradyrhizobium to be low pH tolerant. Tolerance to acidity by rhizobia was correlated with the production of extracellular polysaccharide or polyamines/ glutamate concentration in the cell. Muglia et al. (2007) highlighted the role of glutathione, a tripeptide for the growth of R. tropici under low pH conditions. Watkin et al. (2003) reported the ability of acid tolerant R. legumin-osarum bv. trifolii in accumulating higher level of potassium and phosphorous than an acid sensitive strain. The effect of acid shock on S. meliloti 1021 was analyzed via oligo-based whole genome microarrays which demonstrated that within 20 min of the shock, the cells had started to respond by either up-regulating or down-regulating the specific genes of various cellular functions or hypothetical proteins of unknown functions (Hellweg et al. 2009). In a re-vegetation program on acidic soils, Bradyrhizobium sp. was found to enhance the nodule number and plant growth when six shrubby legumes, such as Cytisus balansae, C. multiflorus, C. scoparius, C. striatus, Genista hystrix and Retama sphaerocarpa, were inoculated with (Rodríguez-Echevarría and Perez-Fernandez 2005).

Heavy metal resistance of rhizobia

Pollution of the biosphere by the toxic metals had increased dramatically since the beginning of industrial revolution by the dumping of solid wastes and the use of industrial waste waters for irrigation. Ever increasing demand for lands, forced the farmers to use contaminated sites for crop cultivation. Heavy metals are the key pollutants causing serious illness to plants, ecosystem and humans by their

¿1 Snrinopr

KACSToaüüJlg ^WJ ^ springer

non-degradable nature. For the reclamation and removal of heavy metals, phytoremediation is suggested to be practiced as it preserves natural soil properties and microbial biomass (Gianfreda and Rao 2004). The use of microorganisms such as Bacillus sp., Pseudomonas sp., Azoto-bacter sp., Enterobacter sp., and Rhizobium sp. were also proposed to speed up the phytoremediation process had been reviewed in detail by Ma et al. (2011).

Rhizobia multiply slowly in soil until they infect a compatible host. Rapid growth of rhizobia occurs only after successful infection by a single cell and formation of a nitrogen-fixing nodule on the host-root which consists of over 108 cycles of bacterial progeny (Downie, 1997). In heavy metal contaminated sites, after the successful establishment of symbiosis with the host plant, the heavy metals tend to accumulate in the nodules. This would be an alternative and less expensive mode to (1) remove heavy metal from soil (2) enhance soil and plant health with an enhanced nitrogen fixation and other plant growth promoting pathways and (3) help to grow heavy metal-free plant components in any contaminated site contributing to food and nutritional security. However, despite demonstrating the extent of benefits through the use of PGPR in remediation of contaminated sites, there had been very few field studies while most of the successful studies are either from greenhouse or growth chambers (Lucy et al. 2004).

Effects of heavy metals on growth, abundance, morphology and physiology of various strains of R. legumin-osarum have been well documented (Castro et al. 1997; Chaudhary et al. 2004; Chaudri et al. 2000; Lakzian et al. 2002; Smith 1997). Continuous exposure to heavy metals leads the viable bacterial cells not only to transform into a non-viable form, but also adversely affects the genetic diversity and nodulation of the host plants (Paton et al. 1997; Hirsch et al. (1993). Reductions in bacterial counts of Rhizobium sp. have been reported with the increasing concentrations of heavy metals such as Cu, Zn and Pb, either sole or in combinations, and variations in the expression of symbiotic genes including nod genes (Stan et al. 2011).

Studies on effectiveness of rhizobia isolated from long-term contaminated sites (over 40 years) and un-polluted sites revealed that only 15 % of the active isolates were effective in polluted sites while 94 % of them were from un-polluted sites. A great diversity in terms of plasmid types has been observed in isolates of un-polluted soil than the isolates from polluted soils. In addition, the dominant plasmid groups present in un-polluted soils were found to be absent in isolates of polluted soils and vice versa (Castro et al. 1997). However, these negative impacts had been specific to exposure time and metal type. This hypothesis had been demonstrated by Carrasco et al. (2005), who isolated 41 heavy metal resistant isolates of rhizoibia from

¿1 Snrinopr

kacstolloUIq rnqjiU ^ springer

a total of 100 isolates in a site contaminated since 6 years by a toxic spill. Genetic diversity among the rhizobial population had also been observed through differences in size of nodC fragment, heavy metal resistance and symbiotic properties. Some strains had been observed to have a broad spectrum resistance and therefore are symbiotically effective even under combined stress conditions. Changes in physiology were found to lead to the variations in protein profiles that serve as a marker for stress response analysis in R. leguminosarum bv. viciae isolated from heavy metal polluted sites (Pereira et al. 2006a).

Similar to the non-nodulating bacterial species, rhizobia also has its own features such as EPS and LPS for influencing heavy metal resistance. EPS are biopolymers that possess negatively charged ligands which instantly form complexes with metal ions through electrostatic interactions (Liu et al. 2001; Sutherland 2001). EPS from R. etli (strain M4), isolated from an acid mine drainage, was shown to impact ecosystem near a manganese mine in Northern Australia (Foster et al. 2000; Pulsawat et al. 2003). Lakzian et al. (2002) identified that plasmids are the major contributing factor for this as highly tolerant strains were noticed to have 6-9 plasmids whereas moderately tolerant strains have only three plasmids. However, an alternate view was reported by Pereira et al. (2006b) on cadmium (Cd) resistance as he found similar number (a maximum of four) plasmids in all the tolerant, moderately tolerant and sensitive isolates. Pereira et al. (2006b) also observed that some highly tolerant strains have had no plasmids and therefore had concluded that, the heavy metal resistance may be related to the plasmids, but there could also be some other mechanisms conferring metal resistance. This study also reported that the concentration of intracellular Cd varies within the groups, where highly tolerant strains have higher quantity. Reports of Figueira et al. (2005), Purchase et al. (1997), and Purchase and Miles (2001), are also support this view.

Natural resistance is not sufficient when soils are contaminated heavily and for a longer period. Use of recombinant rhizobia could play a major role in remediation measures. Microorganisms equipped with high metal-binding capacity through metallothionins for enhancing the tolerance, sequestration of heavy metals have been widely exploited. Metallothionins (MTs) are the low-molecular weight, cysteine-rich, metal binding proteins produced by higher organisms (Kagi 1991). Sriprang et al. (2002) engineered the expression of tetrameric human MTL-4 gene under nifH and nolB promoters in M. huakuii subsp. rengei B3 that had been shown to establish symbiosis with Astragalus sinicus in Cd-polluted soils and to enhance Cd uptake by twofolds.

Phytochelatins (PCs), a naturally occurring peptide, having metal binding properties and found in a variety of

Table 3 Rhizobia and their effects on host plant/in vitro at metal stress conditions


Crop Heavy Growth condition Remarks species metal stress


R. leguminosarum

bv. trifolii NZP561 Bradyrhizobium

RM8 Rhizobium sp.

Green Ni, Zn gram

In vitro

Pot experiments

Mesorhizobium RC3 Chickpea Cr

Hyacinth Co, Cu, Pot experiments bean Zn, Cd and field conditions Pot experiments

M. metallidurans sp. -nov.

R. leguminosarum Maize S. meliloti Rhizobium RL9

Black Cu

medic Lentil

Cd, Zn In vitro Pb Pot experiments

Pot experiments Pb, Ni Pot experiments

Sequestration of Cd

Enhanced growth performance

Greater HM accumulation in nodules than in roots and shoots

Increased growth, nodulation, chlorophyll, leghaemoglobin, nitrogen content, seed protein and seed yield

Heavy metal resistance

Enhanced plant growth and biomass

Enhanced biomass production

Increased growth, nodulation, chlorophyll, leghaemoglobin, nitrogen, seed protein and seed yield

Robinson et al. (2001) Wani et al.

(2007c) Younis (2007)

Wani et al. (2008b)

Vidal et al.


Hadi and Bano

(2010) Fan et al.

(2011) Wani and Khan (2012, 2013)

plants and microorganisms (Cobbett 2000), can accumulate higher concentration of heavy metals than MTs due to their unique structural features (Mehra and Mulchandani 1995). Sriprang et al. (2003) transformed PCs gene from Arabi-dopsis thaliana (AtPCS) to M. huakuii subsp. rengei B3 strain for enhancing the adsorption of heavy metals. They found that the free-living state of the recombinant bacterium had a higher Cd accumulation while the symbiotic state had a higher accumulation of Cu and As than Cd and Zn (Ike et al. 2008).

It is necessary to isolate and study the native rhizobial strains from heavy metals contaminated soils, to identify the potential of rhizobium-legume symbiosis of particular strain for the remediation of the affected area. Such studies with their contribution are presented in Table 3. Rhizobia, such as R. fredii, R. meliloti, R. etli, R. legu-minosarum bv. viceae, R. leguminosarum bv. trifolii, Bradyrhizobium sp. and B. japonicum had been evaluated for heavy metal resistance and of which R. fredii and R. meliloti alone were found to exhibit higher metal tolerance against Tellurium (Te) and Selenium (Se) (Kinkle et al. 1994). Nonnoi et al. (2012) demonstrated differences in the heavy metal resistance spectrum of S. medi-cae and R. leguminosarum bv. trifolii strains isolated from mercury-contaminated soils. Heavy metals are reported to cause harm not only to benefiting microbes, but also to host plants. Paudyal et al. (2007) reported the negative effect of heavy metals such as Al, Fe and Mo on two Rhizobium strains and their symbiotic efficiency on host

plants. Chaudri et al. (2000) observed greatly reduced symbiosis of R. leguminosarum bv. viciae with pea and R. leguminosarum bv. trifolii with white clover under Zn toxicity as a consequence of reduced numbers of free-living rhizobia in the soil indirectly affecting N fixation and Zn phytotoxicity. Severe yellowing of plants, small leaves, lack of nodules and reduced rhizobial counts has also been observed as the symptoms of heavy metal tox-icity in these toxicity affected plants.

Besides nitrogen fixation and heavy metal resistance, some rhizobia exhibit PGP traits under contaminated conditions as reported in soybean cv. Curringa and its rhizobial symbiont B. japonicum at higher arsenic (As) concentrations (Reichman 2007). Guo and Chi (2014) reported cadmium (Cd) tolerant Bradyrhizobium sp. to exhibit several PGP traits including synthesis of IAA, ACC deaminase, siderophores, increased shoot dry weights and high level accumulation of Cd in roots of Lolium multiflorum than in un-inoculated control. They also reported that the strain enhanced the extractable Cd concentrations in the rhizosphere, whereas it decreased the Cd accumulation in root and shoot of G. max by increasing Fe availability.

