Quatrano R S
Department of Biology, University of North Carolina, Chapel Hill 27599-3280, USA.
Cold Spring Harb Symp Quant Biol. 1997;62:65-70.
External gradients, such as unilateral light, applied to apolar zygotes of Fucus result in a cortical asymmetry expressed as the actin-dependent translocation of existing plasma membrane molecules (e.g., DHP receptors) to the shaded side (Fig. 1a). This process corresponds to the alignment of the polar axis. The localized cortical domain identified by the accumulation of DHP receptors, F-actin, and free calcium forms a target site for Golgi vesicle (F granule) secretion. Localized secretion of F granules is essential to stabilize the polar axis (Fig. 1b), and to complete a structural complex at the site for polar growth, postulated to span the plasma membrane, from the actin cytoskeleton to the cell wall (Fig. 2). Furthermore, targeted secretion of the contents of F granules into the plasma membrane and/or cell wall appears to provide localized positional information required to orient the first cell division plane and to differentiate the rhizoid and thallus cells of the two-celled embryo (Fig. 1c). Our cytological approaches using Fucus zygotes point to the importance of directed vesicle movement and secretion in creating asymmetries in the plasma membrane/cell wall during embryogenesis, which appear to have a critical role in cell morphogenesis. Conclusions drawn from these results may provide a useful paradigm for the study of cell morphogenesis and pattern formation in higher plant embryos and vegetative tissues.
施加于墨角藻无极性合子的外部梯度，如单侧光，会导致一种皮层不对称性，表现为现有质膜分子（如二氢吡啶受体）依赖肌动蛋白向背光侧的转运（图1a）。这一过程与极轴的排列相对应。由二氢吡啶受体、F - 肌动蛋白和游离钙的积累所确定的局部皮层区域形成了高尔基体囊泡（F颗粒）分泌的靶位点。F颗粒的局部分泌对于稳定极轴（图1b）以及在极生长位点完成一个假定跨越质膜、从肌动蛋白细胞骨架到细胞壁的结构复合体至关重要（图2）。此外，F颗粒内容物向质膜和/或细胞壁的靶向分泌似乎提供了确定第一个细胞分裂平面以及区分二细胞胚的假根和叶状体细胞所需的局部位置信息（图1c）。我们利用墨角藻合子的细胞学方法表明，在胚胎发育过程中，定向囊泡运动和分泌对于在质膜/细胞壁中产生不对称性很重要，而这似乎在细胞形态发生中起关键作用。从这些结果得出的结论可能为研究高等植物胚胎和营养组织中的细胞形态发生和模式形成提供一个有用的范例。
