Investigating the Molecular Basis of Volatile-mediated Plant Indirect Defense against Herbivorous Insects Using Functional and Comparative Genomics

Volatile organic compounds (VOCs) play important roles in plant indirect defense against herbivorous insects by attracting the natural enemies. I first used a tritrophic model system involving rice, rice fall armyworm (Spodoptera frugiperda), and the parasitoid Cotesia marginiventris to discover and characterize the volatile terpenoids and TPS genes involved in the indirect defense of rice against the insect herbivory with integrated functional genomics analyses. Seven rice TPS genes were found to be significantly up-regulated by both microarray and real-time PCR analyses, with one characterized as a linalool synthase and two as sesquiterpene synthases. The products of all three characterized genes covered most of the volatile terpenoids emitted by the fall armyworm-damaged rice. The products of all three characterized genes covered most of the volatile terpenoids emitted by the fall armyworm-damaged rice. In addition to the insect treatment, responses induced by defense hormone jasmonic acid were also examined with volatile analysis, gene expression profiling and enzyme assays. Only one TPS gene in the microarray analysis was up-regulated during the early response to jasmonic acid, and the gene was characterized as OsLMS (rice limonene synthase). Moreover, a homolog gene with similar sequence was characterized with limonene synthase activity and named OsLMS2. Both genes exhibited a time-dependent expression upregulation upon jasmonic acid treatment. The regulation of terpenoid volatile emission was also examined from the perspective of diurnal cycle. Our results showed that the emission of volatile terpenoids was linked to the diurnal cycle; however, the base level emissions were different among the products of three genes. The volatile terpenoid biosynthesis and emission were regulated at both TPS gene expression level and the substrate level. To further understand the molecular evolution of plant indirect defense against herbivorous insects, I also investigated the evolution of the TPS gene family using comparative genome analyses. These analyses revealed both a rapid evolution of the TPS gene family and a conserved group of monoterpene synthase with a deep evolutionary origin. Biochemical analysis of one of the poplar TPS genes in the conserved group showed linalool synthase activity