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 Regulated at the post-translational level in response to alterations of the sphingolipid and the sterol biosynthetic pathways. Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Restricted to young seedlings, roots, and inflorescences. Expressed in root tips, shoot apex, secretory zone of the stigma, microspores, mature pollen grains, gynoecium vascular tissue and fertilized ovules. Not regulated by myriocin, squalestatin or terbinafine. No visible phenotype under normal growth conditions, but 25% lower levels in triterpenoids content and 15% lower levels in sterol content. Hmg1 and hmg2 double mutants are lethal during male gametophyte development. Belongs to the HMG-CoA reductase family.