Legume nodule senescence: a coordinated death mechanism between bacteria and plant cells

In the legume-rhizobia symbiosis, atmospheric dinitrogen (N-2) fixation by rhizobia takes place in a specific root organ, called the nodule. During biological N-2 fixation, the plant provides photosynthetic carbohydrates to bacteria and, in exchange, bacteria feed the plant with reduced nitrogen (N) under ammonia (NH3) form. Like most other organs, the root nodule has a limited lifespan, and eventually enters a senescence process characterized by a decline of N-2 fixation and the coordinated death of both bacteria and plant cells. This senescence process characterized by a metabolic switch from a carbon sink to a nutrient source either results from nodule aging (developmental nodule senescence) or can be triggered prematurely by adverse environmental conditions. Accumulating evidence suggests that redox and hormone signaling contribute to