Mechanism of Establishment of Cellular Polarity in the Brown Alga Fucus

植物形态发生的一个关键因素是控制细胞的三维生长特征，即现有细胞扩展的方向(S)和/或新细胞的相对位置，如分裂平面的放置所决定的。因此，植物细胞必须对外界信号做出反应，才能定向扩张或分裂。细胞极性与植物不同的发育过程有关，从花粉管生长到产生气孔复合体和双细胞胚胎的不对称细胞分裂。通过细胞质、质膜或细胞壁中的特殊结构域建立极性是指导后续形态发生提供三维信息的重要手段。最近在高等植物和岩藻(Fucus和Pelvetia)受精卵中的数据支持这一假说，即肌动蛋白细胞骨架和分泌在确定植物细胞极性方面都发挥着重要作用。褐藻状受精卵在研究细胞极性方面有一个独特的优势：在其第一个24小时细胞周期的过程中，它们在培养液中建立定向的分子和发育不对称，以响应外部梯度(例如单边光)。最初无极的受精卵相对于外部载体定位细胞顶端的局部生长以及第一分裂平面，产生具有两种不同细胞类型的胚胎。完整的肌动蛋白细胞骨架和定向分泌是这一过程所必需的。然而，指导植物细胞分泌和细胞骨架组织的分子机制却知之甚少。相关的高度保守的Rho和Rab家族的小GTP酶分别在真菌和动物细胞中对细胞骨架的组织和分泌起着重要的调节作用。例如，酿酒酵母(Rho家族)的CDC42P蛋白是肌动蛋白在酵母芽中定位和极地生长所必需的；哺乳动物的Rab8蛋白参与了上皮细胞中囊泡到质膜的极化运输。这些蛋白质充当分子开关，在活跃的GTP结合形式和不活跃的GDP结合形式之间循环。GTPase中某些保守氨基酸的突变产生了显性的活性或非活性形式的蛋白质，这些蛋白质已被引入细胞(通过显微注射或转化表达载体)，以研究GTPase在体内的功能。因此，Rho和Rab GTP酶是研究细胞骨架和分泌在植物细胞极性中作用的有希望的靶标。

A crucial element of plant morphogenesis is the control of the three-dimensional growth characteristics of cells; that is, the direction(s) in which existing cells expand and/or the relative position of new cells, as dictated by the placement of division planes. Thus, plant cells must respond to external signals to expand or divide in an oriented fashion. Cell polarity is implicated in diverse plant developmental processes, from pollen tube growth to the asymmetric cell divisions that generate stomatal complexes and two-celled embryos. The establishment of polarity, via specialized domains in the cytoplasm, plasma membrane, or cell wall of a cell, is an important means of providing three-dimensional information to direct subsequent morphogenesis. Recent data in higher plants and in zygotes of fucoid algae (Fucus and Pelvetia) support the hypothesis that the actin cytoskeleton and secretion both play important roles in establishing plant cell polarity. Fucoid zygotes have a unique advantage for investigations into cell polarity: they establish oriented molecular and developmental asymmetries in culture media in response to external gradients (e.g., unilateral light) over the course of their first 24-hour cell cycle. Zygotes that are initially apolar orient the localized tip growth of the cell, as well as the first plane of division, with respect to the external vector, producing an embryo with two distinct cell types. An intact actin cytoskeleton and directed secretion are necessary for this process. However, the molecular mechanisms that direct secretion and cytoskeletal organization in plant cells are poorly understood. The related, highly-conserved Rho and Rab families of small GTPases play important roles in the regulation of cytoskeletal organization and secretion, respectively, in fungal and animal cells. For example, the Cdc42p protein of S. cerevisiae (in the Rho family) is necessary for actin localization and polar growth at the yeast bud; the mammalian Rab8 protein is involved in t he polarized transport of vesicles to the plasma membrane in epithelial cells. These proteins act as molecular switches, cycling between an active, GTP-bound form and an inactive, GDP-bound form. Mutations in certain conserved amino acids in the GTPases generate either dominant active or inactive forms of the proteins, which have been introduced into cells (either by microinjection or transformation of expression constructs) to investigate GTPase function in viva. Thus, Rho and Rab GTPases are promising targets for investigation of the role of the cytoskeleton and secretion in plant cell polarity.