Chemotypes and terpene synthase genes in Thymus genus: State of the art

Aromatic plants belonging to the Lamiaceae have been used in folk medicine as well as flavouring herbs. This family includes numerous species which are rich in essential oils, that have been studied from both the volatiles point of view and using molecular tools. Thymus is one important genus from this family, comprising about 215 species, whose essential oils have been used by the pharmaceutical industry given their biological activities, namely antibacterial, antifungal and anticancer activities. Essential oils are composed of a mixture of monoterpenes, among other constituents. Terpenoids show high structural variability and over 23.000 different metabolites are known. The occurrence of chemotypes in selected thymus species is listed and organized, and the concept of chemotype is addressed. A multiple approach, considering multivariate analysis of essential oil components and their metabolic relationship, is proposed for chemotype determination. Terpene synthase genes are described, with particular emphasis to the monoterpene synthases, its conserved motifs and features. The role of site directed mutagenesis and its contribution for understanding terpene synthases, revealing the role of some amino acids in the protein activity and final product synthesis is also explored. T. caespititius, a thyme species with carvacrol/thymol, α-terpineol chemotypes, among others, was chosen as an example to describe the state of the art on this topic. The high content of the phenolic monoterpenes makes this species interesting to explore as a source of compounds with antioxidant and antimicrobial properties. Two γ-terpinene synthases were identified in this thyme species, revealing the existence of two isogenes, one isoform directed to the plastids and the other probably cytosolic, indicating the complexity of terpene synthesis. Functional analysis suggests the existence of additional levels of regulation that might involve direct temperature effect, regulating protein activity and thereby affecting the final product. This opens new layers of regulation that will undoubtedly bring complexity to essential oil composition on aromatic plants. In this review, data available both on essential oils characterization and on monoterpene synthase genes for Thymus species is summarized, stressing the importance of integrating molecular and biochemical data on chemotype determination.