Exogenous methyl jasmonate promotes the graft union healing process of melon grafted onto squash and enhances low-temperature tolerance of grafted melon seedlings by activating the transcription factors CmWRKY23 and CmWRKY69

The involvement of plant hormones in the graft union healing process is significant, yet how exogenous MeJA affects graft union healing and stress tolerance in cucurbit seedlings remains poorly understood at the molecular level. Exogenous MeJA treatment significantly accelerated the healing process of oriental melon grafted onto squash rootstocks. This treatment led to increased expression of CmWRKY23, CmWRKY69, and the cell adhesion-related gene CmGH9B3 at the graft union, providing evidence for the participation of the CmWRKY23/CmWRKY69-CmGH9B3 module in the MeJA-facilitated graft union-healing process. Furthermore, CmWRKY23 and CmWRKY69 function as transcriptional activators by directly binding to the promoter of CmAOC2, a key jasmonic acid (JA) biosynthesis gene, thereby upregulating CmAOC2 expression. Knockout mutants of CmWRKY23 or CmWRKY69 significantly decreased the low-temperature tolerance of grafted oriental melon seedlings, accompanied by decreased antioxidant enzyme (SOD, POD, CAT) activity, downregulated CmAOC2 expression, and reduced endogenous JA levels. These findings further supported the role of this transcriptional module in regulating low-temperature tolerance through the JA signaling pathway. Overall, our work established that exogenous MeJA promoted graft union healing and enhanced the low-temperature tolerance in grafted oriental melon seedlings. This coordination was achieved via the CmWRKY23/CmWRKY69-CmGH9B3 module for graft union healing and a CmWRKY23/CmWRKY69-CmAOC2 positive feedback loop for low-temperature adaptation. This study revealed a dual regulatory mechanism by which MeJA coordinated both graft union healing and low-temperature resistance, providing new theoretical foundations and practical targets for improving the quality and environmental adaptability of grafted oriental melon seedlings.