MaMPK3-2 Phosphorylating and Stabilizing Transcriptional Repressor MaAS1 by Counteracting MaRNF4-Mediated Ubiquitination During Banana Fruit Ripening.

The ripening of fleshy fruits is strongly regulated by a complex interplay between transcriptional regulation and post-translational modifications (PTMs). However, the specific PTM regulatory networks involved in fruit ripening remain poorly understood. In this study, we identify MaAS1, an R2R3-type MYB transcription factor, as a key negative regulator of banana fruit ripening. MaAS1 directly binds to the promoters of the ripening-associated genes MaACS1, MaPME23 and MaEXPA15 and represses their transcription. The mitogen-activated protein kinase MaMPK3-2 phosphorylates MaAS1 at three specific sites (Ser61, Thr92 and Ser120), thereby enhancing its transcriptional repressor activity and protein stability and ultimately suppressing fruit ripening. In contrast, the ethylene-induced E3 ubiquitin ligase MaRNF4 mediates the ubiquitination and degradation of MaAS1, thereby promoting ripening. Notably, phosphorylation by MaMPK3-2 antagonises MaRNF4-mediated ubiquitination, maintaining MaAS1 levels and regulating ripening progression. Collectively, these findings reveal a regulatory module involving MaMPK3-2, MaAS1 and MaRNF4, ultimately highlighting the intricate interplay between phosphorylation and ubiquitination in controlling fruit ripening, offering critical insights into the integrated regulatory network interconnecting transcription and PTMs during fruit ripening.