| Summary: | Abstract Microbial‐assisted biosynthesis of silver nanoparticles (AgNPs) has been developed rapidly in recent years, but their mechanism of synthesis and production are still unknown. This study investigated the roles of nitrate reductase produced by the fungus Trichoderma asperellum in the synthesis of AgNPs. The nitrate reductase activity is activated by nitrate and showed a positive correlation with the quantity of AgNPs, which may lead to the development of a fast bioprocess of AgNPs synthesis. Treatment of silver nitrate with the fungal supernatant containing nitrate reductase produced stable, monodispersed, and spherical AgNPs with an average size of 20 nm, which were characterized by UV–visible spectroscopy, scanning electron microscopy and X‐ray diffraction. SDS‐PAGE implied that nitrate reductases were involved in the synthesis of AgNPs. The antifungal activities of synthesized AgNPs solution were compared with those of silver nitrate solution and AgNPs‐free supernatant solution. The results showed that AgNPs solution exhibited the highest antifungal activity against several selected plant pathogens by the agar well diffusion method. This study firstly concluded that extracellular biosynthesis of AgNPs could be promoted by potassium nitrate in Trichoderma asperellum Q1 media and the AgNPs provided a potential enhancement of biocontrol activity derived from the fungi.
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