Exopolysaccharide II production is regulated by salt in the halotolerant strain Rhizobium meliloti EFB1

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Methanotrophs as a reservoir for bioactive secondary metabolites: Pitfalls, insights and promises

Methanotrophs are potent natural producers of several bioactive secondary metabolites (SMs) including isoprenoids, polymers, peptides, and vitamins. Cryptic biosynthetic gene clusters identified from these microbes via genome mining hinted at the vast and hidden SM biosynthetic potential of these microbes. Central carbon metabolism in methanotrophs offers rare pathway intermediate pools that could be further diversified using advanced synthetic biology tools to produce valuable SMs; for example, plant polyketides, rare carotenoids, and fatty acid-derived SMs. Recent advances in pathway reconstruction and production of isoprenoids, squalene, ectoine, polyhydroxyalkanoate copolymer, cadaverine, indigo, and shinorine serve as proof-of-concept. This review provides theoretical guidance for developing methanotrophs as microbial chassis for high-value SMs. We summarize the distinct secondary metabolic potentials of type I and type II methanotrophs, with specific attention to products relevant to biomedical applications. This review also includes native and non-native SMs from methanotrophs, their therapeutic potential, strategies to induce silent biosynthetic gene clusters, and challenges.

Characterization of an exopolysaccharide produced by Enterobacter sp. YU16-RN5 and its potential to alleviate cadmium induced cytotoxicity in vitro

Natural biopolymers have gained remarkable attention for bioremediation particularly in heavy metal removal and oil degradation due to their non-toxic nature and lack of secondary pollution. The exopolysaccharides (EPS) produced by the bacteria have become an important class of biopolymers that are employed in bioremediation. The bacteria isolated from the rhizospheric soil have higher metal tolerance and their EPS are effective in biosorption of heavy metals. Here, we report the characterization of an EPS (EPS-RN5) isolated from the root nodule-associated bacteria, Enterobacter cancerogenus strain YU16-RN5 and its heavy metal biosorption abilities. The bacteria isolated from the West coast of India was cultured in yeast extract mannitol (YEM) medium for EPS extraction and to study the production kinetics on a temporal scale. The biochemical composition, rheological properties and thermostability of EPS-RN5 was characterized by standard methods. The biosorption potential of EPS-RN5 against the selected heavy metals was analyzed by employing the inductively coupled plasma atomic emission spectroscopy (ICP-AES) technique. Further, cell culture experiments were used to test the role of EPS-RN5 in reducing the cytotoxicity exerted by the heavy metals in vitro using a human embryonic kidney cell line (HEK 293T). The bacteria showed good growth in YEM media and the maximum EPS yield was 1800 mg/L at 96 h. The molecular weight of EPS-RN5 was 0.7 × 106 Da and it contained 61.5% total sugars and 14.5% proteins. The monosaccharide composition of the EPS included glucose, sorbose and galactose in the ratio 0.25:0.07:1.0. The EPS-RN5 showed high thermal stability with a degradation temperature of 273 °C. Rheological analysis revealed the non-Newtonian behavior, with pseudoplastic characteristics. The EPS-RN5 efficiently absorbed cadmium and other heavy metals such as mercury, strontium, copper, arsenic, and uranium. In vitro studies revealed significant protective effect against the cadmium-induced cytotoxicity in HEK 293T cells. These results indicate the potential applications of EPS-RN5.

Bacterial Exopolysaccharides: Insight into Their Role in Plant Abiotic Stress Tolerance

Various abiotic stressors like drought, salinity, temperature, and heavy metals are major environmental stresses that affect agricultural productivity and crop yields all over the world. Continuous changes in climatic conditions put selective pressure on the microbial ecosystem to produce exopolysaccharides. Apart from soil aggregation, exopolysaccharide (EPS) production also helps in increasing water permeability, nutrient uptake by roots, soil stability, soil fertility, plant biomass, chlorophyll content, root and shoot length, and surface area of leaves while also helping maintain metabolic and physiological activities during drought stress. EPS-producing microbes can impart salt tolerance to plants by binding to sodium ions in the soil and preventing these ions from reaching the stem, thereby decreasing sodium absorption from the soil and increasing nutrient uptake by the roots. Biofilm formation in high-salinity soils increases cell viability, enhances soil fertility, and promotes plant growth and development. The third environmental stressor is presence of heavy metals in the soil due to improper industrial waste disposal practices that are toxic for plants. EPS production by soil bacteria can result in the biomineralization of metal ions, thereby imparting metal stress tolerance to plants. Finally, high temperatures can also affect agricultural productivity by decreasing plant metabolism, seedling growth, and seed germination. The present review discusses the role of exopolysaccharide-producing plant growth-promoting bacteria in modulating plant growth and development in plants and alleviating extreme abiotic stress condition. The review suggests exploring the potential of EPS-producing bacteria for multiple abiotic stress management strategies.

Expression of genes involved in exopolysaccharide synthesis in Lactiplantibacillus plantarum VAL6 under environmental stresses

Environmental factors can alter exopolysaccharide biosynthesis in lactic acid bacteria (LAB). To further clarify this potential relationship, the mRNA expression of genes involved in exopolysaccharide synthesis such as glmU, pgmB1, cps4E, cps4F, cps4J, and cps4H in Lactiplantibacillus plantarum VAL6 under different conditions including temperature, pH, sodium chloride (NaCl), and carbon dioxide (CO₂) intensification culture was studied. The transcriptomic data revealed that the exposure of L. plantarum VAL6 at pH 3 increased the expression level of cps4H but decreased the expression levels of pgmB1 and cps4E. Under pH 8, cps4F and cps4E were significantly upregulated, whereas pgmB1 was downregulated. Similarly, the expression levels of cps4F, cps4E, and cps4J increased sharply under stresses at 42 or 47 °C. In the case of NaCl stress, glmU, pgmB1, cps4J, and cps4H were downregulated in exposure to NaCl at 7 and 10% concentrations while cps4E and cps4F were upregulated at 1 h of 10%-NaCl treatment and at 5 h of 4%-NaCl treatment. Remarkably, CO₂ intensification culture stimulated the expression of all tested genes. In addition, simultaneous changes in expression of cps4E and cps4F under environmental challenges may elicit the possibility of an association between the two genes. These findings indicated that the expression level of eps genes is responsible for changes in the yield and monosaccharide composition of exopolysaccharides under environmental stresses.

