Antifungal activity and mechanisms of AgNPs and their combination with azoxystrobin against Magnaporthe oryzae

Abstract

Silver nanoparticles (AgNPs) exhibit a broad spectrum of antimicrobial activity against various fungal pathogens, including the devastating rice blast fungus Magnaporthe oryzae, which threatens rice production worldwide. However, the antifungal molecular mechanisms of AgNPs against M. oryzae are still poorly understood. Here, it was found that the mycelial growth and virulence of M. oryzae strains were significantly reduced by AgNPs. Two nm-AgNPs at an EC₅₀ concentration of 2.0 μg mL⁻¹ almost impaired 100% virulence of M. oryzae. Further studies revealed that AgNP treatment changes cell wall morphology and suppresses appressorium formation. The defects in appressorium development upon AgNP treatment were caused by the reduced phosphorylation level of the MAPK MoPmk1 and impaired conidial autophagy. Importantly, the combination of AgNPs and azoxystrobin at appropriate ratios enhanced fungitoxicity to azoxystrobin-sensitive/resistant M. oryzae strains. Due to the excellent control effect of AgNPs on rice blast, we established a method for the biosynthesis of AgNPs by using the biocontrol Bacillus strain Tu27 to reduce waste environmental pollution and synthesis costs. Infection assays showed that the biosynthesized AgNPs displayed similar fungal toxicity. Taken together, our results revealed that AgNPs display the antifungal activity by a unique mechanism of damaging cell wall integrity, reducing phosphorylation of the MAPK MoPmk1, and impairing autophagy, and have an additive synergy with azoxystrobin on M. oryzae.