Biosynthesis pp 207-243
Biosynthesis and Metabolism of Pyrrolizidine Alkaloids in Plants and Specialized Insect Herbivores
Pyrrolizidine alkaloids (PAs) encompass a typical class of plant secondary compounds. During recent years PAs have proved to be an excellent choice to exemplify various mechanistic and functional aspects of plant secondary metabolism. PAs appear to play an important role in constitutive plant chemical defense, particularly, in plant-herbivore interactions. Biochemical and physiological aspects of PA biosynthesis, allocation, and accumulation are reviewed with particular emphasis on key enzymatic steps and chemical diversification of PA backbone structures. A number of taxonomically unrelated specialized insect herbivores sequester PAs from their food plants. The biochemistry of PA sequestration in lepidopterans and leaf beetles is outlined along with the mechanisms of PA absorption, storage, pheromone biosynthesis, and insect-specific PA transformations (i.e., formation of insect alkaloids). The unique feature of PAs, that they exist in two easily interchangeable forms, the pro-toxic free base and the nontoxic N-oxide, is stressed and related to the function of PAs as defensive compounds. The first molecular evidence concerning the phylogenetic origin of PAs is presented.
KeywordsPyrrolizidine alkaloids (plants) Chemical diversification Alkaloid sequestration (insects) Chemical defense Chemical ecology
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