Coping with toxic plant compounds – The insect’s perspective on iridoid glycosides and cardenolides

Specializing on host plants with toxic secondary compounds enforces specific adaptation in insect herbivores. In this review, we focus on two compound classes, iridoid glycosides and cardenolides, which can be found in the food plants of a large number of insect species that display various degrees of adaptation to them. These secondary compounds have very different modes of action: Iridoid glycosides are usually activated in the gut of the herbivores by β-glucosidases that may either stem from the food plant or be present in the gut as standard digestive enzymes. Upon cleaving, the unstable aglycone is released that unspecifically acts by crosslinking proteins and inhibiting enzymes. Cardenolides, on the other hand, are highly specific inhibitors of an essential ion carrier, the sodium pump. In insects exposed to both kinds of toxins, carriers either enabling the safe storage of the compounds away from the activating enzymes or excluding the toxins from sensitive tissues, play an important role that deserves further analysis. To avoid toxicity of iridoid glycosides, repression of activating enzymes emerges as a possible alternative strategy. Cardenolides, on the other hand, may lose their toxicity if their target site is modified and this strategy has evolved multiple times independently in cardenolide-adapted insects. Insects have adapted to tolerate and use iridoid glycosides and cardenolides despite their usually toxic effects. We here review which physiological processes underlie these adaptations. ⺠Iridoid glycosides in the food plant unspecifically harm insects by crosslinking dietary and tissue proteins. ⺠Insects may avoid toxicity by suppressing gut enzymes that activate iridoid glycosides. ⺠Cardenolides act much more specifically by blocking an essential ion carrier, the Na+/K+-ATPase. ⺠Insects on cardenolide plants have repeatedly evolved target site insensitivity to prevent this toxic effect.