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. Expressed in flower primordia and anthers. Induced by wounding. This enhancement is greatly reduced by treatment with arachidonic acid or after inoculation with the fungal pathogen, P.infestans. Belongs to the HMG-CoA reductase family.