NO-dependent H2O2 signaling regulates melatonin-induced resistance to <i>Fusarium oxysporum</i> f. sp. <i>niveum</i> race 1 in watermelon

Fusarium wilt, caused by Fusarium oxysporum f. sp. niveum (FON), is a widespread and destructive fungal disease in watermelon. While melatonin helps plants defend against various pathogens, its role and mechanism in combating FON are not well understood. Here, we demonstrated that overexpressing the melatonin biosynthesis gene ClCOMT1 enhances watermelon (Citrullus lanatus) resistance to FON race 1 (FON1), accompanied by increased production of nitric oxide (NO) mediated by nitrate reductase 1 (ClNR1) and H2O2 mediated by respiratory burst oxidase homologue D (ClRBOHD). Conversely, knockout of ClCOMT1 exhibited contrasting effects compared with its overexpression. Deletion or silencing of ClNR1 and ClRBOHD abolished defense responses and resistance induced by exogenous melatonin or ClCOMT1 overexpression. However, exogenous SNP (a NO donor) or H2O2 restored FON1 resistance in ClCOMT1 knockout plants. Upon FON1 infection, exogenous SNP induced H2O2 production, whereas ClNR1 deletion eliminated melatonin-induced H2O2 generation. Furthermore, ClRBOHD deletion inhibited SNP-induced resistance, while H2O2 supplementation rescued resistance in ClNR1 deletion mutants. Overall, these findings indicate that NO-dependent H2O2 signaling is crucial for melatonin-induced watermelon defense responses and resistance to FON1. Given the growing emphasis on reducing pesticide use, this mechanism holds remarkable potential for the sustainable control of Fusarium wilt.