The taxonomy of rhizobia: an overview

The taxonomy of rhizobia, bacteria capable of nodulating leguminous plants, has changed considerably over the last 20 years, with the original genus Rhizobium, a member of the alpha-Proteobacteria, now divided into several genera. The study of new geographically dispersed host plants, has been a source of many new species and is expected to yield many more. Here we provide an overview of the history of the rhizobia, but focus on the Rhizobium-Allorhizobium-Agrobacterium relationship. Finally, we review recent reports of nodulation and nitrogen fixation with legume hosts by bacteria that are outside the traditional rhizobial phylogenetic lineages. They include species of Methylobacterium and Devosia in the alpha-Proteobacteria and of Burkholderia and Ralstonia in the beta-Proteobacteria.