In the Arabidopsis genome, approximately 120 UDP-glycosyltransferases (UGTs) have been annotated. They generally cataklyze the transfer of sugars to various acceptor molecules, including flavonoids. To better understand their physiological roles, we analyzed a tandemly located putative flavonoid UGT cluster comprising UGT73B1, UGT73B2, and UGT73B3 on Chromosome IV. We then isolated four loss-of-function mutations-ugt73b1-1, ugt73b2-1, ugt73b3-1, and ugt73b3-2. In our expression analysis, the closely related UGTs exhibited tissue-specific patterns of expression that were severely altered in their respective mutant plants. For example, UGT73B2 was up-regulated in ugt73b1-1, whereas UGT73B1 was highly expressed in ugt73b2-1, ugt73b3-1, and ugt73b3-2. Interestingly, each recessive mutant was resistant to methyl viologen (paraquat), an herbicide thought to cause oxidative stress. Our results suggest that UGTs play an important role in the glycosylation pathways when responding to oxidative stress.