Abscisic Acid and Regulation of the Sugar Transporter Gene MdSWEET9b Promote Apple Sugar Accumulation.

Enhancing fruit sugar contents, especially for high-flavonoid apples with a sour taste, is one of the main goals of horticultural crop breeders. This study analyzed sugar accumulation and the underlying mechanisms in the F2 progenies of a hybridization between the high-sugar apple (Malus × domestica) variety 'Gala' and high-flavonoid apple germplasm 'CSR6R6'. We revealed that MdSWEET9b (sugars will eventually be exported transporter) helps mediate sugar accumulation in fruits. Functional characterization of MdSWEET9b in yeast mutants lacking sugar transport as well as in overexpressing and CRISPR/Cas9 knockdown apple calli revealed MdSWEET9b could transport sucrose specifically, ultimately promoting normal yeast growth and accumulation of total sugar contents. Moreover, MdWRKY9 bound to the MdSWEET9b promoter and regulated its activity, which responded to abscisic acid (ABA) signaling. Furthermore, MdWRKY9 interacted with MdbZIP23 (basic leucine zipper) and MdbZIP46, key ABA signal transducers, at the protein and DNA levels to enhance its regulatory effect on MdSWEET9b expression, thereby influencing sugar accumulation. Based on the contents of ABA in lines with differing sugar contents and the effects of ABA treatments on fruits and calli, we revealed ABA as one of the main factors responsible for the diversity in apple fruit sugar content. The results of this study have clarified how MdSWEET9b influences fruit sugar accumulation, while also further elucidating the regulatory effects of the ABA signaling network on fruit sugar accumulation. This work provides a basis for future explorations of the cross-talk between hormone and sugar metabolism pathways.