The celC gene, a new phylogenetic marker useful for taxonomic studies in Rhizobium

Phylogenetic diversity of rhizobia nodulating Phaseolus vulgaris in Croatia and definition of the symbiovar phaseoli within the species Rhizobium pisi

Phaseolus vulgaris is a legume indigenous to America which is currently cultivated in Europe including countries located at the Southeast of this continent, such as Croatia, where several local landraces are cultivated, most of them of Andean origin. In this work we identify at species and symbiovar levels several fast-growing strains able to form effective symbiosis with P. vulgaris in different Croatian soils. The identification at species level based on MALDI-TOF MS and core gene sequence analysis showed that most of these strains belong to the species R. leguminosarum, R. hidalgonense and R. pisi. In addition, several strains belong to putative new species phylogenetically close to R. ecuadorense and R. sophoriradicis. All Croatian strains belong to the symbiovar phaseoli and harbour the α and γ nodC alleles typical for American strains of this symbiovar. Nevertheless, most of Croatian strains harboured the γ nodC gene allele supporting its Andean origin since it is also dominant in other European countries, where Andean cultivars of P. vulgaris are traditionally cultivated, as occurs in Spain. The only strains harbouring the α nodC allele belong to R. hidalgonense and R. pisi, this last only containing the symbiovars viciae and trifolii to date. This is the first report about the presence in Europe of the species R. hidalgonense, the nodulation of P. vulgaris by R. pisi and the existence of the symbiovar phaseoli within this species. These results significantly increase the knowledge of the biogeography of Rhizobium-P. vulgaris symbiosis.

Heterologous Expression of Rhizobial CelC2 Cellulase Impairs Symbiotic Signaling and Nodulation in Medicago truncatula

The infection of legume plants by rhizobia is tightly regulated to ensure accurate bacterial penetration, infection, and development of functionally efficient nitrogen-fixing root nodules. Rhizobial Nod factors (NF) have key roles in the elicitation of nodulation signaling. Infection of white clover roots also involves the tightly regulated specific breakdown of the noncrystalline apex of cell walls in growing root hairs, which is mediated by Rhizobium leguminosarum bv. trifolii cellulase CelC2. Here, we have analyzed the impact of this endoglucanase on symbiotic signaling in the model legume Medicago truncatula. Ensifer meliloti constitutively expressing celC gene exhibited delayed nodulation and elicited aberrant ineffective nodules, hampering plant growth in the absence of nitrogen. Cotreatment of roots with NF and CelC2 altered Ca²⁺ spiking in root hairs and induction of the early nodulin gene ENOD11. Our data suggest that CelC2 alters early signaling between partners in the rhizobia-legume interaction.

Genomic basis of symbiovar mimosae in Rhizobium etli

Background

Symbiosis genes (nod and nif) involved in nodulation and nitrogen fixation in legumes are plasmid-borne in Rhizobium. Rhizobial symbiotic variants (symbiovars) with distinct host specificity would depend on the type of symbiosis plasmid. In Rhizobium etli or in Rhizobium phaseoli, symbiovar phaseoli strains have the capacity to form nodules in Phaseolus vulgaris while symbiovar mimosae confers a broad host range including different mimosa trees.

Results

We report on the genome of R. etli symbiovar mimosae strain Mim1 and its comparison to that from R. etli symbiovar phaseoli strain CFN42. Differences were found in plasmids especially in the symbiosis plasmid, not only in nod gene sequences but in nod gene content. Differences in Nod factors deduced from the presence of nod genes, in secretion systems or ACC-deaminase could help explain the distinct host specificity. Genes involved in P. vulgaris exudate uptake were not found in symbiovar mimosae but hup genes (involved in hydrogen uptake) were found. Plasmid pRetCFN42a was partially contained in Mim1 and a plasmid (pRetMim1c) was found only in Mim1. Chromids were well conserved.

Conclusions

The genomic differences between the two symbiovars, mimosae and phaseoli may explain different host specificity. With the genomic analysis presented, the term symbiovar is validated. Furthermore, our data support that the generalist symbiovar mimosae may be older than the specialist symbiovar phaseoli.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-575) contains supplementary material, which is available to authorized users.

Role of Rhizobium endoglucanase CelC2 in cellulose biosynthesis and biofilm formation on plant roots and abiotic surfaces

Background

The synthesis of cellulose is among the most important but poorly understood biochemical processes, especially in bacteria, due to its complexity and high degree of regulation. In this study, we analyzed both the production of cellulose by all known members of the Rhizobiaceae and the diversity of Rhizobium celABC operon predicted to be involved in cellulose biosynthesis. We also investigated the involvement in cellulose production and biofilm formation of celC gene encoding an endoglucanase (CelC2) that is required for canonical symbiotic root hair infection by Rhizobium leguminosarum bv. trifolii.

Results

ANU843 celC mutants lacking (ANU843ΔC2) or overproducing cellulase (ANU843C2⁺) produced greatly increased or reduced amounts of external cellulose micro fibrils, respectively. Calcofluor-stained cellulose micro fibrils were considerably longer when formed by ANU843ΔC2 bacteria rather than by the wild-type strain, in correlation with a significant increase in their flocculation in batch culture. In contrast, neither calcofluor-stained extracellular micro fibrils nor flocculation was detectable in ANU843C2⁺ cells. To clarify the role of cellulose synthesis in Rhizobium cell aggregation and attachment, we analyzed the ability of these mutants to produce biofilms on different surfaces. Alteration of wild-type CelC2 levels resulted in a reduced ability of bacteria to form biofilms both in abiotic surfaces and in planta.

Conclusions

Our results support a key role of the CelC2 cellulase in cellulose biosynthesis by modulating the length of the cellulose fibrils that mediate firm adhesion among Rhizobium bacteria leading to biofilm formation. Rhizobium cellulose is an essential component of the biofilm polysaccharidic matrix architecture and either an excess or a defect of this “building material” seem to collapse the biofilm structure. These results position cellulose hydrolytic enzymes as excellent anti-biofilm candidates.