Salt stress poses a substantial challenge to melon cultivation, but grafting techniques have shown promise in enhancing salt tolerance. This study aims to identify key genes involved in salt tolerance within melon rootstocks. The salt tolerance of four melon cultivars was evaluated, revealing that ‘ST2’ exhibited salt sensitivity, whereas ‘XZM17’ demonstrated salt tolerance. Grafting experiments indicated that salt-sensitive melons benefit significantly from being grafted onto salt-tolerant rootstocks. Transcriptome analysis further identified the CmDUF239-1 gene as a critical factor contributing to improved salt tolerance in grafted melons. Functional studies demonstrated that knocking out CmDUF239-1 reduces salt tolerance, reflected in decreased activities of antioxidant enzymes (SOD, POD, CAT) and diminished expression levels of related genes (CmSOD1, CmPRX53-1, CmPRX53-2, CmCAT2). Conversely, overexpression of CmDUF239-1 leads to enhanced enzyme activity and gene expression, along with improved Na+/K+ homeostasis, evidenced by decreased Na+ accumulation and increased K+ absorption. Furthermore, CmDUF239-1 overexpression upregulated Na+/K+ transport-related genes (CmSOS1, CmNHX6, CmKUP3, CmSKOR), whereas CmDUF239-1 knockout had the opposite effect. These findings indicate that CmDUF239-1 plays a dual role in promoting salt tolerance by regulating antioxidant defenses and ion transport, contributing to our understanding of the molecular mechanisms behind grafting-induced salt tolerance and providing insights for the breeding of resilient melon varieties.

High temperature stress is a critical factor affecting the growth and yield of melons (Cucumis melo L.), and improving heat tolerance is therefore crucial for stable production. While the overexpression of the CmDUF239-1 gene is known to improve salt tolerance in melons, its impact on heat tolerance remains unexplored. The role of the CmDUF239-1 gene in enhancing heat tolerance and its underlying mechanisms was investigated in this study. Melon seedlings overexpressing CmDUF239-1 (OEDUF239-1), generated via root transformation, exhibited significantly lower reductions in fresh and dry mass under heat stress compared to controls, indicating enhanced heat tolerance. One day post-stress, antioxidant enzyme activities (SOD, POD, CAT, APX, and GR) increased significantly in OEDUF239-1, while malondialdehyde (MDA) levels decreased. Additionally, proline content and the activity of its synthesizing enzyme (P5CS) rose, whereas the activity of proline dehydrogenase (ProDH) dropped. Transcriptomic and qPCR analyses revealed that CmDUF239-1 overexpression upregulated antioxidant enzyme-related genes (e.g., CmCSD1, CmPOD1) and proline-related genes (e.g., CmP5CS), as well as Heat Shock Protein (HSP) genes (e.g., CmHSP17.6II, CmHSP18.2). In summary, the enhancement of heat tolerance in melon by the CmDUF239-1 gene was mediated through the upregulation of genes involved in antioxidant defense and proline metabolism, together with increased accumulation of HSPs, providing a mechanistic basis for heat-resilient breeding programs.

Low-temperature stress is a major factor that limits the productivity and geographical distribution of melon (Cucumis melo L.). This study elucidates that CmDUF239-1 is a positive regulator of cold stress, and its underlying mechanisms are investigated using root-specific overexpression lines. Seedlings overexpressing CmDUF239-1 exhibited increased biomass and reduced relative electrical conductivity under cold stress. CmDUF239-1 overexpression promoted the accumulation of soluble sugar and proline, which was accompanied by enhanced activity of the proline biosynthetic enzyme Δ¹-pyrroline-5-carboxylate synthase (P5CS) and suppressed activity of the proline-degrading enzyme proline dehydrogenase (PDH). Molecular analysis revealed that CmDUF239-1 overexpression upregulated antioxidant enzyme-related genes, sugar metabolism related genes, and proline-related genes. Furthermore, it activated key genes in the ICE-CBF-COR signaling pathway, including CmCBF1, CmCBF2, and the downstream effector gene CmCOR413-2. In conclusion, the CmDUF239-1 gene enhances melon cold tolerance by modulating antioxidant defense, enhancing osmolyte (sugar and proline) metabolism and activating a core signaling pathway. This study not only characterizes a novel function for a DUF family gene but also provides a promising candidate gene for the genetic improvement of cold resilience in melon and other related crops.