HMG-CoA reductase from Camphor Tulsi (Ocimum kilimandscharicum) regulated MVA dependent biosynthesis of diverse terpenoids in homologous and heterologous plant systems

Abstract

Ocimum kilimandscharicum is unique in possessing terpenoids whereas other Ocimum species are renowned for phenylpropanoids as major constituents of essential oil. The key enzyme of MVA/terpenoid metabolic pathway viz 3-hydroxy-3-methylglutaryl Co-A reductase (OkHMGR) of 1.7-Kb ORF encoding ~60-kDa protein was cloned from O. kilimandscharicum and its kinetic characteristics revealed the availability of HMG-CoA as a control point of MVA-pathway. Transcript profiling of the OkHMGR elucidated tissue-specific functions of the gene in flower and leaf tissues in accumulation of terpenoidal essential oil. OkHMGR was differentially regulated in response to exposure to methyl-jasmonate, salicylic-acid, and stress conditions such-as salt and temperature stress, demonstrating its key role in managing signaling and stress-responses. To elucidate its functional role, OkHMGR was transiently over-expressed in homologous and heterologous plants such as O. sanctum, O. basilicum, O. gratissimum, Withania somnifera and Artemisia annua. The over-expression and inhibition dual strategy revealed that the additional OkHMGR in-planta could afford endogenous flow of isoprenoid units towards synthesis of terpenoids. The present study provides in-depth insight of OkHMGR in regulation of biosynthesis of non-plastidal isoprenoids. This is first report on any gene of MVA/isoprenoid pathway from under-explored Camphor Tulsi belonging to genus Ocimum. Studies also suggested that OkHMGR could be a potential tool for attempting metabolic engineering for enhancing medicinally important terpenoidal metabolites in plants.