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 sphingolipid and sterol biosynthetic pathways. Negatively regulated by a PP2A-dependent dephosphorylation occurring at a site different than Ser-577. Completely inhibited by mevinolin (IC(50) = 12.5 nM). Reversibly inactivated by phosphorylation at Ser-577 by spinach or Brassica oleracea HMGR kinases in a cell-free system (PubMed:7588795, PubMed:10318703). Down-regulated by KIN10 through its phosphorylation at Ser-577 (PubMed:28263378). Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Interacts (via N-terminus) with B''ALPHA and B''BETA. Also observed within spherical structures located along the endoplasmic reticulum strands. Found in all tissues. Isoform Short is expressed at low levels specifically in flowers. Expressed in both the tapetum and microspores. Down-regulated by light in immature leaves, but not in roots. Not regulated by myriocin, squalestatin or terbinafine. The N-terminal domain (1-178) of the short isoform is necessary and sufficient for directing the protein to the endoplasmic reticulum and to spherical structures. Inactivated by phosphorylation at Ser-577 by KIN10 activated form (PubMed:28263378). Probably also phosphorylated at additional sites (PubMed:7588795, PubMed:10318703). Dwarfing, early senescence, and sterility. 65% lower levels in triterpenoids content and 50% lower levels in sterol content. Hmg1 and hmg2 double mutants are lethal during male gametophyte development. Belongs to the HMG-CoA reductase family.