The pssB gene product of Rhizobium leguminosarum bv. trifolii is homologous to a family of inositol monophosphatases

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes that involves the rhizobial infection of roots and the bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role for EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia, and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium-legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

Proteomic analysis of free-living Bradyrhizobium diazoefficiens: highlighting potential determinants of a successful symbiosis

BACKGROUND

Strain CPAC 7 (=SEMIA 5080) was recently reclassified into the new species Bradyrhizobium diazoefficiens; due to its outstanding efficiency in fixing nitrogen, it has been used in commercial inoculants for application to crops of soybean [Glycine max (L.) Merr.] in Brazil and other South American countries. Although the efficiency of B. diazoefficiens inoculant strains is well recognized, few data on their protein expression are available.

RESULTS

We provided a two-dimensional proteomic reference map of CPAC 7 obtained under free-living conditions, with the successful identification of 115 spots, representing 95 different proteins. The results highlighted the expression of molecular determinants potentially related to symbiosis establishment (e.g. inositol monophosphatase, IMPase), fixation of atmospheric nitrogen (N2) (e.g. NifH) and defenses against stresses (e.g. chaperones). By using bioinformatic tools, it was possible to attribute probable functions to ten hypothetical proteins. For another ten proteins classified as "NO related COG" group, we analyzed by RT-qPCR the relative expression of their coding-genes in response to the nodulation-gene inducer genistein. Six of these genes were up-regulated, including blr0227, which may be related to polyhydroxybutyrate (PHB) biosynthesis and competitiveness for nodulation.

CONCLUSIONS

The proteomic map contributed to the identification of several proteins of B. diazoefficiens under free-living conditions and our approach-combining bioinformatics and gene-expression assays-resulted in new information about unknown genes that might play important roles in the establishment of the symbiosis with soybean.

The inositol monophosphatase All2917 (IMPA1) is involved in osmotic adaptation in Anabaena sp. PCC7120

Inositol monophosphatase (IMPase; EC 3.1.3.25) acts at the last step in the inositol biosynthesis pathway by hydrolysing inositol-1-phosphate into inositol. In this study, an IMPase encoding gene, all2917 from Anabaena sp. PCC7120, was characterized. We found that All2917 exhibits a specific activity on inositol-1-phosphate, in a typical Mg(2+) -dependent, Li(+) -sensitive manner. The deletion of all2917 in Anabaena made the cells more sensitive to osmotic stress caused by sucrose or sorbitol, while its overexpression led to an increased resistance to such stress. Consistent with these phenotypes, the transcription of all2917 was significantly upregulated upon the sucrose-mediated osmotic stress. Phylogenic analysis using 134 IMPase homologues from 36 cyanobacterial strains shows that members of IMPase family form three major distinct clades, suggesting that multiple copies of IMPase family proteins have been maintained in Cyanobacteria during a long history of evolution, and they may play important roles in cyanobacterial physiology.

Environmental signals and regulatory pathways that influence exopolysaccharide production in rhizobia

Rhizobia are Gram-negative bacteria that can exist either as free-living bacteria or as nitrogen-fixing symbionts inside root nodules of leguminous plants. The composition of the rhizobial outer surface, containing a variety of polysaccharides, plays a significant role in the adaptation of these bacteria in both habitats. Among rhizobial polymers, exopolysaccharide (EPS) is indispensable for the invasion of a great majority of host plants which form indeterminate-type nodules. Various functions are ascribed to this heteropolymer, including protection against environmental stress and host defense, attachment to abiotic and biotic surfaces, and in signaling. The synthesis of EPS in rhizobia is a multi-step process regulated by several proteins at both transcriptional and post-transcriptional levels. Also, some environmental factors (carbon source, nitrogen and phosphate starvation, flavonoids) and stress conditions (osmolarity, ionic strength) affect EPS production. This paper discusses the recent data concerning the function of the genes required for EPS synthesis and the regulation of this process by several environmental signals. Up till now, the synthesis of rhizobial EPS has been best studied in two species, Sinorhizobium meliloti and Rhizobium leguminosarum. The latest data indicate that EPS synthesis in rhizobia undergoes very complex hierarchical regulation, in which proteins engaged in quorum sensing and the regulation of motility genes also participate. This finding enables a better understanding of the complex processes occurring in the rhizosphere which are crucial for successful colonization and infection of host plant roots.

