Transposon-induced methylation of the RsMYB1 promoter disturbs the anthocyanin accumulation in red-fleshed radish (Raphanus sativus L.)

First author: Qingbiao Wang; Affiliations:  Beijing Academy of Agriculture and Forestry Sciences (北京农林科学院): Beijing, China

Corresponding author: Li Zhang

Red-fleshed radish is a unique cultivar that is rich in anthocyanins beneficial to human health in taproot. However, the frequent occurrence of white-fleshed mutants affects the purity of commercially produced radish and mechanism has puzzled breeders for many years. In this study, we combined QTL-seq and transcriptome analyses to identify a candidate gene (RsMYB1) responsible for the anthocyanin accumulation in red-fleshed radish. However, no sequence variation was found in the coding and regulatory regions of the RsMYB1 genes of the red-fleshed (MTH01) and white-fleshed (JC01) lines, and a 7,372-bp CACTA transposon in the RsMYB1 promoter region occurred in both lines. A subsequent analysis suggested that the taproot 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 downregulated, which inhibited anthocyanin biosynthesis in white-fleshed mutants. An examination of transgenic radish calli and the results of a virus-induced gene silencing experiment confirmed the RsMYB1 is responsible for anthocyanins accumulation. Moreover, the mutant phenotype was partially eliminated by a treatment with a demethylating agent. This study explained the molecular regulation mechanism of appearance white-fleshed mutant in red-fleshed radish.

红色果肉的萝卜是非常独特的栽培种，其直根中富含花青素，对于人类健康十分有益。然而，频繁出现的白色果肉突变影响着商业化的萝卜生产，且潜在的遗传机制已经困扰了育种者们多年。本文中，作者结合了QTL-seq和转录组分析鉴定到了一个候选基因RsMYB1作用于红色果肉萝卜中的花青素积累。然而，红色果肉品系MTH01与白色果肉品系JC01的RsMYB1基因在编码区和调控区均没有检测到序列变异，而是在其启动子区域发现了一个长7372bp的CACTA转座子。进一步的分析显示白色果肉的突变体是由于RsMYB1基因启动子区DNA甲基化的状态改变所导致的。这种可遗传的表观变化是由超甲基化的CACTA转座子引起的，其将甲基化扩散到了RsMYB1基因的启动子区域。因此，RsMYB1基因的表达量大大地下调，从而抑制了白色果肉突变体中花青素的生物合成。利用愈伤组织进行的遗传转化试验与病毒诱导的基因沉默试验均表明RsMYB1基因作用于花青素的积累。此外，用去甲基化剂处理白色果肉植株，能够部分消除该突变体的表型。本文的研究揭示了红色果肉胡萝卜中白色果肉突变的分子调控机制。

通讯：张丽

研究方向：萝卜遗传育种。

doi: https://doi.org/10.1093/jxb/eraa010

Journal: Journal of Experimental Botany

Published date: January 21, 2020