POTENTIAL ROLE OF ASCORBATE OXIDASE AS A PLANT DEFENSE PROTEIN AGAINST INSECT HERBIVORY

Ascorbic acid is essential for both nutritive and antioxidant function s in phytophagous insects; however, maintaining sufficient quantities of reduced ascorbate may be problematical for them. In this investigat ion, we show that the plant enzyme ascorbate oxidase retains activity in the digestive system of the herbivore Helicoverpa zea. High levels of the enzyme are present in several host plants of H. zea, including cotton, tomato, soybean, crimson clover, and vetch. The enzyme oxidize s L-ascorbic acid to dehydro-L-ascorbic acid, a potentially toxic prod uct. The oxidation of ascorbic acid also produces active oxygen specie s such as the highly reactive hydroxyl radical. The nutritional qualit y of protein for larval H. zea was significantly reduced by treatment with ascorbate and ascorbate oxidase. Oxidative damage to the protein was indicated by decreased lysine content, increased carbonyl formatio n, and the occurrence of protein fragmentation and polymerization. Fur thermore, the oxidative loss of ascorbate in the herbivore's digestive system prevents ascorbate from functioning as an important antioxidan t against a plethora of dietary prooxidants.