Plant and Soil

, Volume 186, Issue 1, pp 45–52 | Cite as

Phylogeny and taxonomy of rhizobia

  • J. P. W. Young


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 probably orders of magnitude higher. The named species are listed and briefly 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 within the alpha subdivision of the Proteobacteria, but on quite distinct branches, each of which also includes many bacterial species that are not rhizobia. It has been clear for several years that Rhizobium, on this definition, is still too broad and is polyphyletic: there are many non-rhizobia within this radiation. Recently, therefore, it has been suggested that this genus should be split into four genera, namely Rhizobium (R. leguminosarum, R. tropici, R. etli), Sinorhizobium (S. fredii, S. meliloti, S. teranga, S. saheli), Mesorhizobium (M. loti, M. huakuii, M ciceri, M. tianshanense, M. mediterraneum), and a fourth, unnamed, genus for the current R. galegae. The evidence and pros and cons are reviewed.

Key words

Azorhizobium Bradyrhizobium Mesorhizobium phylogeny Rhizobium Sinorhizobium species taxonomy 


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Copyright information

© Kluwer Academic Publishers 1996

Authors and Affiliations

  • J. P. W. Young
    • 1
  1. 1.Department of BiologyUniversity of YorkYorkUK

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