Genetic and biochemical analyses of glycosyltransferases involved in Arabidopsis sinapate ester metabolism

Sinapoylmalate is a major phenylpropanoid accumulated in Arabidopsis thaliana. Its presence causes 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 hyperfluorescent. The results from genetic mapping and complementation analyses showed that BRT1 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. Reverse transcriptase-polymerase chain reaction 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. Identification of other glycosyltransferases that may be redundant with BRT1 has been initiated by analyses of EMS-mutagenized plants in the brt1-1 background and knockouts of UGT84A3 and UGT88A1. UV screening of EMS-mutagenized M2 plants has identified enhancers and suppressors of the brt1 mutation. A method for isolation of the compound(s) responsible for the hyperfluorescent trichome phenotype is reported