Wang Qingbiao, Wang Yanping, Sun Honghe, Sun Liang, Zhang Li
National Engineering Research Center for Vegetables, Beijing Academy of Agriculture and Forestry Sciences, Beijing, China.
Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (North China), Ministry of Agriculture, Beijing, China.
J Exp Bot. 2020 May 9;71(9):2537-2550. doi: 10.1093/jxb/eraa010.
Red-fleshed radish (Raphanus sativus L.) is a unique cultivar whose taproot is rich in anthocyanins beneficial to human health. However, the frequent occurrence of white-fleshed mutants affects the purity of commercially produced radish and the underlying mechanism has puzzled breeders for many years. In this study, we combined quantitative trait location by genome resequencing and transcriptome analyses to identify a candidate gene (RsMYB1) responsible for anthocyanin accumulation in red-fleshed radish. However, no sequence variation was found in the coding and regulatory regions of the RsMYB1 genes of red-fleshed (MTH01) and white-fleshed (JC01) lines, and a 7372 bp CACTA transposon in the RsMYB1 promoter region occurred in both lines. A subsequent analysis suggested that the white-fleshed mutant was the result of altered DNA methylation in the RsMYB1 promoter. This heritable epigenetic change was due to the hypermethylated CACTA transposon, which induced the spreading of DNA methylation to the promoter region of RsMYB1. Thus, RsMYB1 expression was considerably down-regulated, which inhibited anthocyanin biosynthesis in the white-fleshed mutant. An examination of transgenic radish calli and the results of a virus-induced gene silencing experiment confirmed that RsMYB1 is responsible for anthocyanin accumulation. Moreover, the mutant phenotype was partially eliminated by treatment with a demethylating agent. This study explains the molecular mechanism regulating the appearance of white-fleshed mutants of red-fleshed radish.
红肉萝卜（Raphanus sativus L.）是一种独特的栽培品种，其主根富含对人体健康有益的花青素。然而，白肉突变体的频繁出现影响了商业化生产萝卜的纯度，其潜在机制多年来一直困扰着育种者。在本研究中，我们结合基因组重测序的数量性状定位和转录组分析，鉴定出一个负责红肉萝卜花青素积累的候选基因（RsMYB1）。然而，在红肉（MTH01）和白肉（JC01）品系的RsMYB1基因的编码区和调控区未发现序列变异，且两个品系的RsMYB1启动子区域均出现了一个7372 bp的CACTA转座子。随后的分析表明，白肉突变体是RsMYB1启动子中DNA甲基化改变的结果。这种可遗传的表观遗传变化是由于CACTA转座子的高甲基化，它诱导了DNA甲基化向RsMYB1启动子区域的扩散。因此，RsMYB1的表达被显著下调，从而抑制了白肉突变体中的花青素生物合成。对转基因萝卜愈伤组织的检测和病毒诱导基因沉默实验的结果证实，RsMYB1负责花青素的积累。此外，用去甲基化剂处理可部分消除突变体表型。本研究解释了调控红肉萝卜白肉突变体出现的分子机制。
