Max Planck Institute for Chemical Ecology, Tatzendpromenade 1a, 07745 Jena, Germany.
Maize (Zea mays) emits a mixture of volatile compounds upon herbivore attack that attracts herbivore enemies to the plant. One of the major components of this mixture is an unusual acyclic C11 homoterpene, (3E)-4,8- dimethyl-1,3,7-nonatriene (DMNT), which is released by the attacked leaves as well as systemically by younger undamaged leaves of the same plant. Many other plant species emit DMNT following herbivore damage as well. DMNT biosynthesis has been previously shown to proceed via the sesquiterpene alcohol, (E)-nerolidol. Here we demonstrate an enzyme activity that converts farnesyl diphosphate, the universal precursor of sesquiterpenes, to (3S)-(E)-nerolidol, in cell-free extracts of maize leaves that had been fed upon by Spodoptera littoralis. The properties of the (E)-nerolidol synthase resemble those of other terpene synthases. Evidence for its participation in DMNT biosynthesis includes the direct incorporation of deuterium-labeled (E)-nerolidol into DMNT and the close correlation between increases in (E)-nerolidol synthase activity and DMNT emission after herbivore damage. Since farnesyl diphosphate has many other metabolic fates, (E)-nerolidol synthase may represent the first committed step of DMNT biosynthesis in maize. However, the formation of this unusual acyclic terpenoid appears to be regulated at both the level of (E)-nerolidol synthase and at later steps in the pathway. We are currently isolating terpene synthase genes in maize, attempting to identify genes relevant for DMNT synthesis. Utilizing molecular methods, the manipulation of the release of DMNT and other herbivore-induced plant volatiles will help to evaluate their roles in plant defense.