ClaDREB14 enhances the salt tolerance of watermelon by positively regulating the expression of ClaPOD6

Watermelon ( Citrullus lanatus ) is sensitive to salt stress. For breeding applications, it is of great significance to explore the genetic mechanism underlying salt tolerance in watermelon by analyzing the dehydration responsive element–binding (DREB) factor family members. However, they are rarely studied in watermelon. In this study, we identified ClaDREB gene family members in watermelon based on whole genome data; analyzed the physicochemical properties, evolution, and phylogeny; and studied their expression patterns under salt stress in two watermelon varieties with varying salt tolerance. In total, 57 DREB family members were identified in watermelon, and most of them were located in the nucleus. ClaDREBs were divided into six subgroups I–VI. The promoter region of ClaDREBs from subgroup II contained many defense-related and stress responsive elements. Among them, ClaDREB14 was significantly upregulated by salt stress and exhibited differential expression in salt-tolerant and salt-sensitive varieties. Moreover, overexpression of ClaDREB14 in watermelon roots significantly improved the salt tolerance of transgenic plants; mainly, it significantly increased the activities of POD, SOD, and CAT and significantly reduced MDA content. However, the results from gene-edited watermelon roots obtained using CRISPR/Cas9 vectors showed the opposite trend. Furthermore, we demonstrated that ClaDREB14 directly binds to the cis -acting element ACCGAC in the promoter region of ClaPOD6 and promotes its expression. Therefore, ClaDREB14 may enhance salt tolerance by increasing the activity of antioxidant enzymes in watermelon roots. This study provided valuable information on the DREB gene family in watermelon and laid the foundation for future functional validation and genetic engineering applications.