SELECTIVE SEQUESTRATION AND METABOLISM OF PLANT-DERIVED PYRROLIZIDINEALKALOIDS BY CHRYSOMELID LEAF BEETLES

Pyrrolizidine alkaloids (PAs) are assumed to function as plant defence compounds against herbivory. A number of adapted insects are known to sequester plant derived PAs for their own benefit. Here we summarize the chemical interactions between leaf beetles of the genus Oreina (Co leoptera, Chrysomelidae) and their host plants Adenostyles spp., Senec io nemorensis, and S. fuchsii (Asteraceae, tribe Senecioneae). Senecip hylline N-oxide and senecionine N-oxide, the main PAs of Adenostyles, are sequestered in the bodies and exocrine defensive glands of the lea f beetles. The comparison with the PA patterns of the Senecio host pla nt indicates a selective PA uptake. The uptake into the body (hemolymp h) is less specific, whereas the translocation into the defensive glan ds is highly specific. Only the N-oxides of macrocyclic retronecine es ters of the senecionine type are found in significant amounts in the d efensive secretions. Many other PAs such as monoesters and open-chain diesters as well as PAs of other structural types (e.g. monocrotaline N-oxide and senkirkine) are not transferred into the defensive glands. Leaf beetles sequester PAs exclusively as N-oxides. A novel PA not fo und in the food plants was detected in the defensive secretions of Ore ina elongata; it was identified as 13,19-expoxisenecionine N-oxide (or eine), the epoxidation product of seneciphylline N-oxide. Besides this transformation, leaf beetles are able to catalyse further transformat ions such as the O-dealkylation of heliotrine N-oxide to rinderine N-o xide and the O-deacetylation of acetylseneciphylline N-oxide to seneci phylline N-oxide. The plant-beetle interactions are discussed in the f unctional context of PAs as powerful plant defensive chemicals. (C) 19 97 Elsevier Science Ltd.