Autophagy-related protein 1 orchestrates autophagy initiation and feedback degradation in Alternaria alternata

BACKGROUND: Autophagy plays an essential role in fungal development and stress adaptation, yet its regulatory mechanisms in filamentous fungi remain incompletely understood. We functionally characterized Alternaria alternata Atg1 (AaAtg1), a serine/threonine kinase, and demonstrated its dual roles in autophagy initiation and flux modulation. RESULTS: Loss of the AaAtg1 gene resulted in impaired vegetative growth, reduced conidiation, delayed germination, and defective appressorium formation, all of which were restored by genetic complementation. Deletion of AaAtg1 also abolishes autophagosome formation and autophagic flux, impairs peroxisome degradation, and leads to hypersensitivity to oxidative stress and reduced virulence. AaAtg1 physically interacts with core autophagy proteins AaAtg13 and AaAtg8, and its vacuolar localization and degradation are AaAtg8-dependent. Structure-guided mutagenesis of the Atg8-family interacting motif (AIM) disrupts AaAtg1-AaAtg8 binding in yeast two-hybrid assays but not in bimolecular fluorescence complementation, suggesting partial functional retention in vivo. Intriguingly, AIM mutations do not impair autophagy; instead, some transformants exhibit elevated autophagic activity, suggesting a potential negative regulatory role of AIM in autophagy tuning. CONCLUSIONS: These findings suggest a feedback mechanism in which AaAtg8 may facilitate AaAtg1 vacuolar localization, possibly for degradation, thereby influencing autophagic output. Our study elucidates the structure-function relationship of AaAtg1 and uncovers a dual regulatory mechanism that coordinates autophagy progression and stress adaptation in the plant-pathogenic fungus.