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Insects employ iridoids to deter predatory attacks. Larvae of some Chrysomelina species are capable to produce those cyclopentanoid monoterpenes de novo. The iridoid biosynthesis proceeds via the mevalonate pathway to geranyl diphospate (GDP) subsequently converted into 8-hydroxygeraniol-8-O-beta-D-glucoside followed by the transformation into the defensive compounds. We tested whether the glucoside, its aglycon or geraniol has an impact on the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the key regulatory enzyme of the mevalonate pathway and also the iridoid biosynthesis. To address the inhibition site of the enzyme, initially a complete cDNA encoding full length HMGR was cloned from Phaedon cochleariae. Its catalytic portion was then heterologously expressed in Escherichia coli. Purification and characterization of the recombinant protein revealed attenuated activity in enzyme assays by 8-hydroxygeraniol whereas no effect has been observed by addition of the glucoside or geraniol. Thus, the catalytic domain is the target for the inhibitor. Homology modeling of the catalytic domain and docking experiments demonstrated binding of 8-hydroxygeraniol to the active site and indicated a competitive inhibition mechanism. Iridoid producing larvae are potentially able to sequester glucosidically bound 8-hydroxygeraniol whose cleavage of the sugar moiety results in 8-hydroxygeraniol. Therefore, HMGR may represent a regulator in maintenance of homeostasis between de novo produced and sequestered intermediates of iridoid metabolism. Furthermore, we demonstrated that HMGR activity is not only diminished in iridoid producers but most likely prevalent within the Chrysomelina subtribe and also within the insecta.

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-beta-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.