Functional characterization of the β-amyrin synthase gene involved in platycoside biosynthesis in Platycodon grandiflorum

Platycodon grandiflorum A. DC. contains a variety of triterpene saponins induced from the base structure of oleanane-type in roots. The beta-amyrin synthase (bAS) plays an important role in controlling the biosynthesis of triterpene saponins and we obtained the full-length cDNA of the gene, which is named PlgOSC1 in P. grandiflorum. To identify the PlgOSC1 products, we expressed it in heterologous yeast cells. Results of GC-MS analysis showed a unique peak that was consistent with that of the authentic beta-amyrin standard with exactly the same retention time. Furthermore, the pattern of MS fragments of the peak was also exactly the same as that of the beta-amyrin. In qRT-PCR analysis of four organs of P. grandiflorum, no difference in PlgOSC1 expression level was observed; therefore, platycosides derived from beta-amyrin might be present in all P. grandiflorum tissues. In conclusion, PlgOSC1 encodes a bAS enzyme that catalyzes the first committed step of platycosides biosynthesis in P. grandiflorum.