Genome-Wide Identification, Expression, and Functional Analysis of UDP-Glucose Dehydrogenase Family Genes in Rhus chinensis

Background: Uridine diphosphate glucose (UDP-Glc) is one of the key substrates for the biosynthesis of gallotannins in plants. UDP-glucose dehydrogenase (UGD) catalyzes the irreversible oxidation of UDP-Glc to UDP-glucuronic acid (UDP-GlcA), thus affecting the biosynthesis and accumulation of gallotannins in the Chinese gallnut. Methods and Results: In this study, we identified three members of the RcUGD family from the Rhus chinensis genome. Protein sequence alignment revealed that all three RcUGDs possess the conserved NAD+ coenzyme binding motif GAGYVGG and the catalytic motif GFGGSCFQKDIL. qRT-PCR analysis revealed that the expression levels of RcUGD3 in stem and root tissues were respectively 10-fold and 13-fold greater than that in the leaves, in which gallotannin accumulation was higher. RcUGD3 expression level declined by 63% during early (24 d) gallnut development, suggesting an inverse relationship between RcUGD3 expression level and gallotannin biosynthesis. In addition, subcellular localization analysis using the tobacco transient transformation system showed that RcUGD proteins are broadly distributed throughout the cell. Moreover, an in vitro enzyme activity assay indicated that the recombinant RcUGD3 protein catalyzed UDP-Glc to produce UDP-GlcA as shown by HPLC. Taken together, our results suggested that RcUGD3 protein is responsible for UDP-Glc degradation and probably plays a regulatory role in gallotannin biosynthesis in the Chinese gallnut. Conclusions: This study lays a foundation for further elucidating the function and expression regulation mechanism of the RcUGD gene family and provides new insights for the super-accumulation mechanisms of gallotannins in Chinese gallnuts.