Iridoid biosynthesis in Chrysomelina larvae: Fat body produces early terpenoid precursors

Larvae of the Chrysomelina species Phaedon cochleariae and Gastrophysa viridula produce monoterpenoids (iridoids) to defend themselves against predatory attacks by presenting the toxins upon attack as droplets on the top of nine pairs of dorsal glands. Although the conversion of 8-hydroxygeraniol-8-O-β-d-glucoside into the iridoids in the glandular reservoir has been studied in detail, the synthesis of the glucosidically bound precursor received only limited attention. We compared larvae of the two iridoid producing species with those of Chrysomela populi, a sequestering species producing salicylaldehyde, in terms of the key enzymes 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) and isoprenyl diphosphate synthases involved in the biosynthesis of the iridoid precursor. Increased HMGR transcript abundance, high HMGR activity and accumulation of geraniol indicating geranyl diphosphate synthase activity was observed only in the fat body of the iridoid producing larvae in comparison to other larval tissues and to the tested tissues of C. populi. These results correlate with the identification of glucosidically bound 8-hydroxygeraniol in the fat body of the iridoid producers. We suggest that in P. cochleariae and G. viridula glucosidically bound 8-hydroxygeraniol is produced by the fat body and transferred via the hemolymph into the glandular reservoir for further conversion into iridoids.