Huang et al. (2005) reported developing a multi-process phytoremediation system (MPPS) for petroleum hydrocarbons. This employs the use of both PGP bacteria and specific contaminant-degrading bacteria which metabolize the contaminants into non-toxic substances/readily available compounds while the role of PGP bacteria is still

¿1 Snrinopr

KACSTöojüUIg ^WJ ^ springer

prompting plant growth and increasing the plant tolerance to pollutants. MPPS has also the potential for deployment to enhance rhizobium-host symbiosis and plant growth at heavy metal contaminated sites.

Pesticide tolerance of rhizobia

Pesticide accumulation in soils beyond the recommended level occurs either by consistently repeated application or their slow degradation rate. It affects plant growth by altering plant root's architecture, number of root sites for rhizobial infection, transformation of ammonia into nitrates, transformation of microbial compounds to plants and vice versa. Besides this, growth and activity of free-living or endophytic nitrogen fixing bacteria has also been affected (Mathur 1999). Several studies have documented the effects of various pesticides on the reduction of microbial diversity and density on various soil types (El Abyad and Abou-Taleb 1985; Moorma 1988; Martinez-Toledo et al. 1996). Numerous microorganisms have the capacity to degrade the pesticides by the action of degra-dative genes in plasmids/transposons/chromosomes (Kumar et al. 1996). The influence of broad-spectrum of pesticides on the functional attributes of rhizobia and their tolerance to pesticides are reported in Table 4. From the literature survey, it was recognized that none of the rhi-zobia are found to have pesticide tolerance under field conditions. So research on isolating, identifying and characterizing such resistant rhizobia needs to be vigorously pursued as such rhizobia are very much needed considering the quantum of pesticide residue generated currently.

Synergistic effects of rhizobial co-inoculation

The in-consistency of beneficial results of microbial use, when single microbe was used in the field application, have brought an emphasis on co-inoculation of microbes (Bashan and de Bashan 2005). Certain specific co-inoculation causes synergy by functioning as helper bacteria to improve the performance of the other bacteria. Therefore in such co-inoculations, the combination of PGP bacteria, rhizobia and the host genotype has to be selected after extensively careful evaluations (Remans et al. 2007, 2008). A range of PGP microbes can be used with rhizobium that not only improves legume growth and yield but also cost effective and efficient.

Azospirillum, a free living diazotroph, Azotobacter, Bacillus, Psuedomonas, Serretia, and Enterobacter are some of the genera that are successfully used with rhizobium as co-inoculants. Azospirillum, was found to enhance growth and yield of several leguminous crops upon inoculation (Roseline et al. 2008). Improved nodulation was found when

A. lipoferum and R. leguminosarum bv. trifolii were co-inoculated in white clovers (Tchebotar et al. 1998), pigeon-pea and chickpea (Deanand et al. 2002). It was found that Azospirillum can increase the infection site providing a space for Rhizobium resulting in higher nodule formation (Tche-bolas et al. 1988). Co-inoculation with Azospirillum and Rhizobium were shown to increase phytohormones, vitamins and siderophore production (Cassan et al. 2009; Dardanelli et al. 2008). Co-inoculation of common bean with Azospir-illum-Rhizobium was also shown to increase the fixed nitrogen quantity (Reman et al. 2008). Azotobacter was found to be a potential co-inoculant with rhizobium that enhanced the production of phytohormones and vitamins and increase the nodulation (Akhtar et al. 2012; Chandra and Pareek 2002; Dashadi et al. 2011; Qureshi et al. 2009). Bacillus sp. was also been reported to induce the PGP ability, yield (Mishra et al. 2009; Ahmad et al. 2008) and uptake of nutrients (Stajkovic et al. 2011) including phosphorous (Singh et al. 2011). Significant increase in weight of the root and seed yield of chickpea were reported upon inoculation of Rhizobium with B. subtilis OSU-142 and B. megaterium M-3 (Elkoca et al. 2008). Enhanced nodulation and nitrogen fixation was noticed upon inoculation of Bacillus and Azo-spirillum sp. along with rhizobial inoculants in pigeonpea (Rajendran et al. 2008; Roseline et al. 2008). Interaction between Streptomyces lydius WYEC108 and Rhizobium of pea were shown to promote growth of the plant (Tokala et al. 2002) including nodule number and growth, probably by the root and nodule colonization of Streptomyces. Enterobacter is another most abundant PGP bacteria that increased the yield of nodules on green gram when co-inoculated with Bradyrhizobium sp. (Gupta et al. 1998). When R. tropici CIAT899 was co-inoculated with C. balustinum Aur9 it resulted in increased root hair formation and infection sites leading to early nodule development and increased nodule formation (Estevez et al. 2009). Similar result was obtained when Medicago truncatula cv. Caliph was co-inoculated with Pseudomonas fluorescens WSM3457 and Ensifer (Sinorhizobium) medicae WSM419 (Fox et al. 2011). Recently, it was found that nodulation, root and shoot dry weight, grain and straw yield, nitrogen and phosphorus uptake were significantly increased in chickpea upon co-inoculation with Mesorhizobium sp. and P. aeruginosa (Verma et al. 2013). Similar plant growth effects along with the antagonistic activities against F. oxysporum and R. solani has been observed on chickpea by co-inoculation of Meso-rhizobium, Azotobacter chroococcum, P. aeruginosa and Trichoderma harzianum (Verma et al. 2014). Mehboob et al. (2013) had a recent detailed review highlighting the effects of co-inoculation of rhizobia with various rhizospheric bacteria. Although there are many combinations of bacteria were explored for use, still there is a need for an advanced comprehensive research in the area.

Table 4 Rhizobia and their beneficial attributes exerted on host plant/in vitro at pesticide stress conditions


Crop species


Concentrations Condition



Rhizobium MRP1 Pea

Rhizobium MRL3 Lentil

Mesorhizobium Chickpea MRC4

Rhizobium MRP1 -Rhizobium MRL3 Mesorhizobium

MRC4 Bradyrhizobium

MRM6 R. -

leguminosarum RCR 1045


Quizalafop-p- 40, 80 and Pot

ethyl Clodinafop

Metribuzin Glyphosate

120 ig/kg soil

400, 800 and 1,200 ig/kg soil

850, 1,700 and

2,550 ig/L 1,444, 2,888 and

4,332 ig/L

Enhanced biomass, nodulation, experiments leghaemoglobin content, root and shoot N, root and shoot P, seed yield and seed protein

Ahemad and Khan (2009a, 2010b, d)

In vitro

Terbuthylazine 4, 8, 16, 32

and 64 mg/L

In vitro

Concentration-dependent progressive decline in PGP substances except exo-polysaccharides

Growth decline was in the order of Terbuthylazine > Prometryn > Simazine

Ahemad and Khan (2011a, b, 2012a, c)

Singh and Wright (2002)


Prometryn Bentazon

Insecticides Rhizobium MRL3 Chickpea, Fipronil

R. Pea,

leguminosarum Lentil

Mesorhizobium MRC4

Rhizobium MRP1 -Rhizobium MRL3 Mesorhizobium

MRC4 Bradyrhizobium MRM6

Rhizobium MRP1 -Rhizobium MRL3 Mesorhizobium

MRC4 Bradyrhizobium MRM6

Rhizobium MRP1 Pea

4.3, 8.6, 17.2, 34.4 and 68.8 mg/L 4, 8, 16, 32

and 64 mg/L 4.1, 8.2, 16.4, 32.8,

65.6 mg/L 200, 400, and Pot

No adverse effects on growth

Enhanced the biomass, nodulation,

600 mg/kg experiments leghaemoglobin content, root and shoot N,

Ahemad and Khan




Fungicides Hexaconazole


Kitazin Tebuconazole

soil 1,300, 2,600, and

3,900 mg/kg soil 100, 200 and

300 ig/L 25, 50 and 75 ig/L

40, 80 and 120 ig/L 1,500, 3,000 and

4,500 ig/L 96, 192 and 288 ig/L 100, 200 and 300 ig/L

100, 200 and 300 ig/kg soil

root and shoot P, seed yield and seed protein (2009b,

2010a, 2011f)

In vitro

Concentration-dependent progressive decline in PGP substances except exo-polysaccharides

In vitro

Concentration-dependent progressive decline in PGP substances except exo-polysaccharides

Ahemad and Khan (2011a, b, 2012a, c)

Ahemad and Khan (2011a, b, 2012a,c)

In vitro

Pot experiments

Concentration-dependent progressive decline Ahemad and in PGP substances except exo- Khan

polysaccharides, HCN and ammonia (2011d)

Enhanced the biomass, nodulation, leghaemoglobin content, root and shoot N, root and shoot P, seed yield and seed protein


KAcsTöojüUig (D^jiiJ ^ springer


Rhizosphere is a unique niche that provides habitation and nutrition to PGP microorganisms. In turn, these microorganisms produce multiple benefits of induced plant growth, defense against diseases and survival under stress with many other unknown benefits. The present review documents the potential of PGP rhizobia and highlights the unique properties of plant growth induction, defense pathways and the resistance spectrum available against various abiotic stresses on a variety of agricultural crops. However, the extent of success in realizing the benefits of PGP tends to diminish as it moves from laboratory to greenhouse and to fields, which reflects the scarcity of research on the beneficial effects of PGP microbes under field conditions. Therefore, generation of comprehensive knowledge on screening strategies and intense selection of best rhizobacterial strain for rhizosphere competence and survival is the current need to enhance the field level successes. Identification of such potential rhizo-bial strains and developing a robust technology for the use by smallholder farmers is still in its infancy. Thus, additional comprehensive research to exploit the potential of PGP rhi-zobia would provide for expansion of this research area, commercialization and improve sustainability in agricultural production.

Conflict of interest Authors hereby declare no conflict of interest.

Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.


