Divergent evolution of oxidosqualene cyclases in plants

Triterpenes are one of the largest classes of plant metabolites and have important functions. A diverse array of triterpenoid skeletons are synthesized via the isoprenoid pathway by enzymatic cyclization of 2,3-oxidosqualene. The genomes of the lower plants Chlamydomonas reinhardtii and moss (Physcomitrella patens) contain just one oxidosqualene cyclase (OSC) gene (for sterol biosynthesis), whereas the genomes of higher plants contain nine to 16 OSC genes. Here we carry out functional analysis of rice OSCs and rigorous phylogenetic analysis of 96 OSCs from higher plants, including Arabidopsis thaliana, Oryza sativa, Sorghum bicolor and Brachypodium distachyon. The functional analysis identified an amino acid sequence for isoarborinol synthase (OsIAS) (encoded by Os11g35710/OsOSC11) in rice. Our phylogenetic analysis suggests that expansion of OSC members in higher plants has occurred mainly through tandem duplication followed by positive selection and diversifying evolution, and consolidated the previous suggestion that dicot triterpene synthases have been derived from an ancestral lanosterol synthase instead of directly from their cycloartenol synthases. The phylogenetic trees are consistent with the reaction mechanisms of the protosteryl and dammarenyl cations which parent a wide variety of triterpene skeletal types, allowing us to predict the functions of the uncharacterized OSCs.