Sinapate ester metabolism in Brassica and Arabidopsis
The accumulation of sinapate esters (SE) in organ- and tissue-specific patterns is considered a hallmark of Brassicaceae plants. Seeds of Arabidopsis and Brassica napus contain sinapine (sinapoylcholine) as major phenolic compound whereas the UV-shielding sinapoylmalate accumulates in epidermal cells. The accumulation kinetics of SE is based on transcriptional regulation of the enzymes UDP glucose:sinapate glucosyltransferase (SGT), sinapoylglucose:choline sinapoyltransferase (SCT), sinapoylglucose:malate sinapoyltransferase (SMT) and sinapine esterase (SCE). Enzymatic SGT activity is mediated by UDP glucosyltransferases of the UGT84A clade. In B. napus, the enzyme UGT84A9 is limiting for SE biosynthesis during seed development. Arabidopsis employs, besides the SGT homolog UGT84A2, three other hydroxycinnamate UGTs (UGT84A1, -A3, -A4) without pronounced specificity. In the allotetraploid genome of B. napus, UGT84A9 is represented by four loci of which UGT84A9a and -b are involved in SE biosynthesis. The sinapoyltransferases SMT and SCT were derived from hydrolases of the serine carboxypeptidase type (SCPs). Arabidopsis SMT adopted the functional elements of SCPs - catalytic triad, oxyanion hole and hydrogen bond network for substrate recognition - to catalyze glucose ester-dependent acyltransfer reactions. Sinapine esterase evolved by recruitment of lipase-like ancestors. To decrease the amount of antinutritive SE compounds in seeds of B. napus, targeted metabolic engineering was proven as the most efficient strategy. Silencing of UGT84A9 suppresses the biosyntheses of sinapine and related SE. Seed-specific overexpression of sinapine esterase BnSCE3 prevents sinapine accumulation by synchronization of biosynthesis and degradation. Combination of biosynthesis suppression with induced degradation of accumulating SE bears the potential to generate low sinapine B. napus lines.