Root-Specific Overexpression of the CmDUF239-1 Gene Enhances Heat Tolerance in Melon Seedlings by Upregulating Antioxidant Enzymes Activities, Proline Content, and Expression of Heat Shock Protein-Related Genes

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.