# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 3 Potential Agricultural Biotechnology Applications  
Terpenoids play vital roles in different biological processes, not only for plant defenses but also for insect development. Sesquirepenoids of insects include the well-known farnesoic acid (FA), methyl farnesoate (MF), and juvenile hormone (JH) [19; 92; 93]. The production of sesquiterpenoids in both plants and insects starts with the formation of the C5 building units IPP and DMAPP by the mevalonate (MVA) pathway and the alternate plastidial methylerythnitol phosphate (MEP) pathway in plants. The C5 building units are further assembled into farnesyl diphosphate (FPP) and modified into C15 sesquiterpenoid products with the help of different TPSs [94; 95; 96; 19; 97]. Yet, diversity can be found in the production of sesquiterpenoids. For instance, different groups of insects use different pathways to produce JH. While most insects like cockroaches and locust utilize epoxidase CYP15A1 to produce JH, lepidopterans first convert FA to JH-III acid with epoxidase CYP15C1 and subsequently use juvenile hormone acid O-methyltransferasearse (JHAMT) to form JH [92; 96; 97; 98]. Besides its role in regulating various endogenous processes, JH is also involved in mediating insect responses to the environment. For instance, in response to seasonal changes of the photoperiod, aphids switch their reproductive mode between parthenogenesis and sexual reproduction, during which, JH appears to have a role, because transcriptome analysis found that genes (JH acid methyltransferase, JH binding protein-like, circadian clock-controlled protein) responsible for JH synthesis and transport are down-regulated when days are short [99]. Despite the similar chemical structures of terpenoids in plants and sesquiterpenoids in insects, there are currently no studies showing a direct crosstalk between the two kingdoms. Nevertheless, terpenoids that attract herbivores could also function as bioherbicides allowing management of weed growth in paddies or any agricultural fields. _Altica cyanea_ females are attracted by elevated emissions of the volatile blend from damaged _Ludwigia octavalys_ (Jacq.) Raven (Onagraceae), a rice-field weed, in which four terpenoids were identified, i.e., \(\alpha\)-pinene, linalool oxide, geraniol, and phytol [85]. To keep pests away, about 20 years ago a project proposed to transfer the (S)-limonene synthase gene from wildflower _Clarkia breweri_ to petunia. Linalool, which can repel aphids, successfully increased [100]. A similar overexpression of linalool synthase gene from strawberry in chrysanthemum also decreases the tendency of thrips and aphids to attack the transgenic plants with respect control plants [101]. Metabolic engineering involved the strawberry _FaNES1 (Fragaria ananassa_ Nerolidol Synthas 1) gene expressed in other crops such as potato. FaNES1 recombinant protein catalyzes the formation of S-linalool, resulting in a much higher level of linalool [102]. Interestingly, targeting the FaNES1 protein in the mitochondria of transgenic _Arabidopsis_ rather than the protein in plastid can lead to a higher yield of the sesquiterpene nerolidol [103] The homoterpene derivative of nerolidol known as 4,8-dimethyl-1,3(E),7-nonatriene further contributes to plant defense by attracting insect herbivores predators [104]. Another indirect defense example of maize, listed in the above Table 2, consisted in inducing the sesquiterpene synthase gene (_TPS10_) from maize into _Arabidopsis_ to help the female parasitoid wasp _C. marginvertris_ to locate the host caterpillars [105]. Genetically modified crops can be more attractive to pollinators due to their richness in terpene compounds. Comparing Cry3 Bb-expressing genetically modified (GM) eggplants and their near-isogenic control, the GM eggplant contains an abundant amount of terpenes, such as (+)-limonene, Z-jasmone, p-cymene, \(\alpha\)-pinene, and (-)-limonene, which explains the increased visitation by the commercial pollinator bumblebees (_Bombus terrestris_) [106].