A systematic review of the Uridine diphosphate-Galactose/Glucose-4-epimerase (UGE) in plants

Uridine diphosphate (UDP)-Galactose/Glucose-4-epimerase (UGE) is the third enzyme involved in the Leloir pathway. It catalyzes the conversion of UDP-Galactose to UDP-Glucose, which is a rate-limiting step for polysaccharides biosynthesis. As the main cell wall materials, polysaccharides play an irreplaceable role throughout the whole life history of plant. In this review, 1243 UGE mRNA sequences registered in NCBI were obtained and their evolutionary relationship was analyzed by constructing a neighbor-joining tree based on representative sequences. The physicochemical properties of 15 specific UGEs were analyzed, and four UGE sequences of Setaria italica , Zea mays , Ricinus Communis , and Oryza sativa were selected for further secondary structure, three-dimensional protein modeling, transmembrane structure, and signal peptide prediction analyses. The results demonstrate that UGEs are mostly conservative without signal peptides and exert activities in cytoplasm. Then, studies related to the catalytic reaction, characteristic structure, function, and regulation of UGE were summarized, which provide strong evidence that UGE plays a crucial role in cell growth, cell differentiation, cell-to-cell communication, primary metabolism, secondary metabolism, and defense responses. However, the molecular mechanisms whereby UGE regulates the plant stress resistance and useful secondary metabolites accumulation are far from clear. This paper will lay a foundation for further studies and applications of UGE.