The molecular regulation of terpene polymorphism in Thymus vulgaris
Thymus vulgaris, the common garden thyme, is an aromatic plant known and valued for its essential oil since antiquity. In natural populations, the terpenoid composition of the essential oil, which is dominated by monoterpenoids and their derivatives, can vary strongly among individual plants which are otherwise morphologically identical. This variation takes the form of so-called chemotypes named after the dominant monoterpene alcohols produced in the glandular trichomes on the surface of the leaves: geraniol (G), α-terpineol (A), sabinene hydrate (thujanol, U), linalool (L), carvacrol (C), thymol (T), and 1,8-cineole (E). A genetically distinct epistatic series of five loci, with a set order of dominance (G > A > U > L > C > T), has been previously shown to be involved in controlling the monoterpenoid production of individual plants. In this study, some pivotal genes of terpenoid biosynthesis were isolated and identified, and their role in monoterpenoid chemotype formation examined, to gain a better understanding of the control mechanisms underlying the chemical polymorphism in T. vulgaris. Several terpene synthases were isolated: a γ-terpinene synthase, two linalool synthases, two sabinene hydrate synthases, and a (E)-β-caryophyllene synthase. Two cytochrome P450 gene sequences are thought to be involved in the reactions from γ-terpinene to thymol and carvacrol, respectively. The terpene production observed in the plants correlated with differences in the transcript levels of these terpenoid biosynthetic genes. A regeneration protocol for T. vulgaris was established based on that for lavandin (Lavandula x intermedia) which made it possible for the first time to regenerate T. vulgaris plantlets from leaf explants by inducing callus formation followed by subsequent shoot induction on media with different hormone concentrations.