Reactive oxygen species and antioxidants in legume nodules

Reactive oxygen species are a ubiquitous danger for aerobic organisms. This risk is especially elevated in legume root nodules due to the strongly red ucing conditions, the high rates of respiration, the tendency of leghemoglo bin to autoxidize, the abundance of nonprotein Fe and the presence of sever al redox proteins that leak electrons to O-2. Consequently, nodules are par ticularly rich in both quantity and diversity of antioxidant defenses. Thes e include enzymes such as superoxide dismutase (EC 1.15.1.1) and ascorbate peroxidase (EC 1.11.1.11) and metabolites such as ascorbate and thiol tripe ptides, Nodule antioxidants have been the subject of intensive molecular, b iochemical and functional studies that are reviewed here. The emerging them e is that antioxidants are especially critical for the protection and optim al functioning of N-2 fixation, We hypothesize that this protection occurs at least at two levels: the O-2 diffusion barrier in the nodule parenchyma (inner cortex) and the infected cells in the central zone.