Abd-Alla MH (1994a) Solubilization of rock phosphates by Rhizobium and Bradyrhizobium. Folia Microbiol 39:53-56 Abd-Alla MH (1994b) Use of organic phosphorus by Rhizobium leguminosarum biovar. viceae phosphatases. Biol Fertil Soils 18:216-218

Afzal A, Bano A (2008) Rhizobium and phosphate solubilizing bacteria improve the yield and phosphorus uptake in wheat (Triticum aestivum L.). Int J Agric Biol 10:85-88 Ahemad M, Khan MS (2009a) Toxicity assessment of herbicides quizalafop-p-ethyl and clodinafop towards Rhizobium pea symbiosis. Bull Environ Contam Toxicol 82:761-766 Ahemad M, Khan MS (2009b) Effect of insecticide-tolerant and plant growth promoting Mesorhizobium on the performance of chickpea grown in insecticide stressed alluvial soils. J Crop Sci Biotech 12:217-226 Ahemad M, Khan MS (2010a) Comparative toxicity of selected insecticides to pea plants and growth promotion in response to insecticide-tolerant and plant growth promoting Rhizobium leguminosarum. Crop Prot 29:325-329 Ahemad M, Khan MS (2010b) Ameliorative effects of Mesorhizobium sp. MRC4 on chickpea yield and yield components under

1 Чпп'пстрг

KAcsTöLLöUiq rnqjiU ^ springer

different doses of herbicide stress. Pestic Biochem Physiol 98:183-190

Ahemad M, Khan MS (2010c) Insecticide-tolerant and plant growth promoting Rhizobium improves the growth of lentil (Lens esculentus) in insecticide-stressed soils. Pest Manag Sci 67:423-429

Ahemad M, Khan MS (2010d) Growth promotion and protection of lentil (Lens esculenta) against herbicide stress by Rhizobium species. Ann Microbiol 60:735-745 Ahemad M, Khan MS (2010e) Improvement in the growth and symbiotic attributes of fungicide-stressed chickpea plants following plant growth promoting fungicide-tolerant Mesorhizobi-um inoculation. Afr J Basic Appl Sci 2:111-116 Ahemad M, Khan MS (2011a) Effect of pesticides on plant growth promoting traits of green gram-symbiont, Bradyrhizobium sp. strain MRM6. Bull Environ Contam Toxicol 86:384-388 Ahemad M, Khan MS (2011b) Ecotoxicological assessment of pesticides towards the plant growth promoting activities of Lentil (Lens esculentus)-specific Rhizobium sp. strain MRL3. Ecotoxicology 20:661-669 Ahemad M, Khan MS (2011c) Insecticide-tolerant and plant growth promoting Bradyrhizobium sp. (vigna) improves the growth and yield of green gram [Vigna radiata (L.) Wilczek] in insecticide stressed soils. Symbiosis 54:17-27 Ahemad M, Khan MS (2011d) Effect of tebuconazole-tolerant and plant growth promoting Rhizobium isolate MRP1 on pea-Rhizobium symbiosis. Sci Hortic 129:266-272 Ahemad M, Khan MS (2011e) Plant growth promoting fungicide tolerant Rhizobium improves growth and symbiotic characteristics of lentil (Lens esculentus) in fungicide-applied soil. J Plant Growth Regul 30:334-342 Ahemad M, Khan MS (2011f) Insecticide-tolerant and plant growth promoting Rhizobium improves the growth of lentil (Lens esculentus) in insecticide-stressed soils. Pest Manag Sci 67:423-429

Ahemad M, Khan MS (2012a) Ecological assessment of biotoxicity of pesticides towards plant growth promoting activities of pea (Pisum sativum)-specific Rhizobium sp. strain MRP1. Emir J Food Agric 24:334-343 Ahemad M, Khan MS (2012b) Productivity of green gram in tebuconazole-stressed soil, by using a tolerant and plant growth promoting Bradyrhizobium sp. MRM6 strain. Acta Physiol Plant 34:245-254

Ahemad M, Khan MS (2012c) Effects of pesticides on plant growth promoting traits of Mesorhizobium strain MRC4. J Saudi Soc Agric Sci 11:63-71 Ahmad F, Ahmad I, Khan MS (2008) Screening of free-living rhizospheric bacteria for their multiple plant growth promoting activities. Microbiol Res 163:173-181 Akhtar N, Qureshi MA, Iqbal A, Ahmad MJ, Khan KH (2012) Influence of Azotobacter and IAA on symbiotic performance of Rhizobium and yield parameters of lentil. J Agric Res 50:361-372 Alexandre A, Oliveira S (2011) Most heat-tolerant rhizobia show high induction of major chaperone genes upon stress. FEMS Micro-biol Ecol 75:28-36 Alloing G, Travers I, Sagot B, Le Rudulier D, Dupont L (2006) Proline betaine uptake in Sinorhizobium meliloti: characterization of Prb, an opp-like ABC transporter regulated by both proline betaine and salinity stress. J Bacteriol 188:6308-6317 Antoun H, Beauchamp CJ, Goussard N, Chabot R, Lalande R (1998) Potential of Rhizobium and Bradyrhizobium species as plant growth promoting rhizobacteria on non-legumes: effects on radish (Raphanus sativus L.). Plant Soil 204:57-67 Aouani ME, Mhamdi R, Mars M, Ghrir R (1998) Nodulation and growth of common bean under NaCl stress. Soil Biol Biochem 30:1473-1475

Arora NK, Kang SC, Maheshwari DK (2001) Isolation of siderophore producing strains of Rhizobium meliloti and their biocontrol potential against Macrophomina phaseolina that causes charcoal rot of groundnut. Curr Sci 81:673-677 Bardin SD, Huang HC, Pinto J, Amundsen EJ, Erickson RS (2004) Biological control of Pythium damping-off of pea and sugar beet by Rhizobium leguminosarum bv. viceae. Can J Bot 82:291-296 Barra L, Pica N, Gouffi K, Walker GC, Blanco C, Trautwetter A (2003) Glucose 6-phosphate dehydrogenase is required for sucrose and trehalose to be efficient osmoprotectants in Sinorhizobium meliloti. FEMS Microbiol Lett 229:183-188 Bashan Y, de Bashan LE (2005) Bacterial plant growth promotion. In: Hillel D (ed) Encyclopedia of soils in the environment. Elsevier, Oxford, pp 103-115 Beltra R, Del-Solar G, Sanchez-Serrano JJ, Alonso E (1988) Mutants of Rhizobium phaseoli HM Mel (2) obtained by means of elevated temperatures. Zentbl Mikrobiol 143:529-532 Berraho EL, Lesueur D, Diem HG, Sasson A (1997) Iron requirement and siderophore production in Rhizobium ciceri during growth on an iron-deficient medium. World J Microbiol Biotechnol 13:501-510 Bhattacharyya PN, Jha DK (2012) Plant growth-promoting rhizobac-teria (PGPR): emergence in agriculture. World J Microbiol Biotechnol 28:1327-1350 Biswas JC, Ladha JK, Dazzo FB (2000) Rhizobial inoculation improves nutrient uptake and growth of lowland rice. Soil Sci Soc Am J 64:1644-1650 Bodek I, Lyman WJ, Reehl WF, Rosenblatt DH (1988) Environmental inorganic chemistry: properties, processes, and estimation methods. In: Walton BT, Conway RA (eds) SETAC special publication series. Pergamon Press, New York Bohlool BB, Ladha JK, Garrity DP, George T (1992) Biological nitrogen fixation for sustainable agriculture: a perspective. Plant Soil 141:1-11

Boiero L, Perrig D, Masciarelli O, Penna C, Cassan F, Luna V (2007) Phytohormone production by three strains of Bradyrhizobium japonicum and possible physiological and technological implications. Appl Microbiol Biotechnol 74:874-880 Boscari A, Mandon K, Dupont L, Poggi MC, Le-Rudulier D (2002) BetS is a major glycine betaine/praline betaine transporter required for early osmotic adjustment in Sinorhizobium meliloti. J Bacteriol 184:2654-2663 Boscari A, Mandon K, Poggi MC, Le Rudulier D (2004) Functional expression of Sinorhizobium meliloti BetS, a high-affinity betaine transporter, in Bradyrhizobium japonicum USDA110. Appl Environ Microbiol 70:5916-5922 Buol S, Eswaran H (2000) Oxisols. Adv Agron 68:151-195 Caba JM, Centeno ML, Fernandez B, Gresshoff PM, Ligero F (2000) Inoculation and nitrate alter phytohormone levels in soybean roots: differences between a super nodulating mutant and the wild type. Planta 211:98-104 Camerini S, Senatore B, Lonardo E, Imperlini E, Bianco C, Moschetti G, Rotino GL, Campion B, Defez R (2008) Introduction of a novel pathway for IAA biosynthesis to rhizobia alters vetch root nodule development. Arch Microbiol 190:67-77 Carrasco JA, Armario P, Pajuelo E, Burgos A, Caviedes MA, Lopez R, Chamber MA, Palomares AJ (2005) Isolation and characterization of symbiotically effective Rhizobium resistant to arsenic and heavy metals after the toxic spill at the Aznalcollar pyrite mine. Soil Biol Biochem 37:1131-1140 Carson KC, Meyer JM, Dilworth MJ (2000) Hydroxamate siderophore of root nodule bacteria. Soil Biol Biochem 32:11-21 Cassan F, Perrig D, Sgroy V, Masciarelli O, Penna C, Luna V (2009) Azospirillum brasilense Az39 and Bradyrhizobium japonicum E109, inoculated singly or in combination, promote seed germination and early seedling growth in corn (Zea mays L.) and soybean (Glycine max L.). Eur J Soil Biol 45:28-35

Castro IV, Ferreira EM, McGrath SP (1997) Effectiveness and genetic diversity of Rhizobium leguminosarum bv. trifolii isolates in Portuguese soils polluted by industrial effluents. Soil Biol Biochem 29:1209-1213 Chabot R, Antoun H, Cescas MC (1996) Growth promotion of maize and lettuce by phosphate solubilizing Rhizobium leguminosarum bv. phaseoli. Plant Soil 184:311-321 Chandra R, Pareek RP (2002) Effect of rhizobacteria in urd bean and

lentil. Ind J Pulse Res 15:152-155 Chandra S, Choure K, Dubey RC, Maheshwari DK (2007) Rhizo-sphere competent Mesorhizobium loti MP6 induces root hair curling, inhibits Sclerotinia sclerotiorum and enhances growth of Indian mustard (Brassica campestris). Braz J Microbiol 38:128-130