Response of Lactobacillus plantarum VAL6 to challenges of pH and sodium chloride stresses

To investigate the effect of environmental stresses on the exopolysaccharide biosynthesis, after 24 h of culture at 37 °C with pH 6.8 and without sodium chloride, Lactobacillus plantarum VAL6 was exposed to different stress conditions, including pH (pHs of 3 and 8) and high sodium chloride concentration treatments. The results found that Lactobacillus plantarum VAL6 exposed to stress at pH 3 for 3 h gives the highest exopolysaccharide yield (50.44 g/L) which is 6.4 fold higher than non-stress. Under pH and sodium chloride stresses, the mannose content in exopolysaccharides decreased while the glucose increased in comparison with non-stress condition. The galactose content was highest under stress condition of pH 8 meantime rhamnose content increased sharply when Lactobacillus plantarum VAL6 was stressed at pH 3. The arabinose content in exopolysaccharides was not detected under non-stress condition but it was recorded in great amounts after 3 h of stress at pH 3. In addition, stress of pH 8 triggered the mRNA expression of epsF gene resulting in galactose-rich EPS synthesis. According to our results, the stresses of pH and sodium chloride enhance the production and change the mRNA expression of epsF gene, leading to differences in the monosaccharide composition of exopolysaccharides.

Exopolysaccharide II Is Relevant for the Survival of Sinorhizobium meliloti under Water Deficiency and Salinity Stress

Sinorhizobium meliloti is a soil bacterium of great agricultural importance because of its ability to fix atmospheric nitrogen in symbiotic association with alfalfa (Medicago sativa) roots. We looked into the involvement of exopolysaccharides (EPS) in its survival when exposed to different environmental stressors, as well as in bacteria-bacteria and bacteria-substrate interactions. The strains used were wild-type Rm8530 and two strains that are defective in the biosynthesis of EPS II: wild-type Rm1021, which has a non-functional expR locus, and mutant Rm8530 expA. Under stress by water deficiency, Rm8530 remained viable and increased in number, whereas Rm1021 and Rm8530 expA did not. These differences could be due to Rm8530's ability to produce EPS II. Survival experiments under saline stress showed that viability was reduced for Rm1021 but not for Rm8530 or Rm8530 expA, which suggests the existence of some regulating mechanism dependent on a functional expR that is absent in Rm1021. The results of salinity-induced stress assays regarding biofilm-forming capacity (BFC) and autoaggregation indicated the protective role of EPS II. As a whole, our observations demonstrate that EPS play major roles in rhizobacterial survival.

Overexpression of the Chromosome Partitioning Gene parA in Azorhizobium caulinodans ORS571 Alters the Bacteroid Morphotype in Sesbania rostrata Stem Nodules

Azorhizobium caulinodans ORS571 is a diazotroph that forms N2-fixing nodules on the roots and stems of the tropical legume Sesbania rostrata. Deletion of the parA gene of this bacterium results in cell cycle defects, pleiomorphic cell shape, and formation of immature stem nodules on its host plant. In this study, we constructed a parA overexpression mutant (PnptII-parA) to complement a previous study and provide new insights into bacteroid formation. We found that overproduction of ParA did not affect growth, cell morphology, chromosome partitioning, or vegetative nitrogen fixation in the free-living state. Under symbiosis, however, distinctive features, such as a single swollen bacteroid in one symbiosome, relatively narrow symbiosome space, and polyploid cells were observed. The morphotype of the PnptII-parA bacteroid is reminiscent of terminal differentiation in some IRLC indeterminate nodules, but S. rostrata is not thought to produce the NCR peptides that induce terminal differentiation in rhizobia. In addition, the transcript patterns of many symbiosis-related genes elicited by PnptII-parA were different from those elicited by the wild type. Accordingly, we propose that the particular symbiosome formation in PnptII-parA stem-nodules is due to cell cycle disruption caused by excess ParA protein in the symbiotic cells during nodulation.

Microbial Extracellular Polymeric Substances: Ecological Function and Impact on Soil Aggregation

A wide range of microorganisms produce extracellular polymeric substances (EPS), highly hydrated polymers that are mainly composed of polysaccharides, proteins, and DNA. EPS are fundamental for microbial life and provide an ideal environment for chemical reactions, nutrient entrapment, and protection against environmental stresses such as salinity and drought. Microbial EPS can enhance the aggregation of soil particles and benefit plants by maintaining the moisture of the environment and trapping nutrients. In addition, EPS have unique characteristics, such as biocompatibility, gelling, and thickening capabilities, with industrial applications. However, despite decades of research on the industrial potential of EPS, only a few polymers are widely used in different areas, especially in agriculture. This review provides an overview of current knowledge on the ecological functions of microbial EPSs and their application in agricultural soils to improve soil particle aggregation, an important factor for soil structure, health, and fertility.

