Members of the heat shock protein 20 (HSP20) family of proteins play an important role in responding to various forms of stress. Here, the expression of ClaHSP17.4 was induced by heat stress in watermelon. Then, a floral dipping approach was used to introduce the pCAMBIA1391b-GFP overexpression vector encoding the heat tolerance-related gene ClaHSP17.4 from watermelon into Arabidopsis thaliana, and we obtained ClaHSP17.4-overexpressing Arabidopsis plants. Under normal conditions, the phenotypes of transgenic and wild-type (WT) Arabidopsis plants were largely similar. Following exposure to heat stress, however, the germination rates (96%) of transgenic Arabidopsis plants at the germination stages were significantly higher than those of wild-type idopsis (17%). Specifically, the malondialdehyde (MDA) content of transgenic Arabidopsis was half that of the control group, while the activities of peroxidase (POD) and superoxide dismutase (SOD) were 1.25 times those of the control group after exposure to high temperatures for 12 h at the seedling stages. The proline content in ClaHSP17.4-overexpressing transgenic Arabidopsis increased by 17% compared to WT plants (* p < 0.05), while the soluble sugar content rose by 37% (* p < 0.05). These results suggest that ClaHSP17.4 overexpression indirectly improves the antioxidant capacity and osmotic regulatory capacity of Arabidopsis seedlings, leading to improved survival and greater heat tolerance. Meanwhile, the results of this study provide a reference for further research on the function of the ClHSP17.4 gene and lay a foundation for breeding heat-tolerant watermelon varieties and advancing our understanding of plant adaptation to environmental stress.

Members of the CCT gene family have been shown to play roles in photoperiodic flowering and environmental adaptation under a range of conditions. In this study, 29 CCT genes from watermelon were categorized into three distinct subfamilies. The ClCCT genes were systematically analyzed, focusing on their physicochemical properties, gene duplication, motifs, structural divergence, promoter regions, and collinearity with genes from other species. The responsiveness of these genes to abiotic stressors, hormone treatments, and photoperiod prolongation was also examined. Segmental duplication (gene amplification) significantly influenced the evolution of these genes, with most ClCCT genes containing light-, hormone-, and/or abiotic stimulus-responsive elements. In response to abiotic and hormonal stresses, 5 genes responded to cold, 1 gene to drought, and 4 genes to salt. 6 genes were responsive to ABA, and 13 genes to MeJA. Strikingly, ClCCT17, ClCCT4, and ClCCT28 responded to multiple stressors. A majority of these ClCCT genes and their homologs in other species were responsive to prolonged daylight exposure. The varying expression patterns of these genes suggested a key role for watermelon CCT genes in the regulation of both abiotic stress responses and flowering. Functional diversity was also evident among CCT family genes within a given species as well as among species attributable to changes in the structural features and expression patterns of the genes and the encoded proteins.