Chaudhary P, Dudeja SS, Kapoor KK (2004) Effectiveness of host-Rhizobium leguminosarum symbiosis in soils receiving sewage water containing heavy metals. Microbiol Res 159:121-127 Chaudri AM, Allain CMG, Barbosa-Jefferson VL, Nicholson FA, Chambers BJ, McGrath SP (2000) A study of the impacts of Zn and Cu on two rhizobial species in soils of a long-term field experiment. Plant Soil 221:167-179 Chein CT, Maundu J, Cavaness J, Daudurand LM, Orser CS (1992) Characterization of salt-tolerant and salt-sensitive mutants of Rhizobium leguminosarum biovar viciae strain C1204b. FEMS Microbiol Lett 90:135-140 Chen P (2011) Symbiotic effectiveness, competitiveness and salt

tolerance of lucerne rhizobia. RMIT University, Australia Chen RJ, Bhagwat AA, Yaklich R, Keister DL (2002) Characterization of ndvD, the third gene involved in the synthesis of cyclic beta-(1- > 3) (1- > 6)-D-glucans in Bradyrhizobium japonicum. Can J Microbiol 48:1008-1016 Chi F, Yang P, Han F, Jing Y, Shen S (2010) Proteomic analysis of rice seedlings infected by Sinorhizobium meliloti 1021. Proteo-mics 10:1861-1874 Cobbett CS (2000) Phytochelatins and their roles in heavy metal

detoxification. Plant Physiol 123:825-832 Dardanelli MS, de Cordoba FJF, Espuny MR, Carvajal MAR, Diaz MES, Serrano AMG, OkonY MM (2008) Effect of Azospirillum brasilense co-inoculated with Rhizobium on Phaseolus vulgaris flavonoids and nod factor production under salt stress. Soil Biol Biochem 40:2713-2721 Dashadi M, Khosravi H, Moezzi A, Nadian H, Heidari M, Radjabi R (2011) Co-inoculation of Rhizobium and Azotobacter on growth indices of faba bean under water stress in the green house condition. Adv Stud Biol 3:373-385 Davies PJ (1995) The plant hormones: their nature, occurrence and functions. In: Davies PJ (ed) Plant hormones: physiology, biochemistry and molecular biology. Kluwer Academic Publishers, Dordrecht, pp 1-12 Dazzo FB, Yanni YG, Rizk R, Zidan M, Gomaa M, Abu-Baker, Squartini A, Jing Y, Chi F, Shen SH (2005) Recent studies on the Rhizobium cereal association. In: Wang YP, Lin M, Tian ZX, Elmericj C, Newton WE (eds) Biological nitrogen fixation: sustainable agriculture and the environment. Proceedings of the 14th international nitrogen fixation congress. Springer, Dordrecht, pp 379-380 Deanand BJ, Patil AB, Kulkaarni JH, Algawadi AR (2002) Effect of plant growth promoting rhizobacteria on growth and yield of pigeonpea (Cajanus cajan L.) by application of plant growth promoting rhizobacteria. Microbiol Res 159:371-394 Deshwal VK, Dubey RC, Maheshwari DK (2003a) Isolation of plant growth promoting strains of Bradyrhizobium (Arachis) sp. with biocontrol potential against Macrophomina phaseolina causing charcoal rot of peanut. Curr Sci 84:443-444 Deshwal VK, Pandey P, Kang SC, Maheshwari DK (2003b) Rhizobia as a biological control agent against soil borne plant pathogenic fungi. Ind J Exp Biol 41:1160-1164

¿1 Snrinopr

KAcsTiuiUi, ^wj ^ springer

Dobbelaere S, Vanderleyden J, Okon Y (2003) Plant growth promoting effects of diazotrophs in the rhizosphere. Crit Rev Plant Sci 22:107-149 Downie A (1997) Fixing a symbiotic circle. Nature 387:352-353 Duan J, Muller KM, Charles TC, Vesely S, Glick BR (2009) 1-Aminocyclopropane-1carboxylate (ACC) deaminase gene in Rhizobium from Southern Saskatchewan. Microbial Ecol 57:423-436

Duhan JS, Dudeja SS, Khurana AL (1998) Siderophore production in relation to N2 fixation and iron uptake in pigeonpea-Rhizobium symbiosis. Folia Microbiol 43:421-426 Eaglesham ARJ, Ayanaba A (1984) Tropical stress ecology of rhizobia, root-nodulation and legume fixation. In: Subba Rao NS (ed) Current developments in biological nitrogen fixation. Edward Arnold Publishers, London, pp 1-35 Egamberdiyeva D, Juraeva D, Poberejskaya S, Myachina O, Tery-uhova P, Seydalieva L, Aliev A (2004) Improvement of wheat and cotton growth and nutrient uptake by phosphate solubilizing bacteria. In: Proceeding of 26th annual conservation tillage conference for sustainable agriculture, Auburn, pp 58-65 Ehteshamul-Haque S, Ghaffar A (1993) Use of rhizobia in the control of root rot diseases of sunflower, okra, soybean and mung bean. J Phytopathol 138:157-163 El Abyad MS, Abou-Taleb AM (1985) Effects of the herbicides simazine and bromophenoxim on the microflora of two soil types in Egypt. Zentralblatt für Mikrobiologie 40:607-619 El Sheikh EE, Wood M (1990) Salt effects on survival and multiplication of chickpea and soybean rhizobia. Soil Biol Biochem 22:343-347 Elkoca E, Kantar F, Sahin F (2008) Influence of nitrogen fixing and phosphorus solubilizing bacteria on the nodulation, plant growth and yield of chickpea. J Plant Nutr 31:157-171 Estevez J, Dardanelli MS, Megias M, Rodriguez-Navarro DN (2009) Symbiotic performance of common bean and soybean co-inoculated with rhizobia and Chryseobacterium balustinum Aur9 under moderate saline conditions. Symbiosis 49:29-36 Fan LM, Maa ZQ, Liang JQ, Li HF, Wangc ET, Wei GH (2011) Characterization of a copper-resistant symbiotic bacterium isolated from Medicago lupulina growing in mine tailings. Bioresour Technol 102:703-709 FAO (2011) The state of the world's land and water resources for food and agriculture (SOLAW)—Managing systems at risk. Food and Agriculture Organization of the United Nations, Rome and Earthscan, London Figueira EMAP, Lima AIG, Pereira SIA (2005) Monitoring glutathi-one levels as a marker for cadmium stress in Rhizobium leguminosarum biovar viciae. Can J Microbiol 51:7-14 Figueiredo MVB, Burity HA, Martinez CR, Chanway CP (2008) Alleviation of drought stress in common bean (Phaseolus vulgaris L.) by co-inoculation with Paenibacillus polymyxa and Rhizobium tropici. Appl Soil Ecol 40:182-188 Foster LJR, Moy YP, Rogers PL (2000) Metal binding capabilities of Rhizobium etli and its extracellular polymeric substances. Biotechnol Lett 22:1757-1760 Fox SL, O'Hara GW, Brau L (2011) Enhanced nodulation and symbiotic effectiveness of Medicago truncatula when co-inoculated with Pseudomonas fluorescens WSM3457 and Ensifer (Sinorhizobium) medicae WSM419. Plant Soil 348:245-254 Frankenberger WTJ, Arshad M (1995) Photohormones in soil:

microbial production and function. Dekker, New York Gal SW, Choi YJ (2003) Isolation and characterization of salt tolerance rhizobia from Acacia root nodules. Agric Chem Biotechnol 46:58-62 Gianfreda L, Rao MA (2004) Potential of extra cellular enzymes in remediation of polluted soils: a review. Enzyme Microb Technol 35:339-354

1 Чпп'пстрг

KAcsTöLLöJiq rnqjiU ^ springer

Glick BR (1995) The enhancement of plant growth by free-living

bacteria. Can J Microbiol 41:109-117 Glick BR (2012) Plant growth-promoting bacteria: mechanisms and applications. Scientifica, Article ID 963401. doi:http://dx.doi. org/10.6064/2012/963401 Glick BR (2014) Bacteria with ACC deaminase can promote plant

growth and help to feed the world. Microbiol Res 169:30-39 Gopalakrishnan S, Watanabe T, Pearse SJ, Ito O, Hossian AKMZ, Subbarao GV (2009) Biological nitrification inhibition by Brachiaria humidicola roots varies with soil type and inhibits nitrifying bacteria, but not other major soil microorganisms. Soil Sci Plant Nutr 55:725-733 Graham PH (1992) Stress tolerance in Rhizobium and Bradyrhizobi-um and nodulation under adverse soil conditions. Can J Microbiol 38:475-484 Graham PH, Draeger KJ, Ferrey ML, Conroy MJ, Hammer BE, Martinez E, Aarons SR, Quinto C (1994) Acid pH tolerance in strains of Rhizobium and Bradyrhizobium, and initial studies on the basis for acid tolerance of Rhizobium tropici UMR1899. Can J Microbiol 40:198-207 Gray J, Murphy B (2002) Parent material and world soil distribution,

17th world congress of soil science. Bangkok, Thailand Gray EJ, Smith DL (2005) Intracellular and extracellular PGPR: commonalities and distinctions in the plant-bacterium signaling processes. Soil Biol Biochem 37:395-412 Grover M, Ali SZ, Sandhya V, Rasul A, Venkateswarlu B (2010) Role of microorganisms in adaptation of agriculture crops to abiotic stresses. World J Microbiol Biotechnol 27:1231-1240 Gügi B, Orange N, Hellio F, Burini JF, Guillou C, Leriche F, Guespin-Michel JF (1991) Effect of growth temperature on several exported enzyme activities in the psychrotropic bacterium Pseudomonas fluorescens. J Bacteriol 173:3814-3820 Guo J, Chi J (2014) Effect of Cd-tolerant plant growth promoting Rhizobium on plant growth and Cd uptake by Lolium multiflorum Lam. and Glycine max (L.) Merr. in Cd-contaminated soil. Plant Soil 375:205-214 Gupta A, Saxena AK, Gopal M, Tilak KVBR (1998) Effect of plant growth promoting rhizobacteria on competitive ability of introduced Bradyrhizobium sp. (Vigna) for nodulation. Micro-biol Res 153:113-117 Hadi F, Bano A (2010) Effect of diazotrophs (Rhizobium and Azatobactor) on growth of maize (Zea mays L.) and accumulation of lead (PB) in different plant parts. P J Bot 42:4363-4370 Hafeez FY, Hassan Z, Naeem F, Basher A, Kiran A, Khan SA, Malik KA (2008) Rhizobium leguminosarum bv. viciae strain LC-31: analysis of novel bacteriocin and ACC-deaminase gene(s). In: Dakora FD, Chimphango SBM, Valentine AJ, Elmerich C, Newton WE (eds) Biological nitrogen fixation: towards poverty alleviation through sustainable agriculture. Springer, Dordrecht, pp 247-248