Assessment of Methylobacterium oryzae CBMB20 aggregates for salt tolerance and plant growth promoting characteristics for bio-inoculant development

Salinity is one of the major factors contributing to the loss of crop productivity and thereby impacting livelihood of people in more than 100 countries of the world and the area of land affected by salinity is increasing day by day. This will worsen due to various factors such as drought that might result in high soil salinity. Use of plant growth promoting rhizobacteria is one of the promising eco-friendly strategies for salinity stress management as part of sustainable agricultural practices. However, it requires selecting rhizobacteria with good survivability and adaptation to salt stress. In this study we report aggregation of Methylobacterium oryzae CBMB20 cells grown in media containing high C/N ratio (30:1) than in media containing low C/N ratio (7:1). Aggregated Methylobacterium oryzae CBMB20 cells exhibited enhanced tolerance to UV irradiation, heat, desiccation, different temperature regimes, oxidative stress, starvation and supported higher population in media. Poly-β-hydroxybutyrate accumulation, exopolysaccharide production, proline accumulation and biofilm formation were good at 100 mM salt concentration with good microbial cell hydrophobicity at both 50 and 100 mM than other concentrations. Both the aggregated and non-aggregated cells grown under 0-200 mM salt concentrations produced IAA even at 200 mM salt concentration with a peak at 100 mM concentration with aggregated cells producing significantly higher quantities. ACC deaminase activity was observed in all NaCl concentrations studied with gradual and drastic reduction in aggregated and non-aggregated cells over increased salt concentrations.

Evaluation of the effects of different liquid inoculant formulations on the survival and plant-growth-promoting efficiency of Rhodopseudomonas palustris strain PS3

Biofertilizers can help improve soil quality, promote crop growth, and sustain soil health. The photosynthetic bacterium Rhodopseudomonas palustris strain PS3 (hereafter, PS3), which was isolated from Taiwanese paddy soil, can not only exert beneficial effects on plant growth but also enhance the efficiency of nutrient uptake from applied fertilizer. To produce this elite microbial isolate for practical use, product development and formulation are needed to permit the maintenance of the high quality of the inoculant during storage. The aim of this study was to select a suitable formulation that improves the survival and maintains the beneficial effects of the PS3 inoculant. Six additives (alginate, polyethylene glycol [PEG], polyvinylpyrrolidone-40 [PVP], glycerol, glucose, and horticultural oil) were used in liquid-based formulations, and their capacities for maintaining PS3 cell viability during storage in low, medium, and high temperature ranges were evaluated. Horticultural oil (0.5 %) was chosen as a potential additive because it could maintain a relatively high population and conferred greater microbial vitality under various storage conditions. Furthermore, the growth-promoting effects exerted on Chinese cabbage by the formulated inoculants were significantly greater than those of the unformulated treatments. The fresh and dry weights of the shoots were significantly increased, by 10-27 and 22-40 %, respectively. Horticultural oil is considered a safe, low-cost, and easy-to-process material, and this formulation would facilitate the practical use of strain PS3 in agriculture.

Plant growth promoting rhizobia: challenges and opportunities

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 β-1,3 glucanase productions and enhance plant defense by triggering induced systemic resistance through lipopolysaccharides, 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.

Water-limiting conditions alter the structure and biofilm-forming ability of bacterial multispecies communities in the alfalfa rhizosphere

Biofilms are microbial communities that adhere to biotic or abiotic surfaces and are enclosed in a protective matrix of extracellular compounds. An important advantage of the biofilm lifestyle for soil bacteria (rhizobacteria) is protection against water deprivation (desiccation or osmotic effect). The rhizosphere is a crucial microhabitat for ecological, interactive, and agricultural production processes. The composition and functions of bacterial biofilms in soil microniches are poorly understood. We studied multibacterial communities established as biofilm-like structures in the rhizosphere of Medicago sativa (alfalfa) exposed to 3 experimental conditions of water limitation. The whole biofilm-forming ability (WBFA) for rhizospheric communities exposed to desiccation was higher than that of communities exposed to saline or nonstressful conditions. A culture-dependent ribotyping analysis indicated that communities exposed to desiccation or saline conditions were more diverse than those under the nonstressful condition. 16S rRNA gene sequencing of selected strains showed that the rhizospheric communities consisted primarily of members of the Actinobacteria and α- and γ-Proteobacteria, regardless of the water-limiting condition. Our findings contribute to improved understanding of the effects of environmental stress factors on plant-bacteria interaction processes and have potential application to agricultural management practices.

Identification and characterization of a NaCl-responsive genetic locus involved in survival during desiccation in Sinorhizobium meliloti

The Rhizobiaceae are a bacterial family of enormous agricultural importance due to the ability of its members to fix atmospheric nitrogen in an intimate relationship with plants. Their survival as naturally occurring soil bacteria in agricultural soils as well as popular seed inocula is affected directly by drought and salinity. Survival after desiccation in the presence of NaCl is enabled by underlying genetic mechanisms in the model organism Sinorhizobium meliloti 1021. Since salt stress parallels a loss in water activity, the identification of NaCl-responsive loci may identify loci involved in survival during desiccation. This approach enabled identification of the loci asnO and ngg by their reduced ability to grow on increased NaCl concentrations, likely due to their inability to produce the osmoprotectant N-acetylglutaminylglutamine (NAGGN). In addition, the mutant harboring ngg::Tn5luxAB was affected in its ability to survive desiccation and responded to osmotic stress. The desiccation sensitivity may have been due to secondary functions of Ngg (N-acetylglutaminylglutamine synthetase)-like cell wall metabolism as suggested by the presence of a d-alanine-d-alanine ligase (dAla-dAla) domain and by sensitivity of the mutant to β-lactam antibiotics. asnO::Tn5luxAB is expressed during the stationary phase under normal growth conditions. Amino acid sequence similarity to enzymes producing β-lactam inhibitors and increased resistance to β-lactam antibiotics may indicate that asnO is involved in the production of a β-lactam inhibitor.

