HOW DO INSECT HERBIVORES COPE WITH THE EXTREME OXIDATIVE STRESS OF PHOTOTOXIC HOST PLANTS

Plants of the Asteraceae and Hypericaceae possess secondary compounds that induce photooxidation in insect herbivores that consume them. One of the well-established modes of action of these substances is peroxidation of membrane lipids. Some herbivores counteract these defences by avoidance of light and tissues rich in phototoxins or the ability to detoxify these secondary substances. The cytochrome P-450 polysubstrate monooxygenase systems involved, the metabolic products, and a new putative toxin pump have been described. Dietary antioxidants (beta-carotene, vitamin E, ascorbate) are additional defences against phototoxicity. They reduce mortality in herbivores exposed to phototoxins and some specialist herbivores have high constitutive levels. Adapted specialist insects also have higher constitutive levels of superoxide dismutase (SOD) and respond to phototoxins in their diet by the induction of catalase (CAT), glutathione reductase (CR), and increased levels of reduced glutathione (GSH). Artificial inhibition of the enzymes SOD and CAT had little effect on phototoxicity but inhibition of GSH synthesis in herbivores enhanced photooxidative effects of administered phototoxins on lipid peroxidation. While insects have many mechanisms to overcome plant photooxidants, the Asteraceae appear to have adopted a strategy of counterattack. We suggest and provide preliminary evidence that a second group of secondary substances, the sesquiterpene lactones, occurring in the Asteraceae can attack key antioxidant defences to synergise phototoxins. (C) 1995 Wiley-Liss, Inc.