Nitrogen is an essential nutrient as it makes up many biochemical compounds. It is found in the nucleoside triphosphates and amino acids that form the building blocks of nucleic acids and proteins, respectively. Therefore all organisms must gain access to a supply of nitrogen in a form they can assimilate in order to survive.
Dinitrogen (N2) makes up roughly 78% of the atmosphere by volume . However, despite its abundance, plants are unable to assimilate nitrogen in this form. Dinitrogen has a bond energy of 941.4 kJ mole-1  which makes it an exceptionally stable molecule. Eukaryotes do not have the enzymatic capability to break the triple bond. Only a small number of prokaryotes, the nitrogen fixing bacteria,  have the ability to break the triple bond and fix the nitrogen into ammonium which plants are able to assimilate. Therefore the only way plants can gain access to atmospheric nitrogen is via symbiosis.
Nitrogen fixing bacteria, including rhizobia, account for 90%  of the naturally fixed nitrogen compounds in the soil. Without nitrogen fixing bacteria the soil would be unable to sustain plant population density anywhere near its present level. Rhizobia comprise gram negative soil bacteria of the genera Azorhizobium, Bradyrhizobium, Photorhizobium, Rhizobium, and Sinorhizobium . Most nitrogen fixing bacteria are free-living, however a small but significant number are capable of forming symbiotic relationships with plants. The most common type of symbiosis occurs between plants such as beans, pea, lentils, clover, alfalfa, or vetch, (which are part of the family Leguminosae), and rhizobia . Under nitrogen limited conditions the symbionts seek out one another through an elaborate exchange of signals. This signalling leads to a subsequent infection process involving Ca2+ concentration cytosolic spiking  and root hair curling culminating in the formation of an infection thread. Eventually mature root nodules containing bacteroids, nitrogen fixing rhizobia surrounded by a plant derived membrane, are formed. These nodules capture atmospheric nitrogen for the mutual benefit of the bacteria and plant.
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