Identification and Expression Profiling of the HSP70 Gene Family in Melon Under Combined High Temperature and Humidity Stress

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Background/Objectives: Heat-shock protein 70 is a key plant molecular chaperone whose family response to combined high temperature and humidity stress in melon remains uncharacterized. This study aimed to characterize the CmHSP70 family genome-wide and compare its expression dynamics between cultivars with contrasting responses to HTH stress. Methods: The CmHSP70 genes in melon were identified via bioinformatic analysis, with characterization covering phylogeny, gene structure, cis-elements, synteny, and selection pressure. Expression profiles under HTH stress were compared between the tolerant cultivar JD versus the sensitive cultivar JTL across 0 to 3 days using transcriptome with qRT-PCR validation, supplemented by three-dimensional (3D) structure prediction plus protein interaction network analysis. Results: The 16 CmHSP70 proteins were predicted to show diverse subcellular localizations, predominantly in the cytoplasm, endoplasmic reticulum (ER), and chloroplasts, and were grouped into five subfamilies with closest homology to cucurbit crops. Cis-acting element analysis showed that the promoters of CmHSP70 genes contain various stress-responsive and hormone-responsive elements, implying their broad roles in stress adaptation. Gene duplication analysis revealed only one segmental duplication event and no tandem duplications in the CmHSP70 family, indicating limited expansion during melon evolution. Ka/Ks ratios below 1 for all paralogous pairs further suggest strong purifying selection and highly conserved protein functions. Furthermore, expression profiling under high temperature and humidity (HTH) stress identified CmHSP70-4, -7, -8, and -13 as key candidates, showing sustained upregulation through day 3 in the tolerant cultivar but transient induction followed by decline in the sensitive cultivar. Predicted 3D structures and a STRING-based protein–protein interaction network further highlighted these four genes within the predicted chaperone interaction framework. Conclusions: This study provides candidate gene resources for future functional validation and breeding of HTH-tolerant melon and other cucurbit crops. The association between sustained CmHSP70 induction and the tolerant cultivar response offers a working hypothesis for molecular breeding under climate change.