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). (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, seedlings and roots, and, to a lower extent, in seeds, leaves, stems and flowers. Accumulates progressively in mature roots. Repressed by darkness in roots and leaves, but increased ginsenosides accumulation. Repressed transiently by mevinolin (Mev). Belongs to the HMG-CoA reductase family.