Department of Plant Sciences, Tel Aviv University, Tel Aviv, Israel.
PLoS Biol. 2010 Jan 19;8(1):e1000282. doi: 10.1371/journal.pbio.1000282.
Development in multicellular organisms depends on the ability of individual cells to coordinate their behavior by means of small signaling molecules to form correctly patterned tissues. In plants, a unique mechanism of directional transport of the signaling molecule auxin between cells connects cell polarity and tissue patterning and thus is required for many aspects of plant development. Direction of auxin flow is determined by polar subcellular localization of PIN auxin efflux transporters. Dynamic PIN polar localization results from the constitutive endocytic cycling to and from the plasma membrane, but it is not well understood how this mechanism connects to regulators of cell polarity. The Rho family small GTPases ROPs/RACs are master regulators of cell polarity, however their role in regulating polar protein trafficking and polar auxin transport has not been established. Here, by analysis of mutants and transgenic plants, we show that the ROP interactor and polarity regulator scaffold protein ICR1 is required for recruitment of PIN proteins to the polar domains at the plasma membrane. icr1 mutant embryos and plants display an a array of severe developmental aberrations that are caused by compromised differential auxin distribution. ICR1 functions at the plasma membrane where it is required for exocytosis but does not recycle together with PINs. ICR1 expression is quickly induced by auxin but is suppressed at the positions of stable auxin maxima in the hypophysis and later in the embryonic and mature root meristems. Our results imply that ICR1 is part of an auxin regulated positive feedback loop realized by a unique integration of auxin-dependent transcriptional regulation into ROP-mediated modulation of cell polarity. Thus, ICR1 forms an auxin-modulated link between cell polarity, exocytosis, and auxin transport-dependent tissue patterning.
多细胞生物的发育依赖于单个细胞通过小分子信号来协调其行为的能力，从而形成具有正确模式的组织。在植物中，生长素这种信号分子在细胞间的定向运输的独特机制连接了细胞极性和组织模式的形成，因此是许多植物发育过程所必需的。生长素流的方向取决于 PIN 生长素外排转运蛋白在亚细胞水平上的极性定位。PIN 蛋白的动态极性定位源于质膜的内吞循环，但人们尚不清楚这种机制如何与细胞极性的调节剂相连接。Rho 家族的小 GTPases ROPs/RACs 是细胞极性的主要调节剂，然而，它们在调节极性蛋白运输和极性生长素运输中的作用尚未确定。在这里，通过对突变体和转基因植物的分析，我们表明 ROP 相互作用蛋白和极性调节支架蛋白 ICR1 是将 PIN 蛋白募集到质膜上的极性区域所必需的。icr1 突变体胚胎和植物表现出一系列严重的发育异常，这些异常是由差异生长素分布受损引起的。ICR1 在质膜上发挥作用，需要质膜进行胞吐作用，但不会与 PIN 一起循环。ICR1 的表达很快被生长素诱导，但在垂体中生长素稳定最大值的位置以及随后在胚胎和成熟根分生组织中被抑制。我们的结果表明，ICR1 是一个由生长素调节的正反馈回路的一部分，该回路通过生长素依赖性转录调控与 ROP 介导的细胞极性调节的独特整合来实现。因此，ICR1 在细胞极性、胞吐作用和依赖生长素运输的组织模式形成之间形成了一个生长素调节的连接。
