The Redox State, a Referee of the Legume-Rhizobia Symbiotic Game

The symbiosis between legumes and Rhizobia leads to the formation of a new organ, the nodule, where nitrogen fixation takes place. The plant provides bacteria with energy and a micro-aerobic environment compatible with nitrogenase activity. In exchange, bacteria provide a nitrogen supply to the plant. Therefore, nodules represent a unique model for the study of developmental processes, plant-microbe, and carbon/nitrogen/oxygen metabolism interactions. Reactive oxygen species (ROS) have been described to be produced at every step of the symbiotic association: during the symbiosis establishment, during the nitrogen fixation and during the senescence of the nodule. ROS can be harmful provoking oxidative damage or can have a positive role in signalling processes. To deal with ROS production, nodules are fitted with a large panel of enzymatic and non-enzymatic antioxidant mechanisms. This review summarizes the present knowledge about ROS production and scavenging systems in legume nodules, and about their functional roles during the different steps of the symbiotic interaction.