Biological functions of ent- and syn-copalyl diphosphate synthases in rice: key enzymes for the branch point of gibberellin and phytoalexin biosynthesis.

Rice (Oryza sativa L.) produces ent-copalyl diphosphate (ent-CDP) and syn-CDP as precursors for several classes of phytoalexins and the phytohormones, gibberellins (GAs). It has recently been shown that a loss-of-function mutation of OsCPS1, a gene encoding a putative ent-CDP synthase, results in a severely GA-deficient dwarf phenotype in rice. To clarify the biological functions of the ent- and syn-CDP synthases involved in the biosynthesis of phytoalexins and/or GAs, we isolated two cDNAs, OsCyc1 and OsCyc2, encoding putative diterpene cyclases from ultraviolet (UV)-irradiated rice leaves (cv. Nipponbare). The production of phytoalexins in rice leaves is known to be highly induced by UV treatment. Using a bacterial expression system, we demonstrated that OsCyc1 encodes syn-CDP synthase and that OsCyc2 and OsCPS1 encode ent-CDP synthase. The level of expression of the OsCyc1 and OsCyc2 transcripts in rice leaves increased drastically in response to UV treatment, whereas expression of the OsCPS1 transcript was not induced by UV light. These results suggest that OsCyc1, OsCyc2 and OsCPS1 are responsible for the biosynthesis of momilactones A and B and oryzalexin S, oryzalexins A-F and phytocassanes A-E, and GAs, respectively. Our results strongly suggest the presence of two ent-CDP synthase isoforms in rice, one that participates in the biosynthesis of GAs and a second that is involved in the biosynthesis of phytoalexins.