Biochemical defence of pro-oxidant plant allelochemicals by herbivorous insects

Abstract

A new aspect of interactions among insect herbivores and defensive chemistry of plants in the regulation of oxygen toxicity exerted by pro-oxidant allelochemicals is described. Endogenous oxygen toxicity results from activation of the ground state of molecular oxygen to the superoxide anion radical (O 2· −), hydrogen peroxide (H 2O 2), hydroxyl radical (·OH), lipid hydroperoxides (LOOHs), and peroxyl radicals (LO 2· or RO 2·). The strongly lipid-peroxidizing singlet oxygen ( 1Δg O 2 ) is also produced during light activation of photosensitizers. Ingestion of pro-oxidants exacerbates oxygen toxicity by increasing the production of these deleterious forms of oxygen. The role of ascorbate, α-tocopherol, glutathione, carotenoids and urate as antioxidants in insects is apparent, but needs more work for the elucidation of their roles. The major defence mechanism includes a group of antioxidant enzymes represented by superoxide dismutase (SOD), catalase (CAT), glutathione-S- transferase's peroxidative activity (GSTPX), glutathione reductase (GR), and DT-diaphorase, SOD converts O 2· − radicals to H 2O 2 and O 2, CAT decomposes H 2O 2 to H 2O and O 2, GSTPX reduces LOOHs to LOHs with GSH as reductant, and GSSG formed from GSH during the GSTPX reaction is reduced to GSH by GR. DT-diaphorase is an important antioxidant in that it reduces quinones by a two-electron reduction to stable products, thereby preventing the one-electron reduction to semiquinone radicals which generate O 2· − radicals. Therefore, these enzymes are crucial for insect herbivores for preventing the free-radical cascade of oxygen, and terminating the toxic lipid peroxidation chain reaction, in response to the endogenous and potential exogenous oxidant-induced injury.