Taxonomy
Rhizobia are a paraphyletic group that fall into two classes of the proteobacteria—the alpha- and beta-proteobacteria. As shown below, most belong to the order Rhizobiales, but several rhizobia occur in distinct bacterial orders of the proteobacteria.
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α-proteobacteria
- Rhizobiales
- Bradyrhizobiaceae
- Bradyrhizobium
- B. canariense
- B. elkanii
- B. japonicum
- B. liaoningense
- B. yuanmingense
- Brucellaceae
- Ochrobactrum
- O. cytisi
- O. lupini
- Hyphomicrobiaceae
- Azorhizobium
- A. caulinodans
- A. doebereinerae
- Devosia
- D. neptuniae
- Methylobacteriaceae
- Methylobacterium
- M. nodulans
- Phyllobacteriaceae
- Mesorhizobium
- M. albiziae
- M. amorphae
- M. chacoense
- M. ciceri
- M. huakuii
- M. loti
- M. mediterraneum
- M. plurifarium
- M. septentrionale
- M. temperatum
- M. tianshanense
- Phyllobacterium
- P. ifriqiyense
- P. leguminum
- P. trifolii
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- Rhizobiaceae
- Rhizobium
- R. cellulosilyticum
- R. daejeonense
- R. etli
- R. galegae
- R. gallicum
- R. giardinii
- R. hainanense
- R. huautlense
- R. indigoferae
- R. leguminosarum
- R. loessense
- R. lupini
- R. lusitanum
- R. mongolense
- R. miluonense
- R. sullae
- R. tropici
- R. undicola
- R. yanglingense
- Sinorhizobium (Ensifer)
- S. abri
- S. adhaerens
- S. americanum
- S. arboris
- S. fredii
- S. indiaense
- S. kostiense
- S. kummerowiae
- S. medicae
- S. meliloti
- S. mexicanus
- S. morelense
- S. saheli
- S. terangae
- S. xinjiangense
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β-proteobacteria
- Burkholderiales
- Burkholderiaceae
- Burkholderia
- B. caribensis
- B. dolosa
- B. mimosarum
- B. phymatum
- B. tuberum
- Cupriavidus
- C. taiwanensis
- Oxalobacteraceae
- Herbaspirillum
- H. lusitanum
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These groups include a variety of non-symbiotic bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the Bradyrhizobium that nodulate soybean (and may not really be a separate genus). The genes responsible for the symbiosis with plants, however, may be more closely related than the organisms themselves, acquired by horizontal transfer (via bacterial conjugation) rather than vertical gene transfer (from a common ancestor).