Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:26858282). Mediates the conversion of (2E,6E)-farnesyl diphosphate (FPP) into guaia-4,6-diene (PubMed:26858282). (2E,6E)-farnesyl diphosphate = diphosphate + guaia-4,6-diene Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Highly expressed in glandular trichomes (PubMed:26858282). Expressed in leaves and stems (PubMed:26858282). 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.