Involved in terpene biosynthesis in roots. Possesses sesquiterpene (C15) synthase activity in vitro. Does not seem to be involved in diterpene (C20) biosynthesis. Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Predominantly expressed in roots but also in flowers. 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.