Relevance of fucose-rich extracellular polysaccharides produced by Rhizobium sullae strains nodulating Hedysarum coronarium l. legumes

Specific and complex interactions between soil bacteria, known as rhizobia, and their leguminous host plants result in the development of root nodules. This process implies a complex dialogue between the partners. Rhizobia synthesize different classes of polysaccharides: exopolysaccharides (EPS), Kdo-rich capsular polysaccharides, lipopolysaccharides, and cyclic β-(1,2)-glucans. These polymers are actors of a successful symbiosis with legumes. We focus here on studying the EPS produced by Rhizobium sullae bacteria that nodulate Hedysarum coronarium L., largely distributed in Algeria. We describe the influence of the carbon source on the production and on the composition of EPS produced by R. sullae A6 and RHF strains. High-molecular-weight EPS preserve the bacteria from desiccation. The structural characterization of the EPS produced by R. sullae strains has been performed through sugar analysis by gas chromatography-mass spectrometry. The low-molecular-weight EPS of one strain (RHF) has been totally elucidated using nuclear magnetic resonance and quantitative time-of-flight tandem mass spectrometry analyses. An unusual fucose-rich EPS has been characterized. The presence of this deoxy sugar seems to be related to nodulation capacity.

Bacterial exopolysaccharide and biofilm formation stimulate chickpea growth and soil aggregation under salt stress

To compensate for stress imposed by salinity, biofilm formation and exopolysaccharide production are significant strategies of salt tolerant bacteria to assist metabolism. We hypothesized that two previously isolated salt-tolerant strains Halomonas variabilis (HT1) and Planococcus rifietoensis (RT4) have an ability to improve plant growth, These strains can form biofilm and accumulate exopolysacharides at increasing salt stress. These results showed that bacteria might be involved in developing microbial communities under salt stress and helpful in colonizing of bacterial strains to plant roots and soil particles. Eventually, it can add to the plant growth and soil structure. We investigated the comparative effect of exopolysacharide and biofilm formation in two bacterial strains Halomonas variabilis (HT1) and Planococcus rifietoensis (RT4) in response to varying salt stress. We found that biofilm formation and exopolysaccharide accumulation increased at higher salinity. To check the effect of bacterial inoculation on the plant (Cicer arietinum Var. CM-98) growth and soil aggregation, pot experiment was conducted by growing seedlings under salt stress. Inoculation of both strains increased plant growth at elevated salt stress. Weight of soil aggregates attached with roots and present in soil were added at higher salt concentrations compared to untreated controls. Soil aggregation was higher at plant roots under salinity. These results suggest the feasibility of using above strains in improving plant growth and soil fertility under salinity.

Biofilm formation in moderately halophilic bacteria is influenced by varying salinity levels

Bacteria in a biofilm have a co-dependent lifestyle resulting in a harmonized and complex coordination of the bacterial cells within an exopolysaccharide (EPS) matrix. We hypothesized that biofilm formation and EPS production in salt-tolerant bacteria are helpful for plant growth improvement in saline soil, but that they are influenced differently. To investigate this hypothesis, we tested the effect of different salinity levels on the biofilm formation of the bacterial strains PAa6 (Halomonas meridiana), HT2 (Kushneria indalinina) and ST2 (Halomonas aquamarina) on different abiotic and biotic surfaces. Maximum biofilm formation was established at 1 M salt concentration. However, EPS production was maximal at 0-1 M NaCl stress. We also studied the effect of salt stress on EPS produced by the bacterial strains and confirmed the presence of EPS on Cicer arietinum var. CM 98 roots and in soil at different salinity levels, using Alcian blue staining. Overall, the strain PAa6 was more effective in biofilm formation and EPS production. Under saline and non-saline conditions, this strain also colonized the plant roots more efficiently as compared to the other two strains. We conclude that the strain PAa6 has the potential of biofilm formation and EPS production at different salinity levels. The presence of EPS in the biofilm helped the bacterial strains to better colonize the roots.

Modification of exopolysaccharide composition and production by three cyanobacterial isolates under salt stress

BACKGROUND, AIM, AND SCOPE

Polysaccharides are renewable resources representing an important class of polymeric materials of biotechnological interest, offering a wide variety of potentially useful products to mankind. Exopolysaccharides (EPSs) of microbial origin with a novel functionality, reproducible physico-chemical properties, stable cost and supply, became a better alternative to polysaccharides of algal origin. EPSs are believed to protect bacterial cells from desiccation, heavy metals or other environmental stresses, including hostimmune responses, and to produce biofilms, thus enhancing the cells chances of colonising special ecological niches. One of the most important stress factor is salt stress for microorganisms. The present investigation is aimed to determine correlation between salt resistance and EPS production by three cyanobacterial isolates (Synechocystis sp. BASO444, Synechocystis sp. BASO507 and Synechocystis sp. BASO511). It is also aimed to investigate the effect of salt concentrations on EPS production by cyanobacteria and effect of salt on monosaccharide composition of EPS.

MATERIALS AND METHODS

Cyanobacterial isolates were identified by 16 S rRNA analysis. Its salt (NaCl) tolerance and association with exopolysaccharides (EPSs) production in three cyanobacterial isolates were investigated. Also, EPS was analysed by HPLC for monomer characterization.

