Insect-induced gene expression at the core of volatile terpene release in Medicago truncatula

The blends of induced volatiles released by higher plants in response to herbivory regularly contain terpenoids. The precursors of volatile terpenoids can be synthesized via two pathways, the mevalonate (MVA) and the methyl erythritol 4-phosphate (MEP) pathways localized in the cytosol and in plastids, respectively. Terpenes are important players in interactions between plants and herbivorous insects, by acting in both direct and indirect defenses. We recently characterized a gene encoding an (E)-beta-ocimene synthase (MtEBOS) in the legume Medicago truncatula Gaertn. Compared to undamaged plants, caterpillar-damaged M. truncatula emitted (E)-beta-ocimene at an elevated level and this increase is associated with high levels of expression of MtEBOS mRNA. Exogenous treatment with jasmonic acid also increases transcript accumulation of MtEBOS. These results indicate that transcript accumulation is used as a tightly regulated mechanism to control (E)-beta-ocimene emission. The data, along with additional findings in other species, illustrate that like most plant families legumes regulate the final steps of volatile terpene biosynthesis at the level of transcript induction.