[The pssA gene encodes UDP-glucose: polyprenyl phosphate-glucosyl phosphotransferase initiating biosynthesis of Rhizobium leguminosarum exopolysaccharide]

Symbiotic nitrogen-fixing bacteria Rhizobium leguminosarum by. viciae VF39 secrete an acidic heteropolysaccharide, the biosynthesis of which involves the stage of polyprenyl diphosphate octasaccharide formation, with its carbohydrate fragment corresponding to the repeating polymer unit. The amino acid analysis of the product of the pssA gene, we have earlier identified, showed its homology to bacterial polyisoprenyl phosphate hexose 1-phosphate transferases catalyzing the formation of phosphodiester bonds between polyprenyl phosphates and hexose 1-phosphates, whose donors are nucleotide sugars. The immunoblotting demonstrated that Rhizobium cells synthesize a protein with a molecular mass of 25 kDa, which implies the translation of the open reading frame occurring from the second initiating codon followed by the protein processing. It was shown that PssA is an integral membrane-bound protein involved in glucose 1-phosphate transfer from UDP-glucose to polyprenyl phosphate to form polyprenyl diphosphate glucose. These results suggest that the pssA gene encodes UDP-glucose:polyprenyl phosphate-glucosyl phosphotransferase.

Rhizobial exopolysaccharides: genetic control and symbiotic functions

Specific complex interactions between soil bacteria belonging to Rhizobium, Sinorhizobium, Mesorhizobium, Phylorhizobium, Bradyrhizobium and Azorhizobium commonly known as rhizobia, and their host leguminous plants result in development of root nodules. Nodules are new organs that consist mainly of plant cells infected with bacteroids that provide the host plant with fixed nitrogen. Proper nodule development requires the synthesis and perception of signal molecules such as lipochitooligosaccharides, called Nod factors that are important for induction of nodule development. Bacterial surface polysaccharides are also crucial for establishment of successful symbiosis with legumes. Sugar polymers of rhizobia are composed of a number of different polysaccharides, such as lipopolysaccharides (LPS), capsular polysaccharides (CPS or K-antigens), neutral β-1, 2-glucans and acidic extracellular polysaccharides (EPS). Despite extensive research, the molecular function of the surface polysaccharides in symbiosis remains unclear.

This review focuses on exopolysaccharides that are especially important for the invasion that leads to formation of indetermined (with persistent meristem) type of nodules on legumes such as clover, vetch, peas or alfalfa. The significance of EPS synthesis in symbiotic interactions of Rhizobium leguminosarum with clover is especially noticed. Accumulating data suggest that exopolysaccharides may be involved in invasion and nodule development, bacterial release from infection threads, bacteroid development, suppression of plant defense response and protection against plant antimicrobial compounds. Rhizobial exopolysaccharides are species-specific heteropolysaccharide polymers composed of common sugars that are substituted with non-carbohydrate residues. Synthesis of repeating units of exopolysaccharide, their modification, polymerization and export to the cell surface is controlled by clusters of genes, named exo/exs, exp or pss that are localized on rhizobial megaplasmids or chromosome. The function of these genes was identified by isolation and characterization of several mutants disabled in exopolysaccharide synthesis. The effect of exopolysaccharide deficiency on nodule development has been extensively studied. Production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen or sulphur. There is a strong suggestion that production of a variety of symbiotically active polysaccharides may allow rhizobial strains to adapt to changing environmental conditions and interact efficiently with legumes.

Environmental modulation of the pssTNOP gene expression in Rhizobium leguminosarum bv. trifolii

Exopolysaccharide production by Rhizobium leguminosarum bv. trifolii is required for successful establishment of nitrogen-fixing symbiosis with clover (Trifolium pratense L.). Using plasmid-borne transcriptional fusions of promoters of pss genes with promoterless lacZ the effect of root exudate, phosphate, and ammonia on expression of pssT, pssN, pssO, and pssP genes in wild-type strain RtTA1 background was determined. A stimulating effect of these environmental factors on pssO and pssP gene expression was observed. The putative pssO gene promoter was determined to be a strong promoter within which the divergent nod-box element was identified. The pssO promoter was slightly inducible in a flavonoid-dependent manner in wild-type R. leguminosarum bv. trifolii strains RtTA1 and ANU843 and very weakly active in a mutant of strain ANU843 that lacks the regulatory nodD gene. The expression of pssO and pssP genes in planta was investigated using plasmid-borne pssO-gusA and pssP-gusA fusions under different phosphate availability to clover. The level of pssO-gusA fusion expression was shown to be dependent on phosphate concentration in the plant growth medium.

Exopolysaccharide synthesis in Rhizobium leguminosarum bv. trifolii is related to various metabolic pathways

Rhizobium leguminosarum bv. trifolii synthesizes extracellular polysaccharide (EPS) that is postulated to be a biologically active signalling molecule in clover symbiosis. A group of seven exopolysaccharide-deficient (Exo), non-nitrogen-fixing mutants of R. leguminosarum bv. trifolii strain 24.1 isolated by transposon mutagenesis were complemented to mucoid phenotype by a low-copy plasmid carrying the pssA gene encoding the first glucosyl-IP-transferase. Some of these mutants were not corrected in their symbiotic defect by the pssA gene. Precise localization of Tn5 insertion sites by subcloning and sequencing the adjacent genomic DNA in the Exo mutants identified the disrupted genes and their possible functions. Only one mutant (Rt74) was mutated in pssA gene; others were mutated in diverse genes that were not directly involved in EPS biosynthesis. The suppression of EPS deficiency in these mutants by additional copies of pssA indicated a possible connection between exopolysaccharide biosynthesis and various metabolic pathways.

