Studies on the mechanism of the accumulation of positional information at the division site in higher plants

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The preprophase band (PPB) is an array of microtubules in plant cells that appears in G2 and disappears in prometaphase. Its function appears to be to mark the site on the plasma membrane where the cell plate will fuse at the end of cytokinesis. Little is known about how the plasma membrane becomes modified during PPB formation. To understand the mechanism of how a PPB memory site is generated, we have re-analyzed the structural changes in membrane architecture associated with PPB formation using epidermal cells of high pressure frozen onion cotyledons. Our micrographs demonstrate that the forming PPB regions contain large numbers of clathrin-coated pits and clathrin-coated vesicles.To compare the density of coated vesicles in PPB and non-PPB regions, we have prepared serial thin sections both in the nuclear region and in the region far from the nucleus in prophase and interphase cells. The density of coated vesicles was significantly higher in the PPB regions than in the adjacent non-PPB regions. A more detailed morphological analysis of the vesicles by means of dual-axis EM tomography (7 nm 3-D resolution) shows that plasma membrane associated clathrin-coated pits and two types of vesicles, clathrin-coated and noncoated. The latter had more densely staining contents and were distributed even further away from the plasma membrane than the clathrin-coated vesicles. This suggests the involvement of endocytosis in the development of PPBs. Outside the forming PPB regions in a prophase cell and in the cortical region of an interphase cell we also saw the same structures, but they were less numerous, confirming the notion that PPB formation involves an upregulation of endocytosis in the future PPB region.