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 peroxi dation of membrane lipids. Some herbivores counteract these defences b y avoidance of light and tissues rich in phototoxins or the ability to detoxify these secondary substances. The cytochrome P-450 polysubstra te monooxygenase systems involved, the metabolic products, and a new p utative toxin pump have been described. Dietary antioxidants (beta-car otene, vitamin E, ascorbate) are additional defences against phototoxi city. They reduce mortality in herbivores exposed to phototoxins and s ome specialist herbivores have high constitutive levels. Adapted speci alist insects also have higher constitutive levels of superoxide dismu tase (SOD) and respond to phototoxins in their diet by the induction o f catalase (CAT), glutathione reductase (CR), and increased levels of reduced glutathione (GSH). Artificial inhibition of the enzymes SOD an d CAT had little effect on phototoxicity but inhibition of GSH synthes is in herbivores enhanced photooxidative effects of administered photo toxins on lipid peroxidation. While insects have many mechanisms to ov ercome plant photooxidants, the Asteraceae appear to have adopted a st rategy of counterattack. We suggest and provide preliminary evidence t hat a second group of secondary substances, the sesquiterpene lactones , occurring in the Asteraceae can attack key antioxidant defences to s ynergise phototoxins. (C) 1995 Wiley-Liss, Inc.