Figure 1. The biosynthetic pathway of diterpenoids in plants.

Proposed biosynthetic pathway for momilactones in rice. Both syn-pimara-7,15-dine and ent-kaurene, the key intermediates for momilactone and gibberellin, respectively, are synthesized from geranylgeranyl diphosphate (GGDP) by sequential enzymatic reactions of type B and type A cyclases. In mosses and liverworts, cyclic diterpenes, such as ent-kaurene, are constructed by a bifunctional cyclase with type B and A activity in a single polypeptide.