ADP-ribosylation Factor Machinery Mediates Endocytosis in Plant Cells

Endocytosis is crucial for various cellular functions and development of multicellular organisms. In mammals and yeast, ADP-ribosylation factor (ARF) GTPases, key components of vesicle formation, and their regulators ARF-guanine nucleotide exchange factors (GEFs) and ARF-GTPase-activating protein (GAPs) mediate endocytosis. A similar role has not been established in plants, mainly because of the lack of the canonical ARF and ARF-GEF components that are involved in endocytosis in other eukaryotes. In this study, we revealed a regulatory mechanism of endocytosis in plants based on ARF GTPase activity. We identified that ARF-GEF GNOM and ARF-GAP vascular network defective 3 (VAN3), both of which are involved in polar auxin transport-dependent morphogenesis, localize at the plasma membranes as well as in intracellular structures. Variable angle epifluorescence microscopy revealed that GNOM and VAN3 localize to partially overlapping discrete foci at the plasma membranes that are regularly associated with the endocytic vesicle coat clathrin. Genetic studies revealed that GNOM and VAN3 activities are required for endocytosis and internalization of plasma membrane proteins, including PIN-FORMED auxin transporters. These findings identified ARF GTPase-based regulatory mechanisms for endocytosis in plants. GNOM and VAN3 previously were proposed to function solely at the recycling endosomes and trans-Golgi networks, respectively. Therefore our findings uncovered an additional cellular function of these prominent developmental regulators.