Halder AK, Chakrabarty PK (1993) Solubilization of inorganic

phosphate by Rhizobium. Folia Microbiol 38:325-330 Halder AK, Mishra AK, Bhattacharya P, Chakrabarthy PK (1990) Solubilization of rock phosphate by Rhizobium and Bradyrhizobium. J Gen Appl Microbiol 36:1-92 Hall AE (2004) Breeding for adaptation to drought and heat in

cowpea. Eur J Agron 21:447-454 Han MJ, Yun H, Lee SY (2008) Microbial small heat shock proteins

and their use in biotechnology. Biotechnol Adv 26:591-609 Hartel PG, Alexander M (1984) Temperature and desiccation

tolerance of cowpea rhizobia. Can J Microbiol 30:820-823 Hartl FU, Hayer-Hartl M (2009) Converging concepts of protein

folding in vitro and in vivo. Nat Struct Mol Biol 16:574-581 Hellweg C, Puhler A, Weidner S (2009) The time course of the transcriptomic response of Sinorhizobium meliloti 1021 following a shift to acidic pH. BMC Microbiol 9:37

Hirsch PR, Jones MJ, McGrath SP, Giller KE (1993) Heavy metals from past applications of sewage sludge decrease the genetic diversity of Rhizobium leguminosarum biovar trifolii populations. Soil Biol Biochem 25:1485-1490 Huang XD, El Alawi Y, Gurska J, Glick BR, Greenberg BM (2005) A multi-process phytoremediation system for decontamination of persistent total petroleum hydrocarbons (TPHs) from soils. Microchem J 81:139-147 Hungria M, Franco AA (1993) Effects of high temperature on nodulation and nitrogen fixation by Phaseolus vulgaris L. Plant Soil 149:95-102

IFPRI (2012) Global food policy report. International Food Policy

Research Institute, Washington Ike A, Sriprang R, Ono H, Murooka Y, Yamashita M (2008) Promotion of metal accumulation in nodule of Astragalus sinicus by the expression of the iron-regulated transporter gene in Mesorhizobium huakuii subsp. rengei B3. J Biosci Bioeng 105:642-648

Islam MZ, Sattar MA, Ashrafuzzaman M, Berahim Z, Shamsuddoha ATM (2013) Evaluating some salinity tolerant rhizobacterial strains to lentil production under salinity stress. Int J Agric Biol 15:499-504

Jarvis SC (1996) Future trends in nitrogen research. Plant Soil 181:47-56

Jebara M, Mhamdi R, Aouani ME, Ghrir R, Mars M (2001) Genetic diversity of Sinorhizobium populations recovered from different Medicago varieties cultivated in Tunisian soils. Can J Microbiol 47:139-147

Jenson JB, Peters NK, Bhuvaneswari TV (2002) Redundancy in periplasmic binding protein-dependent transport systems for trehalose, sucrose, and maltose in Sinorhizobium meliloti. J Bacteriol 184:2978-2986 Jiang JQ, Wei W, Du BH, Li XH, Wang L, Yang SS (2004) Salt-tolerance genes involved in cation efflux and osmoregulation of Sinorhizobium fredii RT19 detected by isolation and characterization of Tn5 mutants. FEMS Microbiol Lett 239:139-146 Johri BN, Sharma A, Virdi JS (2003) Rhizobacterial diversity in India and its influence on soil and plant health. Adv Biochem Eng Biotechnol 84:49-89 Joseph B, Patra RR, Lawrence R (2007) Characterization of plant growth promoting rhizobacteria associated with chickpea (Cicer arietinum L.). Int J Plant Prod 2:141-152 Kagi JHR (1991) Overview of metallothionein. Methods Enzymol 205:613-626

Kaneko T, Nakamura Y, Sato S, Asamizu E, Kato T, Sasamoto S, Watanabe A, Idesawa K, Ishikawa A, Kawashima K, Kimura T, Kishida Y, Kiyokawa C, Kohara M, Matsumoto M, Matsuno A, Mochizuki Y, Nakayama S, Nakazaki N, Shimpo S, Sugimoto M, Takeuchi C, Yamada M, Tabata S (2000) Complete genomic sequence of nitrogen-fixing symbiotic bacterium Mesorhizobium loti. DNA Res 7:331-338 Karanja NK, Wood M (1988) Selecting Rhizobium phaseoli strains for use with beans (Phaseolus vulgaris L.) in Kenya. Tolerance of high temperature and antibiotic resistance. Plant Soil 112:15-22

Khan MS, Zaidi A, Aamil M (2002) Biocontrol of fungal pathogens by the use of plant growth promoting rhizobacteria and nitrogen fixing microorganisms. Ind J Bot Soc 81:255-263 Kinkle BK, Sadowsky MJ, Johnstone K, Kokinen WC (1994) Tellurium and selenium resistance in rhizobia and its potential use for direct isolation of Rhizobium meliloti from Soil. Appl Environ Microbiol 60:1674-1677 Krasova-Wade T, Diouf O, Ndoye I, Sall CE, Braconnier S, Neyra M (2006) Water condition effects on rhizobia competition for cowpea nodule occupancy. African J Biotech 5:1457-1463

Kulkarni S, Nautiyal CS (2000) Effects of salt and pH stress on temperature tolerant Rhizobium sp. NBRI330 nodulating Pros-opis juliflora. Curr Microbiol 40:221-226 Kumar P (2012) Ph.D. thesis. Gurukul Kangri University, Haridwar, India

Kumar S, Mukerji KG, Lai R (1996) Molecular aspects of pesticide

degradation by microorganisms. Crit Rev Microbiol 22:1-26 Lakzian A, Murphy P, Turner A, Beynon JL, Giller KE (2002) Rhizobium leguminosarum bv. viciae populations in soils with increasing heavy metal contamination: abundance, plasmid profiles, diversity and metal tolerance. Soil Biol Biochem 34:519-529

Laranjo M, Alexandre A, Oliveira S (2014) Legume growth-promoting rhizobia: an overview on the Mesorhizobium genus. Microbiol Res 169:2-17 Lindström K, Lipsanen P, Kaijalainen S (1990) Stability of markers used for identification of two Rhizobium galegae inoculant strains after five years in the field. Appl Environ Microbiol 56:444-450

Liu Y, Lam MC, Fang HHP (2001) Adsorption of heavy metals by

EPS of activated sludge. Water Sci Technol 43:59-66 Lloret J, Bolanos L, Lucas MM, Peart JM, Brewin NJ, Bonilla I, Rivilla R (1995) Ionic stress and osmotic pressure induce different alterations in the lipopolysaccharide of a Rhizobium meliloti strain. Appl Environ Microbiol 61:3701-3704 Lloret J, Wulff BBH, Rubio JM, Downie JA, Bonilla I, Rivilla R (1998) Exo-polysaccharide II production is regulated by salt in the halotolerant strain Rhizobium meliloti EFB1. J Bacteriol 179:5366-5371

Lucy M, Reed E, Glick BR (2004) Applications of free living plant growth promoting rhizobacteria. Antonie Van Leeuwenhoek 86:1-25

Lugtenberg B, Kamilova F (2009) Plant growth promoting rhizobac-

teria. Ann Rev Microbiol 63:541-556 Ma W, Guinel FC, Glick BR (2003a) The Rhizobium leguminosarum bv. viciae ACC deaminase protein promotes the nodulation of pea plants. Appl Environ Microbiol 69:4396-4402 Ma W, Sebestianova S, Sebestian J, Burd GI, Guinel F, Glick BR (2003b) Prevalence of 1-aminocyclopropaqne-1-carboxylate in deaminase in Rhizobiaum sp. Antonie Van Leeuwenhoek 83:285-291

Ma W, Charles TC, Glick BR (2004) Expression of an exogenous 1 aminocyclopropane-1-carboxylate deaminase gene in Sinorhizo-bium meliloti increases its ability to nodulate alfalfa. Appl Environ Microbiol 70:5891-5897 Ma Y, Prasad MNV, Rajkumar M, Freitas H (2011) Plant growth promoting rhizobacteria and endophytes accelerate phytoreme-diation of metalliferous soils. Biotechnol Adv 29:248-258 Madhaiyan M, Poonguzhali S, Ryu JH, Sa TM (2006) Regulation of ethylene levels in canola (Brassica campestris) by 1-aminocy-clopropane-1-carboxylate deaminase-containing Methylobacte-rium fujisawaense. Planta 224:268-278 Martinez-Toledo MV, Salmeron V, Rodelas B, Pozo C, GonzalezLopez J (1996) Studies on the effects of the herbicide simazine on microflora of four agricultural soils. Environ Toxicol Chem 15:1115-1118

Mathur SC (1999) Future of Indian pesticides industry in next

millennium. Pestic Inf 24:9-23 Mauseth JD (1991) Botany: an introduction to plant biology.

