RECENT DEVELOPMENTS IN RHIZOBIUM TAXONOMY

Recent developments in Rhizobium taxonomy are presented from a molecular and evolutionary point of view. Analyses of ribosomal RNA gene sequences provide a solid basis to infer phylogenies in the Rhizobiaceae family. These studies confirmed that Rhizobium and Bradyrhizobium are only distantly related and showed that Rhizobium and Bradyrhizobium are related to other groups of bacteria that are not plant symbionts. Rhizobium and Agrobacterium species are intermixed. Differences in plasmid content may explain to a good extent the different behavior of Rhizobium and Agrobacterium as symbionts or pathogens. Other approaches to identify and classify bacteria such as DNA-DNA hybridization, fatty acid analysis, RFLP and RPD-PCR techniques and phylogenies derived from other genes are in general agreement to the groupings derived by ribosomal sequences. Only a small proportion of nodulated legumes have been sampled for their symbionts and more knowledge is required on the systematics and taxonomy of Rhizobium and Bradyrhizobium species.