Catalyzes the synthesis of mevalonate, the specific precursor of all isoprenoid compounds present in plants. Possible role in plant defense mechanisms as well as in the cell cycle. (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. Found in protoplasts and leaves submitted to stress. Low levels found in apexes, anthers and roots. Expressed at highest levels during the G0-G1 transition in the cell cycle of mesophyll protoplasts. In leaves, by wounding, elicitation, bacterial or fungal infection. Belongs to the HMG-CoA reductase family.