Sinapoylmalate is a major phenylpropanoid that is accumulated in Arabidopsis. Its presence causes the adaxial surface of leaves to fluoresce blue under UV light, and mutations that lead to lower levels of sinapoylmalate decrease UV-induced leaf fluorescence. The Arabidopsis bright trichomes 1 (brt1) mutant was first identified in a screen for mutants that exhibit a reduced epidermal fluorescence phenotype; however, subsequent examination of the mutant revealed that its trichomes are hyper-fluorescent. The results from genetic mapping and complementation analyses showed that BRT1 (At3g21560) encodes UGT84A2, a glucosyltransferase previously shown to be capable of using sinapic acid as a substrate. Residual levels of sinapoylmalate and sinapic acid:UDP-glucose glucosyltransferase activity in brt1 leaves suggest that BRT1 is one member of a family of partially redundant glycosyltransferases that function in Arabidopsis sinapate ester biosynthesis. RT-PCR analysis showed that BRT1 is expressed through all stages of plant life cycle, a result consistent with the impact of the brt1 mutation on both leaf sinapoylmalate levels and seed sinapoylcholine content. Finally, the compound accumulated in brt1 trichomes was identified as a sinapic acid-derived polyketide, indicating that when sinapic acid glycosylation is reduced, a portion of it is instead activated to its CoA thioester, which then serves as a substrate for chalcone synthase.