Autophagosomes arise in yeast and animals from the sealing of a cup-shaped double-membrane precursor, the phagophore. The concerted action of about 30 evolutionarily conserved autophagy related (ATG) proteins lies at the core of this process. However, the mechanisms allowing phagophore generation and its differentiation into a sealed autophagosome are still not clear in detail, and very little is known in plants. This is due in part to the scarcity of structurally informative, real-time imaging data of ATG proteins at the phagophore site. Among these, the ATG5 complex directs anchoring of ATG8 to the phagophore, an event required for membrane expansion. Detailed real-time and 3D imaging of ATG5, ATG8, and an ER marker at the expanding phagophore allowed us to propose a model for autophagosome formation in plants. This model implies tight connections of the growing phagophore with the outer face of the cortical endoplasmic reticulum and prompts new questions on the mechanism of autophagosome biogenesis.
Keywords: Arabidopsis; autophagosome; autophagy-related protein 5; endoplasmic reticulum; phagophore.