RESULTS

Increased EPS production was associated with NaCl tolerance. The most tolerant isolate, Synechocystis sp. BASO444, secreted the most EPS (500 mg/L). EPS production by Synechocystis sp. BASO444, Synechocystis sp. BASO507 and Synechocystis sp. BASO511 was investigated following exposure to 0.2 and 0.4 M NaCl. Also, flasks containing medium without NaCl were inoculated in the same manner to serve as controls. The monosaccharide compositions of EPS produced by the three isolates following exposure to 0.2 M NaCl were analysed by HPLC. Control EPS of BASO444 was composed of glucose (97%) and galacturonic acid (3%). The composition of BASO511 (control) was glucose (95%), xylose (4.80%), arabinose (0.13%), glucuronic acid (0.03%) and galacturonic acid (0.04%). However, the composition of BASO507 (control) was glucose (0.98%), xylose (98.00%), arabinose (1.00%), glucuronic acid (0.01%) and galacturonic acid (0.01%). In the presence of 0.2 M NaCl, EPS compositions and ratios of three cyanobacterial isolates changed.

DISCUSSION

Although hyperproduction of EPS in response to starvation, antiviral activity, thickening agent and cosmetic industry for product formulations has been reported for cyanobacteria, the effect of NaCl on EPS production in cyanobacteria is not a popular area of study. There are no clear reports correlating EPS production and NaCl tolerance. The gap in the data about the effect of NaCl on cyanobacterial EPS production was filled by this investigation, and the results of our study have important implications in both the industrial and environmental arenas.

CONCLUSIONS

Our results indicate that 1) exposure to elevated concentrations of NaCl affects the composition of EPS produced by Synechocystis sp. BASO444, Synechocystis sp. BASO507 and Synechocystis sp. BASO511, and 2) there is a correlation between NaCl tolerance and EPS production in some cyanobacteria.

RECOMMENDATIONS AND PERSPECTIVES

Differences in the monosaccharide composition and ratios of EPS may promote NaCl tolerance in these microorganisms. As well, these alternative composition polysaccharides may be important for industrial applications.

Production of succinoglycan polymer in Sinorhizobium meliloti is affected by SMb21506 and requires the N-terminal domain of ExoP

The protein tyrosine kinase ExoP, consisting of an N-terminal periplasmic and a C-terminal cytoplasmic domain, is important for polymerization of the exopolysaccharide succinoglycan (EPS I) in Sinorhizobium meliloti. We analyzed the contribution of the ExoP paralogs ExoP2 and SMb21506 to the production of the high molecular weight (HMW) form of EPS I. ExoP2, though not contributing to EPS I or lipopolysaccharide biosynthesis, showed increased expression at high osmolarity and was expressed in Medicago sativa nodules, suggesting an involvement in the synthesis of an as-yet-unidentified polysaccharide. Furthermore, a mutation in SMb21506 affected the production of HMW EPS I, particularly in the absence of the C-terminal ExoP domain. High salinity induced the production of HMW EPS I by the wild type and mutants whereas high osmolarity had the opposite effect. It was shown that ExoP localizes at the inner membrane of S. meliloti cells. Tyrosine phosphorylation of the C-terminal domain was strongly increased by amino acid substitutions in the polysaccharide co-polymerase motif (formerly proline-rich motif) located in the N-terminal domain, suggesting that this phosphorylation could be modulated by conformational changes of the N-terminal domain. Moreover, deletion of a coiled-coil motif present in the N-terminal domain abolished phosphorylation and EPS I production and, consequently, the ability to nodulate M. sativa.

Importance of trehalose biosynthesis for Sinorhizobium meliloti Osmotolerance and nodulation of Alfalfa roots

The disaccharide trehalose is a well-known osmoprotectant, and trehalose accumulation through de novo biosynthesis is a common response of bacteria to abiotic stress. In this study, we have investigated the role of endogenous trehalose synthesis in the osmotolerance of Sinorhizobium meliloti. Genes coding for three possible trehalose synthesis pathways are present in the genome of S. meliloti 1021: OtsA, TreYZ, and TreS. Among these, OtsA has a major role in trehalose accumulation under all of the conditions tested and is the main system involved in osmoadaptation. Nevertheless, the other two systems are also important for growth in hyperosmotic medium. Genes for the three pathways are transcriptionally responsive to osmotic stress. The presence of at least one functional trehalose biosynthesis pathway is required for optimal competitiveness of S. meliloti to nodulate alfalfa roots.

GeneSMb21071of plasmid pSymB is required for osmoadaptation ofSinorhizobium meliloti1021 and is implicated in modifications of cell surface polysaccharides structure in response to hyperosmotic stress

Megaplasmid pSymB of the nitrogen-fixing symbiont Sinorhizobium meliloti , implicated in adaptation to hyperosmotic stress, contains 11 gene clusters that apparently encode surface polysaccharides. However, only 2 of these clusters, containing the exo and exp genes, have been associated with the synthesis of the acidic exopolysaccharides succinoglycan and galactoglucan, respectively. The functions of the other 9 clusters remain unsolved. The involvement of one of those regions, pSymB cluster 3, on surface polysaccharide synthesis and its possible implication in osmoadaptation were investigated. In silico analysis of cluster 3 showed that it putatively encodes for the synthesis and transport of a methylated surface polysaccharide. Mutants affected in this cluster were symbiotically effective but showed defects in growth under saline and nonsaline osmotic stress. The gene SMb21071, encoding a putative initiating glycosyltransferase, is transcriptionally induced under hyperosmotic conditions. Sodium dodecyl sulfate – polyacrylamide gel electrophoresis and silver staining showed that osmotic stresses changed the profiles of surface polysaccharides of wild-type and mutants strains in different ways. The overall results suggest that cluster 3 is important for growth under saline stress and essential for growth under nonsaline hyperosmotic stress, and it appears to be implicated in maintaining and (or) modifying surface polysaccharides in response to osmotic stress.

