Functional characterization of the rice kaurene synthase-like gene family

The rice (Oryza sativa) genome contains a family of kaurene synthase-like genes (OsKSL) presumably involved in diterpenoid biosynthesis. While a number of OsKSL enzymes have been functionally characterized, several have not been previously investigated, and the gene family has not been broadly analyzed. Here we report cloning of several OsKSL genes and functional characterization of the encoded enzymes. In particular, we have verified the expected production of ent-kaur-16-ene by the gibberellin phytohormone biosynthesis associated OsKS1 and demonstrated that OsKSL3 is a pseudo-gene, while OsKSL5 and OsKSL6 produce ent-(iso)kaur-15-ene. Similar to previous reports, we found that our sub-species variant of OsKSL7 produces ent-cassa-12,15-diene, OsKSL10 produces ent-(sandaraco)pimar-8(14),15-diene, and OsKSL8 largely syn-stemar-13-ene, although we also identified syn-stemod-12-ene as an alternative product formed in approximately 20% of the reactions catalyzed by OsKSL8. Along with our previous reports identifying OsKSL4 as a syn-pimara-7,15-diene synthase and OsKSL11 as a syn-stemod-13(17)-ene synthase, this essentially completes biochemical characterization of the OsKSL gene family, enabling broader analyses. For example, because several OsKSL enzymes are involved in phytoalexin biosynthesis and their gene transcription is inducible, promoter analysis was used to identify a pair of specifically conserved motifs that may be involved in transcriptional up-regulation during the rice plant defense response. Also examined is the continuing process of gene evolution in the OsKSL gene family, which is particularly interesting in the context of very recently reported data indicating that a japonica sub-species variant of OsKSL5 produces ent-pimara-8(14),15-diene, rather than the ent-(iso)kaur-15-ene produced by the indica sub-species variant analyzed here.