ObjectiveTo screen and identify uridine diphosphate （UDP）-rhamnose synthase （RHM） genes from Ornithogalum saundersiae and verify their UDP-rhamnose （UDP-Rha） biosynthetic functions， thereby providing reliable candidate genes for the efficient biosynthesis of neohesperidin and other rhamnoside natural products. MethodsBased on previously reported plant RHM sequences， two candidate RHM genes， OsRHM1 and OsRHM2， were identified from O. saundersiae through transcriptome screening and sequence alignment. The two genes were cloned and subjected to bioinformatics analysis and multiple sequence alignment. An in vitro RHM-Cm1，2RhaT cascade reaction was established， and the ability of OsRHM1 and OsRHM2 to catalyze the conversion of UDP-glucose （UDP-Glc） to UDP-Rha was evaluated based on neohesperidin production. Real-time fluorescence quantitative polymerase chain reaction PCR （Real-time PCR） was performed to analyze the relative expression levels of the two genes in roots， bulbs， and leaves. ResultsThe open reading frame （ORF） of OsRHM1 was 2 076 bp and encoded 692 amino acids， with a predicted protein molecular mass of 77.97 kDa and a theoretical isoelectric point of 7.25. The ORF of OsRHM2 was 2 019 bp and encoded 673 amino acids， with a predicted protein molecular mass of 75.82 kDa and a theoretical isoelectric point of 7.24. Multiple sequence alignment showed that OsRHM1 and OsRHM2 contained the typical conserved motifs GxxGxxG/A and YxxxK of the plant RHM family and shared high amino acid sequence identity with previously reported RHMs. The in vitro cascade reaction demonstrated that both OsRHM1 and OsRHM2 were capable of catalyzing the conversion of UDP-Glc to UDP-Rha. The neohesperidin production in the OsRHM2 cascade reaction system was approximately 2.58-fold that in the OsRHM1 system. Real-time PCR analysis showed that the relative expression levels of OsRHM1 and OsRHM2 were higher in roots than in bulbs and leaves. ConclusionOsRHM2 exhibited a relatively high capacity for UDP-Rha supply and may serve as a candidate enzyme resource for the subsequent biosynthesis of rhamnoside natural products and the construction of UDP-Rha supply systems.