Characterization and symbiotic importance of acidic extracellular polysaccharides of Rhizobium sp. strain GRH2 isolated from acacia nodules

What If Root Nodules Are a Guesthouse for a Microbiome? The Case Study of Acacia longifolia

Acacia longifolia is one of the most aggressive invaders worldwide whose invasion is potentiated after a fire, a common perturbation in Mediterranean climates. As a legume, this species establishes symbioses with nitrogen-fixing bacteria inside root nodules; however, the overall microbial diversity is still unclear. In this study, we addressed root nodules' structure and biodiversity through histology and Next-Generation Sequencing, targeting 16S and 25S-28S rDNA genes for bacteria and fungi, respectively. We wanted to evaluate the effect of fire in root nodules from 1-year-old saplings, by comparing unburnt and burnt sites. We found that although having the same general structure, after a fire event, nodules had a higher number of infected cells and greater starch accumulation. Starch accumulated in uninfected cells can be a possible carbon source for the microbiota. Regarding diversity, Bradyrhizobium was dominant in both sites (ca. 77%), suggesting it is the preferential partner, followed by Tardiphaga (ca. 9%), a non-rhizobial Alphaproteobacteria, and Synechococcus, a cyanobacteria (ca. 5%). However, at the burnt site, additional N-fixing bacteria were included in the top 10 genera, highlighting the importance of this process. Major differences were found in the mycobiome, which was diverse in both sites and included genera mostly described as plant endophytes. Coniochaeta was dominant in nodules from the burnt site (69%), suggesting its role as a facilitator of symbiotic associations. We highlight the presence of a large bacterial and fungal community in nodules, suggesting nodulation is not restricted to nitrogen fixation. Thus, this microbiome can be involved in facilitating A. longifolia invasive success.

Genotypic and symbiotic diversity studies of rhizobia nodulating Acacia saligna in Tunisia reveal two novel symbiovars within the Rhizobium leguminosarum complex and Bradyrhizobium

Acacia saligna is an invasive alien species that has the ability to establish symbiotic relationships with rhizobia. In the present study, genotypic and symbiotic diversity of native rhizobia associated with A. saligna in Tunisia were studied. A total of 100 bacterial strains were selected and three different ribotypes were identified based on rrs PCR-RFLP analysis. Sequence analyses of rrs and four housekeeping genes (recA, atpD, gyrB and glnII) assigned 30 isolates to four putative new lineages and a single strain to Sinorhizobium meliloti. Thirteen slow-growing isolates representing the most dominant IGS (intergenic spacer) profile clustered distinctly from known rhizobia species within Bradyrhizobium with the closest related species being Bradyrhizobium shewense and Bradyrhizobium niftali, which had 95.17% and 95.1% sequence identity, respectively. Two slow-growing isolates, 1AS28L and 5AS6L, had B. frederekii as their closest species with a sequence identity of 95.2%, an indication that these strains could constitute a new lineage. Strains 1AS14I, 1AS12I and 6AS6 clustered distinctly from known rhizobia species but within the Rhizobium leguminosarum complex (Rlc) with the most closely related species being Rhizobium indicum with 96.3% sequence identity. Similarly, the remaining 11 strains showed 96.9 % and 97.2% similarity values with R. changzhiense and R. indicum, respectively. Based on nodC and nodA phylogenies and cross inoculation tests, these 14 strains of Rlc species clearly diverged from strains of Sinorhizobium and Rlc symbiovars, and formed a new symbiovar for which the name sv. "salignae" is proposed. Bacterial strains isolated in this study that were taxonomically assigned to Bradyrhizobium harbored different symbiotic genes and the data suggested a new symbiovar, for which sv. "cyanophyllae" is proposed. Isolates formed effective nodules on A. saligna.

A role for exopolysaccharides in the protection of microorganisms from desiccation

Mucoid strains of Escherichia coli, Acinetobacter calcoaceticus, and Erwinia stewartii were significantly more resistant to desiccation than corresponding isogenic nonmucoid mutants (survival rates of up to 35% in mucoid strains and between 0.7 and 5% in nonmucoid variants), even in colonies containing both cell types. Desiccation was found to bring about an induction of beta-galactosidase in Lon strains of E. coli K-12 carrying transcriptional lac fusions in the capsule biosynthetic (cps) regulon. This induction was dependent on the transcriptional activators RcsA and RcsB. Induction was lower in cells carrying mutations in the membrane sensor protein RcsC.

