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

前期带（PPB）是植物细胞中出现在G2期并在前中期消失的微管阵列。它的功能似乎是在细胞质分裂结束时标记细胞板将融合的质膜上的位点。关于PPB形成过程中质膜是如何被修饰的知之甚少。为了了解PPB记忆位点是如何产生的机制，我们重新分析了与PPB形成相关的膜结构的结构变化，使用高压冷冻洋葱子叶的表皮细胞。我们的显微照片表明，形成PPB区域包含大量的网格蛋白包被的小凹和网格蛋白包被的囊泡。为了比较PPB和非PPB区域中包被的囊泡的密度，我们制备了一系列的薄切片，在核区域和远离核的区域，在前期和间期细胞。PPB区域的包被囊泡密度显著高于相邻的非PPB区域。一个更详细的形态分析的囊泡通过双轴EM断层扫描（7纳米3-D分辨率）显示，质膜相关网格蛋白涂层的坑和两种类型的囊泡，网格蛋白涂层和noncoated。后者有更密集的染色内容物和分布，甚至更远离质膜比网格蛋白包被的小泡。这表明内吞作用参与PPBs的发育。在前期细胞中形成PPB区域外和间期细胞的皮质区域中，我们也看到了相同的结构，但它们数量较少，证实了PPB形成涉及未来PPB区域内吞作用上调的观点。

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.