Involved in the biosynthesis of labdane-type diterpenoid including marrubiin and other labdane-related furanoid diterpenoids with potential applications as anti-diabetics, analgesics or vasorelaxants (Probable). Terpene synthase that produces (+)-copalyl diphosphate ((+)-CPP) from geranylgeranyl diphosphate (GGPP) (PubMed:24990389). (2E,6E,10E)-geranylgeranyl diphosphate = (+)-copalyl diphosphate Secondary metabolite biosynthesis; terpenoid biosynthesis. Present in both leaves and flowers, with higher levels in leaves. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.