Evolution of Labdane-Related Diterpene Synthases in Cereals

Gibberellins (GAs) are labdane-related diterpenoid phytohormones that regulate various aspects of higher plant growth. A biosynthetic intermediate of GAs is ent-kaurene, a tetra-cyclic diterpene that is produced through successive cyclization of geranylgeranyl diphosphate catalyzed by the two distinct monofunctional diterpene synthases - ent-copalyl diphosphate synthase (ent-CPS) and ent-kaurene synthase (KS). Various homologous genes of the two diterpene synthases have been identified in cereals including rice (Oryza sativa), wheat (Triticum aestivum) and maize (Zea mays), and are believed to have been derived from GA-biosynthetic ent-CPS and KS genes through duplication and neofunctionalization. They play roles in specialized metabolism, giving rise to diverse labdane-related diterpenoids for defense, because a variety of diterpene synthases generate diverse carbon-skeleton structures. This review mainly describes the diterpene synthase homologs that have been identified and characterized in rice, wheat and maize, and shows the evolutionary history of the various homologs in rice inferred by comparative genomics studies using wild rice species such as Oryza rufipogon and Oryza brachyantha. Additionally, we introduce labdane-related diterpene synthases in bryophytes and gymnosperms to illuminate the macroscopic evolutionary history of diterpene synthases in the plant kingdom - bifunctional enzymes possessing both CPS and KS activity are present in bryophytes; gymnosperms possess monofunctional CPS and KS responsible for GA biosynthesis and also possess bifunctional diterpene synthases facilitating specialized metabolism for defense.

Keywords: Biosynthesis; Cereal; Diterpene synthase; Evolution; Gibberellin; Phytoalexin.