Mechanism of Establishment of Cellular Polarity in the Brown Alga Fucus

植物形态发生的一个关键要素是控制细胞的三维生长特征;即，现有细胞扩展的方向和/或新细胞的相对位置，如由分裂平面的位置所决定的。 因此，植物细胞必须响应外部信号，以定向方式扩展或分裂。细胞极性与植物的发育过程密切相关，从花粉管生长到产生气孔复合体和二细胞胚的不对称细胞分裂。通过细胞质、质膜或细胞壁中的专门结构域建立极性是提供三维信息以指导随后的形态发生的重要手段。 最近在高等植物和岩藻（墨角藻和Pelvetia）的受精卵中的数据支持肌动蛋白细胞骨架和分泌物在建立植物细胞极性中起重要作用的假设。 岩藻样合子在研究细胞极性方面具有独特的优势：它们在培养基中建立定向的分子和发育不对称性以响应外部梯度（例如，单侧光）在其第一个24小时细胞周期的过程中。 最初是非极性的合子使细胞的局部尖端生长以及第一分裂平面相对于外部载体定向，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个过程所必需的。然而，指导植物细胞分泌和细胞骨架组织的分子机制知之甚少。 小GTP酶的相关的高度保守的Rho和Rab家族分别在真菌和动物细胞中的细胞骨架组织和分泌的调节中起重要作用。例如，S.在酵母芽中，Rab 8蛋白是肌动蛋白定位和极性生长所必需的;哺乳动物Rab 8蛋白参与上皮细胞中囊泡向质膜的极化运输。这些蛋白质充当分子开关，在活性的GTP结合形式和非活性的GDP结合形式之间循环。GTP酶中某些保守氨基酸的突变产生显性活性或非活性形式的蛋白质，其已被引入细胞（通过显微注射或表达构建体的转化）以研究体内GTP酶功能。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的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.