Browning is a pervasive problem in horticultural products, substantially diminishing the appearance, flavor and nutritional value of fruit, including important fruits like apple (Malus × domestica Borkh.). In this study, we compared the physiological characteristics of the browning-resistant line ‘Rb-18’ with the susceptible variety ‘Fuji’ and found that the polyphenol oxidase (PPO) enzyme activity and phenolic content of Rb-18 were significantly lower than those in Fuji. In addition, the PPO enzyme in Fuji showed a stronger affinity for its substrate, catechol, compared to Rb-18. Through transcriptome and RT-qPCR analyses, MdPPO7 expression was identified as contributing to flesh browning after cutting. Subsequent fruit injection and stable genetic transformation of the MdPPO7 gene into apple fruit and calli determined that syringic acid, procyanidin, phloridzin, chlorogenic acid, gallic acid, catechin, and caffeic act as its catalytic substrates in the process involved in browning. Furthermore, luciferase reporter, yeast one-hybrid, β-glucuronidase reporter assays and chip-qPCR analysis demonstrated that a WRKY transcription factor(MdWRKY3) binds to the promoter region of polyphenol oxidase gene (MdPPO7) and positively regulates its expression to promote apple flesh browning. This study provides insights into the molecular regulatory mechanisms of fruit browning in fresh-cut apples and provides a theoretical basis for the generation of high-quality apple germplasm resources.

中文翻译：

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多酚氧化酶基因 MdPPO7 的表达受 MdWRKY3 调节，以调节切片苹果果实的褐变


褐变是园艺产品中普遍存在的问题，它大大降低了水果的外观、风味和营养价值，包括苹果 （Malus × domestica Borkh.） 等重要水果。本研究比较了抗褐变系 'Rb-18' 与感病品种 'Fuji' 的生理特性，发现 Rb-18 的多酚氧化酶 （PPO） 酶活性和酚类含量显著低于 Fuji。此外，与 Rb-18 相比，Fuji 中的 PPO 酶对其底物儿茶酚的亲和力更强。通过转录组和 RT-qPCR 分析，确定 MdPPO7 表达导致切割后肉质褐变。随后的果实注射和 MdPPO7 基因向苹果果实和愈伤组织的稳定遗传转化决定了丁香酸、原花青素、根皮苷、绿原酸、没食子酸、儿茶素和咖啡酸在涉及褐变的过程中充当其催化底物。此外，荧光素酶报告基因、酵母单杂交、β-葡萄糖醛酸酶报告基因检测和 chip-qPCR 分析表明，WRKY 转录因子 （MdWRKY3） 与多酚氧化酶基因 （MdPPO7） 的启动子区结合，并正向调节其表达以促进苹果果肉褐变。本研究为鲜切苹果果实褐变的分子调控机制提供了见解，为生成优质苹果种质资源提供了理论依据。