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The symbiotic interaction between legumes and bacteria of Rhizobium type leads to the formation of new organs, called nodules, which provides a niche for bacterial nitrogen (N2) fixation. In the nodules, bacteria differentiate into bacteroids able to fix atmospheric N2 through nitrogenase activity. As nitrogenase is strongly inhibited by oxygen, N2-fixation is made possible thanks to microaerophilic conditions prevailing in the nodules, characterized by a balance between low oxygen contents and adjusted nitric oxide (NO) contents. NO was shown to be produced by both the plant and bacterial partners during symbiosis, from early interaction steps between the plant and the bacteria to N2-fixing and senescence steps in mature nodules. NO is required for an optimal establishment of the symbiotic interaction. Transcriptomic analysis at early stage of the symbiosis showed that NO regulates about 2030 genes including genes involved in the induction of cell dedifferentiation and organogenesis, and in the repression of plant defense reactions, favouring the establishment of the plant–microbe interaction. In mature nodules, NO was found to act as both an inhibitor of N2-fixation, a regulator of nitrogen metabolism, a beneficial metabolic intermediate for the maintenance of the energy metabolism under hypoxic conditions and a signal triggering nodule senescence. The present review provides an overview of NO sources and degradation pathways and of its multifaceted functions throughout the whole symbiotic process. It additionally analyses NO crosstalks with reactive oxygen species and hormones and presents particularly promising issues to decipher the roles of NO in N2-fixing symbioses.

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.