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Both sorbitol and sucrose are synthesized in source leaves and transported to fruit for supporting fruit growth in tree fruit species of the Rosaceae family. In apple (Malus domestica), antisense suppression of aldose-6-phosphate reductase, the key enzyme for sorbitol synthesis, significantly decreased the sorbitol concentration but increased the sucrose concentration in leaves, leading to a lower sorbitol but a higher sucrose supply to fruit in these plants. In response to this altered carbon supply, the transgenic fruit had lower concentration of sorbitol and much higher concentration of glucose but similar levels of fructose, sucrose, and starch throughout fruit development relative to the untransformed control. Activities of sorbitol dehydrogenase, fructokinase, and sucrose phosphate synthase were lower, whereas activities of neutral invertase, sucrose synthase, and hexokinase were higher in the transgenic fruit during fruit development. Transcript levels of MdSOT1, MdSDHs, MdFK2, and MdSPS3/6 were downregulated, whereas transcript levels of MdSUC1/4, MdSUSY1-3, MdNIV1/3, MdHKs, and MdTMT1 were upregulated in the transgenic fruit. These findings suggest that the Sucrose cycle and the sugar transport system are very effective in maintaining the level of fructose and provide insights into the roles of sorbitol and sucrose in regulating sugar metabolism and accumulation in sorbitol-synthesizing species.

Summary: In plants under drought stress, sugar content in roots increases, which is important for drought resistance. However, the molecular mechanisms for controlling the sugar content in roots during response to drought remain elusive. Here, we found that the MdDOF3‐MdHT1.2 module‐mediated glucose influx into the root is essential for drought resistance in apple (Malus × domestica). Drought induced glucose uptake from the rhizosphere and up‐regulated the transcription of hexose transporter MdHT1.2. Compared with the wild‐type plants, overexpression of MdHT1.2 promoted glucose uptake from the rhizosphere, thereby facilitating sugar accumulation in root and enhancing drought resistance, whereas silenced plants showed the opposite phenotype. Furthermore, ATAC‐seq, RNA‐seq and biochemical analysis demonstrated that MdDOF3 directly bound to the promoter of MdHT1.2 and was strongly up‐regulated under drought. Overexpression of MdDOF3 in roots improved MdHT1.2‐mediated glucose transport capacity and enhanced plant resistance to drought, but MdDOF3‐RNAihr apple plants showed the opposite phenotype. Moreover, overexpression of MdDOF3 in roots did not attenuate drought sensitivity in MdHT1.2‐RNAi plants, which was correlated with a lower glucose uptake capacity and glucose content in root. Collectively, our findings deciphered the molecular mechanism through which glucose uptake from the rhizosphere is mediated by MdDOF3‐MdHT1.2, which acts to modulate sugar content in root and promote drought resistance. [ABSTRACT FROM AUTHOR]

Summary: RAD5B belongs to the Rad5/16‐like group of the SNF2 family, which often functions in chromatin remodelling. However, whether RAD5B is involved in chromatin remodelling, histone modification, and drought stress tolerance is largely unclear. We identified a drought‐inducible chromatin remodeler, MdRAD5B, which positively regulates apple drought tolerance. Transposase‐accessible chromatin with high‐throughput sequencing (ATAC‐seq) analysis showed that MdRAD5B affects the expression of 466 drought‐responsive genes through its chromatin remodelling function in response to drought stress. In addition, MdRAD5B interacts with and degrades MdLHP1, a crucial regulator of histone H3 trimethylation at K27 (H3K27me3), through the ubiquitin‐independent 20S proteasome. Chromatin immunoprecipitation‐sequencing (ChIP‐seq) analysis revealed that MdRAD5B modulates the H3K27me3 deposition of 615 genes in response to drought stress. Genetic interaction analysis showed that MdRAD5B mediates the H3K27me3 deposition of drought‐responsive genes through MdLHP1, which causes their expression changes under drought stress. Our results unravelled a dual function of MdRAD5B in gene expression modulation in apple in response to drought, that is, via the regulation of chromatin remodelling and H3K27me3. [ABSTRACT FROM AUTHOR]

Summary: Understanding the molecular regulation of plant cold response is the basis for cold resistance germplasm improvement. Here, we revealed that the apple histone deacetylase MdHDA6 can perform histone deacetylation on cold‐negative regulator genes and repress their expression, leading to the positive regulation of cold tolerance in apples. Moreover, MdHDA6 directly interacts with the transcription factor MdTCP15. Phenotypic analysis of MdTCP15 transgenic apple lines and wild types reveals that MdTCP15 negatively regulates cold tolerance in apples. Furthermore, we found that MdHDA6 can facilitate histone deacetylation of MdTCP15 and repress the expression of MdTCP15, which positively contributes to cold tolerance in apples. Additionally, the transcription factor MdTCP15 can directly bind to the promoter of the cold‐negative regulator gene MdABI1 and activate its expression, and it can also directly bind to the promoter of the cold‐positive regulator gene MdCOR47 and repress its expression. However, the co‐expression of MdHDA6 and MdTCP15 can inhibit MdTCP15‐induced activation of MdABI1 and repression of MdCOR47, suggesting that MdHDA6 suppresses the transcriptional regulation of MdTCP15 on its downstream genes. Our results demonstrate that histone deacetylase MdHDA6 plays an antagonistic role in the regulation of MdTCP15‐induced transcriptional activation or repression to positively regulate cold tolerance in apples, revealing a new regulatory mechanism of plant cold response. [ABSTRACT FROM AUTHOR]