Rhizobia

Rhizobia are soil bacteria that fix nitrogen (diazotrophs) after becoming established inside root nodules of legumes (Fabaceae). In order to express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are Gram-negative, motile, non-sporulating rods.

The first known species of rhizobia, Rhizobium leguminosarum, was identified in 1889, and all further species were initially placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classification, and now there are many in other genera. Most research has been done on crop and forage legumes such as clover, alfalfa, beans, peas, and soy; more research is being done on North American legumes.

The word rhizobia comes from the Ancient Greek , rhíza, meaning "root" and , bios, meaning "life". The word rhizobium is still sometimes used as the singular form of rhizobia

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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