Divergence in tissue-specific expression patterns of genes associated with the terpeniod biosynthesis in two oregano species Origanum vulgare L., and Origanum majorana

Oregano (Origanum vulgare) and marjoram (O. majorana) are two discrete (in terms of sensory characteristics) species within the genus Origanum (Lamiaceae). Their aroma, flavor, and pharmaceutical value are a result of their essential oils, which comprise mainly of monoterpenes and sesquiterpenes. Marjoram is rich in bicyclic monoterpene cis-sabinene hydrate derived from the biosynthetic "sabinyl" pathway while the phenolic monoterpene carvacrol, arising from the "cymyl" pathway, is the distinctive feature of oregano. To investigate differences between the terpene biosynthetic pathways of both species, we identified key enzymes of terpene biosynthesis from the two species of genus Origanum. The heterologous expression of these enzymes showed that each formed multiple mono- or sesquiterpene products and, in combination, were responsible for the direct production of almost all terpenes found in Origanum essential oils. The correlation between essential oil composition and relative terpene synthase transcript concentrations in O. vulgare and O. majorana demonstrated that monoterpene synthase activity is predominantly regulated at the level of transcription and that the phenolic monoterpene alcohol thymol is derived from a-terpinene, the product of a single monoterpene synthase. The combination of heterologously expressed terpene synthases for in vitro assays resulted in blends of mono- and sesquiterpene products that strongly resembled those found in vivo, indicating that terpene synthase expression levels directly control the composition of the essential oils. These results will facilitate the metabolic engineering, directed breeding of Origanum cultivars with higher quantities of essential oils, and improved oil compositions.