Defense by pyrrolizidine alkaloids: Developed by plants and recruited by insects

Pyrrolizidine alkaloids are constitutively expressed toxic plant secondary compounds with sporadic occurrence in distantly related angiosperm families. Physiological and ecological aspects of their role in defense against herbivores are discussed. In Senecio species senecionine N-oxide is synthesized as backbone structure in roots, distributed through the phloem all over the plant and diversified by peripheral reactions yielding the species-specific alkaloid profiles. Except structural diversification the alkaloids do not exhibit any turnover or degradation. They are, however, spatially mobile, slowly allocated and accumulated at strategic important sites of defense, e.g. inflorescences and epidermal tissues. Based on the molecular evolution of the first pathway-specific enzyme, evidence is presented that the pathways of pyrrolizidine alkaloids evolved independently in the various angiosperm taxa. Pyrrolizidine alkaloids are the best studied example for a plant defense system recruited by herbivorous insects. Various adapted insects sequester the alkaloids and utilize them for their own protection against predators and parasitoids. Adapted arctiids (Lepidoptera) are chosen to illustrate the specific integration of a plant defense system into an insect's biology. This integration concerns sensory recognition and efficient exploitation of plant alkaloid sources as well as maintenance and allocation of the alkaloids in a non-toxic and metabolically safe state. These biochemical adaptations assure the unique role of the alkaloids in the insect's behavior, for instance, male-to-female transfer of the alkaloids that guarantees an efficient protection of the eggs by coating with alkaloids from both parents.