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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 host plant provides the bacteria with amino acids so they do not need to assimilate ammonia. [5] The amino acids are then shuttled back to the plant with newly fixed nitrogen. Nitrogenase is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions.
The symbiosis between nitrogen fixing rhizobia and the legume family has emerged and evolved over the past 66 million years. [30] [31] Although evolution tends to swing toward one species taking advantage of another in the form of noncooperation in the selfish-gene model, management of such symbiosis allows for the continuation of cooperation. [32]
Rhizobium: Symbiotic nitrogen fixation by Rhizobium with legumes contributes substantially to total nitrogen fixation. Rhizobium inoculation is a well-known agronomic practice to ensure adequate nitrogen. [5] [6] One of the most widespread species is R. leguminosarum. Bradyrhizobium spp. (in particular Bradyrhizobium japonicum). [7]
Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N 2) to ammonia (NH 3) in a process called nitrogen fixation.
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.
This initiates root hair curling. The rhizobia then penetrate the root hairs and proliferate to form an infection thread. Through the infection thread, the bacteria move toward the main root. The bacteria develop into bacteroids within newly formed root nodules and perform nitrogen fixation for the plant.
In response to root exudates, rhizobia produce Nod signalling factors that are recognized by legumes and induce the formation of nodules on plant roots. [28] Within these structures, Rhizobium fix atmospheric nitrogen into ammonia that is then used by the plant. In turn, plants provide the bacteria with a carbon source to energize the nitrogen ...