Targeted Inhibition of Photosystem II Electron Transport Using Bioherbicide-Loaded Ultrasmall Nanodevices

Usnic acid (UA) is a promising bioherbicide with a mode of action targeting photosystem II (PSII) inhibition. This study investigates the enhancement of UA's herbicidal efficacy through a novel nanoformulation using ultrasmall superparamagnetic iron oxide nanoparticles (USPIONs) as a smart delivery system. USPIONs presenting a sub-10 nm mean particle diameter were synthesized and thoroughly characterized for agricultural applications, with the objective of improving UA delivery and achieving controlled release. The basal release kinetic results revealed that c.a. 1086 min were required to release 50% of the UA release (t_50%) and when nanoparticle solution was exposed to an external alternating magnetic field (AMF) exposure, the time to 50% UA release (t50%) was about 41.03 min. In vivo chlorophyll fluorescence analysis revealed that the nanoenabled formulation enhanced PSII inhibition, enhancing suppression of electron flow at the quinone A (Q_A) to quinone B (Q_B) interface. The uncapped and oleic acid-capped USPIONs exhibited reduced Fv/Fm values, to 18.93% and 27.34%, respectively, compared to free usnic acid. Furthermore, gene expression analysis showed a 2.5-fold upregulation in the photosynthetic genes psbA and petA, compared to that in untreated control plants, indicating a robust physiological response. Enzyme assays demonstrated an upregulation in activities of superoxide dismutase and catalase (SOD, CAT) in treated lettuce leaves, underscoring the induction of oxidative stress. Molecular docking simulations highlighted the preferential binding of UA within the Q_B-binding domain, suggesting a strong interaction potential at the catalytic site. Additionally, USPIONs were predicted to interact near the center of the D1 protein. These findings indicate that USPIONs enhance the PSII-inhibitory action of UA relative to its nonloaded form, supporting their feasibility as targeted bioherbicide carriers pending broader agronomic and environmental validation.