CONICET | Buscador de Institutos y Recursos Humanos

Rhizobia are soil, rhizospheric and nodule bacteria. A large proportion of their genome, nearly 50 %, is plasmid borne. Nodulation and nitrogen fixation capabilities as well as host-legume specificity is plasmid encoded in Rhizobium in the so called symbiotic plasmids. The term symbiovar (sv.) has been proposed to refer to symbiotic specificity and accounts for the existence of different specificities in a single species and for the same specificity in different rhizobial lineages, in relation to plasmid lateral transfer (1). Most Phaseolus vulgaris nodulating bacteria correspond to ?tropici? or ?phaseoli? symbiovars and those pSyms have a high frequency of transfer among rhizobia. The ?phaseoli? pSym is found in distantly related species. We have analyzed the genomes of a diversity of rhizobia (6, 7), including Rhizobium grahamii that represents a new group of rhizobia that has been poorly characterized until now (2, 3). R. grahamii has a broad-host-range that includes Leucaena leucocephala and P. vulgaris bean although it is a poor competitor for bean nodulation in the presence of Rhizobium etli or Rhizobium phaseoli strains. In R. grahamii CCGE502 chromosome an integrated plasmid was found, supporting the already described genomic plasticity (4). The genomic comparison of symbiotic plasmids suggests that these plasmids are species-specific in the grahamii group. In sv. ?phaseoli? or sv. ?tropici?, pSyms display high conservation, around 90% and 99%, respectively (5, 6). We were unable to transfer the R. grahamii CCGE502 pSym and a larger plasmid to other rhizobial hosts but we could transfer the symbiotic plasmid to Agrobacterium tumefaciens with transfer dependent on homoserine lactones. The conjugative transfer properties of R. grahamii CCGE502 plasmids may explain their divergence and narrow distribution in related rhizobial species