Identification of the Mesorhizobium loti gene responsible for glycerophosphorylation of periplasmic cyclic beta-1,2-glucans

Periplasmic cyclic beta-1,2-glucans play a crucial role in symbiosis as well as in hypo-osmotic adaptation for rhizobia. These glucans are modified in many species by anionic substituents such as glycerophosphoryl and succinyl ones, but their role remains to be examined. In this work, the cgmA homolog is shown to be responsible for glycerophosphorylation of cyclic beta-1,2-glucans in Mesorhizobium loti. The mutation in cgmA converted most anionic glucans into neutral ones, leaving a small amount of succinylated ones. An additional mutation in opgC, which encodes a succinyltransferase homolog, abolished the residual succinyl substituents in the cgmA-mutant background. The double mutant in cgmA and opgC did not show any significant phenotypic differences from the wild type during both vegetative growth and symbiosis. It is concluded that the anionic substituents make a minor contribution, if any, to the effectiveness of cyclic beta-1,2-glucans in M. loti.

The Brucella abortus cyclic beta-1,2-glucan virulence factor is substituted with O-ester-linked succinyl residues

Brucella periplasmic cyclic beta-1,2-glucan plays an important role during bacterium-host interaction. Nuclear magnetic resonance spectrometry analysis, thin-layer chromatography, and DEAE-Sephadex chromatography were used to characterize Brucella abortus cyclic glucan. In the present study, we report that a fraction of B. abortus cyclic beta-1,2-glucan is substituted with succinyl residues, which confer anionic character on the cyclic beta-1,2-glucan. The oligosaccharide backbone is substituted at C-6 positions with an average of two succinyl residues per glucan molecule. This O-ester-linked succinyl residue is the only substituent of Brucella cyclic glucan. A B. abortus open reading frame (BAB1_1718) homologous to Rhodobacter sphaeroides glucan succinyltransferase (OpgC) was identified as the gene encoding the enzyme responsible for cyclic glucan modification. This gene was named cgm for cyclic glucan modifier and is highly conserved in Brucella melitensis and Brucella suis. Nucleotide sequencing revealed that B. abortus cgm consists of a 1,182-bp open reading frame coding for a predicted membrane protein of 393 amino acid residues (42.7 kDa) 39% identical to Rhodobacter sphaeroides succinyltransferase. cgm null mutants in B. abortus strains 2308 and S19 produced neutral glucans without succinyl residues, confirming the identity of this protein as the cyclic-glucan succinyltransferase enzyme. In this study, we demonstrate that succinyl substituents of cyclic beta-1,2-glucan of B. abortus are necessary for hypo-osmotic adaptation. On the other hand, intracellular multiplication and mouse spleen colonization are not affected in cgm mutants, indicating that cyclic-beta-1,2-glucan succinylation is not required for virulence and suggesting that no low-osmotic stress conditions must be overcome during infection.

Nodulation of Mimosa spp. by the beta-proteobacterium Ralstonia taiwanensis

Several beta-proteobacteria have been isolated from legume root nodules and some of these are thought to be capable of nodulating and fixing N2. However, in no case has there been detailed studies confirming that they are the active symbionts. Here, Ralstonia taiwanensis LMG19424, which was originally isolated from Mimosa pudica nodules, was transformed to carry the green fluorescent protein (gfp) reporter gene before being used to inoculate axenically-grown seedlings of M. pudica and M. diplotricha. Plants were harvested at various intervals for 56 days after inoculation, then examined for evidence of infection and nodule formation. Nodulation of both Mimosa spp. was abundant, and acetylene reduction assays confirmed that nodules had nitrogenase activity. Confocal laser scanning microscopy (CLSM) showed that fresh M. pudica nodules with nitrogenase activity had infected cells containing bacteroids expressing gfp. In parallel, fixed and embedded nodules from both Mimosa spp. were sectioned for light and electron microscopy, followed by immunogold labeling with antibodies raised against gfp and nitrogenase Fe (nifH) protein. Significant immunolabeling with these antibodies confirmed that R. taiwanensis LMG19424 is an effective N2-fixing symbiont of Mimosa spp. Both species were infected via root hairs and, in all respects, the nodule ontogeny and development was similar to that described for other mimosoid legumes. The nodules were indeterminate with a persistent meristem, an invasion zone containing host cells being invaded via prominent infection threads, and an N2-fixing zone with infected cells containing membrane-bound symbiosomes.

The symbiotic phenotypes of exopolysaccharide-defective mutants of Rhizobium sp. strain TAL1145 do not differ on determinate- and indeterminate-nodulating tree legumes

