Texture softening is a physiological indicator of fruit ripening, which eventually contributes to fruit quality and consumer’s acceptance. Despite great progress has been made in identification of the genes related to fruit softening, the upstream transcriptional regulatory pathways of these softening-related genes are not fully elucidated. Here, a novel bHLH gene, designated as MabHLH28, was identified because of its significant upregulation in banana fruit ripening. DAP-Seq analysis revealed that MabHLH28 bound to the core sequence of ‘CAYGTG’ presented in promoter regions of fruit softening-associated genes, such as the genes related to cell wall modification (MaPG3, MaPE1, MaPL5, MaPL8, MaEXP1, MaEXP2, MaEXPA2 and MaEXPA15) and starch degradation (MaGWD1 and MaLSF2), and these bindings were validated by EMSA and DLR assays. Transient overexpression and knockdown of MabHLH28 in banana fruit resulted in up- and down-regulation of softening-related genes, thereby hastening and postponing fruit ripening. Furthermore, overexpression of MabHLH28 in tomato accelerated the ripening process by elevating the accumulation of softening-associated genes. In addition, MabHLH28 showed interaction withMaWRKY49/111 and itself to form protein complexes, which could combinatorically strengthen the transcription of softening-associated genes. Taken together, our findings suggest that MabHLH28 mediates fruit softening by upregulating the expression of softening-related genes either alone or in combination with MaWRKY49/111.

中文翻译：

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Banana MabHLH28独立或与MaWRKY49/111协同正向调节软化相关基因的表达介导果实成熟


质地软化是水果成熟的生理指标，最终有助于水果质量和消费者的接受度。尽管在鉴定与果实软化相关的基因方面取得了很大进展，但这些软化相关基因的上游转录调控途径尚未完全阐明。在此，鉴定出一种新的 bHLH 基因，命名为 MabHLH28，因为它在香蕉果实成熟过程中显着上调。 DAP-Seq 分析显示，MabHLH28 与果实软化相关基因启动子区域的“CAYGTG”核心序列结合，例如与细胞壁修饰相关的基因（MaPG3、MaPE1、MaPL5、MaPL8、MaEXP1、MaEXP2、MaEXPA2）和 MaEXPA15）和淀粉降解（MaGWD1 和 MaLSF2），并且这些结合通过 EMSA 和 DLR 测定进行了验证。香蕉果实中 MabHLH28 的瞬时过表达和敲低导致软化相关基因的上调和下调，从而加速和推迟果实成熟。此外，番茄中 MabHLH28 的过度表达通过增加软化相关基因的积累来加速成熟过程。此外，MabHLH28与MaWRKY49/111及其自身相互作用形成蛋白质复合物，可以组合增强软化相关基因的转录。综上所述，我们的研究结果表明，MabHLH28 通过单独或与 MaWRKY49/111 组合上调软化相关基因的表达来介导果实软化。