Morphological, phytochemical, and transcriptome analyses provide insights into the biosynthesis of monoterpenes in Monarda citriodora.

ABSTRACT

MAIN CONCLUSION: Morphological, phytochemical, and transcriptome analyses revealed candidate genes involved in the biosynthesis of volatile monoterpenes and development of glandular trichomes in Monarda citriodora. Monarda citriodora Cerv. ex Lag. is a valuable aromatic plant due to the presence of monoterpenes as major constituents in its essential oil (EO). Thus, it is of sheer importance to gain knowledge about the site of the biosynthesis of these terpenoid compounds in M. citriodora, as well as the genes involved in their biosynthesis. In this study, we studied different types of trichomes and their relative densities in three different developmental stages of leaves, early stage of leaf development (L1), mid-stage of leaf development (L2), and later stage of leaf development (L3) and the histochemistry of trichomes for the presence of lipid and terpenoid compounds. Further, the phytochemical analysis of this plant through GC-MS indicated a higher content of monoterpenes (thymol, thymoquinone, γ-terpinene, p-cymene, and carvacrol) in the L1 stage with a substantial decrease in the L3 stage of leaf development. This considerable decrease in the content of monoterpenes was attributed to the decrease in the trichome density from L1 to L3. Further, we developed a de novo transcriptome assembly by carrying out RNA sequencing of different plant parts of M. citriodora. The transcriptome data revealed several putative unigenes involved in the biosynthesis of specialized terpenoid compounds, as well as regulatory genes involved in glandular trichome development. The data generated in the present study build a strong foundation for further improvement of M. citriodora, in terms of quantity and quality of its essential oil, through genetic engineering.