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Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells to form root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase.
The legume–rhizobium symbiosis is a classic example of mutualism—rhizobia supply ammonia or amino acids to the plant and, in return, receive organic acids (mainly malate and succinate, which are dicarboxylic acids) as a carbon and energy source.
Some bacteria found in the family are used for plant nutrition and collectively make up the rhizobia. Other bacteria such as Agrobacterium tumefaciens and Rhizobium rhizogenes severely alter the development of plants in their ability to induce crown galls or hairy roots, respectively. [2]
Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature. [1] Generally, about 2–5% of rhizosphere bacteria are PGPR. [2] They are an important group of microorganisms used in biofertilizer ...
A bacteroid depends on the plant for survival. [19] Leghemoglobin, produced by leguminous plants after colonization of S. meliloti, interacts with the free oxygen in the root nodule where the rhizobia reside. Rhizobia are contained within symbiosomes in the root nodules of leguminous plants. The leghemoglobin reduces the amount of free oxygen ...
Rhizobium leguminosarum is a bacterium which lives in a mutualistic symbiotic relationship with legumes, and has the ability to fix free nitrogen from the air. [2] R. leguminosarum has been very thoroughly studied—it has been the subject of more than a thousand publications.
Rhizobium rhizogenes (formerly Agrobacterium rhizogenes) is a Gram-negative soil bacterium that produces hairy root disease in dicotyledonous plants. R. rhizogenes induces the formation of proliferative multiple-branched adventitious roots at the site of infection, so-called 'hairy roots'. [ 3 ]
Rhizobium lentis are rod-shaped bacteria found in the soil. They require oxygen and do not form spores. [1] Rhizobium lentis grow well on YEMA medium agar, where they form colonies which are circular, convex and creamy white. These bacteria grow at 12–37 °C but can survive at temperatures as low as 4 °C. [1] Strains grow well at pH 5.5 to 8.2.