This enzyme condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA, which is the substrate for HMG-CoA reductase. Devoided of acetoacetyl-CoA thiolase (AACT) activity. Required for the development of both tapetosomes and elaioplasts in tapetal cells and for pollen viability during pollen tube elongation. acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 2/3. Highly expressed in flower buds, pollen grains and anthers in the tapetal cells, at an intermediate levels in roots and weakly in leaves. Defect in pollen adhesion to the stigma resulting in male sterility under normal growth conditions. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.