Hydrogen peroxide as a multifaceted regulator of the Atg4 protease and autophagy in the pathogenic fungus Alternaria alternata.

Introduction: Autophagy-related protease AaAtg4 was previously identified as a key regulator in the pathogenic fungus Alternaria alternata, orchestrating a complex interplay among autophagy, oxidative stress resistance, iron homeostasis, and ACT toxin biosynthesis. The underlying mechanisms of AaAtg4 in relation to oxidative stress response remain unknown.

Methods: Genetic and biochemical analyses.

Results: In this study, we examined the effect of hydrogen peroxide (H₂O₂) on AaAtg4. Functioning as a cysteine protease, AaAtg4 directly interacts with the AaAtg8 ubiquitin-like protein and is indispensable for AaAtg8 processing and autophagosome formation, with its enzymatic activity modulated by oxidative cues. H₂O₂ differentially impacts AaAtg4 activity, phosphorylation, binding with AaAtg8, AaAtg8 lipidation/delipidation, and autophagy. H₂O₂ has biphasic effects on AaAtg4. Moderate H₂O₂ levels enhance AaAtg4 activity and autophagy, whereas excessive H₂O₂ suppresses both, revealing a threshold-dependent redox regulation. Furthermore, AaAtg4 interacts with the stress-responsive mitogen-activated protein kinase AaHog1, which modulates its phosphorylation under conditions less conducive to autophagy and thus, reinforces a dynamic signaling axis.

Discussion: These findings indicate that H₂O₂ has multifaceted effects on AaAtg4 in a dosage-dependent or threshold-specific manner. These regulatory mechanisms may position AaAtg4 as a central integrator of cellular stress responses and secondary metabolism, thereby advancing our understanding of fungal pathogenicity and environmental adaptation.