Cytochrome P-450 in plant/insect interactions: geraniol 10-hydroxylase and the biosynthesis of iridoid monoterpenoids.

The interactions between plant secondary metabolites (particularly monoterpenes) and insects are discussed. Such metabolites are likely to have influenced the evolution of cyt P450-linked detoxification systems in animals, through animal/plant coevolution. The biosynthesis of many classes of plant secondary metabolites involves cyt P450 enzymes. Of these, one of the best characterised is the geraniol/nerol 10-hydroxylase which catalyses a key step in the biosynthesis of the iridoid class of plant terpenes. It would appear that these monoterpenoids are synthesised (via cyt P450 hydroxylation) from different precursors in different plant species, namely geraniol, its isomer nerol, or the related monoterpenoid, citronellol. We show that cyt P450 from the plants Catharanthus roseus and Nepeta racemosa are capable of hydroxylating geraniol, nerol and citronellol, and thus do not impose precursor specificity on iridoid biosynthesis in plants.