Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:21818683). Mediates the conversion of (2E,6E)-farnesyl diphosphate (FPP) into gamma-gurjunene, (E)-beta-farnesene and (+)-valencene, and of (2Z,6Z)-farnesyl diphosphate ((ZZ)-FPP) into (E)-alpha-bergamotene and (Z)-gamma-bisabolene as well as beta-bisabolene, (Z)-alpha-bergamotene and (E)-gamma-bisabolene to a lower extent (PubMed:21818683). Can act with a low efficiency as a monoterpene synthase with geranyl diphosphate (GPP) as substrate, thus producing beta-myrcene, (E)-beta-ocimene, limonene, terpinolene, gamma-terpinene and (Z)-beta-ocimene (PubMed:21818683). (2Z,6Z)-farnesyl diphosphate = beta-bisabolene + diphosphate (2E,6E)-farnesyl diphosphate = (+)-valencene + diphosphate (2E,6E)-farnesyl diphosphate = (E)-beta-farnesene + diphosphate (2E,6E)-farnesyl diphosphate = diphosphate + gamma-gurjunene (2Z,6Z)-farnesyl diphosphate = (E)-gamma-bisabolene + diphosphate (2E)-geranyl diphosphate = diphosphate + limonene (2E)-geranyl diphosphate = beta-myrcene + diphosphate (2E)-geranyl diphosphate = (E)-beta-ocimene + diphosphate (2E)-geranyl diphosphate = diphosphate + terpinolene (2E)-geranyl diphosphate = diphosphate + gamma-terpinene (2Z,6Z)-farnesyl diphosphate = (Z)-gamma-bisabolene + diphosphate (2E,6E)-farnesyl diphosphate = (1S,5S,6R)-alpha-bergamotene + diphosphate (2Z,6Z)-farnesyl diphosphate = (1S,5S,6S)-alpha-bergamotene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Mostly expressed in stem and trichomes, to a lower extent in leaves, flowers and roots and, at low levels, in fruits. Lower levels upon jasmonic acid treatment. 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.