Antioxidants and antioxidant enzymes as biochemical defenses against phototoxin ingestion by insect herbivores.
Creators: name: Aucoin, Richard R.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1991
Subjects: subject: Biology, Entomology.

description: Many secondary plant compounds are capable of photoactivation resulting in the production of toxic species of oxygen. One mechanism of defense for insects feeding on phototoxic plants may be the presence of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPOX), and glutathione reductase (GR). The activities of these enzymes were examined in larvae of three lepidoptera: Ostrinia nubilalis, Manduca sexta, and Anaitis plagiata. Highest levels of antioxidant enzyme activity were found in A. plagiata, a specialist feeder on Hypericum perforatum, which contains high levels of the phototoxin hypericin. Larvae of A. plagiata fed leaf discs treated with hypericin exhibited a short-term, concentration-dependent decline in enzyme activity. Longer term studies with A. plagiata fed either the phototoxic H. perforatum, or the closely related but non phototoxic H. calycinum, resulted in increased CAT and GR activity in larvae fed the phototoxic plant whereas superoxide dismutase activity was not significantly different. These results suggest that CAT and GR may be inducible defenses against phototoxins. Other insect defenses against phototoxins include specific biochemical defenses such as antioxidants. These antioxidant defenses eliminate or quench the deleterious singlet oxygen and free radicals formed by these phototoxins. We examined the role of dietary antioxidants in protecting the phototoxin-sensitive insect herbivore M. sexta. Elevated dietary levels of the lipid-soluble antioxidants beta-carotene and vitamin E resulted in a concentration-dependent reduction in the mortality associated with treatment of larvae with the phototoxic thiophene $\alpha$-T. Elevated levels of dietary ascorbic acid had no effect whereas reduced levels greatly increased the toxicity of $\alpha$-T. Tissue levels of antioxidants were shown to increase substantially in larvae fed antioxidant-supplemented diets. The results suggest that the ability to absorb and utilize plant-derived antioxidants could be an important defense against photo-activated plant secondary compounds and may have allowed some insects to exploit phototoxic plants. The effects of oxidative stress induced by $\alpha$-T at the biochemical level and the protective effect of antioxidants and antioxidant enzymes were also examined. The phototoxin $\alpha$-T strongly induced lipid peroxidation (LPO) in midgut tissues of the phototoxin-sensitive M. sexta in a UV-dependent manner, however this LPO was prevented when the compound was administered to larvae raised on high vitamin E diets. In the absence of UV, $\alpha$-T caused a significant increase in GPOX, GR, and non-GSH-dependent PER activity over 72 h. However in the presence of UV, $\alpha$-T strongly inhibited GPOX and GR and prevented the increase in PER. $\alpha$-T also affected cellular thiol status with approximately a 50% increase in total and GSH content in midgut tissue, although this was not UV-dependent. The effectiveness of antioxidant enzymes and the antioxidant GSH in providing protection against phototoxins were also examined. Neither the SOD inhibitor DEDC nor the CAT inhibitor 3AT affected the acute toxicity of topically applied $\alpha$-T to M. sexta larvae. The GSH-depleting agent BSO also had no effect on acute toxicity. In contrast, GSH depletion strongly enhanced the chronic (72 h) toxicity of $\alpha$-T when the phototoxin was incorporated into diets. GSH depletion also enhanced LPO in midgut tissue of $\alpha$-T-treated larvae. Implications of the results are discussed in terms of the role antioxidants and antioxidant enzymes may have played in the successful adaptation of some insect species to phototoxin-containing plants. The interrelationships of biochemical, physiological, physical, and behavioural mechanisms of defense are considered.