Comparative profiling of polyphenols and antioxidants and analysis of antiglycation activities in rapeseed (Brassica napus L.) under different moisture regimes.

Genotype, growth stages, and moisture regimes affect polyphenols as beneficial compounds in rapeseed with edible and medicinal properties. The aims of this study were to assess the effects of tissue, genotype background and moisture on growth, pigment composition, phenolic acids, flavonoids, antioxidant, and antiglycation activities in rapeseed. Treatments included two moisture regimes (10% field capacity as drought-treated and 30% field capacity as control), tissue (leaf, flower and seed), and 12 rapeseed genotypes. The range of loss in growth traits under drought compared with control was between 23% and 47%. Drought reduced number of leaves, leaf area, fresh and dry weights by 23%, 31%, 37%, and 36%, respectively whilst increased chlorophylls, carotenoids, total pigment, phenolic compounds, flavonoids, and antioxidant activities. Analysis for antiglycation properties in two genotypes (G01 and G08) which accumulated higher phenolic compounds showed that higher antiglycation property was associated with higher epicatechin, and caffeic, and syringic acids. Flower extracts showed higher phenolics than leaf and seed suggesting flowering stage is a preferred timing to harvest a higher polyphenols from rapeseed. Overall, our results demonstrated role of specific polyphenols in antiglycation activities and the importance of growth stage and genotype in attaining higher polyphenols and antioxidants that affect edible and medicinal values of rapeseed under water limited conditions.

Light induces gene expression to enhance the synthesis of storage reserves in Brassica napus L. embryos.

KEY MESSAGE: In this manuscript, we disclosed the influence of light on the accumulation of storage reserves in B. napus embryos.1.Light induced the gene expression in the developing embryos of B. napus.2.Light promoted the starch synthesis in chloroplasts of B. napus embryos.3.Light enhanced the metabolic activity of storage reserve synthesis in B. napus embryos. Light influences the accumulation of storage reserves in embryos, but the molecular mechanism was not fully understood. Here, we monitored the effects of light on reserve biosynthesis in Brassica napus by comparing embryos from siliques grown in normal light conditions to those that were shaded or masked (i.e., darkened completely). Masked embryos developed more slowly, weighed less, and contained fewer proteins and lipids than control embryos. They also had fewer and smaller oil bodies than control embryos and lacked chloroplasts, where starch grains are usually synthesized. The levels of most amino acids, carbohydrates, and fatty acids were higher in masked embryos than in control or shaded embryos, whereas the levels of these metabolites in the masked endosperms were lower than those in control and shaded endosperm. Transcriptome analysis indicated that genes involved in photosynthesis (42 genes), amino acid biosynthesis (51 genes), lipid metabolism (61 genes), and sugar transport (13 genes) were significantly repressed in masked embryos. Our results suggest that light contributes to reserve accumulation in embryos by inducing the expression of metabolic genes, thereby enhancing the biosynthesis of storage reserves.

Correlation analysis of the transcriptome and metabolome reveals the regulatory network for lipid synthesis in developing Brassica napus embryos.

KEY MESSAGE: In this manuscript, we explored the key molecular networks for oil biosynthesis with the transcriptome and metabolome of B. napus embryo at different developmental stages. Brassica napus (B. napus) is an important oil crop worldwide, yet the molecular pathways involved in oil biosynthesis in seeds are not fully understood. In this study, we performed a combined investigation of the gene expression profiles and metabolite content in B. napus seeds at 21, 28 and 35 days after flowering (DAF), when seed oil biosynthesis takes place. The total triacylglycerol (TAG) content in seed embryos increased over the course of seed maturation, and was accompanied by changes in the fatty acid profile, an increase in lipid droplets, and a reduction in starch grains. Metabolome analysis showed that the total amino acid, free fatty acid and organic acid contents in seed embryos decreased during seed maturation. In total, the abundance of 76 metabolites was significantly different between 21 and 28 DAF, and 68 metabolites changed in abundance between 28 and 35 DAF. Transcriptome analysis showed that the set of genes differentially expressed between stages was significantly enriched in those related to lipid metabolism, transport, protein and RNA metabolism, development and signaling, covering most steps of plant lipid biosynthesis and metabolism. Importantly, the metabolite and gene expression profiles were closely correlated during seed development, especially those associated with TAG and fatty acid biosynthesis. Further, the expression of major carbohydrate metabolism-regulating genes was closely correlated with carbohydrate content during seed maturation. Our results provide novel insights into the regulation of oil biosynthesis in B. napus seeds and highlights the coordination of gene expression and metabolism in this process.

Integrated metabolite profiling and transcriptome analysis reveals a dynamic metabolic exchange between pollen tubes and the style during fertilization of Brassica napus.

