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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: High‐temperature stress results in protein misfolding/unfolding and subsequently promotes the accumulation of cytotoxic protein aggregates that can compromise cell survival. Heat shock proteins (HSPs) function as molecular chaperones that coordinate the refolding and degradation of aggregated proteins to mitigate the detrimental effects of high temperatures. However, the relationship between HSPs and protein aggregates in apples under high temperatures remains unclear. Here, we show that an apple (Malus domestica) chloroplast‐localized, heat‐sensitive elongation factor Tu (MdEF‐Tu), positively regulates apple thermotolerance when it is overexpressed. Transgenic apple plants exhibited higher photosynthetic capacity and better integrity of chloroplasts during heat stress. Under high temperatures, MdEF‐Tu formed insoluble aggregates accompanied by ubiquitination modifications. Furthermore, we identified a chaperone heat shock protein (MdHsp70), as an interacting protein of MdEF‐Tu. Moreover, we observed obviously elevated MdHsp70 levels in 35S: MdEF‐Tu apple plants that prevented the accumulation of ubiquitinated MdEF‐Tu aggregates, which positively contributes to the thermotolerance of the transgenic plants. Overall, our results provide new insights into the molecular chaperone function of MdHsp70, which mediates the homeostasis of thermosensitive proteins under high temperatures. Significance Statement: Our results provide new insights into the connections between molecular chaperones and denatured protein aggregates, which is required to mediate the homeostasis of thermosensitive proteins under high temperature. [ABSTRACT FROM AUTHOR]

SUMMARY: Nitrogen (N) is an essential nutrient for crop growth and development, significantly influencing both yield and quality. Melatonin (MT), a known enhancer of abiotic stress tolerance, has been extensively studied. However, its relationship with nutrient stress, particularly N deficiency, and the underlying regulatory mechanisms of MT on N absorption remain unclear. In this study, exogenous MT treatment was found to improve the tolerance of apple plants to N deficiency. Apple plants overexpressing the MT biosynthetic gene N‐acetylserotonin methyltransferase 9 (MdASMT9) were used to further investigate the effects of endogenous MT on low‐N stress. Overexpression of MdASMT9 improved the light harvesting and heat transfer capability of apple plants, thereby mitigating the detrimental effects of N deficiency on the photosynthetic system. Proteomic and physiological data analyses indicated that MdASMT9 overexpression enhanced the trichloroacetic acid cycle and positively modulated amino acid metabolism to counteract N‐deficiency stress. Additionally, both exogenous and endogenous MT promoted the transcription of MdHY5, which in turn bound to the MdNRT2.1 and MdNRT2.4 promoters and activated their expression. Notably, MT‐mediated promotion of MdNRT2.1 and MdNRT2.4 expression through regulating MdHY5, ultimately enhancing N absorption. Taken together, these findings shed light on the association between MdASMT9‐mediated MT biosynthesis and N absorption in apple plants under N‐deficiency conditions. Significance Statement: In this study, both exogenous and endogenous melatonin improved the tolerance of apple plants to N deficiency. Melatonin promoted the expression of MdNRT2.1 and MdNRT2.4 through regulating MdHY5, thereby improving nitrate absorption. [ABSTRACT FROM AUTHOR]

SUMMARY: Sorbitol is a critical photosynthate and storage substance in the Rosaceae family. Sorbitol transporters (SOTs) play a vital role in facilitating sorbitol allocation from source to sink organs and sugar accumulation in sink organs. While prior research has addressed gene duplications within the SOT gene family in Rosaceae, the precise origin and evolutionary dynamics of these duplications remain unclear, largely due to the complicated interplay of whole genome duplications and tandem duplications. Here, we investigated the synteny relationships among all identified Polyol/Monosaccharide Transporter (PLT) genes in 61 angiosperm genomes and SOT genes in representative genomes within the Rosaceae family. By integrating phylogenetic analyses, we elucidated the lineage‐specific expansion and syntenic conservation of PLTs and SOTs across diverse plant lineages. We found that Rosaceae SOTs, as PLT family members, originated from a pair of tandemly duplicated PLT genes within Class III‐A. Furthermore, our investigation highlights the role of lineage‐specific and synergistic duplications in Amygdaloideae in contributing to the expansion of SOTs in Rosaceae plants. Collectively, our findings provide insights into the genomic origins, duplication events, and subsequent divergence of SOT gene family members. Such insights lay a crucial foundation for comprehensive functional characterizations in future studies. Significance Statement: We found that Rosaceae SOTs, as PLT family members, originated from a pair of tandemly duplicated PLT genes within Class III‐A. Furthermore, our investigation highlights the role of lineage‐specific and synergistic duplications in Amygdaloideae in contributing to the expansion of SOTs in Rosaceae plants. [ABSTRACT FROM AUTHOR]

SUMMARY: Salt stress adversely affects the yield and quality of crops and limits their geographical distribution. Studying the functions and regulatory mechanisms of key genes in the salt stress response is important for breeding crops with enhanced stress resistance. Autophagy plays an important role in modulating the tolerance of plants to various types of abiotic stressors. However, the mechanisms underlying salt‐induced autophagy are largely unknown. Cation/Ca2+ exchanger proteins enhance apple salt tolerance by inhibiting Na+ accumulation but the mechanism underlying the response to salt stress remains unclear. Here, we show that the autophagy‐related gene MdATG18a modulated apple salt tolerance. Under salt stress, the autophagic activity, proline content, and antioxidant enzyme activities were higher and Na+ accumulation was lower in MdATG18a‐overexpressing transgenic plants than in control plants. The use of an autophagy inhibitor during the salt treatment demonstrated that the regulatory function of MdATG18a depended on autophagy. The yeast‐one‐hybrid assay revealed that the homeodomain‐leucine zipper (HD‐Zip) transcription factor MdHB7‐like directly bound to the MdATG18a promoter. Transcriptional regulation and genetic analyses showed that MdHB7‐like enhanced salt‐induced autophagic activity by promoting MdATG18a expression. The analysis of Na+ efflux rate in transgenic yeast indicated that MdCCX1 expression significantly promoted Na+ efflux. Promoter binding, transcriptional regulation, and genetic analyses showed that MdHB7‐like promoted Na+ efflux and apple salt tolerance by directly promoting MdCCX1 expression, which was independent of the autophagy pathway. Overall, our findings provide insight into the mechanism underlying MdHB7‐like‐mediated salt tolerance in apple through the MdHB7‐like‐MdATG18a and MdHB7‐like‐MdCCX1 modules. These results will aid future studies on the mechanisms underlying stress‐induced autophagy and the regulation of stress tolerance in plants. [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]