Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:21818683). Mediates the conversion of (2E,6E)-farnesyl diphosphate ((EE)-FPP) into beta-bisabolene, and of (2Z,6Z)-farnesyl diphosphate ((ZZ)-FPP) into alpha-bisabolene, but also smaller amounts of (Z)-gamma-bisabolene, (E)-gamma-bisabolene and nerolidol (PubMed:21813655). (2Z,6Z)-farnesyl diphosphate = (E)-alpha-bisabolene + diphosphate (2Z,6Z)-farnesyl diphosphate = beta-bisabolene + diphosphate (2E,6E)-farnesyl diphosphate = beta-bisabolene + diphosphate (2E,6E)-farnesyl diphosphate = (Z)-gamma-bisabolene + diphosphate (2E,6E)-farnesyl diphosphate = (E)-gamma-bisabolene + diphosphate (2Z,6Z)-farnesyl diphosphate = (E)-gamma-bisabolene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Mostly expressed in roots, to a lower extent in flowers and, at low levels, in fruits. 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.