Genomic Analysis and Functional Characterization of the Terpene Synthase Gene Family in Brachypodium distachyon

Brachypodium distachyon is a small, temperate grass native to the Mediterranean region. While not agriculturally important, it possesses many characteristics such as rapid generation time, self-pollination, a small genome size, and ease of genetic transformation, that make it an attractive model organism for genetic studies. Brachypodium is closely related to other grasses of economical importance such as rice, sorghum, and especially wheat, and in fact it is the first member of the Pooideae grass sub-family to have its genome fully sequenced. Cereal grasses such as rice, sorghum, and maize have been reported to produce a class of secondary metabolites called terpenes as volatile gases during insect herbivory and after the application of the plant defense signaling molecule, methyl jasmonate (MeJA), as part of an indirect defense mechanism. The terpenes are attractive to predators and parasitoids of the attacking herbivore. The aims of this thesis are to identify the volatile terpenes that Brachypodium plants produce after insect damage and other stresses, to identify the terpene synthases in the Brachypodium genome, and to identify and functionally characterize the particular terpene synthases that are responsible for the production of the volatile products. Upon damage by fall armyworms (Spodoptera frugiperda, a lepidopteran pest of grasses), the plants produce a few terpenes. However, upon the application of exogenous MeJA, Brachypodium plants produce seven sesquiterpenes, including β-caryophyllene. 18 putative full-length terpene synthases were identified from the Brachypodium distachyon genome. Amongst the eight putative mono- and sesquiterpene synthases that were cloned, two were found to be functional in in vitro enzyme assays: BdTPS6 and BdTPS7. BdTPS6 produced β-caryophyllene as its major product, and BdTPS7 produced three sesquiterpenes of unknown identity. Therefore, Brachypodium distachyon retains a β-caryophyllene synthase that is important in the indirect defense of other previously studied cereal grasses.