Antioxidant defenses in the peripheral cell layers of legume root nodules

Ascorbate peroxidase (AP) is a key enzyme that scavenges potentially harmful H2O2 and thus prevents oxidative damage in plants, especially in N-2-fixing legume root nodules. The present study demonstrates that the nodule endodermis of alfalfa (Medicago sativa) root nodules contains elevated levels of AP protein, as well as the corresponding mRNA transcript and substrate (ascorbate). Enhanced AP protein levels were also found in cells immediately peripheral to the infected region of soybean (Glycine max), pea (Pisum sativum), clover (Trifolium pratense), and common bean (Phaseolus vulgaris) nodules. Regeneration of ascorbate was achieved by (homo)glutathione and associated enzymes of the ascorbate-glutathione pathway, which were present at high levels. The presence of high levels of antioxidants suggests that respiratory consumption of O-2 in the endodermis or nodule parenchyma may be an essential component of the O-2-diffusion barrier that regulates the entry of O-2 into the central region of nodules and ensures optimal functioning of nitrogenase.