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

In the legume-rhizobia symbiosis, atmospheric dinitrogen (N2) fixation by rhizobia takes place in a specific root organ, called the nodule. During biological N2 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 N2 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 the triggering of senescence. Indeed, nodule senescence is a developmentally programmed process in which antioxidants, reactive oxygen and N species (ROS/RNS), hormones and proteinases play key roles. This chapter reviews recent knowledge on nodule senescence and proposes models by which ROS, RNS, and likely hormones intervene in the orchestration of this process.