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A banana R2R3‐MYB transcription factor MaMYB3 is involved in fruit ripening through modulation of starch degradation by repressing starch degradation‐related genes and MabHLH6

Fan, Zhong‐Qi, Ba, Liang‐Jie, Shan, Wei, Xiao, Yun‐Yi, Lu, Wang‐Jin, Kuang, Jian‐Fei, Chen, Jian‐Ye
Theplant journal 2018 v.96 no.6 pp. 1191-1205
bananas, fruit quality, gene expression regulation, gene overexpression, genes, phosphotransferases (kinases), repressor proteins, ripening, starch, tomatoes, transcription (genetics)
Starch degradation is a necessary process determining banana fruit quality during ripening. Many starch degradation‐related genes are well studied. However, the transcriptional regulation of starch degradation during banana fruit ripening remains poorly understood. In this study, we identified a MYB transcription factor (TF) termed MaMYB3, as a putative protein binding the promoter of MaGWD1, a member of glucan water dikinase (GWD) family which has been demonstrated as an important enzyme of starch degradation. MaMYB3 was ripening‐ and ethylene‐repressible, and its expression was negatively correlated with starch degradation. Acting as a nucleus‐localized transcriptional repressor, MaMYB3 repressed the transcription of 10 starch degradation‐related genes, including MaGWD1, MaSEX4, MaBAM7‐MaBAM8, MaAMY2B, MaAMY3, MaAMY3A, MaAMY3C, MaMEX1, and MapGlcT2‐1, by directly binding to their promoters. Interestingly, a previously identified activator of starch degradation‐related genes, MabHLH6, was also suppressed by MaMYB3. The ectopic overexpression of MaMYB3 in tomato down‐regulated the expression of starch degradation‐related genes, inhibited starch degradation and delayed fruit ripening. Based on these findings, we conclude that MaMYB3 negatively impacts starch degradation by directly repressing starch degradation‐related genes and MabHLH6, and thereby delays banana fruit ripening. Collectively, our study expands our understanding of the complex transcriptional regulatory hierarchy modulating starch degradation during fruit ripening.