Abstract Browning is an important factor affecting the quality of fresh-cut apples, and studying this factor is valuable. Previous research has found that apple browning is due to an enzymatic reaction caused by polyphenol oxidase (PPO), which results in the darkening of apples through the oxidation of polyphenols to quinones. We screened two Fuji apple types, “flushed-skin color pattern” (Type P) and “striped-skin color pattern” (Type T) from bud mutation cultivars. Type P has slower browning rate than type T. Using physiology, targeted metabolomics and real-time quantitative PCR, we found that polyphenol-related parameters including PPO activities, total phenol contents and expression of five genes (MdF3H, MdPPO, MdUGT88F1/4, MdLAR and MdC4H) did not change during browning in both apple types. However, levels of hyperoside, peroxidase (POD), superoxide dismutase (SOD), hydrogen peroxide (H2O2), ascorbic acid (AsA) and expression of nine genes (MdPAL, MdCHS, MdCHI, MdANS, MdFLS, MdANR, MdUGT71K1s, MdTTG1 and MdMYB1) were higher levels in type P apples. Correlation analysis indicated that the browning was unrelated to total phenol and PPO activities. Browning index (BI) was negatively correlated with hyperoside, whilst malondialdehyde (MDA) and nine up-regulated genes were positively correlated. We therefore conclude that hyperoside may be the most critical indicator in apple browning. In summary, we found a difference of enzymatic browning unrelated to in Fuji apples. This research provides new insight into the phenomenon of fruit browning.

中文翻译：

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从生理学、靶向代谢组学和基因表达分析看富士苹果与PPO无关的酶促褐变差异

摘要 褐变是影响鲜切苹果品质的重要因素，对该因素的研究具有重要意义。此前的研究发现，苹果褐变是由于多酚氧化酶（PPO）引起的酶促反应，通过多酚氧化成醌类导致苹果变黑。我们从芽突变品种中筛选了两种富士苹果类型，“红皮色图案”（P 型）和“条纹皮色图案”（T 型）。P 型的褐变速度比 T 型慢。通过生理学、靶向代谢组学和实时定量 PCR，我们发现多酚相关参数包括 PPO 活性、总酚含量和 5 个基因（MdF3H、MdPPO、MdUGT88F1/4、 MdLAR 和 MdC4H) 在两种苹果类型的褐变过程中没有变化。然而，金丝桃苷、过氧化物酶 (POD)、P型苹果中超氧化物歧化酶（SOD）、过氧化氢（H2O2）、抗坏血酸（AsA）和九种基因（MdPAL、MdCHS、MdCHI、MdANS、MdFLS、MdANR、MdUGT71K1s、MdTTG1和MdMYB1）的表达水平较高。相关分析表明褐变与总酚和PPO活性无关。布朗宁指数 (BI) 与金丝桃苷呈负相关，而丙二醛 (MDA) 和 9 个上调基因呈正相关。因此我们得出结论，金丝桃苷可能是苹果褐变最关键的指标。总之，我们发现了与富士苹果无关的酶促褐变差异。这项研究提供了对水果褐变现象的新见解。MdUGT71K1s、MdTTG1 和 MdMYB1) 在 P 型苹果中含量较高。相关分析表明褐变与总酚和PPO活性无关。布朗宁指数 (BI) 与金丝桃苷呈负相关，而丙二醛 (MDA) 和 9 个上调基因呈正相关。因此我们得出结论，金丝桃苷可能是苹果褐变最关键的指标。总之，我们发现了与富士苹果无关的酶促褐变差异。这项研究提供了对水果褐变现象的新见解。MdUGT71K1s、MdTTG1 和 MdMYB1) 在 P 型苹果中含量较高。相关分析表明褐变与总酚和PPO活性无关。布朗宁指数 (BI) 与金丝桃苷呈负相关，而丙二醛 (MDA) 和 9 个上调基因呈正相关。因此我们得出结论，金丝桃苷可能是苹果褐变最关键的指标。总之，我们发现了与富士苹果无关的酶促褐变差异。这项研究提供了对水果褐变现象的新见解。而丙二醛（MDA）和九个上调基因呈正相关。因此我们得出结论，金丝桃苷可能是苹果褐变最关键的指标。总之，我们发现了与富士苹果无关的酶促褐变差异。这项研究提供了对水果褐变现象的新见解。而丙二醛（MDA）和九个上调基因呈正相关。因此我们得出结论，金丝桃苷可能是苹果褐变最关键的指标。总之，我们发现了与富士苹果无关的酶促褐变差异。这项研究提供了对水果褐变现象的新见解。