Catalyzes the conversion of geranylgeranyl diphosphate to the gibberellin precursor ent-copalyl diphosphate. (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in developing pollen from 7 days before flowering. 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. Belongs to the terpene synthase family.