Plant-engineered ZnO and CuO nanoparticles exhibit pesticidal activity and mitigate Fusarium infestation in soybean: A mechanistic understanding

Herein, CuO and ZnO nanoparticles (NPs) were biogenically synthesized using plant (Artemisia vulgaris) extracts. The biogenic NPs were subsequently evaluated in vitro for antifungal activity (200 mg/L) against Fusarium virguliforme (FV; the cause of soybean sudden death), and for crop protection (200-500 mg/L) in FV-infested soybean. ZnONPs exhibited 3.8-, 2.5-, and 4.9 -fold greater in vitro antifungal activity, compared to Zn or Cu acetate salt, the Artemisia extract, and a commercial fungicide (Medalion Fludioxon), respectively. The corresponding CuONP values were 1.2-, 1.0-, and 2.2 -fold, respectively. Scanning electron microscopy (SEM) revealed significant morpho-anatomical damage to fungal mycelia and conidia. NP-treated FV lost their hyphal turgidity and uniformity and appeared structurally compromised. ZnONP caused shriveled and broken mycelia lacking conidia, while CuONP caused collapsed mycelia with shriveled and disfigured conidia. In soybean, 200 mg/L of both NPs enhanced growth by 13%, compared to diseased controls, in both soil and foliar exposures. Leaf SEM showed fungal colonization of different infection sites, including the glandular trichome, palisade parenchyma, and vasculature. Foliar application of ZnONP resulted in the deposition of particulate ZnO on the leaf surface and stomatal interiors, likely leading to particle and ion entry via several pathways, including ion diffusion across the cuticle/stomata. SEM also suggested that ZnO/CuO NPs trigger structural reinforcement and anatomical defense responses in both leaves and roots against fungal infection. Collectively, these findings provide important insights into novel and effective mechanisms of crop protection against fungal pathogens by plant-engineered metal oxide nanoparticles, thereby contributing to the sustainability of nano-enabled agriculture.