The SnRK2.3-AREB1-TST1/2 cascade activated by cytosolic glucose regulates sugar accumulation across tonoplasts in apple and tomato

Soluble sugars are the core components of fruit quality, and the degree of sugar accumulation is largely determined by tonoplast-localized sugar transporters. We previously showed that two classes of tonoplast sugar transporters, MdERDL6 and MdTST1/2, coordinately regulate sugar accumulation in vacuoles. However, the mechanism underlying this coordination remains unknown. Here we discovered that two transcription factors, MdAREB1.1/1.2, regulate MdTST1/2 expression by binding their promoters in apple. The enhanced MdAREB1.1/1.2 expression in MdERDL6-1-overexpression plants resulted in an increase in MdTST1/2 expression and sugar concentration. Further studies established that MdSnRK2.3, whose expression could be regulated by expressing MdERDL6-1, could interact with and phosphorylate MdAREB1.1/1.2, thereby promoting the MdAREB1.1/1.2-mediated transcriptional activation of MdTST1/2. Finally, the orthologous SlAREB1.2 and SlSnRK2.3 exhibited similar functions in tomato fruit as in their apple counterparts. Together, our findings provide insights into the regulatory mechanism of tonoplast sugar transport exerted by SnRK2.3-AREB1-TST1/2 for fruit sugar accumulation.