Involved in defensive oleoresin formation in conifers in response to insect attack or other injury. Involved in 52 sesquiterpene (C15) olefins biosynthesis. (2E,6E)-farnesyl diphosphate = diphosphate + gamma-humulene (2E,6E)-farnesyl diphosphate = diphosphate + sibirene (2E,6E)-farnesyl diphosphate = diphosphate + longifolene (2E,6E)-farnesyl diphosphate = beta-himachalene + diphosphate (2E,6E)-farnesyl diphosphate = diphosphate + gamma-himachalene (2E,6E)-farnesyl diphosphate = alpha-himachalene + diphosphate Binds 3 Mg(2+) ions per subunit. Terpene metabolism; oleoresin biosynthesis. Constitutive expression. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). In sesquiterpene synthases utilizing farnesyl diphosphate as substrate, magnesium is more efficient as cofactor than is manganese. Belongs to the terpene synthase family. Tpsd subfamily.