The CmMAPK6-CmbHLH100 module regulates melon senescence by modulating iron homeostasis.

Leaf senescence is a finely regulated cellular process orchestrated by developmental programs and environmental factors. While mitogen-activated protein kinase (MAPK) cascades have been documented to participate in the regulation of leaf senescence, their functional involvement in this process in melon (Cucumis melo L.) remains largely uncharacterized. Here, we identified that CmMAPK6 acts as a positive regulator of leaf senescence in melon. Overexpression of CmMAPK6 led to a typical premature senescence phenotype, including leaf yellowing, decreased chlorophyll content, and chloroplast disintegration. Furthermore, the iron content in leaves of CmMAPK6-OE lines was significantly reduced, and the transcript levels of key iron uptake-related genes, CmFRO2 and CmIRT1, were significantly downregulated in these leaves. CmMAPK6 interacted with the iron homeostasis regulator CmbHLH100 and phosphorylated it at Thr-115 and Ser-159. Taken together, these findings reveal a novel CmMAPK6-CmbHLH100 regulatory module that regulates iron deficiency-induced leaf senescence in melon. The work provides a theoretical foundation for understanding the regulatory network of cucurbit crop senescence and developing strategies to delay the senescence process through nutrient homeostasis management.