PHYLOGENY AND TAXONOMY OF RHIZOBIA

Rhizobia are bacteria that form nitrogen-fixing nodules on the roots, or occasionally the shoots, of legumes. There are currently more than a dozen validly named species, but the true number of species is proba bly orders of magnitude higher. The named species are listed and brief ly discussed. Sequences of the small subunit ribosomal RNA (SSU or 16S rRNA) support the well-established subdivision of rhizobia into three genera: Rhizobium, Bradyrhizobium, and Azorhizobium. These all lie wi thin the alpha subdivision of the Proteobacteria, but on quite distinc t branches, each of which also includes many bacterial species that ar e not rhizobia. It has been clear for several years that Rhizobium, on this definition, is still too broad and is polyphyletic: there are ma ny non-rhizobia within this radiation. Recently, therefore, it has bee n suggested that this genus should be split into four genera, namely R hizobium (R. leguminosarum, R, tropici, R. etli), Sinorhizobium (S. fr edii, S. meliloti, S. teranga, S. saheli), Mesorhizobium (M. loti, M. huakuii, M ciceri, M. tianshanense, M. mediterraneum), and a fourth, u nnamed, genus for the current R. galegae. The evidence and pros and co ns are reviewed.