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 (MdERDL6–1, MdERDL6–2, MdTST1 and MdTST2) showed higher expression under drought, suggesting their roles in response to drought stress. Promoter cis-elements analyses, yeast one-hybrid and dual-luciferase tests confirmed that the drought-induced transcription factor MdDREB2A could promote the expression of MdERDL6–1/−2 and MdTST1/2 by binding to their promoter regions. Moreover, overexpressing of each of these four MdVSTs alone in transgenic apple or Arabidopsis plants accumulated more soluble sugars and abscisic acid, and enhanced drought resistance. Furthermore, apple plants overexpressing MdERDL6–1 also showed reduced water potential, facilitated stomatal closure and reactive oxygen species scavenging under drought condition 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.

中文翻译：

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MdDREB2A 调控的苹果液泡糖转运蛋白通过促进可溶性糖积累和激活 ABA 信号传导增强抗旱性


可溶性糖不仅是水果品质的重要贡献者，而且还可以作为应对非生物胁迫的渗透调节剂。早期干旱胁迫促进糖积累，而特定糖转运蛋白控制糖的细胞分布。在这里，我们发现苹果幼苗在干旱胁迫下在叶子组织中积累可溶性糖。胁迫和对照植株的转录谱揭示了几种质膜或液泡膜定位的糖转运蛋白基因的差异表达。其中，四个先前鉴定的液泡糖转运蛋白（VST）基因（MdERDL6-1、MdERDL6-2、MdTST1和MdTST2）在干旱条件下表现出较高的表达，表明它们在应对干旱胁迫中的作用。启动子顺式元件分析、酵母单杂交和双荧光素酶测试证实，干旱诱导的转录因子MdDREB2A可以通过与MdERDL6–1/−2和MdTST1/2的启动子区域结合来促进MdERDL6–1/−2和MdTST1/2的表达。此外，在转基因苹果或拟南芥植物中单独过度表达这四种MdVST可以积累更多的可溶性糖和脱落酸，并增强抗旱性。此外，与对照植物相比，过表达 MdERDL6-1 的苹果植物在干旱条件下还表现出水势降低、促进气孔关闭和活性氧清除。总的来说，我们的结果提出了一种通过操纵参与液泡糖运输的基因来增强水果的抗旱性和糖积累的潜在策略。