ANTIOXIDANT DEFENSES IN THE PERIPHERAL CELL-LAYERS OF LEGUME ROOT-NODULES

Ascorbate peroxidase (AP) is a key enzyme that scavenges potentially h armful H2O2 and thus prevents oxidative damage in plants, especially i n N-2-fixing legume root nodules. The present study demonstrates that the nodule endodermis of alfalfa (Medicago sativa) root nodules contai ns elevated levels of AP protein, as well as the corresponding mRNA tr anscript and substrate (ascorbate). Enhanced AP protein levels were al so found in cells immediately peripheral to the infected region of soy bean (Glycine max), pea (Pisum sativum), clover (Trifolium pratense), and common bean (Phaseolus vulgaris) nodules. Regeneration of ascorbat e was achieved by (homo)glutathione and associated enzymes of the asco rbate-glutathione pathway, which were present at high levels. The pres ence of high levels of antioxidants suggests that respiratory consumpt ion 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.