MdHY5 and MdHY5S form a positive transcription loop to regulate browning and phenolics synthesis in fresh-cut apple under purple LED light

Abstract

ABSTRACTEnzymatic browning significantly affects the appearance and quality of fresh-cut fruit. Light treatment can effectively inhibit fresh-cut apple browning via unknown molecular mechanisms. Here, we found that the application of purple LED light decreased the browning index of fresh-cut apple, delaying browning as compared to that of fresh-cut apple placed in the dark, and suggesting that purple LED light suppresses browning. In addition, the expression levels ofMdHY5andMdHY5S, important BASIC LEUCINE ZIPPER DOMAIN (bZIP) transcription factors that are involved in the light signaling pathway, were increased by purple LED light treatment. SilencingMdHY5andMdHY5Sin apple meant that purple LED light treatment no longer inhibited fresh-cut apple browning, the expression levels of the browning-related genesPOLYPHENOL OXIDASE(MdPPO) andPEROXIDASE(MdPOD) increased, and the expression of the phenolic synthesis genePHENYLALANINE AMMONIALYASE(MdPAL) decreased. Further study revealed thatMdHY5andMdHY5Sbind to theMdPPOandMdPODpromoters, reducing their transcription. In contrast,MdHY5andMdHY5Sbind to theMdPALpromoter, enhancing transcription. Further research revealed that MdHY5 and MdHY5S also bind directly to each other’s promoters to form a positive transcriptional loop that activates their transcription. Our findings revealed that purple LED light inhibits browning and increases phenolics synthesis in fresh-cut apple by activatingMdHY5andMdHY5S. The results provide a theoretical basis upon which the new methods for improving the appearance and quality of fresh-cut apple can be based.