MYB73 modulates apple flesh browning by repressing the expression of three polyphenol oxidase genes

Apple flesh browning results from the oxidation of phenolic compounds to quinones by polyphenol oxidases (PPOs), affecting the quality of fresh-cut apple products. Both the activity of PPOs and the concentration of their substrates contribute to browning; however, how the MdPPO genes are transcriptionally regulated remains largely unclear. Here, we show that overexpression of the coding sequence of Ma1 (cMa1-OE), a major gene controlling apple fruit acidity, leads to a significant increase in flesh browning in 'Royal Gala' apple. By measuring PPO activities and phenolic concentrations, we found that while slightly elevated concentrations of chlorogenic acid and catechin may contribute to enhanced flesh browning in cMa1-OE fruit at the early stages of fruit development, increased flesh browning at harvest is primarily caused by up-regulated PPO activities. Gene expression analysis throughout fruit development identified MdPPO3 as the gene primarily responsible for the enhanced PPO activity in cMa1-OE fruits at the early stages of fruit development, whereas both MdPPO15 and MdPPO16 are responsible at fruit harvest. An R2R3-MYB transcription factor, MdMYB73, repressed the expression of MdPPO3, MdPPO15, and MdPPO16 by binding to their promoters. RNAi suppression of MdMYB73 increased MdPPO3, MdPPO15, and MdPPO16 expression and total PPO activity, leading to enhanced flesh browning; however, this effect was blocked by RNAi of MdPPO3/MdPPO15/MdPPO16. These results indicate that MdMYB73 regulates apple flesh browning via suppressing MdPPO3, MdPPO15, and MdPPO16 expression; overexpression of Ma1 decreases MdMYB73 expression in fruit, which releases the suppression of the MdPPO genes, leading to enhanced flesh browning.