Involved in sesquiterpene (C15) biosynthesis. The major product is (Z)-gamma-bisabolene with minor amounts of (E)-nerolidol and alpha-bisabolol. (2E,6E)-farnesyl diphosphate = (Z)-gamma-bisabolene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Predominantly expressed in roots. Expressed in the cortex and the sub-epidermal layers of roots. Also detected in leaf hydathodes and flower stigmata. By wounding. 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.