Genomics-based design of defined growth media for the plant pathogen Xylella fastidiosa

Based on the genetic analysis of the phytopathogen Xylella fastidiosa genome, five media with defined composition were developed and the growth abilities of this fastidious prokaryote were evaluated in liquid media and on solid plates. All media had a common salt composition and included the same amounts of glucose and vitamins but differed in their amino acid content. XDM(1) medium contained amino acids threonine, serine, glycine, alanine, aspartic acid and glutamic acid, for which complete degradation pathways occur in X. fastidiosa; XDM(2) included serine and methionine, amino acids for which biosynthetic enzymes are absent, plus asparagine and glutamine, which are abundant in the xylem sap; XDM(3) had the same composition as XDM(2) but with asparagine replaced by aspartic acid due to the presence of complete degradation pathway for aspartic acid; XDM(4) was a minimal medium with glutamine as a sole nitrogen source; XDM(5) had the same composition as XDM(4), plus methionine. The liquid and solidified XDM(2) and XDM(3) media were the most effective for the growth of X. fastidiosa. This work opens the opportunity for the in silico design of bacterial defined media once their genome is sequenced.

Rhizobium leguminosarum bv. trifolii PssP protein is required for exopolysaccharide biosynthesis and polymerization

Rhizobium leguminosarum bv. trifolii produces an acidic exopolysaccharide (EPS) that is important for the induction of nitrogen-fixing nodules on clover. Recently, three genes, pssN, pssO, and pssP, possibly involved in EPS biosynthesis and polymerization were identified. The predicted protein product of the pssP gene shows a significant sequence similarity to other proteins belonging to the PCP2a family that are involved in the synthesis of high-molecular-weight EPS. An R. leguminosarum bv. trifolii TA1 mutant with the entire coding region of pssP deleted did not produce the EPS. A pssP mutant with the 5' end of the gene disrupted produced exclusively low-molecular-weight EPS. A mutant that synthesized a functional N-terminal periplasmic domain but lacked the C-terminal part of PssP produced significantly reduced amounts of EPS with a slightly changed low to high molecular form ratio. Mutants affected in the PssP protein carrying a stable plasmid with a constitutively expressed gusA gene induced nodules on red clover that were not fully occupied by bacteria. A mutant with the entire pssP gene deleted infected only a few plant cells in the nodule. The pssP promoter-gusA reporter fusion was active in bacteroids during nodule development.

Regulation of Raoultella terrigena comb.nov. phytase expression

Phytases catalyze the release of phosphate from phytate (myo-inositol hexakisphosphate) to inositol polyphosphates. Raoultella terrigena comb.nov. phytase activity is known to increase markedly after cells reach the stationary phase. In this study, phytase activity measurements made on single batch cultures indicated that specific enzyme activity was subject to catabolite repression. Cyclic AMP (cAMP) showed a positive effect in expression during exponential growth and a negative effect during stationary phase. RpoS exhibited the opposite effect during both growth phases; the induction to stationary phase decreased twofold in the rpoS::Tn10 mutant, but the effect of RpoS was not clearly determined. Two phy::MudI1734 mutants, MW49 and MW52, were isolated. These formed small colonies in comparison with the MW25 parent strain when plated on Luria-Bertani (LB) or LB supplemented with glucose. They did not grow in minimal media or under anaerobiosis, but did grow aerobically on LB and LB glucose at a lower rate than did MW25. The beta-galactosidase activity level in these mutants increased three to four fold during stationary growth in LB glucose and during anaerobiosis. Addition of cAMP during the exponential growth of MW52 on LB glucose provoked a decrease in beta-galactosidase activity during the stationary phase, confirming its negative effect on phytase expression during stationary growth.

Isolation and sequencing of Rhizobium leguminosarum Bv. Trifolii PssN, PssO and PssP genes encoding the proteins involved in polymerization and translocation of exopolysaccharide

Rhizobium leguminosarum bv. trifolii produces an acidic exopolysaccharide (EPS) that plays an important role in symbiotic interaction with clover plants. The sequence of 6.0-kb DNA fragment located upstream of the previously described prsDEorf3 and pssCDE genes involved in exopolysaccharide biosynthesis revealed three new genes designated pssN, pssO and pssP. The predicted protein product of pssP gene shares a significant homology to members of the membrane-periplasmic auxiliary (MPA1) family, that are involved in polymerization of the repeating subunits of EPS. The putative pssN protein product is highly homologous to the family of the outer membrane auxiliary (OMA) proteins engaged in translocation of polysaccharides in bacteria. The PssO did not reveal homology to the known bacterial proteins, but showed characteristic features of outer membrane proteins, and with PssN and PssP, it might be a part of the system involved in polymerization and translocation of EPS across the bacterial membranes.