The expression of monoterpene synthase genes and their respective end products are affected by gibberellic acid in Thymus vulgaris.

Thymus vulgaris L. (Lamiaceae), a well-known aromatic medicinal herb, has many important essential constituents in its oil, including γ-terpinene, carvacrol, thymol, and p-cymene. Gibberellins comprise hundreds of components, which regulate several various growths and underlying developmental processes, such as cell division and elongation, shoot elongation, seed germination, and gene expression. In this study, we investigated the influence of sprayed gibberellic acid (GA) treatments on the internode length, leaf morphology, length of new shoot, expression of monoterpene synthase genes and monoterpenes content during two plant growth stages. Our results showed that increasing of internode length was a clear effect of GA that was varied with internode position. The results also showed that all internodes displayed a dramatic increase in the highest concentration of GA. Also, the foliar application of GA resulted in not only an increased expression level of monoterpene synthase genes, but also the improved production of a monoterpene, especially in the moderate concentration of GA that they were up-regulated. In the lowest GA concentrations, relative expression levels were similar or lower than the control plants and a notable downregulation in those genes was observed in the application of the highest concentration of GA rather than the moderate concentrations. Overall, the expression of two out of five monoterpene synthase genes, TPS and CYP71D181, showed a correlation with the level of γ-terpinene and carvacrol, respectively, indicating that they are regulated at the transcriptional levels.