Metabolism and Compartmentation of Carbon and Nitrogen in Legume Nodules

Symbiotic association of legume plants with soil bacteria of the genera Rhizobium, Bradyrhizobium, and Azorhizobium leads to the formation of nodules in which the microsymbionts are capable of fixing atmospheric dinitrogen. Symbiotic bacteria are fully dependent for their respiration and nitrogen fixation on the supply of photosynthate from the host plants. The nitrogen fixed in the bacteroids is exported to the plant cell cytosol as ammonium and assimilated to specific forms for translocation to the plant shoots. The nodule is a specialized symbiotic organ in which both the plant cell and microsymbiont develop specific structures and metabolic functions required for nitrogen fixation.