Getting there: vesicles en route for plant cytokinesis

In dividing plant cells, membranous vesicles (60-80 nm in diameter) are transported to the site where a new cell wall that separates the daughter cells is formed. In this thesis the physical parameters size and stiffness that vesicles require to reach the forming cell plate were studied. Synthetic lipid vesicles and polystyrene beads were injected into dividing cells during cytokinesis. At this stage, the cell has a structure, the phragmoplast, which contains a specific configuration of microtubules and actin filaments. Vesicles of up to 150 nm in diameter were injected and all of them moved through the phragmoplast towards the forming cell plate. The cell plate is made from the fusing vesicles and becomes the cell wall between the two daughter cells. Since smaller beads of 20 and 40 nm in diameter did not move through the phragmoplast to the cell plate, the conclusion is that not size but stiffness is a limiting parameter. In order to reach the phragmoplast, vesicles have to move from the site of injection to the cell center. This instigated the question whether hydrodynamic flow occurs in the cytoplasm of plant cells. Theoretically, with a simple lattice model, as well as experimentally, with the FRAP (fluorescence recovery after photobleaching) method, free GFP molecules in the cytoplasm of tobacco BY-2 suspension cells were shown to exhibit hydrodynamic flow: a drag of molecules caused by active molecular motor-driven transport of organelles along (bundles of) actin filaments. Such a flow distributes free cytosolic molecules in the large plant cells faster than diffusion.