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Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes . To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. [1] In general, they are gram negative, motile, non-sporulating rods.
Rhizobacteria, through nitrogen fixation, are able to convert gaseous nitrogen (N 2) to ammonia (NH 3) making it an available nutrient to the host plant which can support and enhance plant growth. The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia. [ 5 ]
A method for nitrogen fixation was first described by Henry Cavendish in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the Birkeland–Eyde process of 1903. [67] The fixation of nitrogen by lightning is a very similar natural occurring process.
Association with animals—although diazotrophs have been found in many animal guts, there is usually sufficient ammonia present to suppress nitrogen fixation. [3] Termites on a low nitrogen diet allow for some fixation, but the contribution to the termite's nitrogen supply is negligible.
Nod factors (nodulation factors or NF), are signaling molecules produced by soil bacteria known as rhizobia in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing nodulation.
Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin.
The most commonly applied rhizobacteria are Rhizobium and closely related genera. Rhizobium are nitrogen-fixing bacteria that form symbiotic associations within nodules on the roots of legumes . This increases host nitrogen nutrition and is important to the cultivation of soybeans, chickpeas and many other leguminous crops.
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.