FOLIAR OXIDATIVE STRESS AND INSECT HERBIVORY - PRIMARY COMPOUNDS, SECONDARY METABOLITES, AND REACTIVE OXYGEN SPECIES AS COMPONENTS OF INDUCED RESISTANCE

Oxidative responses of plants to pathogens and other environmental str esses have received considerable recent attention. We propose that an oxidative response also occurs following attack by herbivores. Our dat a strongly indicate a shift in the oxidative status of soybean followi ng herbivory by the insect Helicoverpa tea. Herbivory caused significa nt increases in lipid peroxidation and (OH)-O-. radical formation. The activity of several oxidative enzymes including lipoxygenases, peroxi dase, diamine oxidase, ascorbate oxidase, and NADH oxidase I increased after herbivory on soybean. The enhanced production of phenolic compo unds is indicated by an increase in the activity of phenylalanine ammo nia lyase in wounded tissues. On the other hand, the level of soybean foliar antioxidants such as ascorbic acid, total carotenoids, nonprote in thiols, and catalase decreased significantly following herbivory. T hese results implicate primary compounds (e.g., ascorbic acid, protein s), secondary metabolites (e.g., phenolics), and reactive oxygen speci es (e.g., hydroxyl radical, hydrogen peroxide) as multiple components of induced resistance. The oxidative changes in the host plant corresp ond with increased oxidative damage in the midgut of insects feeding o n previously wounded plants. Decreases in nonprotein thiols and reduce d ascorbic acid occurred in midgut epithelial tissue from insects feed ing on wounded plants compared to the insects on control plants. In co ntrast, midgut hydroperoxides and dehydroascorbic acid concentrations were greater in insects on wounded plants compared to their counterpar ts on control plants. We conclude that oxidative responses in soybean may have both positive and negative effects upon the host plant: a dec rease in herbivory and an increase in oxidative damage to the plant. T he salient benefit to the plant, in terms of insect resistance, is the relative balance between these opposing effects.