Salt-Induced Remodeling of Spatially Restricted Clathrin-Independent Endocytic Pathways in Arabidopsis Root

Endocytosis is a ubiquitous cellular process that is characterized well in animal cells in culture but poorly across intact, functioning tissue. Here, we analyze endocytosis throughout the Arabidopsis thaliana root using three classes of probes: a lipophilic dye, tagged transmembrane proteins, and a lipid-anchored protein. We observe a stratified distribution of endocytic processes. A clathrin-dependent endocytic pathway that internalizes transmembrane proteins functions in all cell layers, while a sterol-dependent, clathrin-independent pathway that takes up lipid and lipid-anchored proteins but not transmembrane proteins is restricted to the epidermal layer. Saline stress induces a third pathway that is clathrinindependent, nondiscriminatory in its choice of cargo, and operates across all layers of the root. Concomitantly, small acidic compartments in inner cell layers expand to form larger vacuole-like structures. Plants lacking function of the Rab-GEF (guanine nucleotide exchange factor) VPS9a (vacuolar protein sorting 9A) neither induce the third endocytic pathway nor expand the vacuolar system in response to salt stress. The plants are also hypersensitive to salt. Thus, saline stress reconfigures clathrin-independent endocytosis and remodels endomembrane systems, forming large vacuoles in the inner cell layers, both processes correlated by the requirement of VPS9a activity.