Mechanistic insight into the anti-alternaria activity of bimetallic zinc oxide and silver/zinc oxide nanoparticles

Alternaria alternata is an opportunistic phytopathogen that negatively impact the growth and production of a wide variety of host plants. In this study, we evaluated the antifungal potential of biogenic ZnO, and bimetallic silver and zinc oxide (Ag/ZnO) nanoparticles synthesized using seed extract of Abrus precatorious and characterized using different analytical tools. In vitro antifungal potentials of ZnO and Ag/ZnO nanoparticles were carried out using the food poison technique. Morphological and ultrastructure of the A. alternata treated with the nanoparticles were carried out using high resolution scanning and transmission electron microscopy (HRSEM and HRTEM). In addition, changes in polysaccharide production, chitin content and enzymatic (cellulase and lipase) activities of A. alternata were assayed. Double peak signifying a UVmax of 353.88 and 417.25 nm representing Ag and ZnO respectively was formed in the bimetallic nanoparticles. HRSEM and HRTEM results shows agglomerated nanoparticles with particle and crystallite size of 23.94 and 16.84 nm for ZnO nanoparticles, 35.12 and 28.99 nm for Ag/ZnO nanoparticles respectively. In vitro antifungal assay shows a significant concentration-dependent inhibition (p < 0.05) of A. alternata mycelia with highest percentage inhibition of 73.93 % (ZnO nanoparticles) and 68.26 % (Ag/ZnO nanoparticles) at 200 ppm. HRSEM and HRTEM micrographs of the treated A. alternata mycelia shows alteration of the cellular structure, clearance of the cytoplasmic organelles and localization of the nanoparticles within the cell. A. alternata treated with 200 ppm nanoparticles show a significant decrease (p < 0.05) in the polysaccharides and chitin contents, cellulase and lipase activities. The results suggests that ZnO and Ag/ZnO nanoparticles mode of action may be via alteration of the fungal cell wall through the inhibition of polysaccharides, chitin, cellulases and lipases synthesis. ZnO and Ag/ZnO nanoparticles may be a promising tool for the management and control of disease causing fungal phytopathogens.