Role of potassium uptake systems in Sinorhizobium meliloti osmoadaptation and symbiotic performance

Stimulation of potassium uptake is the most rapid response to an osmotic upshock in bacteria. This cation accumulates by a number of different transport systems whose importance has not been previously addressed for rhizobia. In silico analyses reveal the presence of genes encoding four possible potassium uptake systems in the genome of Sinorhizobium meliloti 1021: Kup1, Kup2, Trk, and Kdp. The study of the relevance of these systems under a number of different growth conditions and in symbiosis showed that the integrity of Kup1 or Trk is essential for growth under laboratory conditions even in osmotically balanced media and the absence of both systems leads to a reduced infectivity and competitiveness of the bacteria in alfalfa roots. Trk is the main system involved in the accumulation of potassium after an osmotic upshift and the most important system for growth of S. meliloti under hyperosmotic conditions. The other three systems, especially Kup1, are also relevant during the osmotic adaptation of the cell, and the relative importance of the Kdp system increases at low potassium concentrations.

Salt-inducible bionylon polymer from Bacillus megaterium

Poly-gamma-glutamate (PGA) is a chiral polyamide material that possesses a nylon-like backbone, a bionylon polymer. We examined the PGA productivity of Bacillus megaterium and found NaCl-responsive PGA production in the bacterium. In the system of B. megaterium, salt would be significant in controlling the yield, molecular size, and stereochemistry of bionylon.

Isolation of salt-sensitive mutants of Sinorhizobium meliloti strain Rm1021

The determinants necessary for adaptation to high NaCl concentrations and competition for nodule occupancy in Sinorhizobium meliloti were investigated genetically. Mutations in fabG as well as smc02909 (transmembrane transglycosylase), trigger factor (tig) and smc00717 (probably ftsE) gave rise to strains that were unable to tolerate high salt and were uncompetitive for nodule occupancy relative to the wild-type. Moreover exoF1, exoA and pgm determinants were determined to be necessary for strain Rm1021 to survive high NaCl and/or MgCl(2) concentrations. The introduction of an expR(+) allele was capable of suppressing the Mg(2+) sensitivity associated with the exoF1, but not the exoA, mutation in a manner independent of exopolysaccharide II (EPS II)-associated mucoidy. The results also show that the EPS II-associated mucoid phenotype was affected by either Mg(2+)or K(+), but not by Li(+), Ca(2+), or high osmolarity.

Transcriptome profiling reveals the importance of plasmid pSymB for osmoadaptation of Sinorhizobium meliloti

In this work, DNA microarrays were used to investigate genome-wide transcriptional responses of Sinorhizobium meliloti to a sudden increase in external osmolarity elicited by addition of either NaCl or sucrose to exponentially growing cultures. A time course of the response within the first 4 h after the osmotic shock was established. We found that there was a general redundancy in the differentially expressed genes after NaCl or sucrose addition. Both kinds of stress resulted in induction of a large number of genes having unknown functions and in repression of many genes coding for proteins with known functions. There was a strong replicon bias in the pattern of the osmotic stress response; whereas 64% of the upregulated genes had a plasmid localization, 85% of the downregulated genes were chromosomal. Among the pSymB osmoresponsive genes, 83% were upregulated, suggesting the importance of this plasmid for S. meliloti osmoadaptation. Indeed, we identified a 200-kb region in pSymB needed for adaptation to saline shock which has a high density of osmoregulated genes.

High temperature-induced changes in exopolysaccharides, lipopolysaccharides and protein profile of heat-resistant mutants of Rhizobium sp. (Cajanus)

A thermosensitive wild-type strain (PP201) of Rhizobium sp. (Cajanus) and its 14 heat-resistant mutants were characterized biochemically with regard to their cell surface (exopolysaccharides (EPSs) and lipopolysaccharides (LPSs)) properties and protein profile. Differences were observed between the parent strain and the mutants in all these parameters under high temperature conditions. At normal temperature (30 degrees C), only half of the mutant strains produced higher amounts of EPSs than the parent strain, but at 43 degrees C, all the mutants produced higher quantities of EPS. The LPS electrophoretic pattern of the parent strain PP201 and the heat-resistant mutants was almost identical at 30 degrees C. At 43 degrees C, the parent strain did not produce LPS but the mutants produced both kinds of LPSs. The protein electrophoretic pattern showed that the parent strain PP201 formed very few proteins at high temperature, whereas the mutants formed additional new proteins. A heat shock protein (Hsp) of 63-74 kDa was overproduced in all mutant strains.

Cloning and expression of afpA, a gene encoding an antifreeze protein from the arctic plant growth-promoting rhizobacterium Pseudomonas putida GR12-2

The Arctic plant growth-promoting rhizobacterium Pseudomonas putida GR12-2 secretes an antifreeze protein (AFP) that promotes survival at subzero temperatures. The AFP is unusual in that it also exhibits a low level of ice nucleation activity. A DNA fragment with an open reading frame encoding 473 amino acids was cloned by PCR and inverse PCR using primers designed from partial amino acid sequences of the isolated AFP. The predicted gene product, AfpA, had a molecular mass of 47.3 kDa, a pI of 3.51, and no previously known function. Although AfpA is a secreted protein, it lacked an N-terminal signal peptide and was shown by sequence analysis to have two possible secretion systems: a hemolysin-like, calcium-binding secretion domain and a type V autotransporter domain found in gram-negative bacteria. Expression of afpA in Escherichia coli yielded an intracellular 72-kDa protein modified with both sugars and lipids that exhibited lower levels of antifreeze and ice nucleation activities than the native protein. The 164-kDa AFP previously purified from P. putida GR12-2 was a lipoglycoprotein, and the carbohydrate was required for ice nucleation activity. Therefore, the recombinant protein may not have been properly posttranslationally modified. The AfpA sequence was most similar to cell wall-associated proteins and less similar to ice nucleation proteins (INPs). Hydropathy plots revealed that the amino acid sequence of AfpA was more hydrophobic than those of the INPs in the domain that forms the ice template, thus suggesting that AFPs and INPs interact differently with ice. To our knowledge, this is the first gene encoding a protein with both antifreeze and ice nucleation activities to be isolated and characterized.

