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( Rhizobia)
Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia cannot independently fix nitrogen, and requires a plant host. Morphologically they are generally gram negative, motile, non-sporulating rods. The first species (R. leguminosarum) was identified in 1889, and all further species placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classification and now there are many in other genera. Rhizobium is still sometimes used as the singular of rhizobia. Most research has been done on crop and forage legumes such as clover, beans, and soy. However, recently more work is occurring on North American legumes. Rhizobia consist of 57 species found in 12 genera.[1] Most belong to the Rhizobiales, a probably-monophyletic group of proteobacteria. Within that group, however, they are scattered among several different families These groups also include a variety of other bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the rhizobia that nodulate soybean (and may not really be a separate genus). The genes responsible for the symbiosis with plants, however, may be closer than the organisms themselves, acquired by horizontal transfer (via bacterial conjugation) rather than from a common ancestor.
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