Involved in the biosynthesis of diterpenes in roots. Possesses dolabella-3,7-dien-18-ol synthase activity and dolathalia-3,7,11-triene synthase activity in vitro. Catalyzes the formation of dolabella-3,7-dien-18-ol and dolathalia-3,7,11-triene from geranygeranyl diphosphate (GGPP). Does not seem to be involved in sesquiterpene biosynthesis. (2E,6E,10E)-geranylgeranyl diphosphate + H2O = (3E,7E)-dolabella-3,7-dien-18-ol + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate = (3E,7E)-dolathalia-3,7,11-triene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. kcat is 0.00083 sec(-1) with geranylgeranyl diphosphate as substrate. Secondary metabolite biosynthesis; terpenoid biosynthesis. 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. TPS20 in Arabidopsis ecotype Columbia lacks dolabellane-type diterpene synthase activity because of a 17 amino acid deletion and several mutations in its sequence. TPS20 protein in ecotype Cvi is functional and possesses dolabellane-type diterpene synthase activity.