Construction and validation of a Sinorhizobium meliloti whole genome DNA microarray: genome-wide profiling of osmoadaptive gene expression

Based on the complete Sinorhizobium meliloti genome sequence we established DNA microarrays as a comprehensive tool for systematic genome-wide gene expression analysis in S. meliloti 1021. For these PCR fragment-based microarrays, called Sm6kPCR, a collection of probes for the 6207 predicted protein-coding genes consisting of 6046 gene-specific PCR fragments and 161 70 mer oligonucleotides was arrayed in high density on glass slides. To obtain these PCR fragments primer pairs were designed to amplify internal gene-specific DNA fragments of 80-350 bp. Additionally, these primers were characterized by a 5' extension that allowed for reamplification using standard primers after the first amplification employing the specific primers. In order to ascertain the quality of the Sm6kPCR microarrays and to validate gene expression studies in S. meliloti parallel hybridizations based on RNA samples obtained from cells cultured under identical conditions were performed. In addition, gene expression in S. meliloti in response to an osmotic upshift imposed by the addition of 0.38 M NaCl was monitored. 137 genes were identified showing significant changes in gene expression resulting from the osmotic upshift. From these genes 52 were induced and 85 genes were repressed. Among the genes displaying different RNA levels some functional groups could be identified that are particularly remarkable. Repression was observed for 8 genes related to motility and chemotaxis, 7 genes encoding amino acid biosynthesis enzymes and 15 genes involved in iron uptake whereas 14 genes involved in transport of small molecules and 4 genes related to polysaccharide biosynthesis were induced.

Cell surface characteristics of two halotolerant strains of Sinorhizobium meliloti

The halotolerant Sinorhizobium meliloti strain Rmd201 and its variant Rmd201 a were examined for their cell surface properties. The variant strain formed rough colonies and was found to be more hydrophobic. Growth of the variant strain was not affected appreciably when NaCl concentration of the medium was increased from 2 mM to 700 mM. Exopolysaccharide (EPS) and the lipopolysaccharide (LPS) content of the variant strain was found to be 7 and 14 times less, respectively, than the parent strain. However, enhanced synthesis of high molecular weight LPS bands were observed in SDS-PAGE analysis in the variant strain when the NaCl concentration was raised from 2 mM to 700 mM. Ribose and glucosamine were present in the variant LPS only. Mannose appeared as a major LPS constituent of the variant when grown in high salt containing medium. All these cell surface characteristics indicated that there were significant differences between the halotolerant strains of S. meliloti. The changes in the cell surface of the variant strain indicated the possible mutation in the gene(s) of cell surface polysaccharide biosynthesis.

Specific binding of the regulatory protein ExpG to promoter regions of the galactoglucan biosynthesis gene cluster of Sinorhizobium meliloti--a combined molecular biology and force spectroscopy investigation

Specific protein-DNA interaction is fundamental for all aspects of gene transcription. We focus on a regulatory DNA-binding protein in the Gram-negative soil bacterium Sinorhizobium meliloti 2011, which is capable of fixing molecular nitrogen in a symbiotic interaction with alfalfa plants. The ExpG protein plays a central role in regulation of the biosynthesis of the exopolysaccharide galactoglucan, which promotes the establishment of symbiosis. ExpG is a transcriptional activator of exp gene expression. We investigated the molecular mechanism of binding of ExpG to three associated target sequences in the exp gene cluster with standard biochemical methods and single molecule force spectroscopy based on the atomic force microscope (AFM). Binding of ExpG to expA1, expG-expD1, and expE1 promoter fragments in a sequence specific manner was demonstrated, and a 28 bp conserved region was found. AFM force spectroscopy experiments confirmed the specific binding of ExpG to the promoter regions, with unbinding forces ranging from 50 to 165 pN in a logarithmic dependence from the loading rates of 70-79000 pN/s. Two different regimes of loading rate-dependent behaviour were identified. Thermal off-rates in the range of k(off)=(1.2+/-1.0) x 10(-3)s(-1) were derived from the lower loading rate regime for all promoter regions. In the upper loading rate regime, however, these fragments exhibited distinct differences which are attributed to the molecular binding mechanism.

O-antigen structural variation: mechanisms and possible roles in animal/plant-microbe interactions

Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

Rhizobium tropici genes involved in free-living salt tolerance are required for the establishment of efficient nitrogen-fixing symbiosis with Phaseolus vulgaris

Characterization of nine transposon-induced mutants of Rhizobium tropici with decreased salt tolerance (DST) allowed the identification of eight gene loci required for adaptation to high external NaCl. Most of the genes also were involved in adaptation to hyperosmotic media and were required to overcome the toxicity of LiCl. According to their possible functions, genes identified could be classified into three groups. The first group included two genes involved in regulation of gene expression, such as ntrY, the sensor element of the bacterial ntrY/ntrX two-component regulatory system involved in regulation of nitrogen metabolism, and greA, which encodes a transcription elongation factor. The second group included genes related to synthesis, assembly, or maturation of proteins, such as alaS coding for alanine-tRNA synthetase, dnaJ, which encodes a molecular chaperone, and a nifS homolog probably encoding a cysteine desulfurase involved in the maturation of Fe-S proteins. Genes related with cellular build-up and maintenance were in the third group, such as a noeJ-homolog, encoding a mannose-1-phosphate guanylyltransferase likely involved in lipopolysaccharide biosynthesis, and kup, specifying an inner-membrane protein involved in potassium uptake. Another gene was identified that had no homology to known genes but that could be conserved in other rhizobia. When inoculated on Phaseolus vulgaris growing under nonsaline conditions, all DST mutants displayed severe symbiotic defects: ntrY and noeJ mutants were impaired in nodulation, and the remaining mutants formed symbiosis with very reduced nitrogenase activity. The results suggest that bacterial ability to adapt to hyperosmotic and salt stress is important for the bacteroid nitrogen-fixing function inside the legume nodule and provide genetic evidence supporting the suggestion that rhizobia face severe environmental changes after their release into plant cells.

