Catalyzes the synthesis of mevalonate. The specific precursor of all isoprenoid compounds present in plants. (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.