Saunders, Philadelphia, pp 348-415 McGrath JW, Hammerschmidt F, Quinn JP (1998) Biodegradation of phosphonomycin by Rhizobium huakuii PMY1. Appl Environ Microbiol 64:356-358 Mehboob I, Naveed M, Zahir ZA, Ashraf M (2012) Potential of rhizobia for sustainable production of non-legumes. In: Ashraf

¿1 Snrinopr

KAcsTöojüUig ^laU ^ springer

M, Öztürk M, Ahmad M, Aksoy A (eds) Crop production for agricultural improvement. Springer, Netherlands, pp 659-704 Mehboob I, Naveed M, Zahir ZA, Sessitch A (2013) Potential or rhizosphere bacteria for improving Rhizobium-Legume symbiosis. In: Arora NK (ed) Plant microbe symbiosis: fundamentals and advances. Springer, India, pp 305-349 Mehra RK, Mulchandani P (1995) Glutathione-mediated transfer of

Cu (I) into phytochelatins. Biochem J 307:697-705 Mhadhbi H, Jebara M, Limam F, Aouani ME (2004) Rhizobial strain involvement in plant growth, nodule protein composition and antioxidant enzyme activities of chickpea-rhizobia symbioses: modulation by salt stress. Plant Physiol Biochem 42:717-722 Mhadhbi H, Jebara M, Zitoun A, Limam F, Aouani ME (2008) Symbiotic effectiveness and response to mannitol-mediated osmotic stress of various chickpea-rhizobia associations. World J Microbiol Biotech 24:1027-1035 Michiels J, Verreth C, Vanderleyden J (1994) Effects of temperature stress on bean nodulating Rhizobium strains. Appl Environ Microbiol 60:1206-1212 Miller-Williams M, Loewen PC, Öresnik IJ (2006) Isolation of salt sensitive mutants of Sinorhizobium meliloti strain Rm1021. Microbiol 152:2049-2059 Mishra RPN, Singh RK, Jaiswal HK, Kumar V, Maurya S (2006) Rhizobium-mediated induction of phenolics and plant growth promotion in rice (Oryza sativa L.). Curr Microbiol 52:383-389 Mishra PK, Mishra S, Selvakumar G, Bisht JK, Kundu S, Gupta HS (2009) Co-inoculation of Bacillus thuringeinsis -KR1 with Rhizobium leguminosarum enhances plant growth and nodulation of pea (Pisum sativum L.) and lentil (Lens culinaris L.). World J Microbiol Biotechnol 25:753-761 Moorma TB (1988) A review of pesticide effects on microorganisms and microbial processes related to soil fertility. J Prod Agric 2:14-23

Mpepereki SF, Makoneses Wollum AG (1997) Physiological characterization of indigenous rhizobia nodulating Vigna unguiculata in Zimbabwean soils. Symbiosis 22:275-292 Mrabet M, Mhamdi R, Tajini F, Tiwari R, Trabelsi M, Aouani ME (2005) Competitiveness and symbiotic effectiveness of a R. gallicum strain isolated from root nodules of Phaseolus vulgaris. Europ J Agron 22:209-216 Muglia CI, Grasso DH, Aguilar ÖM (2007) Rhizobium tropici response to acidity involves activation of glutathione synthesis. Microbiol 153:1286-1296 Münchbach M, Nocker A, Narberhaus F (1999) Multiple small heat

shock proteins in Rhizobia. J Bacteriol 181:83-90 Munns DN, Keyser HH, Fogle VW, Hohenberg JS, Righetti TL, Lauter DL, Zaruog MG, Clarkin KL, Whitacre KW (1979) Tolerance of soil acidity in symbiosis of mung bean with rhizobia. Agron J 71:256-260 Naidu VSGR, Panwar JDS, Annapurna K (2004) Effect of synthetic auxins and Azorhizobium caulinodans on growth and yield of rice. Ind J Microbiol 44:211-213 Nandal K, Sehrawat AR, Yadav AS, Vashishat RK, Boora KS (2005) High temperature-induced changes in exo-polysaccharides, lipo-polysaccharides and protein profile of heat-resistant mutants of Rhizobium sp. (Cajanus). Microbiol Res 160:367-373 Nascimento F, Brlgido C, Alho L, Glick BR, Oliveira S (2012) Enhanced chickpea growth promotion ability of a mesorhizobia expressing an exogenous ACC deaminase gene. Plant Soil 353:221-230

Neubauer U, Furrer G, Kayser A, Schulin R (2000) Siderophores, NTA, and citrate: potential soil amendments to enhance heavy metal mobility in phytoremediation. Int J Phytoremed 2:353-368 Nieto KF, Frankenberger WT Jr (1990) Influence of adenine, isopentyl alcohol and Azotobacter chroococcum on the growth of Raphanus sativus. Plant Soil 127:147-156

¿1 Snrinopr

KAcsTöLLöUiq rnqjiU ^ springer

Nieto KF, Frankenberger WT Jr (1991) Influence of adenine, isopentyl alcohol and Azotobacter chroococcum on the vegetative growth of Zea mays. Plant Soil 135:213-221 Noel TC, Sheng C, Yost CK, Pharis RP, Hynes MF (1996) Rhizobium leguminosarum as a plant growth promoting rhizobacterium: direct growth promotion of canola and lettuce. Can J Microbiol 42:279-283

Nogales J, Campos R, Ben-Abdelkhalek H, Olivares J, Lluch C, Sanjuan J (2002) Rhizobium tropici genes involved in free-living salt tolerance are required for the establishing of efficient nitrogen-fixing symbiosis with Phaseolus vulgaris. Mol Plant Microbe Interact 15:225-232 Nonnoi F, Chinnaswamy A, de la Torre VSG, de la Pena TC, Lucas MM, Pueyo JJ (2012) Metal tolerance of rhizobial strains isolated from nodules of herbaceous legumes (Medicago spp. and Trifolium spp.) growing in mercury-contaminated soils. Appl Soil Ecol 61:49-59 Ohtake H, Wu H, Imazu K, Ambe Y, Kato J, Kuroda A (1996) Bacterial phosphonate degradation, phosphite oxidation and polyphosphate accumulation. Res Conserv Recycl 18:125-134 Okazaki S, Sugawara M, Minamisawa K (2004) Bradyrhizobium elkanii rtxC gene is required for expression of symbiotic phenotypes in the final step of rhizobitoxine biosynthesis. Appl Environ Microbiol 70:535-541 Ozkoc I, Deliveli MH (2001) In vitro inhibition of the mycelial growth of some root rot fungi by Rhizobium leguminosarum biovar phaseoli isolates. Turk J Biol 25:435-445 Parker JH (1972) How fertilizer moves and reacts in soil. Crops Soils 72:7-11

Pathak DV, Sharma MK, Sushil K, Naresh K, Sharma PK (2007) Crop improvement and root rot suppression by seed bacterization in chickpea. Arch Agron Soil Sci 53:287-292 Paton GI, Palmer G, Burton M, Rattray EAS, McGrath SP, Glover LA, Killham K (1997) Development of an acute and chronic ecotoxicity assay using lux-marked Rhizobium leguminosarum biovar trifolii. Lett Appl Microbiol 24:296-300 Patten CL, Glick BR (1996) Bacterial biosynthesis of indole-3-acetic

acid. Can J Microbiol 42:207-220 Paudyal SP, Aryal RR, Chauhan SVS, Maheshwari DK (2007) Effect of heavy metals on growth of rhizobium strains and symbiotic efficiency of two species of tropical legumes. Sci World 5:27-32 Payakapong W, Tittabutr P, Teaumroong N, Boonkerd N, Singleton PW, Borthakur D (2006) Identification of two clusters of genes involved in salt tolerance in Sinorhizobium sp. strain BL3. Symbiosis 41:47-53 Peix A, Rivas-Boyero AA, Mateos PF, Rodriguez-Barrueco C, Martínez-Molina E, Velazquez E (2001) Growth promotion of chickpea and barley by a phosphate solubilizing strain of Mesorhizobium mediterraneum under growth. Soil Biol Biochem 33:103-110

Peng S, Biswas JC, Ladha JK, Gyaneshwar P, Chen Y (2002) Influence of rhizobial inoculation on photosynthesis and grain yield of rice. Argon J 94:925-929 Penning de Vries FWT, Van Laar HH, Chardon MCM (1983) Bioenergetics of growth of seeds, fruits and storage organs. Proceedings on Potential productivity of field crops under different environments on 22-26th September 1980. International Rice Research Institute, Los Banos, pp 37-59 Peoples MB, Brockwell J, Herridge DF, Rochester IJ, Alves BJR, Urquiaga S, Boddey RM, Dakora FD, Bhattarai S, Maskey SL, Sampet C, Rerkasem B, Khan DF, Hauggaard-Nielsen H, Jensen ES (2009) The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis 48:1-17 Peoples MB, Crasswell ET (1992) Biological nitrogen fixation: investments, expectations and actual contributions to agriculture. Plant Soil 141:13-39

Pereira SIA, Lima AIG, Figueira EMAP (2006a) Heavy metal toxicity in Rhizobium leguminosarum biovar viciae isolated from soils subjected to different sources of heavy-metal contamination: effects on protein expression. Appl Soil Ecol 33:286-293 Pereira SIA, Lima AIG, Figueira EMAP (2006b) Screening possible mechanisms mediating cadmium resistance in Rhizobium legu-minosarum bv. viciae isolated from contaminated Portuguese soils. Microbial Ecol 52:176-186 Prabha C, Maheshwari DK, Bajpai VK (2013) Diverse role of fast growing rhizobia in growth promotion and enhancement of psoralen content in Psoralea corylifolia L. Pharmacogn Mag 9:S57-S65

Pulsawat W, Leksawasdi N, Rogers PL, Foster LJR (2003) Anions effects on biosorption of Mn(II) by extracellular polymeric substance (EPS) from Rhizobium etli. Biotechnol Lett 25:1267-1270

Purchase D, Miles RJ (2001) Survival and nodulating ability of indigenous and inoculated Rhizobium leguminosarum biovar trifolii in sterilized and unsterilized soil treated with sewage sludge. Curr Microbiol 42:59-64 Purchase D, Miles RJ, Young TWK (1997) Cadmium uptake and nitrogen fixing ability in heavy-metal-resistant laboratory and field strains of Rhizobium leguminosarum biovar trifolii. FEMS Microbiol Ecol 22:85-93 Qureshi MA, Ahmad MJ, Naveed M, Iqbal A, Akhtar N, Niazi KH (2009) Co-inoculation with Mesorhizobium ciceri and Azoto-bacter chroococcum for improving growth, nodulation and yield of chickpea (Cicer arietinum L.). Soil Environ 28:124-129 Rajendran G, Sing F, Desai AJ, Archana G (2008) Enhanced growth and nodulation of pigeonpea by co-inoculation of Bacillus strains with Rhizobium sp. Bioresour Technol 99:544-550 Rajkumar M, Ae N, Prasad MNV, Freitas H (2010) Potential of siderophore-producing bacteria for improving heavy metal phytoextraction. Trends Biotechnol 28:142-149 Reddy PP (2013) Plant growth promoting rhizobacteria (PGPR).

