The regulation of non-climacteric fruit ripening by the transcription factor NON-RIPENING (NOR) is poorly understood. Here, we identified that the NOR homolog in the non-climacteric fruit watermelon (Citrullus lanatus) is located within the selective sweep and sweetness quantitative trait locus that was selected during domestication from landraces to cultivars. ClNOR knockout substantially delayed fruit ripening, and the fruits of the knockout plants had lower abscisic acid (ABA) levels, lighter colored flesh, and were less sweet compared to wild type. Transcriptome analysis and DNA affinity purification sequencing revealed that ClNOR targeted the Basic Leucine Zipper gene ClbZIP1, which links ClNOR to genes that do not have a ClNOR-binding motif in their promoters, such as the ABA biosynthesis gene, 9-cis-epoxycarotenoid dioxygenase ClNCED1 and the chromoplast phosphate transporter gene ClPHT4;2. The double mutant Clnor Clbzip1 exhibited delayed fruit ripening, lower ABA levels, and lighter colored flesh. Its delayed ripening phenotype was stronger than that of the Clbzip1 single mutant. Additionally, the ClNORT,T haplotype in cultivated watermelon resulted in higher ClbZIP1 expression, but ClNORC,T from landraces and ClNORC,G from ancestral watermelon did not. Heterologous ClNORT,T expression rescued the delayed ripening phenotype of the Slnor knockout in tomato (Solanum lycopersicum). This natural variant (564T/C) of ClNOR promoted fruit ripening by enhancing target gene transcription. Overall, these findings will help elucidate the evolutionary mechanisms of non-climacteric fruit ripening.

中文翻译：

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NON-RIPENING 的天然变体促进西瓜果实成熟


转录因子 NON-RIPENING （NOR） 对非更年期果实成熟的调节知之甚少。在这里，我们确定了非更年期水果西瓜 （Citrullus lanatus） 中的 NOR 同源物位于从地方品种到栽培品种的驯化过程中选择的选择性扫描和甜度数量性状基因座内。ClNOR 敲除大大延迟了果实的成熟，与野生型相比，敲除植株的果实具有较低的脱落酸 （ABA） 水平，果肉颜色较浅，甜度较低。转录组分析和 DNA 亲和纯化测序显示，ClNOR 靶向碱性亮氨酸拉链基因 ClbZIP1，该基因将 ClNOR 与其启动子中没有 ClNOR 结合基序的基因联系起来，例如 ABA 生物合成基因、9-顺式-环氧类胡萝卜素双加氧酶 ClNCED1 和染色体磷酸盐转运蛋白基因 ClPHT4;2。双突变体 Clnor Clbzip1 表现出果实成熟延迟、ABA 水平较低和果肉颜色较浅。其延迟成熟表型比 Clbzip1 单突变体强。此外，培养西瓜中的 ClNORT，T 单倍型导致较高的 ClbZIP1 表达，但来自地方品种的 ClNORC，T 和来自祖先西瓜的 ClNORC，G 则没有。异源 ClNORT，T 表达挽救了番茄 （Solanum lycopersicum） 中 Slnor 敲除的延迟成熟表型。ClNOR 的这种自然变体 （564T/C） 通过增强靶基因转录促进果实成熟。总的来说，这些发现将有助于阐明非更年期果实成熟的进化机制。