Molecular regulation of sinapate ester metabolism inBrassica napus: expression of genes, properties of the encoded proteins and correlation of enzyme activities with metabolite accumulation

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Abstract
Summary: Members of the Brassicaceae family accumulate specific sinapate esters, i.e. sinapoylcholine (sinapine), which is considered as a major antinutritive compound in seeds of important crop plants likeBrassica napus, and sinapoylmalate, which is implicated in UV‐B tolerance in leaves. We have studied the molecular regulation of the sinapate ester metabolism inB. napus, and we describe expression of genes, some properties of the encoded proteins and profiles of the metabolites and enzyme activities. The cloned cDNAs encoding the key enzymes of sinapine biosynthesis, UDP‐glucose (UDP‐Glc):B. napussinapate glucosyltransferase (BnSGT1) and sinapoylglucose:B. napuscholine sinapoyltransferase (BnSCT), were functionally expressed. BnSGT1 belongs to a subgroup of plant GTs catalysing the formation of 1‐O‐hydroxycinnamoyl‐β‐d‐glucoses. BnSCT is another member of serine carboxypeptidase‐like (SCPL) family of acyltransferases. TheB. napusgenome contains at least twoSGTandSCTgenes, each derived from its progenitorsB. oleraceaandB. rapa. BnSGT1 and BnSCT activities are subjected to pronounced transcriptional regulation.BnSGT1transcript level increases throughout early stages of seed development until the early cotyledonary stage, and stays constant in later stages. The highest level ofBnSGT1transcripts is reached in 2‐day‐old seedlings followed by a dramatic decrease. In contrast, expression ofBnSCTis restricted to developing seeds. Regulation of gene expression at the transcript level seems to be responsible for changes of BnSGT1 and BnSCT activities during seed and seedling development ofB. napus. Together with sinapine esterase (SCE) and sinapoylglucose:malate sinapoyltransferase (SMT), activities of BnSGT1 and BnSCT show a close correlation with the accumulation kinetics of the corresponding metabolites.