SmERF6 promotes the expression of terpenoid pathway in Salvia officinalis and improves the production of high-value abietane diterpenes, carnosol and carnosic acid.

Carnosol (CO) and carnosic acid (CA) are pharmaceutically important diterpenes predominantly produced in members of Lamiaceae, Salvia officinalis (garden sage), Salvia fruticosa, and Rosmarinus officinalis. Nevertheless, the availability of these compounds in plant systems is very low. In an effort to improve the in planta content of these diterpenes in garden sage, SmERF6 (Salvia miltiorrhiza Ethylene Responsive Factor 6) transcription factor was expressed heterologously. Bai et al. (The ethylene response factor SmERF6 co-regulates the transcription of SmCPS1 and SmKSL1 and is involved in tanshinone biosynthesis in Salvia miltiorrhiza hairy roots. Planta 2018; 248:243-55.) proved that SmERF6 binds to the promoter regions of Copalyl pyrophosphate synthase and Kaurene synthase-like genes and improves transcription, thereby augmenting ferruginol levels, a common precursor for abietane diterpenes in Salvia genus; moreover, transgenic hairy roots of S. miltiorrhiza displayed 4-fold improved tanshinone content. In our study, heterologous transient expression of SmERF6 in S. officinalis exhibited inter-specific activity in promoting differential accumulation of diterpenes. Overexpression studies showed elevation in the levels of CO (2-fold) and CA (5-fold). Furthermore, in infiltrated leaves levels of ferruginol (50%) and CA derivatives (rosmanol, epirosmanol, and methyl CA) were significantly upregulated along with the other signature terpenes. Finally, stable transgenic lines of S. officinalis developed through Agrobacterium-mediated in planta genetic transformation accumulated significant amounts of CO (4-folds) and CA (3-folds), as compared to wild plants. Overall, the present study is the first report on improving the content of pharmaceutically important diterpenes in S. officinalis by overexpressing pathway-specific transcription factors. The current findings showed convincing evidence for the concept of improving specialized metabolite(s) content in medicinal plants by manipulating the expression of transcriptional regulators.