Recent advances in crop protection. Springer, India, pp 131-145 Reichman SM (2007) The potential use of the legume-rhizobium symbiosis for the remediation of arsenic contaminated sites. Soil Biol Biochem 39:2587-2593 Reitz M, Rudolph K, Schroder I, Hoffmann-Hergarten S, Hallmann J, Sikora RA (2000) Lipopolysaccharides of Rhizobium etli strain G12 act in potato roots as an inducing agent of systemic resistance to infection by the cyst nematode Globodera pallida. Appl Environ Microbiol 66:3515-3518 Remans R, Croonenborghs A, Gutierrez RT, Michiels J, Vanderley-den J (2007) Effects of plant growth-promoting rhizobacteria on nodulation of Phaseolus vulgaris L. are dependent on plant P nutrition. Eur J Plant Pathol 119:341-351 Remans R, Ramaekers L, Schelkens S, Hernandez G, Garcia A, Reyes JL, Mendez N, Toscano V, Mulling M, Galvez L, Vanderleyden J (2008) Effect of Rhizobium-Azospirillum coinoculation on nitrogen fixation and yield of two contrasting Phaseolus vulgaris L. genotypes cultivated across different environments in Cuba. Plant Soil 312:25-37 Richardson AE, Hadobas PA (1997) Soil isolates of Pseudomonas sp.

that utilize inositol phosphates. Can J Microbiol 43:509-516 Richardson AE, Henderson AP, James GS, Simpson RJ (1988) Consequences of soil acidity and the effect of lime on the nodulation of Trifolium subterraneum L. growing in an acid soil. Soil Biol Biochem 20:439-445 Robinson B, Russell C, Hedley M, Clothier B (2001) Cadmium adsorption by rhizobacteria: implications for New Zealand pastureland. Agric Ecosyst Environ 87:315-321 Rodrigues C, Laranjo M, Oliveira S (2006) Effect of heat and pH stress in the growth of chickpea mesorhizobia. Curr Microbiol 53:1-7

Rodríguez-Echevarría S, Perez-Fernandez MA (2005) Potential use of Iberian shrubby legumes and rhizobia inoculation in re-vegetation projects under acidic soil conditions. Appl Soil Ecol 29:203-208

Romdhane SB, Trabelsi M, Aouani ME, de Lajudie P, Mhamdi R (2009) The diversity of rhizobia nodulating chickpea (Cicer arietinum) under water deficiency as a source of more efficient inoculants. Soil Biol Biochem 41:2568-2572 Roseline R, Lara R, Sarah S, German H, Aurelio G, Jorge R, Nancy M, Vidalina T, Miguel M, Lazaro G, Jos V (2008) Effect of Rhizobium-Azospirillum co-inoculation on nitrogen fixation and yield of two contrasting Phaseolus vulgaris L. genotypes cultivated across different environments in Cuba. Plant Soil 312:25-37

Sanginga N, Danso SKA, Mulongoy K, Ojeifo AA (1994) Persistence and recovery of introduced Rhizobium 10 years after inoculation on Leucaena leucocephala grown on an Alfisol in Southwestern Nigeria. Plant Soil 159:199-204 Sardesai N, Babu CR (2001) Cold stress induced high molecular weight membrane polypeptides are responsible for cold tolerance in Rhizobium DDSS69. Microbiol Res 156:279-284 Saxena D, Amin M, Khanna S (1996) Modulation of protein profiles in Rhizobium sp. under salt stress. Can J Microbiol 42:617-620 Senthilkumar M, Madhaiyan M, Sundaram SP, Kannaiyan S (2009) Intercellular colonization and growth promoting effects of Methylobacterium sp. with plant-growth regulators on rice (Oryza sativa L. CvCO-43). Microbiol Res 164:92-104 Serraj R, Roy G, Drevon JJ (1994) Salt stress induces a decrease in the oxygen uptake of soybean nodules and in their permeability to oxygen diffusion. Physiol Plant 191:161-168 Shaharoona B, Arshad M, Zahir ZA (2006) Effect of plant growth promoting rhizobacteria containing ACC-deaminase on maize (Zea mays L.) growth under axenic conditions and on nodulation in mung bean. Lett Appl Microbiol 42:155-159 Sharma P, Padh H, Shrivastava N (2013) Hairy root cultures: a suitable biological system for studying secondary metabolic pathways in plants. Eng Life Sci 13:62-75 Shaukat SS, Siddqui IA (2003) The influence of mineral and carbon sources on biological control of charcoal rot fungus, Macropho-mina phaseolina by fluorescent pseudomonads in tomato. Lett Appl Microbiol 36:392-398 Siddiqui ZA, Mahmood I (2001) Effects of rhizobacteria and root symbionts on the reproduction of Meloidogyne javanica and growth of chickpea. Bioresour Technol 79:41-45 Siddiqui IA, Ehteshamul-Haque S, Ghaffar A (1998) Effect of rhizobia and fungal antagonists in the control of root infecting fungi on sun flower and chickpea. Pak J Bot 30:279-286 Siddiqui IA, Ehteshamul-Haque S, Zaki MJ, Ghaffar A (2000) Effect of urea on the efficacy of Bradyrhizobium sp. and Trichoderma harzianum in the control of root infecting fungi in mungbean and sunflower. Sarhad J Agric 16:403-406 Sinclair TR, de Wit CT (1975) Photosythate and nitrogen requirement

for seed production of various crops. Science 189:565-567 Sindhu SS, Dadarwal KR (2001) Chitinolytic and cellulolytic Pseudomonas sp. antagonistic to fungal pathogen enhances nodulation by Mesorhizobium sp. Cicer in chickpea. Microbiol Res 156:353-358 Singh G, Wright D (2002) In vitro studies on the effects of herbicides

on the growth of rhizobia. Lett Appl Microbiol 35:12-16 Singh RK, Mishra RPN, Jaiswal HK, Kumar V, Pandev SP, Rao SB, Annapurna K (2006) Isolation and identification of natural endophytic rhizobia from rice (Oryza sativa L.) through rDNA PCR-RFLP and sequence analysis. Curr Microbiol 52:345-349 Singh G, Sekhon HS, Sharma P (2011) Effect of irrigation and biofertilizer on water use, nodulation, growth and yield of chickpea (Cicer arietinum L.). Arch Agron Soil Sci 57:715-726

¿1 Snrinopr

KAcsTöajüUig ^wj ^ springer

Skrary FA, Cameron DC (1998) Purification and characterization of a Bacillus licheniformis phosphatase specific for D-alphaglycer-phosphate. Arch Biochem Biophys 349:27-35 Smith SR (1997) Rhizobium in soils contaminated with copper and zinc following the long-term application of sewage sludge and other organic wastes. Soil Biol Biochem 29:1475-1489 Smith LT, Smith GM, D'souza MR, Pocard JA, Le Rudulier D, Madkour MA (1994) Osmoregulation in Rhizobium meliloti: mechanism and control by other environmental signals. J Exp Zool 268:162-165 Soussi M, Santamaria M, Ocana A, Lluch C (2001) Effects of salinity on protein and lipopolysaccharide pattern in a salt-tolerant strain of Mesorhizobium ciceri. J Appl Microbiol 90:476-481 Spaepen S, Vanderleyden J (2011) Auxin and plant-microbe interactions. Cold Spring Harb Perspect Biol 3:a001438 Spaepen S, Vanderleyden J, Remans R (2007) Indole-3-acetic acid in microbial and microorganism-plant signaling. FEMS Microbiol Rev 31:425-448

Sriprang R, Hayashi M, Yamashita M, Ono H, Saeki K, Murooka Y

(2002) A novel bioremediation system for heavy metals using the symbiosis between leguminous plant and genetically engineered rhizobia. J Biotech 99:279-293

Sriprang R, Hayashi M, Ono H, Takagi M, Hirata K, Murooka Y

(2003) Enhanced accumulation of Cd2+ by a Mesorhizobium sp. transformed with a gene from Arabidopsis thaliana coding for phytochelatin synthase. Appl Environ Microbiol 69:1791

Stajkovic O, Delic D, Josic D, Kuzmanovic D, Rasulic N, Knezevic-Vukcevic J (2011) Improvement of common bean growth by co-inoculation with Rhizobium and plant growth promoting bacteria. Rom Biotechnol Lett 16:5919-5926 Stan V, Gament E, Cornea CP, Voaides C, Dusa M, Plopeanu G (2011) Effects of heavy metal from polluted soils on the Rhizobium diversity. Not Bot Hort Agrobot Cluj 39:88-95 Streeter JG (2003) Effect of trehalose on survival of Bradyrhizobium

japonicum during desiccation. J Appl Microbiol 95:484-491 Suarez R, Wong A, Ramirez M, Barraza A, Orozco MC, Cevallos MA, Lara M, Hernandez G, Iturriaga G (2008) Improvement of drought tolerance and grain yield in common bean by over-expressing trehalose-6-phosphate synthase in rhizobia. Mol Plant Microbe Interact 21:958-966 Subbarao GV, Johansen C, Jana MK, Kumar Rao JVDK (1990) Effects of the sodium/calcium ratio in modifying salinity responses in pigeonpea (Cajanus cajan). Plant Physiol 136:439-443

Subbarao GV, Ito O, Sahrawat KL, Berry WL, Nakahara K, Ishikawa T, Watanabe T, Suenaga K, Rondon M, Rao IM (2006) Scope and strategies for regulation of nitrification in agricultural systems challenges and opportunities. Crit Rev Plant Sci 25:303-335

Subbarao GV, Sahrawat KL, Nakahara K, Ishikawa T, Kishii M, Rao IM, Hash CT, George TS, Srinivasa Rao P, Nardi P, Bonnett D, Berry W, Suenaga K, Lata JC (2012) Bio-logical nitrification inhibition—a novel strategy to regulate nitrification in agricultural systems. Adv Agron 114:249-302 Sugawara M, Cytryn EJ, Sadowsky MJ (2010) Functional role of Bradyrhizobium japonicum trehalose biosynthesis and metabolism genes during physiological stress and nodulation. Appl Environ Microbiol 76:1071-1081 Sullivan JT, Trzebiatowski JR, Cruickshank RW, Gouzy J, Brown SD, Elliot RM, Fleetwood DJ, McCallum NG, Rossbach U, Stuart GS, Weaver JE, Webby RJ, De Bruijin FJ, Ronson CW (2002) Comparative sequence analysis of the symbiosis island of Mesorhizobium loti strain R7A. J Bacteriol 184:3086-3095 Sutherland IW (2001) Exo-polysaccharides in biofilms, flocs and related structure. Water Sci Technol 43:77-86

Tank N, Saraf M (2010) Salinity-resistant plant growth promoting rhizobacteria ameliorates sodium chloride stress on tomato plants. J Plant Interact 5:51-58 Tao G, Tian S, Cai M, Xie G (2008) Phosphate solubilizing and mineralizing abilities of bacteria isolated from soils. Pedosphere 18:515-523

Tchebotar VK, Kang UG, Asis CA Jr, Akao S (1998) The use of GUS-reporter gene to study the effect of Azospirillum-Rhizobium co-inoculation on nodulation of white clover. Biol Fertil Soils 27:349-352

