Apple vacuolar sugar transporters regulated by MdDREB2A enhance drought resistance by promoting accumulation of soluble sugars and activating ABA signaling

Soluble sugars are not only an important contributor to fruit quality, but also serve as the osmotic regulators in response to abiotic stresses. Early drought stress promotes sugar accumulation, while specific sugar transporters govern the cellular distribution of the sugars. Here, we show that apple plantlets accumulate soluble sugars in leaf tissues under drought stress. Transcriptional profiling of stressed and control plantlets revealed differential expression of several plasma membrane-or vacuolar membrane-localized sugar transporter genes. Among these, four previously identified vacuolar sugar transporter (VST) genes (<i>MdERDL6-1</i>, <i>MdERDL6-2</i>, <i>MdTST1</i>, and <i>MdTST2</i>) showed higher expression under drought, suggesting their roles in response to drought stress. Promoter <i>cis</i>-elements analyses, yeast one-hybrid, and dual-luciferase tests confirmed that the drought-induced transcription factor MdDREB2A could promote the expression of <i>MdERDL6-1/-2</i> and <i>MdTST1/2</i> by binding to their promoter regions. Moreover, overexpressing of each of these four <i>MdVSTs</i> alone in transgenic apple or <i>Arabidopsis</i> plants accumulated more soluble sugars and abscisic acid (ABA), and enhanced drought resistance. Furthermore, apple plants overexpressing <i>MdERDL6-1</i> also showed reduced water potential, facilitated stomatal closure, and reactive oxygen species scavenging under drought conditions compared to control plants. Overall, our results suggest a potential strategy to enhance drought resistance and sugar accumulation in fruits through manipulating the genes involved in vacuolar sugar transport.