Catalyzes the conversion of geranylgeranyl diphosphate to the phytoalexin precursor ent-copalyl diphosphate. (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate By UV irradiation. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). 2 different ent-CDP synthases exist in rice, one being involved in gibberellin biosynthesis and the other in phytoalexins biosynthesis. Phytoalexins are diterpenoid secondary metabolites involved in the defense mechanism of the plant and produced in response to attack (by a pathogen, elicitor or UV irradiation). Belongs to the terpene synthase family.