Tejera NA, Soussi M, Lluch C (2006) Physiological and nutritional indicators of tolerance to salinity in chickpea plants growing under symbiotic conditions. Environ Exp Bot 58:17-24 Thami-Alami I, Elboutahiri N, Udupa SM (2010) Variability in natural populations of Sinorhizobium meliloti in Morocco. In: Porqueddu C, Ríos S (eds) The contributions of grasslands to the conservation of Mediterranean biodiversity. Zaragoza, CI-HEAM/CIBIO/FAO/SEEP, pp 265-269 Tittabutr P, Payakapoig W, Teaumroong N, Boonkerd N, Singleton PW, Borthakur D (2006) The alternative sigma factor rpoH2 is required for salt tolerance in Sinorhizobium sp. strain BL3. Res Microbiol 157:811-818 Tokala RK, Strap JL, Jung CM, Crawford DF, Salove MH, Deobald LA, Bailey JF, Morra MJ (2002) Novel plant-microbe rhizo-sphere interaction involving Streptomyces lydicus Wyec 108 and the pea plant (Pisum sativum). Appl Environ Microbiol 68:2161-2171

Turner NC, Wright GC, Siddique KHM (2000) Adaptation of grain legumes (pulses) to water-limited environments. Adv Agron 71:193-231

Uchiumi T, Oowada T, Itakura M, Mitsui H, Nukui N, Dawadi P, Kaneko T, Tabata S, Yokoyama T, Tejima T, Saeki K, Oomori H, Hayashi M, Maekawa T, Sriprang R, Murooka Y, Tajima S, Simomura K, Nomura M, Suzuki A, Shimoda S, Sioya K, Abe M, Minamisawa K (2004) Expression islands clustered on symbiosis island of Mesorhizobium loti genome. J Bacteriol 186:2439-2448

Uma C, Sivagurunathan P, Sangeetha D (2013) Performance of bradyrhizobial isolates under drought conditions. Int J Curr Microbiol App Sci 2:228-232 Uren NC (2007) Types, amounts, and possible functions of compounds released into the rhizosphere by soil-grown plants. In: Pinton R, Varanini Z, Nannipieri P (eds) The rhizosphere: biochemistry and organic substances at the soil-plant interface. CRC Press, Boca Raton, pp 1-22 USDA (2012) Climate change and agriculture in the United States: effects and adaptation. USDA Technical Bulletin 1935. Washington, DC

Vanderlinde EM, Harrison JJ, Muszynski A, Carlson RW, Turner RJ, Yost CK (2010) Identification of a novel ABC transporter required for desicattion tolerance, and biofilm formation in Rhizobium leguminosarum bv. viciae 3841. FEMS Microbiol Ecol 71:327-340

Velazquez E, Peix A, Zurdo-Piñeiro JL, Palomo JL, Mateos PF, Rivas R, Muñoz-Adelantado E, Toro N, García-Benavides P, Martínez-Molina E (2005) The coexistence of symbiosis and pathogenicity determining genes in Rhizobium strains enables them to induce nodules and tumors or hairy roots in plants. Mol Plant Microbe Interact 18:1325-1332 Verma JP, Yadav J, Tiwari KN, Kumar A (2013) Effect of indigenous Mesorhizobium spp. and plant growth promoting rhizobacteria on yields and nutrients uptake of chickpea (Cicer arietinum L.) under sustainable agriculture. Ecol Eng 51:282-286 Verma JP, Yadav J, Tiwari KN, Jaiswal DK (2014) Evaluation of plant growth promoting activities of microbial strains and their

jjjilliir dlloll iújao c •

KAcsTöLLöUiq rnqjiU ^ springer

effect on growth and yield of chickpea (Cicer arietinum L.) in India. Soil Biol Biochem 70:33-37 Vessey KJ (2003) Plant growth promoting rhizobacteria as biofertil-

izers. Plant Soil 255:571-586 Vidal C, Chantreuil C, Berge O, Maure L, Escarree J, Bena G, Brunel B, Cleyet-Marel JC (2009) Mesorhizobium metallidurans sp. nov., a metalresistant symbiont of Anthyllis vulneraria growing on metallicolous soil in Languedoc France. Int J Syst Evol Microbiol 59:850-855 Villacieros M, Power B, Sanchez-Contreras M, Lloret J, Oruezabal RI, Martin M, Fernandez-Pinas F, Bonilla I, Whelan C, Dowling DN, Rivilla R (2003) Colonization behaviour of Pseudomonas fluorescens and Sinorhizobium meliloti in the alfalfa (Medicago sativa) rhizosphere. Plant Soil 251:47-54 Walton DC, Li Y (1995) Abscisic acid biosynthesis and metabolism. Plant hormones: physiology, biochemistry and molecular biology. Kluwer, Dordrecht, pp 140-157 Wani PA, Khan MS (2012) Bioremediaiton of lead by a plant growth

promoting Rhizobium species RL9. Bacteriology J 2:66-78 Wani PA, Khan MS (2013) Nickel detoxification and plant growth promotion by multi metal resistant plant growth promoting Rhizobium species RL9. Bull Environ Contam Toxicol 91:117-124

Wani PA, Khan MS, Zaidi A (2007a) Co-inoculation of nitrogen fixing and phosphate solubilizing bacteria to promote growth, yield and nutrient uptake in chickpea. Acta Agron Hung 55:315-323

Wani PA, Khan MS, Zaidi A (2007b) Effect of metal tolerant plant growth promoting Bradyrhizobium sp. (vigna) on growth, symbiosis, seed yield and metal uptake by green gram plants. Chemosphere 70:36-45 Wani PA, Khan MS, Zaidi A (2007c) Synergistic effects of the inoculation with nitrogen fixing and phosphate solubilizing rhizobacteria on the performance of field grown chickpea. J Plant Nutr Soil Sci 170:283-287 Wani PA, Khan MS, Zaidi A (2008a) Chromium-reducing and plant growth promoting Mesorhizobium improves chickpea growth in chromium-amended soil. Biotechnol Lett 30:159-163 Wani PA, Khan MS, Zaidi A (2008b) Effect of metal-tolerant plant growth promoting Rhizobium on the performance of pea grown in metal-amended soil. Arch Environ Contam Toxicol 55:33-42 Watkin ELJ, O'Hara GW, Glenn AR (1997) Calcium and acid stress interact to affect the growth of Rhizobium leguminosarum bv. trifolii. Soil Biol Biochem 29:1427-1432 Watkin ELJ, O'Hara GW, Glenn AR (2003) Physiological responses to acid stress of an acid-soil tolerant and an acid-soil sensitive strain of Rhizobium leguminosarum biovar trifolii. Soil Biol Biochem 35:621-624 Wei W, Jiang J, Li X, Wang L, Yang SS (2004) Isolation of saltsensitive mutants from Sinorhizobium meliloti and characterization of genes involved in salt tolerance. Lett Appl Microbiol 39:278-283

Weyens N, van der Lelie D, Taghavi S, Vangronsveld J (2009) Phytoremediation: plant-endophyte partnerships take the challenge. Curr Opin Biotechnol 20:248-254

Willems A (2006) The taxonomy of rhizobia: an overview. Plant Soil 287:3-14

Wilson PW, Burris RH (1947) The mechanism of biological nitrogen

fixation. Bacteriol Rev 11(1):41-73 Wittenberg JB, Wittenberg BA, Day DA, Udvardi MK, Appleby CA (1996) Siderophore bound iron in the peribacteroid space of soybean root nodules. Plant Soil 178:161-169 Xu J, Xiao-Lin L, Luo L (2012) Effects of engineered Sinorhizobium meliloti on cytokinin synthesis and tolerance of alfalfa to extreme drought stress. Appl Environ Microbiol 78:8056 Yang J, Kloepper JW, Ryu CM (2009) Rhizosphere bacteria help

plants tolerate abiotic stress. Trends Plant Sci 14(1):1-4 Yanni YG, Rizk RY, Abd El-Fattah FK, Squartini A, Corich V, Giacomini A, De Bruijn F, Rademaker J, Maya-Flores J, Ostrom P, Vega-Hernandez M, Hollingsworth RI, Martinez-Molina E, Mateos P, Velazquez E, Wopereis J, Triplett E, Umali-Gracia M, Anarna JA, Rolfe BG, Ladha JK, Hill J, Mujoo R, Ng PK, Dazzo FB (2001) The beneficial plant growth promoting association of Rhizobium leguminosarum bv. trifolii with rice roots. Aust J Plant Physiol 28:845-870 Yelton MM, Yang SS, Edie SA, Lim ST (1983) Characterization of an effective salt-tolerant fast-growing strain of Rhizobium japonicum. J Gen Microbiol 129:1537-1547 Younis M (2007) Responses of Lablab purpureus-Rhizobium symbiosis to heavy metals in pot and field experiments. World J Agric Sci 3:111-122

Zafar-ul-Hye M, Ahmad M, Shahzad SM (2013) Synergistic effect of rhizobia and plant growth promoting rhizobacteria on the growth and nodulation of lentil seedlings under axenic conditions. Soil Environ 32:79-86 Zahir ZA, Shah MK, Naveed M, Akhter MJ (2010) Substrate dependent auxin production by Rhizobium phaseoli improves the growth and yield of Vigna radiata L. under salt stress conditions. J Microbiol Biotechnol 20:1288-1294 Zahran HH (1999) Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an aird climate. Microbiol Mol Biol Rev 63(4):968-989 Zahran HH, Rasanen LA, Karsisto M, Lindstrom K (1994) Alteration of lipopolysaccharide and protein profiles in SDS-PAGE of rhizobia by osmotic and heat stress. World J Microbiol Biotechnol 10:100-105 Zhan HJ, Lee CC, Leigh JA (1991) Induction of the second exo-polysaccharide (EPS) in Rhizobium meliloti SU47 by low phosphate concentrations. J Bacteriol 173:7391-7394 Zhang XP, Karsisto M, Harper R, LindstroEm K (1991) Diversity of Rhizobium bacteria isolated from the root nodules of leguminous trees. Int J Syst Evol Bacteriol 41:104-113 Zhou K, Binkley D, Doxtader KG (1992) A new method for estimating gross phosphorus mineralization and immobilization rates in soils. Plant Soil 147:243-250 Zhu JK (2001) Plant salt tolerance. Plant Sci 6:66-71

¿1 Snrinopr

KACSTLúüUlg ^WJ ^ springer