Selective sequestration and metabolism of plant derived pyrrolizidine alkaloids 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 <i>Oreina</i> (Coleoptera, Chrysomelidae) and their host plants <i>Adenostyles</i> spp., <i>Senecio nemorensis</i>, and <i>S. fuchsii</i> (Asteraceae, tribe Senecioneae). Seneciphylline <i>N</i>-oxide and senecionine <i>N</i>-oxide, the main PAs of <i>Adenostyles</i>, are sequestered in the bodies and exocrine defensive glands of the leaf beetles. The comparison with the PA patterns of the <i>Senecio</i> host plant indicates a selective PA uptake. The uptake into the body (hemolymph) is less specific, whereas the translocation into the defensive glands is highly specific. Only the <i>N</i>-oxides of macrocyclic retronecine esters of the senecionine type are found in significant amounts in the defensive secretions. Many other PAs such as monoesters and open-chain diesters as well as PAs of other structural types (e.g. monocrotaline <i>N</i>-oxide and senkirkine) are not transferred into the defensive glands. Leaf beetles sequester PAs exclusively as <i>N</i>-oxides. A novel PA not found in the food plants was detected in the defensive secretions of Oreina elongata; it was identified as 13,19-expoxisenecionine <i>N</i>-oxide (oreine), the epoxidation product of seneciphylline <i>N</i>-oxide. Besides this transformation, leaf beetles are able to catalyse further transformations such as the O-dealkylation of heliotrine <i>N</i>-oxide to rinderine <i>N</i>-oxide and the O-deacetylation of acetylseneciphylline <i>N</i>-oxide to seneciphylline <i>N</i>-oxide. The plant-beetle interactions are discussed in the functional context of PAs as powerful plant defensive chemicals.