Genome-wide identification and functional characterization of 2, 3-oxidosqualene cyclase genes in Phellodendron amurense

Phellodendron amurense Rupr., a significant medicinal tree, is known for its content of valuable triterpenoids, including limonoids. However, the biosynthetic pathway of triterpenoids in P. amurense remains poorly understood. In this study, we employed a genome-wide strategy to systematically identify thirteen candidate 2,3-oxidosqualene cyclase (OSC) genes. Through analyzing the gene structure and conserved motifs of the PaOSCs gene family, we discovered that the insertion of long terminal repeat retrotransposons contributed to the structural variation of PaOSC introns. Functional identification of camelliol C synthase (PaOSC1) and cycloartenol synthase (PaOSC5) was achieved using heterologous expression systems. Additionally, the evolutionary analysis revealed that the PaOSC genes are under purifying selection. Furthermore, comparative genomics analysis revealed that the presence of limonoids predates the divergence of Rutaceae and Meliaceae. Our findings provide valuable insights into the catalytic and evolutionary mechanisms of OSCs, contributing to the diversity of triterpenoids in the Sapindales.