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:29378087). Promotes triterpenes accumulation in roots (By similarity). (R)-mevalonate + CoA + 2 NADP(+) = (3S)-hydroxy-3-methylglutaryl-CoA + 2 H(+) + 2 NADPH Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Belongs to the HMG-CoA reductase family.