Vascular plants invariably contain a class II diterpene cyclase (EC 5.5.1.x), as an ent-copalyl diphosphate synthase is required for gibberellin phytohormone biosynthesis. This has provided the basis for evolution of a functionally diverse enzymatic family. A bifunctional diterpene synthase was characterized from the lycophyte Selaginella moellendorffii . The structure of its product, labda-7,13E-dien-15-ol, demonstrates that this enzyme catalyzes a novel class II diterpene cyclization reaction, and clarifies the biosynthetic origins of the family of derived natural products.

All higher plants contain kaurene oxidases (KO), which are multifunctional cytochromes P450 that catalyze oxidation at the C4α methyl, converting ent-kaurene to ent -kaurenoic acid, an early step in gibberellin phytohormone biosynthesis. Arabidopsis produces no labdane-related diterpenoids other than gibberellins, whereas rice produces a wide range of such natural products as antibiotic phytoalexins or allelochemicals. While rice contains several kaurene oxidase homologs, only OsKO2 (CYP701A6) is required for gibberellin biosynthesis. Here, we demonstrate that the KO from Arabidopsis thaliana (CYP701A3 or AtKO) exhibits significantly greater promiscuity than does OsKO2. To further characterize this plasticity of AtKO, we determined the structure of the resulting products, whereas OsKO2 only hydroxylates its substrates on C19, AtKO reacts with labdane-related diterpenes of varied stereochemistry, which further leads to altered hydroxylation regiochemistry. Our results demonstrate greater promiscuity of AtKO, which evolved in the absence of other potential labdane-related diterpene substrates, relative to OsKO2 from rice, which contains many such alternative substrates. Thus, our data are consistent with the hypothesis that enzymatic plasticity is shaped, at least in part, by evolutionary context such as that noted here.