The mechanism of color fading in sunburned apple peel
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The effect of sunburn on apple fruit is important because the appearance reduces the market value. Previous studies have shown that high temperature and strong light are the major causes for its occurrence and severity, and the color change of the peel is an obvious symptom of fruit sunburn. While the changes in the pigments contribute to fruit color, the mechanisms involved in the pigment biosynthesis that takes place during the development of the apple fruit sunburn are not well understood. Our research examined the changes in the content of the major pigments and the transcript levels of the carotenoid and anthocyanin biosynthetic genes. The results showed that sunburn had a significant influence on the chlorophyll, anthocyanins, and carotenoids in apple fruit. When sunburn occurred on the apple, the values of a* and a*/b* were reduced, while L* and b* were augmented, and sunburn promoted carotenoid, phenolic and flavonoid synthesis while inhibiting the accumulation of chlorophyll and anthocyanins. Correspondingly, the apple peel color became light and yellow following sunburn. Sunburned apples underwent changes in the genes for carotenoids and anthocyanins. The PSY and PDS genes were up-regulated in sunburned apple fruits when the apples turned yellow, suggesting that the PSY and PDS genes could have very important roles in the regulation of the accumulation of the carotenoid in sunburn apples. The expression of MdANR, MdFLS, MdUFGT2, MYB1-2, and bHLH33 decreased compared to controls. These results suggest that the regulatory genes MYB1-2 and bHLH33 prevent the accumulation of anthocyanins in sunburned apples by down-regulating the structural genes MdANR and MdFLS.
KeywordsApple Sunburn Color fading
This research was supported by the National Natural Science Foundation of China (31860549) and the Higher Education Fund of Gansu province (2018B-034). We thank Pro. Zhengyang Zhao for his helpful suggestions.
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