A unique COPII population in plant autophagy.

Increasing evidence supports the bona fide function of the coat protein complex II (COPII) machinery in regulating autophagosomes biogenesis during macroautophagy/autophagy induced by nutrient starvation. However, the participation of the COPII machinery in the plant autophagy pathway remains elusive. We recently identified a unique population of COPII vesicles containing AT3G62560/AtSar1d-AT1G02130/AtRabD2a that functions in modulating autuphagosome biogenesis in Arabidopsis thaliana. Proteomic analysis identified the mechanistic connection between autophagy-related (ATG) proteins and a subset of specific COPII paralogs, including AtSar1d. Mutants of AtSar1d affect autophagosome progression and display starvation-related phenotypes. AtSar1d interacts with ATG8 by a non-canonical motif. Cellular and genetic analysis demonstrated that a plant-unique RAB1/Ypt1 homolog AtRabD2a coordinates with AtSar1d to mediate the specific COPII functions in the autophagy pathway. This study identified a plant-specific nexus in regulating autophagosome biogenesis.