Structural switching of Arabidopsis QSOX1 defines stress-dependent chaperone and defense functions.

Quiescin sulfhydryl oxidase 1 (QSOX1) is a redox-responsive enzyme in Arabidopsis thaliana, previously characterized for its chaperone function under abiotic stress. Its role in pathogen defense, however, has remained unclear. Here, we show that recombinant QSOX1 exhibits structure-dependent antifungal activity. The low molecular weight (LMW) form suppressed the growth of diverse fungal pathogens at concentrations of 20-40 μg/mL, whereas the high molecular weight (HMW) form displayed little or no inhibitory effect. Confocal and scanning electron microscopy revealed that LMW-QSOX1 disrupts fungal membranes and induces hyphal fragmentation, while HMW assemblies retain only holdase chaperone activity. These findings indicate that QSOX1 undergoes stress-induced structural switching, with LMW species acting as antifungal effectors and HMW complexes functioning in protein stabilization. Our results show that QSOX1 integrates protein quality control and pathogen resistance, enabling plants to respond effectively to changing environmental conditions.