HB26, a member of HD-Zip I subfamily, is involved in the regulation of hydrolysable tannin biosynthesis in the cupules of Quercus variabilis by transactivation of UGT84A13

Tannins are useful for many industrial applications owing to their strong chelating, antibacterial, and antioxidant properties. Hydrolysable tannins (HTs) are suitable for feed additives to replace antibiotics because of their low anti-nutritional effects. However, regulation of HT biosynthesis in plants remains largely unknown. Here, we found that the HTs are predominant phenols in Chinese cork oak (Quercus variabilis) and their contents in cupules were much higher than those in other tissues; simultaneously, the expression of UGT84A13, a key gene for the first step of HT biosynthesis, was also high. RNA-seq analysis revealed 351 candidate genes whose expressions were correlated with UGT84A13 expressions and HT contents. Comprehensive analysis of the HD-Zip family identified subfamily I members-HB20 and HB26-as candidate genes responsible for regulation of UGT84A13 expression and HT biosynthesis. Subcellular localization assays showed that both HD-Zip proteins were localized to the nucleus. Interestingly, HB20 and HB26 had distinct trans-activating domains. Y1H and luciferase assays finally confirmed that HB26, instead of HB20, interacted with UGT84A13 promoter and activated its expression. This study provides valuable data for different tissues of Q. variabilis and new insights into the transcriptional regulation of UGT84A13 and HT biosynthesis, thereby laying the foundation for molecular breeding and enhancement of HT biosynthesis.