MYB73 NEGATIVELY REGULATES BOTH PEEL COLOR DEVELOPMENT AND FLESH BROWNING IN APPLE
Apple fruit quality encompasses external appearance such as size and color and internal eating quality such as taste and flavor. Fruit acidity, peel color and flesh browning are three important traits for fruit quality, but it is largely unknown if their regulation is linked. When Ma1, the major gene controlling fruit acidity, was overexpressed in ‘Royal Gala’ apple, the transgenic apple had not only altered acidity but also enhanced peel red color and exacerbated flesh browning. The objective of my dissertation research is to identify the molecular mechanism that links peel red color and flesh browning to Ma1 overexpression. Anthocyanin synthesis underlies red color development in apple peel. MYB73, an R2R3-MYB transcription factor that transcriptionally regulates Ma1 expression, is identified as a negative regulator for anthocyanin synthesis. MYB73 binds to the promoter of the gene encoding UDP-glycose: flavonoid-3-O-glycosyltransferase (UFGT), the enzyme that catalyzes the last step in anthocyanin synthesis, to repress its expression. In Ma1-OE fruit, down-regulation of MYB73 releases UFGT from MYB73 repression, leading to enhanced anthocyanin biosynthesis. Apple flesh browning results from the oxidation of phenolic compounds to quinones by polyphenol oxidases (PPOs). While slightly elevated concentrations of chlorogenic acid and catechin may have contributed to enhanced flesh browning in Ma1-OE fruit at the early stages of fruit development, enhanced flesh browning at harvest is primarily caused by increased PPO activities. PPO3 is the gene responsible for the enhanced PPO activity in the Ma1-OE fruits at the early stages of fruit development whereas both PPO15 and PPO16 are responsible at fruit harvest. MdMYB73 negatively regulates the expression of PPO3, PPO15, and PPO16 by binding to their promoters. Overexpression of Ma1 somehow decreases MYB73 expression in fruit, which releases the suppression of the PPO genes, leading to enhanced flesh browning. In conclusion, MYB73 negatively regulates both peel red color development and flesh browning by repressing the expression of UFGT and PPO genes as well as transcriptionally activating Ma1 expression, linking both peel color and flesh browning to fruit acidity. These findings lay the foundation for altering these three important quality traits in apple by using the same transcription factor.
Hua, Jian; Turgeon, Robert
Ph. D., Horticulture
Doctor of Philosophy
dissertation or thesis