Members of the Brassicaceae accumulate complex patterns of sinapate esters, as shown in this communication with seeds of oilseed rape (Brassica napus). Fifteen seed constituents were isolated and identified by a combination of high-field NMR spectroscopy and high resolution electrospray ionisation mass spectrometry. These include glucose, gentiobiose and kaempferol glycoside esters as well as sinapine (sinapoylcholine), sinapoylmalate and an unusual cyclic spermidine amide. One of the glucose esters (1,6-di-O-sinapoylglucose), two gentiobiose esters (1-O-caffeoylgentiobiose and 1,2,6′-tri-O-sinapoylgentiobiose) and two kaempferol conjugates [4′-(6-O-sinapoylglucoside)-3,7-di-O-glucoside and 3-O-sophoroside-7-O-(2-O-sinapoylglucoside)] seem to be new plant products. Serine carboxypeptidase-like (SCPL) acyltransferases catalyze the formation of sinapine and sinapoylmalate accepting 1-O-β-acetal esters (1-O-β-glucose esters) as acyl donors. To address the question whether the formation of other components of the complex pattern of the sinapate esters in B. napus seeds is catalyzed via 1-O-sinapoyl-β-glucose, we performed a seed-specific dsRNAi-based suppression of the sinapate glucosyltransferase gene (BnSGT1) expression. In seeds of BnSGT1-suppressing plants the amount of sinapoylglucose decreased below the HPLC detection limit resulting in turn in the disappearance or marked decrease of all the other sinapate esters, indicating that formation of the complex pattern of these esters in B. napus seeds is dependent on sinapoylglucose. This gives rise to the assumption that enzymes of an SCPL acyltransferase family catalyze the appropriate transfer reactions to synthesize the accumulating esters.

Resveratrol is a phytoalexin produced in various plants like wine, peanut or pine in response to fungal infection or UV irradiation, but it is absent in members of the Brassicaceae. Moreover, resveratrol and its glucoside (piceid) are considered to have beneficial effects on human health, known to reduce heart disease, arteriosclerosis and cancer mortality. Therefore, the introduction of the gene encoding stilbene synthase for resveratrol production in rapeseed is a tempting approach to improve the quality of rapeseed products. The stilbene synthase gene isolated from grapevine (Vitis vinifera L.) was cloned under control of the seed-specific napin promotor and introduced into rapeseed (Brassica napus L.) by Agrobacterium-mediated co-transformation together with a ds-RNA-interference construct deduced from the sequence of the key enzyme for sinapate ester biosynthesis, UDP-glucose:sinapate glucosyltransferase (BnSGT1), assuming that the suppression of the sinapate ester biosynthesis may increase the resveratrol production in seeds through the increased availability of the precursor 4-coumarate. Resveratrol glucoside (piceid) was produced at levels up to 361 μg/g in the seeds of the primary transformants. This value exceeded by far piceid amounts reported from B. napus expressing VST1 in the wild type sinapine background. There was no significant difference in other important agronomic traits, like oil, protein, fatty acid and glucosinolate content in comparison to the control plants. In the third seed generation, up to 616 μg/g piceid was found in the seeds of a homozygous T3-plant with a single transgene copy integrated. The sinapate ester content in this homozygous T3-plant was reduced from 7.43 to 2.40 mg/g. These results demonstrate how the creation of a novel metabolic sink could divert the synthesis towards the production of piceid rather than sinapate ester, thereby increasing the value of oilseed products.

Seeds of oilseed rape (Brassica napus) accumulate high amounts of antinutritive sinapate esters (SE) with sinapoylcholine (sinapine) as major component, accompanied by sinapoylglucose. These phenolic compounds compromise the use of the protein-rich valuable seed meal. Hence, a substantial reduction of the SE content is considered essential for establishing rape as a protein crop. The present work focuses on the suppression of sinapine synthesis in rape. Therefore, rape (spring cultivar Drakkar) was transformed with a dsRNAi construct designed to silence seed-specifically the BnSGT1 gene encoding UDP-glucose:sinapate glucosyltransferase (SGT1). This resulted in a substantial decrease of SE content in T2 seeds with a reduction reaching 61%. In T2 seeds a high and significant correlation between the contents of sinapoylglucose and all other sinapate esters has been observed. Among transgenic plants, no significant difference in other important agronomic traits, such as oil, protein, fatty acid and glucosinolate content in comparison to the control plants was observed. Maximal reduction of total SE content by 76% was observed in seeds of one homozygous T2 plant (T3 seeds) carrying the BnSGT1 suppression cassette as a single copy insert. In conclusion, this study is an initial proof of principle that suppression of sinapoylglucose formation leads to a strong reduction of SE in rape seeds and is thus a promising approach in establishing rape, currently an important oil crop, as a protein crop as well.