Redox Control Of The Legume-Rhizobium Symbiosis

The onset of the nitrogen-fixing legume-rhizobium symbiosis is a complex process that requires elaborate communication between the two partners. Nodule metabolism is very active and continuously generates reactive oxygen species (ROS) and reactive nitrogen species (RNS). During the evolution of aerobic life, ROS and RNS have been recruited as versatile signaling molecules due to their chemical properties and capacity to interact with enzymes and transcription factors. Numerous studies have demonstrated the presence of nitric oxide, hydrogen peroxide and other ROS and RNS at different stages of symbiosis, from early recognition between plant and bacteria to nodule senescence. Antioxidant metabolites and enzymes and other proteins such as phytoglobins finely regulate ROS and RNS concentrations, thereby allowing the beneficial and critical participation of these molecules as signals in many aspects of nodule physiology. Here we review the contribution of ROS, RNS and antioxidants to the redox control of the onset of symbiosis and subsequent nodule development, paying attention to both developmental (aging) and stress-induced senescence. Because of their importance in regulation and signaling, a substantial part of this review is devoted to post-translational redox modifications involving, among others, residues of methionine, cysteine and tyrosine of nodule proteins.