KEY MESSAGE: In this study, we analyzed the transcriptome and metabolite profile of the style to explore the essential metabolites and specific genes for pollen tube growth of B. napus in vivo. For sexual reproduction of flowering plants, pollen must germinate on the stigma and the pollen tube must grow through the style to deliver the sperm nuclei to the female gametophyte cells. During this process, the rapidly growing pollen tube can cover substantial distances. Despite the clear requirements for energy and cellular building blocks in this process, few studies have examined the role of metabolism in the style for pollen tube elongation. In this study, we comprehensively analyzed the transcriptome and metabolite profiles during pollen germination and pollen tube growth in the style in Brassica napus. We profiled the transcripts and metabolites stored in pollen and identified many transcripts related to metabolic pathways. Mature pollen contained low levels of nutrients, whereas the styles contained high levels of diverse nutrients. The levels of most nutrients in the style, especially metabolites for cell wall synthesis and energy metabolism, rapidly decreased at 2 h after pollination, along with pollen germination and pollen tube elongation through the style. A subset of genes involved in cell wall synthesis and nutrient transport were expressed specifically in styles at 1 h after pollination. These results demonstrated that successful fertilization involves the transcripts and nutrients stored in mature pollen, and specific gene expression and stored nutrients in the style. Therefore, these findings enhance our understanding of fertilization in B. napus.

Dynamic Metabolic Profiles and Tissue-Specific Source Effects on the Metabolome of Developing Seeds of Brassica napus.

Canola (Brassica napus) is one of several important oil-producing crops, and the physiological processes, enzymes, and genes involved in oil synthesis in canola seeds have been well characterized. However, relatively little is known about the dynamic metabolic changes that occur during oil accumulation in seeds, as well as the mechanistic origins of metabolic changes. To explore the metabolic changes that occur during oil accumulation, we isolated metabolites from both seed and silique wall and identified and characterized them by using gas chromatography coupled with mass spectrometry (GC-MS). The results showed that a total of 443 metabolites were identified from four developmental stages. Dozens of these metabolites were differentially expressed during seed ripening, including 20 known to be involved in seed development. To investigate the contribution of tissue-specific carbon sources to the biosynthesis of these metabolites, we examined the metabolic changes of silique walls and seeds under three treatments: leaf-detachment (Ld), phloem-peeling (Pe), and selective silique darkening (Sd). Our study demonstrated that the oil content was independent of leaf photosynthesis and phloem transport during oil accumulation, but required the metabolic influx from the silique wall. Notably, Sd treatment resulted in seed senescence, which eventually led to a severe reduction of the oil content. Sd treatment also caused a significant accumulation of fatty acids (FA), organic acids and amino acids. Furthermore, an unexpected accumulation of sugar derivatives and organic acid was observed in the Pe- and Sd-treated seeds. Consistent with this, the expression of a subset of genes involved in FA metabolism, sugar and oil storage was significantly altered in Pe and Sd treated seeds. Taken together, our studies suggest the metabolite profiles of canola seeds dynamically varied during the course of oil accumulation, which may provide a new insight into the mechanisms of the oil accumulation at the metabolite level.

Enhanced seed oil production in canola by conditional expression of Brassica napus LEAFY COTYLEDON1 and LEC1-LIKE in developing seeds.

The seed oil content in oilseed crops is a major selection trait to breeders. In Arabidopsis (Arabidopsis thaliana), LEAFY COTYLEDON1 (LEC1) and LEC1-LIKE (L1L) are key regulators of fatty acid biosynthesis. Overexpression of AtLEC1 and its orthologs in canola (Brassica napus), BnLEC1 and BnL1L, causes an increased fatty acid level in transgenic Arabidopsis plants, which, however, also show severe developmental abnormalities. Here, we use truncated napin A promoters, which retain the seed-specific expression pattern but with a reduced expression level, to drive the expression of BnLEC1 and BnL1L in transgenic canola. Conditional expression of BnLEC1 and BnL1L increases the seed oil content by 2% to 20% and has no detrimental effects on major agronomic traits. In the transgenic canola, expression of a subset of genes involved in fatty acid biosynthesis and glycolysis is up-regulated in developing seeds. Moreover, the BnLEC1 transgene enhances the expression of several genes involved in Suc synthesis and transport in developing seeds and the silique wall. Consistently, the accumulation of Suc and Fru is increased in developing seeds of the transgenic rapeseed, suggesting the increased carbon flux to fatty acid biosynthesis. These results demonstrate that BnLEC1 and BnL1L are reliable targets for genetic improvement of rapeseed in seed oil production.