Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process

Contents  Summary 1 I. Introduction 2 II. The characteristics of legume nodule senescence 3 III. Redox homeostasis and programmed cell death 6 IV. Antioxidants and programmed cell death during the senescence process  10 V. Perspectives for manipulating nodule senescence 13 VI. Conclusions and perspectives 14  Acknowledgements 15  References 15 Summary Research on legume nodule development has contributed greatly to our current understanding of plant–microbe interactions. However, the factors that orchestrate root nodule senescence have received relatively little attention. Accumulating evidence suggests that redox signals contribute to the establishment of symbiosis and senescence. Although degenerative in nature, nodule senescence is an active process programmed in development in which reactive oxygen species (ROS), antioxidants, hormones and proteinases have key roles. Nodules have high levels of the redox buffers, ascorbate and glutathione, which are important in the nodulation process and in senescence. These metabolites decline with N-fixation as the nodule ages but the resultant decrease in redox buffering capacity does not necessarily lead to enhanced ROS or oxidative stress. We propose models by which ROS and antioxidants interact with hormones such as abscisic acid in the orchestration of nodule senescence.