Salicylic acid-doped iron nano-biostimulants potentiate defense responses and suppress Fusarium wilt in watermelon

Chemo- and bio-genic metallic nanoparticles (NPs), as a novel nano-enabled strategy, have demonstrated a great potential in crop health management. The current study aimed to explore the efficacy of advanced nanocomposites (NCs), integrating biogenic (bio) metallic NPs and plant immunity-regulating hormones, in crop disease control. Bio-iron (Fe) NPs were biosynthesized using cell-free supernatant of a Fe-resistant strains, Bacillus marisflavi ZJ-4. Further, salicylic acid-coated bio-FeNPs (SI) NCs were prepared via co-precipitation method under alkaline conditions. Both bio-FeNPs and SINCs were characterized using basic analytical techniques, including Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction analysis, and scanning/transmission electron microscopy. Bio-FeNPs and SINCs had variable shapes with average sizes of 72.35 nm and 65.87 nm, respectively. Under greenhouse conditions, bio-FeNPs and SINCs improved the agronomic traits of the watermelon plants and SINCs outperformed bio-FeNPs, providing the maximum growth promotion of 32.5%. Soil-drenching with bio-FeNPs and SINCs suppressed Fusarium oxysporum f. sp. niveum-caused Fusarium wilt in watermelon, and SINCs provided better protection than bio-FeNPs, through inhibiting the fungal invasive growth within host plants. SINCs improved the antioxidative capacity and primed a systemic acquired resistance (SAR) response via activating the expression of salicylic acid signaling pathway genes. These findings indicate that SINCs can reduce the severity of Fusarium wilt in watermelon by modulating antioxidative capacity and potentiating SAR to restrict in planta fungal invasive growth. This study provides new insights into the potential of bio-FeNPs and SINCs as biostimulants and bioprotectants for growth promotion and Fusarium wilt suppression, ensuring sustainable watermelon production.