Involved in defensive oleoresin formation in conifers in response to insect attack or other injury. Involved in sesquiterpene (C15) olefins biosynthesis. Produces mainly (E)-gamma-bisabolene when used with farnesyl diphosphate as substrate. No activity with geranyl diphosphate or geranylgeranyl diphosphate. (2E,6E)-farnesyl diphosphate = (E)-gamma-bisabolene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Terpene metabolism; oleoresin biosynthesis. Up-regulated by methyl jasmonate. 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. Tpsd subfamily.