Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:26858282). Mediates the conversion of (2E,6E)-farnesyl diphosphate (FPP) into germacrene D, (-)-(E)-beta-caryophyllene and beta-copaene (PubMed:26858282). (2E,6E)-farnesyl diphosphate = diphosphate + germacrene D (2E,6E)-farnesyl diphosphate = (-)-(E)-beta-caryophyllene + diphosphate (2E,6E)-farnesyl diphosphate = beta-copaene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Expressed in leaves and stems. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.