Catalyzes the synthesis of mevalonate, the specific precursor of all isoprenoid compounds present in plants (By similarity). Component of the triterpene saponins (e.g. ginsenosides or panaxosides) and phytosterols biosynthetic pathways (PubMed:24569845, PubMed:29378087). Promotes triterpenes accumulation in roots (PubMed:24569845). (R)-mevalonate + CoA + 2 NADP(+) = (3S)-hydroxy-3-methylglutaryl-CoA + 2 H(+) + 2 NADPH Competitive inhibition by mevinolin (Mev) is leading to a significant reduction of total ginsenoside in adventitious roots. Triggered by darkness. Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Localized in intracellular vesicles. Mostly expressed in the petioles of seedlings and roots (both in main roots and lateral roots), and, to a lower extent, in seeds, seedlings, leaves, stems and flowers. Relatively constant levels in mature roots (PubMed:24569845). Rapid decrease in roots and leaves from the leaf opened to the green fruit stage (PubMed:30577538). At the leaf opened stage, accumulates mostly in roots (PubMed:30577538). Induced in the hypocotyl of a 4 days-etiolated seedling and reversed by light exposure within 2 hours. Inhibited by dark treatment in 7-days-old seedling, where true leaves are emerged. Repressed by darkness in roots and leaves, but increased ginsenosides accumulation. Triggered by methyl jasmonate (MJ). Repressed transiently by mevinolin (Mev) (PubMed:24569845). Influenced in roots by relative humidity and photosynthetically active radiation (PAR), and in leaves by rain and soil water potential (PubMed:30577538). Belongs to the HMG-CoA reductase family.