Review of the chemical ecology of homoterpenes in arthropod–plant interactions

The homoterpenes 4,8-dimethyl-1,3,7-nonatriene (DMNT) and 4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) are volatile products of plant metabolism reported from diverse plant taxa and multiple plant tissues. As such, they have a range of potential ecological functions. Here, we review the key literature to assess evidence for roles in contrasting plant–arthropod interactions. TMTT, and DMNT especially, have been reported as sometimes dominant constituents of floral scents from angiosperm taxa ranging from primitive Magnoliales to more advanced, taxonomic orders of economic significance such as Fabales and Sapindales. Although all taxa producing TMTT and DMNT in floral scents are entomophilous (‘insect pollinated’), experimental evidence for an assumed role of these homoterpenes in pollinator attraction is limited. Representing a trade-off, in some cases, homoterpenes in floral scents have been shown to act as kairomones, attracting herbivores. Additionally, both TMTT and DMNT are released by plant foliage in response to arthropod feeding, mechanical damage simulating feeding, or even egg deposition. Evidence for a functional role in herbivore-induced plant volatile (HIPV) blends comes from a wide range of angiosperm orders, including anemophilous (‘wind pollinated’) taxa, as well as from gymnosperms. We conclude by considering how TMTT and DMNT function in community-level interactions and highlighting research priorities that will reveal how plants avoid trade-offs from contrasting ecological functions of DMNT and TMTT release and how homoterpene production might be exploited to develop improved crop varieties.