MucR and mucS activate exp genes transcription and galactoglucan production in Sinorhizobium meliloti EFB1

When grown under standard conditions, Sinorhizobium meliloti EFB1 simultaneously produces two acidic exopolysaccharides, succinoglycan and galactoglucan, yielding very mucoid colonies. In this strain, MucR is essential for galactoglucan synthesis. A mutation in the mucS gene resulted in less mucoid colonies than in the wild-type EFB1. This mucS- strain was complemented to the wild-type phenotype by the cloned mucS gene, indicating that mucS is necessary for a wild-type level of galactoglucan production. Reverse transcription-polymerase chain reaction analysis of exp genes, which encode the pathway for galactoglucan production, in EFB1 and in the mutants affected in mucS, mucR, and both genes simultaneously, showed that MucS is a transcriptional activator of the exp genes but does not affect its own transcription. Furthermore, MucR is necessary for mucS transcriptional activation. As introduction of a cloned mucS gene in a mucR- strain yielded colonies less mucoid than the wild type, MucR could also activate exp genes transcription through other pathways. Deletion analysis of the expE promoter showed a region important for transcription and MucS activation. This region, containing a palindrome, is present in the putative expA, expC, expD, and expE promoters but not in the mucS promoter, suggesting that it is the target for MucS. A mucR-mucS- mutant, which does not produce galactoglucan, was impaired in competitive nodulation of alfalfa in soil microcosms, indicating another possible role for this exopolysaccharide in symbiosis.

Purification and properties of a glucuronan lyase from Sinorhizobium meliloti M5N1CS (NCIMB 40472)

A glucuronan lyase extracted from Sinorhizobium meliloti strain M5N1CS was purified to homogeneity by anion-exchange chromatography. The purified enzyme corresponds to a monomer with a molecular mass of 20 kDa and a pI of 4.9. A specific activity was found only for polyglucuronates leading to the production of 4,5-unsaturated oligoglucuronates. The enzyme activity was optimal at pH 6.5 and 50 degrees C. Zn(2+), Cu(2+), and Hg(2+) (1 mM) inhibited the enzyme activity. No homology of the enzyme N-terminal amino acid sequence was found with any of the previously published protein sequences. This enzyme purified from S. meliloti strain M5N1CS corresponding to a new lyase was classified as an endopolyglucuronate lyase.

MucR is necessary for galactoglucan production in Sinorhizobium meliloti EFB1

Sinorhizobium meliloti can produce two types of acidic exopolysaccharides, succinoglycan and galactoglucan, that are interchangeable for infection of alfalfa nodules. Strain SU47 and derivatives produce only succinoglycan, unless it grows under phosphate limitation or carries a mutation in either of two regulatory loci, mucR or expR. It has been proposed that MucR acts as a transcriptional repressor that blocks the expression of the exp genes responsible for galactoglucan production. Strain EFB1 simultaneously produces both exopolysaccharides. Heterologous expression of lacZ transcriptional fusions of the expE promoters has shown that genetic background is more important that promoter sequence for exp gene expression, since expE promoters from both strains are expressed at high level in EFB1 and not in SU47. We have found that mucR is present in mucoid and nonmucoid strains, and in EFB1 differs from SU47 in only one conservative amino acid change. MucR proteins from both strains are interchangeable. An mucR mutant of EFB1 cannot produce galactoglucan and does not express mucS.

Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate

Biological N(2) fixation represents the major source of N input in agricultural soils including those in arid regions. The major N(2)-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N(2)-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N(2)-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Expression and regulation of phosphate stress inducible genes in Sinorhizobium meliloti

Sinorhizobium meliloti 104A14 was mutated with transposon Tn5B22, which creates lacZ transcriptional fusions when inserted in the correct orientation relative to the promoter. This promoter reporter allowed us to identify six phosphate stress inducible (psi) genes in S. meliloti that are up-regulated in response to inorganic phosphate (Pi) starvation. The transposon and flanking DNA were cloned from each psi::Tn5B22 reporter mutant and the junction DNA sequenced. High identity/similarity of the inferred peptides with those in major data bases allowed identification of the following genes: dnaK, expC, pssB, ackA, vipC, and prkA. The prkA homolog was also found to be up-regulated in response to carbon starvation and when nitrate replaced ammonium as the nitrogen source. Through allele replacement techniques, PhoB- mutants were generated for the expC, ackA, vipC, and pssB reporter strains. Loss of a functional PhoB resulted in the absence of Pi-sensitive induction in all four genes. These experiments suggest the Pho regulon in S. meliloti includes genes that presumably are not directly linked to Pi acquisition or assimilation. The psi strains were tested for their symbiotic properties under growth conditions that were Pi-limiting or Pi-nonlimiting for the host plant. All were Nod+ and Fix+ except the reporter strain of dnaK transcription, which was less effective than the wild-type strain under both P treatments, indicating DnaK is required for optimum symbiotic function.