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Rhizobia are a "group of soil bacteria that infect the roots of legumes to form root nodules". [2] Rhizobia are found in the soil and, after infection, produce nodules in the legume where they fix nitrogen gas (N 2) from the atmosphere, turning it into a more readily useful form of nitrogen. From here, the nitrogen is exported from the nodules ...
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]
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.
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.
In another part of the cycle, the process of nitrogen fixation constantly puts additional nitrogen into biological circulation. This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as Azotobacter, or by those that live in close symbiosis with leguminous plants, such as rhizobia.
Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses and aquatic ferns such as Azolla. [4] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. [5]
The rhizobacteria commonly applied as inoculants include nitrogen-fixers, phosphate-solubilisers and other root-associated beneficial bacteria which enhance the availability of the macronutrients nitrogen and phosphorus to the host plant. Such bacteria are commonly referred to as plant growth promoting rhizobacteria (PGPR).
This process bleeds into other processes such as nitrogen fixation in plants. [21] The evolutionary advantage of such an interaction allows genetic exchange between both organisms involved to increase the propensity for novel functions as seen in the plant-bacterium interaction ( holobiont formation).