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.
Highlights
Between 0.70–2.0% on dry weight basis[6]
Secondary metabolites of terpenoid origin are synthesized by mevalonate and non-mevalonate pathways and many studies have shown their involvement in the biosynthetic pathway[17,18,19,20]
Has been cloned and characterized in a number of plants such as W. somnifera[12], A. thaliana[17], Andrographis paniculata[19] etc, no reports are available for its cloning, kinetic and functional characterization in any of the Ocimum species
Summary
Between 0.70–2.0% on dry weight basis[6]. Seed and leaf essential oil of O. kilimandscharicum contains minor proportions of other monoterpenoids such as Presence of a wide range of these secondary metabolites makes O. kilimandscharicum a valuable material to be used as medicinal bio-resource[7] Phytomolecules such as methyl chavicol, 1,8-cineole, eugenol, (E)-bisabolene, terpineol, linalool, (Z)-cinnamic acid methyl ester, camphor etc are reported to possess various antimicrobial, antispasmodic, bactericidal, carminative, anti-helminthic, hepatoprotective, antiviral and larvicidal activities[6,7,8]. The terpenoid metabolites are produced by the plant through MEP (2C-methyl-D-erythritol 4-phosphate) and MVA (mevalonic acid) pathway in plastids and cytosol, respectively[9]. Both the pathways are actively involved in the biosynthesis of isoprenoid moieties operative from their respective compartments[10,11]. The study provides a novel OkHMGR which can be used as a powerful sequence to drive MVA led, isoprenoids synthesis in medicinally and aromatically important plants
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