| Endocytosis and vesicle trafficking during tip growth of root hairs |
| Ovecka M, Lang I, Baluška F, Ismail A, Illeš P, Lichtscheidl IK |
| Protoplasma 226, 39-54 (2005) |
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| The directional elongation of root hairs, “tip growth”, depends on the coordinated and highly regulated trafficking of vesicles which fill the tip cytoplasm and are active in secretion of cell wall material. Unfortunately, only little is known about the dynamics of endocytosis in living root hairs. We have analyzed the motile behaviour of vesicles in the apical region of living root hairs of Arabidopsis thaliana and of Triticum aestivum using live cell microscopy. For direct observation of endocytosis and of the fate of endocytotic vesicles and endosomes, we have used the fluorescent endocytosis marker dyes FM1-43 and FM4-64. Rapid endocytosis has been detected mainly in the tip and caused a bright fluorescence of the apical cytoplasm. The internalized membranes proceeded through highly dynamic early endosomes in the clear zone to late endosomal compartments in the sub-apical region that are excluded from the clear zone. In addition, secretory vesicles derived from early endosomes cycled back to the plasma membrane. Alternatively, internalized cargo ended up in the dynamic vacuole by fusion of large endosomal compartments with the tonoplast. Before export to the lytic compartments of late endosomes and the vacuole, early endosomes remained in the apical zone, recycling to the plasma membrane and back into the cytoplasm for more than 30 min. Endoplasmic reticulum was not involved in this recycling pathway. Actin cytoskeleton was needed for the endocytosis itself as well as for the further membrane trafficking. The actin-depolymerizing drug latrunculin B modified the dynamic properties of vesicles and endosomes; they became immobilized and aggregated in the tip. Treatment with brefeldin A inhibited membrane trafficking and caused the disappearance of FM-containing vesicles as well as of early endosomes from the clear zone and accumulation of labelled structures in motile brefeldin A-induced compartments. These large endocytic compartments re-dispersed upon removal of the drug. Our r esults prove the localization of active endocytosis in the tip and indicate specific endomembrane compartments and their secretory recycling involved in the tip growth of root hairs. |
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