Violet LED light-activated MdHY5 positively regulates phenolic accumulation to inhibit fresh-cut apple fruit browning

Fresh-cut fruit browning severely affects the appearance of fruit. Light treatment can effectively inhibit fresh-cut apple fruit browning, but the regulatory mechanism remains unknown. Here, we discovered that violet LED (Light-Emitting-Diode) light treatment significantly reduced fresh-cut apple fruit browning. Metabolomic analysis revealed that violet LED light treatment enhanced the phenolic accumulation of fresh-cut apple fruit. Transcriptomic analysis showed that the expression of phenolic degradation genes <i>POLYPHENOL OXIDASE</i> (<i>MdPPO</i>) and <i>PEROXIDASE</i> (<i>MdPOD</i>) was reduced, and the expression of phenolic synthesis gene <i>PHENYLALANINE AMMONIA LYASE</i> (<i>MdPAL</i>) was activated by violet LED light treatment. Moreover, two <i>ELONGATED HYPOCOTYL 5</i> (<i>MdHY5</i> and <i>MdHYH</i>) transcription factors involved in light signaling were identified. The expression of <i>MdHY5</i> and <i>MdHYH</i> was activated by violet LED light treatment. Violet LED light treatment no longer inhibited fresh-cut apple fruit browning in MdHY5- or MdHYH- silenced fruit. Further experiments revealed that MdHY5 and MdHYH suppressed <i>MdPPO</i> and <i>MdPOD</i> expression and promoted <i>MdPAL</i> expression by binding to their promoters. In addition, MdHY5 and MdHYH bound to each other's promoters and enhanced their expression. Overall, our findings revealed that violet LED light-activated <i>MdHY5</i> and <i>MdHYH</i> formed a positive transcriptional loop to regulate the transcription of <i>MdPPO</i>, <i>MdPOD</i>, and <i>MdPAL</i>, which in turn inhibited the degradation of phenolics and promoted the synthesis of phenolics, thus inhibiting fresh-cut apple fruit browning. These results provide a theoretical basis for improving the appearance and quality of fresh-cut apple fruit.