Arabidopsis AUTOPHAGY-RELATED2 is essential for ATG18a and ATG9 trafficking during autophagosome closure

As a fundamental metabolic pathway, autophagy plays important roles in plant growth and development, particularly under stress conditions. A set of autophagy-related (ATG) proteins is recruited for the formation of a double-membrane autophagosome. Among them, the essential roles of ATG2, ATG18, and ATG9 have been well established in plant autophagy via genetic analysis; however, the underlying molecular mechanism for ATG2 in plant autophagosome formation remains poorly understood. In this study, we focused on the specific role of ATG2 in the trafficking of ATG18a and ATG9 during autophagy in Arabidopsis (Arabidopsis thaliana). Under normal conditions, YFP-ATG18a proteins are partially localized on late endosomes and translocated to ATG8e-labeled autophagosomes upon autophagic induction. Real-time imaging analysis revealed sequential recruitment of ATG18a on the phagophore membrane, showing that ATG18a specifically decorated the closing edges and finally disassociated from the completed autophagosome. However, in the absence of ATG2, most of the YFP-ATG18a proteins are arrested on autophagosomal membranes. Ultrastructural and 3D tomography analysis showed that unclosed autophagosome structures are accumulated in the atg2 mutant, displaying direct connections with the endoplasmic reticulum membrane and vesicular structures. Dynamic analysis of ATG9 vesicles suggested that ATG2 depletion also affects the association between ATG9 vesicles and the autophagosomal membrane. Furthermore, using interaction and recruitment analysis, we mapped the interaction relationship between ATG2 and ATG18a, implying a possible role of ATG18a in recruiting ATG2 and ATG9 to the membrane. Our findings unveil a specific role of ATG2 in coordinating ATG18a and ATG9 trafficking to mediate autophagosome closure in Arabidopsis.