Sinapate esters in brassicaceous plants: biochemistry, molecular biology, evolution and metabolic engineering

Abstract

Brassicaceous plants are characterized by a pronounced metabolic flux toward sinapate, produced by the shikimate/phenylpropanoid pathway, which is converted into a broad spectrum of O-ester conjugates. The abundant sinapate esters in Brassica napus and Arabidopsis thaliana reflect a well-known metabolic network, including UDP-glucose:sinapate glucosyltransferase (SGT), sinapoylglucose:choline sinapoyltransferase (SCT), sinapoylglucose:L-malate sinapoyltransferase (SMT) and sinapoylcholine (sinapine) esterase (SCE). 1-O-Sinapoylglucose, produced by SGT during seed development, is converted to sinapine by SCT and hydrolyzed by SCE in germinating seeds. The released sinapate feeds via sinapoylglucose into the biosynthesis of sinapoylmalate in the seedlings catalyzed by SMT. Sinapoylmalate is involved in protecting the leaves against the deleterious effects of UV-B radiation. Sinapine might function as storage vehicle for ready supply of choline for phosphatidylcholine biosynthesis in young seedlings. The antinutritive character of sinapine and related sinapate esters hamper the use of the valuable seed protein of the oilseed crop B. napus for animal feed and human nutrition. Due to limited variation in seed sinapine content within the assortment of B. napus cultivars, low sinapine lines cannot be generated by conventional breeding giving rise to genetic engineering of sinapate ester metabolism as a promising means. In this article we review the progress made throughout the last decade in identification of genes involved in sinapate ester metabolism and characterization of the encoded enzymes. Based on gene structures and enzyme recruitment, evolution of sinapate ester metabolism is discussed. Strategies of targeted metabolic engineering, designed to generate low-sinapate ester lines of B. napus, are evaluated.

Secondary metabolites during early development in plants

“…The constant presence of coloured flavonoids during the germination of various species (Brassica spp., Vigna radiata, Lycopersicum esculentum, Sinapis alba, Z. mays, A. thaliana, and L. luteus) has permitted speculation and investigation concerning the photoprotective role of these SM (Siegelman and Hendricks 1957;Dumortier and Vendrig 1982a;Rengel and Kordan 1988;Kubasek et al 1992;Kubasek et al 1998;Huub et al 1997;Katagiri et al 2002;Zhou et al 2007;Poustka et al 2007;Pourcel et al 2010;Milkowski and Strack 2010). Huub et al (1997) evaluated this hypothesis with 3-day-old Lycopersicum esculentum plants that were irradiated with red light (681 nm) at various intensities (0.0001 to 10 pmol/m 2 s) after 24 h and with a darkness control.…”

“…The presence of SM during these stages and their spatio-temporal distribution can be important survival strategies. In this section, the roles of SM as reservoirs of nitrogen, defence substances against phytopathogens and insects, allelopathic agents, molecules with photoprotective roles, and attraction signals for nitrogen-fixing microorganisms are documented (Niemeyer et al 1989a;Zuanazzii et al 1998;Weir et al 2003;Gregianini et al 2004;Zheng et al 2008;Milkowski and Strack 2010;Yamaji and Ichihara 2012).…”

“…Zheng and Ashihara 2004;Zheng et al 2005;Poustka et al 2007;Valletta et al 2007;Pourcel 2010;Milkowski and Strack 2010). This increasing and/or decreasing result in a significant characteristic of SM dynamics, namely, that the gene expression of all SM for a species is not a reflection of each particular plant.…”

The Phenylpropanoid Pathway in Arabidopsis

“…Because of the antinutritive properties of sinapoylcholine, engineering sinapate ester metabolism to reduce its biosynthesis in the seeds of B. napus could improve the nutritive quality of canola meal Milkowski and Strack, 2010). Two SCT genes (BnSCT1 and BnSCT2) have been identified in B. napus since the characterization of the Arabidopsis SCT, and metabolic engineering projects are currently underway to eliminate the biosynthesis of choline in B. napus seeds (Milkowski et al, 2004;Huang et al, 2008;Weier et al, 2008;Bhinu et al, 2009;Milkowski and Strack, 2010).…”

Metabolomics reveals organ-specific metabolic rearrangements during early tomato seedling development

“…Some of the most rapid response mechanisms available to a seedling involve changes in secondary plant metabolism, resulting in corrective and/or protective compounds, all parts of the plant's environmental defense arsenal [e.g. in Arabidopsis, (Milkowski and Strack 2010); brassica, (Zhou et al 2007); tomato, (Guo and Wang 2010); sorghum, (Huang and Backhouse 2005)]. …”