The biochemistry of homoterpenes – Common constituents of floral and herbivore-induced plant volatile bouquets

This review presents a survey of the distribution of homoterpene volatiles in land plants and covers recent findings on the biosynthesis and functions of homoterpenes. [Display omitted] ► Homoterpenes are common floral and insect-induced volatiles in angiosperms. ► The biosynthetic pathway of the homoterpenes DMNT and TMTT has been elucidated. ► The P450 CYP82G1 from Arabidopsis thaliana was identified as a DMNT/TMTT synthase. ► Engineering the formation of homoterpenes may decipher their role in plant defense. Volatile organic compounds emitted by plants mediate a variety of interactions between plants and other organisms. The irregular acyclic homoterpenes, 4,8-dimethylnona-1,3,7-triene (DMNT) and 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT), are among the most widespread volatiles produced by angiosperms with emissions from flowers and from vegetative tissues upon herbivore feeding. Special attention has been placed on the role of homoterpenes in attracting parasitoids and predators of herbivores and has sparked interest in engineering homoterpene formation to improve biological pest control. The biosynthesis of DMNT and TMTT proceeds in two enzymatic steps: the formation of the tertiary C 15-, and C 20-alcohols, ( E)-nerolidol and ( E, E)-geranyl linalool, respectively, catalyzed by terpene synthases, and the subsequent oxidative degradation of both alcohols by a single cytochrome P450 monooxygenase (P450). In Arabidopsis thaliana, the herbivore-induced biosynthesis of TMTT is catalyzed by the concerted activities of the ( E, E)-geranyllinalool synthase, AtGES, and CYP82G1, a P450 of the so far uncharacterized plant CYP82 family. TMTT formation is in part controlled at the level of AtGES expression. Co-expression of AtGES with CYP82G1 at wound sites allows for an efficient conversion of the alcohol intermediate. The identified homoterpene biosynthesis genes in Arabidopsis and related genes from other plant species provide tools to engineer homoterpene formation and to address questions of the regulation and specific activities of homoterpenes in plant–herbivore interactions.