White Paper Bagging Promotes Peel Coloration in ‘Mi Yu’ Passion Fruit by Upregulating Anthocyanin Biosynthetic Genes

Abstract Passion fruit ( Passiflora edulis ) is an economically important perennial vine. ‘Mi Yu’ is a hybrid passion fruit cultivar (purple passion fruit × yellow passion fruit) with excellent quality, however, its peel often exhibits poor coloration during summer, which affects both harvest decisions and commercial value. In this study, the effect of paper bagging on the peel coloration and the underlying molecular mechanism of this improvement were explored. Measurements of peel color and pigment-related indicators showed that the white paper bagging treatment significantly increased the a* value and anthocyanin and proanthocyanidin contents of the peel, while decreasing the b* value compared to the control group and other treatment groups, effectively promoting peel coloration. Gene families of key structural genes involved in the anthocyanin biosynthesis pathway in purple passion fruit were identified and validated through clustering analysis. QPCR analysis showed that under white paper bagging conditions, the expression levels of multiple structural genes were significantly increased, included PeCHS1 , and PeCHS7 from the CHS gene family, PeF3H4 and PeF3H5 from the F3H gene family, PeDFR1 and PeDFR2 from the DFR gene family, and PeANS3 , PeANS4 , PeANS5 , and PeANS6 from the ANS gene family. Furthermore, the expression levels of anthocyanin biosynthesis genes were compared between the yellow and purple peel cultivars under natural conditions based on the transcriptome sequencing results. Compared to normal conditions, the expression level of PeCHS1 was found to be particularly significantly increased under white paper bagging treatment. Our results indicate that bagging with white paper bags effectively enhance peel coloration of ‘Mi Yu’ passion fruit during summer by upregulating anthocyanin biosynthetic genes. This provides a practical technical and theoretical foundation for managing peel color in passion fruit production, as well as deeper insight into the molecular mechanism through which bagging treatment improves fruit peel color.