Recent insights into antioxidant defenses of legume root nodules

Summary Legume root nodules are sites of intense biochemical activity and consequently are at high risk of damage as a result of the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). These molecules can potentially give rise to oxidative and nitrosative damage but, when their concentrations are tightly controlled by antioxidant enzymes and metabolites, they also play positive roles as critical components of signal transduction cascades during nodule development and stress. Thus, recent advances in our understanding of ascorbate and (homo)glutathione biosynthesis in plants have opened up the possibility of enhancing N2 fixation through an increase of their concentrations in nodules. It is now evident that antioxidant proteins other than the ascorbate‐glutathione enzymes, such as some isoforms of glutathione peroxidases, thioredoxins, peroxiredoxins, and glutathione S‐transferases, are also critical for nodule activity. To avoid cellular damage, nodules are endowed with several mechanisms for sequestration of Fenton‐active metals (nicotianamine, phytochelatins, and metallothioneins) and for controlling ROS/RNS bioactivity (hemoglobins). The use of ‘omic’ technologies has expanded the list of known antioxidants in plants and nodules that participate in ROS/RNS/antioxidant signaling networks, although aspects of developmental variation and subcellular localization of these networks remain to be elucidated. To this end, a critical point will be to define the transcriptional and post‐transcriptional regulation of antioxidant proteins. Contents Summary 960 I. Introduction 961 II. Ascorbate 961 III. Thiols 963 IV. Ascorbate–glutathione pathway 963 V. Superoxide dismutases and catalases 964 VI. Thiol peroxidases: glutathione peroxidases and peroxiredoxins 966 VII. Protein disulfide reductases: thioredoxins and glutaredoxins 967 VIII. Glutathione S‐transferases 967 IX. Antioxidants and metal sequestration 968 X. Antioxidants of nodule bacteroids 969 XI. Other molecules with antioxidative properties in nodules 969 XII. Antioxidants and oxidative/nitrosative signaling 970 XIII. Antioxidants and oxidative/nitrosative stress 971 XIV. Conclusions and perspectives 972 Acknowledgements 973 References 973