Endocytic Uptake of Nutrients, Cell Wall Molecules, and Fluidized Cell Wall Portions into Heterotrophic Plant Cells

After arrival at the surface of heterotrophic cells, nutrients are taken up by these cells via endocytosis to sustain metabolic processes. Recent advances in plant endocytosis reveal that this is true for their heterotrophic cells, either cultivated in suspension cultures or for intact root apices. Importantly, sucrose appears to act as a specific stimulus for fluid-phase endocytosis. Uptake of extracellular nutrients by endocytosis is not in direct conflict with transport through membrane-bound carriers given that cell homeostasis can be better maintained if both these mechanisms operate in parallel. Besides nutrients, plant cells also accomplish internalization of cell wall molecules, such as xyloglucans and boron/calcium cross-linked pectins. Even large portions of apparently fluidized cell wall together with symbiotic bacteria can be internalized into some plant cells, suggesting that they can perform phagocytosis-like tasks despite their robust cell walls. Internalized cell wall molecules allow effective adaptation to osmotic stress, and also may serve for nutritive purposes. Plant endosomes enriched with the internalized cell wall molecules are used for new cell wall formation during plant cytokinesis. Moreover, rapid remodeling of cell walls through endosomal recycling is likely involved in opening/closing movements of stomata, and perhaps also in the formation of wall papillae during pathogen attacks and in recovery of cells from plasmolysis.