Abscisic acid-responsive transcription factors PavDof2/6/15 mediate fruit softening in sweet cherry

背景回顾：Softening is a key step during fruit ripening that is modulated by the interplay between multiple phytohormones. The antagonistic action of abscisic acid (ABA) and auxin determines the rate of fruit ripening and softening. 

提出问题：However, the transcription factors that integrate ABA and auxin signals to regulate fruit softening remain to be determined. 

主要发现：In this study, we identified several DNA-binding with One Finger (Dof) transcription factors essential for ABA-promoted fruit softening, based on transcriptome analysis of two sweet cherry (Prunus avium L.) varieties with different fruit firmness. 

结果1-Dof6和Dof2/15分别促进和抑制ABA介导的果实软化：We show that PavDof6 directly binds to the promoters of genes encoding cell wall-modifying enzymes to activate their transcription, while PavDof2/15 directly repress their transcription. Transient overexpression of PavDof6 and PavDof2/15 in sweet cherry fruits resulted in precocious and delayed softening, respectively. 

结果2-ARF8促进Dof2/15：In addition, we show that the auxin response factor PavARF8, the expression of whose encoding gene is repressed by ABA, activates PavDof2/15 transcription. 

结果3-ARF8和Dof介导ABA的合成：Furthermore, PavDof2/6/15 and PavARF8 directly bind to the 9-cis-epoxycarotenoid dioxygenase 1 (PavNCED1) promoter and regulate its expression, forming a feedback mechanism for ABA-mediated fruit softening. 

结论：These findings unveil the physiological framework of fruit softening and establish a direct functional link between the ABA–PavARF8–PavDofs module and cell-wall-modifying genes in mediating fruit softening.

Proposed model explaining the function of PavDof2/6/15 in regulating fruit softening in sweet cherry.

软化是果实成熟过程中的关键步骤，由多种植物激素相互作用来共同调节。脱落酸（ABA）和生长素的拮抗作用决定了果实的成熟和软化速度。然而，能够整合ABA和生长素信号以调节果实软化的转录因子仍有待确定。本文中，作者通过对两个果实硬度不同的甜樱桃的比较转录组分析，鉴定了ABA促进果实软化所必需几个Dof转录因子。作者发现，PavDof6直接结合到细胞壁修饰酶编码基因的启动子上以激活其转录，而PavDof2/15则直接抑制该基因的转录。在甜樱桃果实中瞬时过表达PavDof6和PavDof2/15会分别导致过早和推迟软化。此外，作者还发现其基因转录受ABA抑制的生长素响应因子PavARF8能够激活PavDof2/15的转录。再者，PavDof2/6/15和PavARF8能够直接结合PavNCED1基因的启动子上并调节其表达，从而形成了一个ABA介导的果实软化的反馈调节机制。本文的研究结果揭示了果实软化的生理框架，并在ABA–PavARF8–PavDofs模块和细胞壁修饰基因之间建立了直接的功能联系，以介导甜樱桃的果实软化。