MdAP2-like, a new regulator in apple, simultaneously modulates fruit softening and size

Abstract

The texture of apples significantly affects fruit quality, storability, and consumer preferences. Several studies have revealed the mechanisms underlying fruit softening in other fruit species; however, studies on the regulatory mechanisms of apple fruit softening are limited. In this study, we characterized an ethylene response factor (ERF) transcription factor, MdAP2-like, which is associated with apple fruit softening. The expression of MdAP2-like was highly induced by ethylene treatment but inhibited by 1-methylcyclopropene treatment. Further experiments indicated that MdAP2-like could trans-activate and directly bind to the promoter of Mdβ-GAL18, which participates in pectin degradation. Additionally, the transient overexpression or silencing of MdAP2-like in apple fruit correspondingly increased or reduced ethylene production and promoted or delayed the decrease in firmness and fruit softening. Overexpressed MdAP2-like in the calli of apple fruit upregulated the transcript levels of Mdβ-GAL18, promoted the β-galactosidase (β-Gal) activity, and increased the content of free galactose. Moreover, the ectopic expression of MdAP2-like in tomato fruit increased the ethylene production and β-Gal activity, reduced the fruit firmness, and induced the expression levels of key genes associated with ethylene biosynthesis, carotenoid biosynthesis, and cell wall modification. Notably, the overexpression of MdAP2-like in tomato accelerated fruit ripening and softening, as well as shortened fruit size. Collectively, these results reveal that MdAP2-like is a new transcriptional regulator related to the softening of apple fruit by activating the expression of Mdβ-GAL18. This study provides a valuable genetic resource that can simultaneously manipulate the texture and size of apples.