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.