Three classes of exopolysaccharide (EPS) defective mutants were isolated by Tn3Hogus -insertion mutagenesis of Rhizobium sp. strain TAL1145, which nodulates tree legumes. The class I and class III mutants produced 10-22% of the EPS produced by TAL1145 and appeared partially mucoid while the class II mutants formed small, opaque and non-mucoid colonies. Size-fractionation of the soluble EPSs made by these mutants in the culture supernatant indicated that the class I and the class III mutants produced reduced levels of both highland low-molecular-mass EPSs while the class II mutants lacked both these EPSs but produced a small amount of a medium-molecular-mass anthrone-reactive EPS. The succinyl and acetyl substituents observed in the TAL1145 EPS were absent in the EPS of the class II mutants. When examined under UV, the class I and class III mutants grown on Calcofluor-containing YEM agar showed dim blue fluorescence, compared to the bright blue fluorescence of the wild-type strain, whereas the class II mutants did not fluoresce. While the dim blue fluorescence of the class III mutants changed to yellow-green after 10 d, the fluorescence of the class I mutants did not change after prolonged incubation. Unlike the EPS-defective mutants of other rhizobia, these mutants did not show different symbiotic phenotypes on determinate- and indeterminate-nodulating tree legumes. The class I and the class III mutants formed small ineffective nodules on both types of legumes whereas the class II mutants formed normal nitrogen-fixing nodules on both types. The genes disrupted in the class I and class III mutants form a single complementation group while those disrupted in the class II mutants constitute another. All the three classes of EPS-defective mutants were located within a 10.8 kb region and complemented by two overlapping cosmids.

Glycoconjugate and lipid components ofRhizobium"hedysari" IS123, a root-nodule symbiont of the stress-tolerant legumeHedysarum coronarium

We have examined the diversity of glycoconjugates and cellular lipids of Rhizobium "hedysari" IS123, a bacterial symbiont that specifically nodulates the drought-tolerant forage legume Hedysarum coronarium. IS123 develops a complete capsule consisting of a loose fibrillar network of ruthenium-red-staining acidic polymers and produces two different exopolysaccharides (EPS). EPS-A contains glucose, galactose, mannose, and a noncarbohydrate substitution tentatively identified as a lactyl ester. The composition of EPS-B, which includes glucose and galactose, as well as O-acetyl, pyruvyl, and succinyl substituents, is very similar to that of the EPS-II described in Rhizobium meliloti. IS123 also makes an O-acetylated heterooligosaccharide and unsubstituted β-1,2-glucans. The cellular fatty acid composition of IS123 is dominated by 18:1 and also includes 14:0, 16:0, 16:1, 3OH-16:0, 17:0Δ, 18:0, 3OH-18:0, and 19:0Δ. Phospholipids of IS123 include phosphatidylethanolamine, N-methyl phosphatidylethanolamine, N,N-dimethyl phosphatidylethanolamine, phosphatidylcholine, and phosphatidylglycerol.Key words: Rhizobium, capsule, exopolysaccharides, oligosaccharides, lipids.

Induction of nodule primordia on Phaseolus and Acacia by lipo-chitin oligosaccharide nodulation signals from broad-host-range Rhizobium strain GRH2

Rhizobium wild-type strain GRH2 was originally isolated from the tree, Acacia cyanophylla, and has a broad host-range which includes herbaceous legumes, such as Phaseolus and Trifolium species. Here we show that strains of Rhizobium sp. GRH2, into which heterologous nodD alleles have been introduced, produce a large diversity of both sulphated and non-sulphated lipo-chitin oligosaccharides (LCOs). Most of the molecular species contain an N-methyl group on the reducing-terminal N-acetyl-glucosamine. The LCOs vary in the nature of the fatty acyl chain and in the length of the chitin backbone. The majority of the LCOs have an oligosaccharide chain length of five GlcNAc residues, but a few are oligomers having six GlcNAc units. LCOs purified from GRH2 are able to induce root hair formation and deformation on Acacia cyanophylla and A. melanoxylon plants. We show that an N-vaccenoyl-chitopentaose bearing an N-methyl group is able to induce nodule primordia on Phaseolus vulgaris, A. cyanophylla, and A. melanoxylon, indicating that for these plants an N-methyl modification is sufficient for nodule primordia induction.

Cyclic beta-glucans of members of the family Rhizobiaceae

Cyclic beta-glucans are low-molecular-weight cell surface carbohydrates that are found almost exclusively in bacteria of the Rhizobiaceae family. These glucans are major cellular constituents, and under certain culture conditions their levels may reach up to 20% of the total cellular dry weight. In Agrobacterium and Rhizobium species, these molecules contain between 17 and 40 glucose residues linked solely by beta-(1,2) glycosidic bonds. In Bradyrhizobium species, the cyclic beta-glucans are smaller (10 to 13 glucose residues) and contain glucose linked by both beta-(1,6) and beta-(1,3) glycosidic bonds. In some rhizobial strains, the cyclic beta-glucans are unsubstituted, whereas in other rhizobia these molecules may become highly substituted with moieties such as sn-1-phosphoglycerol. To date, two genetic loci specifically associated with cyclic beta-glucan biosynthesis have been identified in Rhizobium (ndvA and ndvB) and Agrobacterium (chvA and chvB) species. Mutants with mutations at these loci have been shown to be impaired in their ability to grow in hypoosmotic media, have numerous alterations in their cell surface properties, and are also impaired in their ability to infect plants. The present review will examine the structure and occurrence of the cyclic beta-glucans in a variety of species of the Rhizobiaceae. The possible functions of these unique molecules in the free-living bacteria as well as during plant infection will be discussed.