Genome-wide analysis of UDP-glycosyltransferase family in Citrus sinensis and characterization of a UGT gene encoding flavonoid 1–2 rhamnosyltransferase

UDP-glycosyltransferases (UGTs) play a crucial role in the glycosylation of secondary metabolites in plants, which is of significant importance for growth and response to biotic or abiotic stress. Despite the wide identification of UGT family members in various species, limited information is available regarding this family in citrus. In this study, we identified 87 UGT genes from the Citrus sinensis genome and classified them into 14 groups. We characterized their gene structures and motif compositions, providing insights into the molecular basis underlying discrepant functions of UGT genes within each evolutionary branch. Tandem duplication events were found to be the main driving force behind UGT gene expansion. Additionally, we identified numerous cis-acting elements in the promoter region of UGT genes, including those responsive to light, growth factors, phytohormones, and stress conditions. Notably, light-responsive elements were found with a frequency of 100 %. We elucidated the expression pattern of UGTs during fruit development in Citrus aurantium using RNA-seq and quantitative real-time PCR (qRT-PCR), revealing that 10 key UGT genes are closely associated with biosynthesis of bitter flavanone neohesperidosides (FNHs). Furthermore, we identified Ca1,2RhaT as a flavonoid 1-2 rhamnosyltransferase (1,2RhaT) involved in FNHs biosynthesis for the first time. Isolation and functional characterization of the gene Ca1,2RhaT from Citrus aurantium in vitro and in vivo indicated that Ca1,2RhaT encoded a citrus 1,2RhaT and possessed rhamnosyl transfer activities. This work provides comprehensive information on the UGT family while offering new insights into understanding molecular mechanisms regulating specific accumulation patterns of FNHs or non-bitter flavanone rutinosides (FRTs) in citrus.