Abstract
Purple turnip Brassica rapa ssp. rapa is highly appreciated by consumers but the metabolites and molecular mechanisms underlying the root skin pigmentation remain open to study. Herein, we analyzed the anthocyanin composition in purple turnip (PT) and green turnip (GT) at five developmental stages. A total of 21 anthocyanins were detected and classified into the six major anthocynanin aglycones. Distinctly, PT contains 20 times higher levels of anthocyanins than GT, which explain the difference in the root skin pigmentation. We further sequenced the transcriptomes and analyzed the differentially expressed genes between the two turnips. We found that PT essentially diverts dihydroflavonols to the biosynthesis of anthocyanins over flavonols biosynthesis by strongly down-regulating one flavonol synthase gene, while strikingly up-regulating dihydroflavonol 4-reductase (DFR), anthocyanidin synthase and UDP-glucose: flavonoid-3-O-glucosyltransferase genes as compared to GT. Moreover, a nonsense mutation identified in the coding sequence of the DFR gene may lead to a nonfunctional protein, adding another hurdle to the accumulation of anthocyanin in GT. We also uncovered several key members of MYB, bHLH and WRKY families as the putative main drivers of transcriptional changes between the two turnips. Overall, this study provides new tools for modifying anthocyanin content and improving turnip nutritional quality.
Highlights
Turnip (Brassica rapa ssp. rapa), belongs to the Cruciferae family and represents one of the most important leaf and root vegetable crops for human consumption and animal fodder in China and throughout East Asia
The goal of this work was to elucidate the molecular and metabolic mechanisms underlying the differential pigmentation in turnips, as a foundation for the development of turnip varieties that are rich in anthocyanin compounds to meet the increasing demand for health-promoting components in our daily diet
We performed the functional annotation of the unigenes in various database, including NR, Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes (KEGG), Clusters of Orthologous Groups (COG), KOG, Gene Ontology (GO) and and Pfam databases, which resulted in 52,449 unigenes successfully annotated (Table 3)
Summary
Turnip (Brassica rapa ssp. rapa), belongs to the Cruciferae family and represents one of the most important leaf and root vegetable crops for human consumption and animal fodder in China and throughout East Asia. Anthocyanins are secondary metabolites with health-promoting virtues, such as anti-oxidation, anti-mutation, prevention of cardiovascular disease, liver protection, and inhibiting the metastasis of tumor cells [7,8,9,10,11,12]. They play important fundamental physiological functions in plants, including UV protection, pigmentation of flowers and fruits to attract pollinators and for seed dispersal, and responses to biotic and environmental stresses [13,14,15,16,17,18,19,20]. The identification, analysis and genetic manipulation of anthocyanin metabolites have become an important topic in plant secondary metabolite research [21]
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