CmDUF239-1 Improves the Salt Tolerance of Grafted Melon by Enhancing Antioxidant Capacity and Na+/K+ Homeostasis

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 <i>CmDUF239-1</i> gene as a critical factor contributing to improved salt tolerance in grafted melons. Functional studies demonstrated that knocking out <i>CmDUF239-1</i> reduces salt tolerance, reflected in decreased activities of antioxidant enzymes (SOD, POD, CAT) and diminished expression levels of related genes (<i>CmSOD1</i>, <i>CmPRX53-1</i>, <i>CmPRX53-2</i>, <i>CmCAT2</i>). Conversely, overexpression of <i>CmDUF239-1</i> leads to enhanced enzyme activity and gene expression, along with improved Na⁺/K⁺ homeostasis, evidenced by decreased Na⁺ accumulation and increased K⁺ absorption. Furthermore, <i>CmDUF239-1</i> overexpression upregulated Na⁺/K⁺ transport-related genes (<i>CmSOS1</i>, <i>CmNHX6</i>, <i>CmKUP3</i>, <i>CmSKOR</i>), whereas <i>CmDUF239-1</i> knockout had the opposite effect. These findings indicate that <i>CmDUF239-1</i> 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.