Characterization of the Key Enzymes and a Gene Pair in Triterpenoid Biosynthesis of Ficus microcarpa

The ornamental plant Ficus microcarpa produces diverse bioactive triterpenoids. We functionally characterized eight oxidosqualene cyclases (FmOSC1–8), which together generate eight distinct triterpenol skeletons, underpinning the species chemical diversity. FmOSC1 was identified as a rare multifunctional ψ-taraxasterol synthase, the first of its kind in Moraceae. Key residues differentiating FmOSC1 from the α-amyrin-predominant FmOSC2 were pinpointed. Furthermore, a chromosomally linked gene pair (FmOSC3/FmCYP716A520) was shown to sequentially produce lupane-type derivatives. This cytochrome P450 (FmCYP716A520) exhibited broad substrate flexibility, processing intermediates from FmOSC1, FmOSC2, and FmOSC6 to form ψ-taraxastane-, ursane-, and oleanane-type products, respectively. Remarkably, CYP716A homologues from five other plant species could also utilize the FmOSC1 product, yielding oxidized derivatives and revealing conserved catalytic versatility within this enzyme subfamily. Our findings provide fundamental genetic insights into triterpenoid biosynthesis in F. microcarpa and offer valuable biocatalytic tools for synthetic biology applications.