Elucidation of the essential oil biosynthetic pathways in Cinnamomum burmannii through identification of six terpene synthases

Cinnamomum burmannii is a traditional plant that has long been used as a spice, food preservative, and food flavoring. Essential oils in C. burmannii, which mainly consist of mono- and sesquiterpenes such borneol, linalool, and caryophyllene, have impressive pharmaceutical properties. Although the transcriptome-based discovery of (+)-bornyl diphosphate synthase (CbTPS1) from C. burmannii was reported in our previous study, the remaining terpene synthases (TPSs) corresponding to various terpene biosynthesis pathways remain unidentified. In this study, we report the results of RNA-sequencing of a borneol type plant and functional characterization of six additional full-length candidate TPS genes (named CbTPS2-7). Phylogenetic analysis revealed that CbTPS2 and CbTPS3 together with the previously identified CbTPS1 protein belong to the TPS-b subfamily, and enzyme assays using geranyl diphosphate (GPP) and farnesyl diphosphate (FPP) as substrates revealed that CbTPS1, CbTPS2 and CbTPS3 catalyze the formation of monoterpenes. CbTPS4, CbTPS5, and CbTPS6, which belong to the TPS-a clade, generated monoterpenes and sesquiterpenes. CbTPS7, which belongs to the TPS-g clade, showed linalool/nerolidol synthase activity. These CbTPSs identified in C. burmannii produced a total of 10 monoterpenes and 14 sesquiterpenes in an in vitro assay. These findings clarify the biosynthesis pathways of 13 monoterpenoids and 12 sesquiterpenoids in the leaf essential oil of C. burmannii and shed light on terpene biosynthesis in Cinnamomum.