Involved in the biosynthesis of ent-kaurene diterpenoids natural products such as oridonin, miltiradiene, eriocalyxin B and nezukol, known to exhibit antitumor, anti-inflammatory and antibacterial activities (PubMed:28381502). Catalyzes the conversion of (2E,6E,10E)-geranylgeranyl diphosphate (GGPP) to ent-copalyl diphosphate (ent-CPP) (PubMed:28381502). (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Secondary metabolite biosynthesis; terpenoid biosynthesis. Highly expressed in leaves, and, at low levels, in stems, but barely in roots and flowers. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Abietane diterpenoids (e.g. miltiradiene, abietatriene and ferruginol) accumulate specifically in the periderm of roots (PubMed:28381502). The ent-kaurene diterpenoid oridonin, main constituent of Isodon rubescens, accumulates in leaves (PubMed:28381502). Belongs to the terpene synthase family. Tpsc subfamily.