Publications - Faculty - Recently Submitted Faculty Publications 2009

Etxeberria - Plant Science

Evidence for two endocytic transport pathways in plant cells

Ed Etxeberria, Pedro Gonzalez and Javier Pozueta

Abstract. Internalization and intracellular routing of endocytic vesicles in eukaryotic cells varies with the nature of the cargo molecules and targeted organelle, and proceeds through an intricate network of internal endosomal compartments.  However, the path of fluid-phase endocytosis (FPE), the internalization of external solutes from the apoplast via plasmalemma generated vesicles, remains unresolved despite some indication of a direct transport route to the vacuole.  To test this hypothesis, we made use of the membrane-impermeable Na-dependent fluorescent marker Coro-Na in combination with the fluorescent membrane marker FM 4-64 and confocal laser scanning microscopy.  When protoplasts from sweet lime juice cells were incubated in Na-free solution, FM 4-64, Coro-Na and 200 mM sucrose, two distinct types of labeled vesicles were evident.  A set of vesicles (≈1 µm in diameter) was intensely labeled with Coro-Na and to a lesser extent with FM 4-64, whereas a second type of 1-7 µm structures appeared exclusively labeled with FM 4-64.  These data demonstrate the parallel functioning of two endocytic entrance pathways in plant cells.  In one system, a set of small endocytic vesicles merge with the endosome whereas a separate set of vesicles fuse to form larger structures independent from the endosome.  Although it is likely that both vesicle systems eventually contribute to solutes reaching the vacuole, given their size (1-7 μm), and based on previous observations of endocytic vesicle formation from the plasmalemma, we conclude that these latter vesicles constitute the primary FPE vesicle system.