Signal Transduction in Root Gravitropism

重力是调节植物生长和形态的基本信号。尽管重力对植物的成功至关重要，但高等植物感知和响应重力信号的细胞和分子事件基本上是未知的。根为研究这些事件提供了几乎独特的优势，因为感知和反应发生在明确定义的、空间上不同的区域。据认为，在根中，根帽的小柱细胞可以感知重力。这些细胞随后产生一个信号，并将其传递到生长区。这个信号随后通过一种可能涉及生长素、H+和Ca2+重新分配的机制引发不对称生长。小柱细胞中导致重力感知的事件仍然知之甚少。在一个被广泛接受的重力感知初始过程模型（淀粉statolith假说）中，小柱细胞中淀粉质体的沉淀启动了导致重力感知的信号系统。然而，帽细胞中重力感知机制的分子成分仍然基本上未知。这个信号系统必须将静石的沉降转化为一个编码重力方向的细胞信号。研究表明，对拟南芥根进行重力刺激后，细胞质pH和小柱细胞壁Ca2+和pH发生了迅速的变化。离子通量的快速诱导表明离子转运体的激活与最初的重力感应事件密切相关。抑制Ca2+或pH的这些变化也会抑制根的重力反应，这表明它们是根冠向地性信号传导过程继续进行所必需的。因此，本研究的目的是利用拟南芥作为模型系统，确定这些离子通量如何在根冠的重力感应细胞中被激活。了解重力导致的激活离子转运蛋白负责这些通量应提供洞察一些最初的分子变化,小柱的重力信号编码细胞的根cap.Several这个问题将调查方法:(1)H +, Ca2 +和K +通量监测在细胞质中,细胞壁和周围介质的小柱细胞完整gravistimulated根冠。这些离子的变化将被监控在生活,使用一系列新颖的荧光离子成像探针对根进行重力响应。(2)在小柱细胞中应用抑制剂和激活剂会改变信号或离子运输活动的细胞质调节因子的活性，如第二信使、肌动蛋白和微管蛋白骨架。然后将评估这些因素对抑制或模拟H+和Ca2+通量的引力调节的影响。(3)由于有大量证据表明钙调蛋白在离子转运体调控和重力反应中的作用，钙调蛋白活性将被操纵，其对重力刺激离子通量的影响将被监测。此外，一种新的基于绿色荧光蛋白的钙调蛋白活性指示器将用于成像小柱细胞质中潜在的重力诱导的钙调蛋白激活域。(4)为了验证重力感知的淀粉静止石模型，将使用激光镊子置换非重力刺激根小柱细胞中的淀粉质体。然后将评估类重力对H+和Ca2+通量调节的影响。淀粉质体位移诱导小柱细胞离子转运的重力样激活，将有力地支持淀粉静止石模型在拟南芥根冠中的重力感知。本研究的结果将扩展对植物重力感知机制的理解。特别是，这些研究将有助于确定植物根系重力感应系统初始元素的分子候选物。

Gravity is a fundamental signal that regulates plant growth and form. Despite its importance to plant success, the cellular and molecular events whereby higher plants sense and respond to the gravity signal are essentially unknown. Roots offer an almost unique advantage for studying these events in that sensing and response occur in well defined, spatially distinct regions. It is thought that in the root gravity is perceived in the columella cells of the root cap. These cells then generate a signal that is translocated to the growth zone. This signal then elicits asymmetrical growth through a mechanism that may involve redistributions of auxin, H+ and Ca2+. The events in the columella cells that lead to gravity perception remain poorly understood. In a widely accepted model for the initial process of gravity sensing (the starch statolith hypothesis), the settling of amyloplasts in the columella cells initiates the signaling systems that lead to gravity perception. However, the molecular components of the gravity perception machinery in the cap cells remain essentially unknown. This signaling system must translate sedimentation of statoliths to a cellular signal encoding the direction of gravity. Changes in cytoplasmic pH and columella cell wall Ca2+ and pH have been shown to occur rapidly after gravistimulation of the root of Arabidopsis thaliana. This rapid induction of ion fluxes suggests activation of ion transporters that are closely associated with the initial gravity sensing events. Inhibition of these changes in Ca2+ or pH also inhibits the graviresponse of the root, suggesting they are required for the gravitropic signaling processes of the root cap to proceed. The goal of this research is therefore to define how these ion fluxes are activated in the gravity sensing cells of the root cap using Arabidopsis thaliana as a model system. Understanding how gravity leads to the activation of the ion transporters responsible for these fluxes should provide insight into some of the initial molecular changes that encode the gravity signal in the columella cells of the root cap.Several approaches to this problem will be investigated:(1) H+, Ca2+ and K+ fluxes will be monitored in the cytoplasm, cell walls and medium around columella cells in the intact gravistimulated root cap. These ionic changes will be monitored in living, graviresponding roots using a range of novel, fluorescent, ion imaging probes.(2) The activities of cytoplasmic regulators of signaling or ion transport activities, such as second messengers and the actin and tubulin cytoskeleton, will be altered by application of inhibitors and activators to the columella cells. The effect of these factors on inhibiting or mimicking the gravitational regulation of H+ and Ca2+ fluxes will then be assessed.(3) As there is extensive evidence for a role of calmodulin in ion transporter regulation as well as in the graviresponse, calmodulin activity will be manipulated and its effect on the gravistimulated ion fluxes monitored. In addition, a novel green fluorescent protein-based indicator of calmodulin activity will be used to image potential gravity-induced calmodulin activation domains within the columella cell cytoplasm.(4) In order to test the starch statolith model of gravity perception, laser tweezers will be used to displace amyloplasts in the columella cells in non-gravity stimulated roots. Gravity-like effects on the regulation of H+ and Ca2+ fluxes will then be assessed. Induction of a gravity-like activation of columella cell ion transport by amyloplast displacement would strongly support the starch statolith model for gravity perception in the Arabidopsis root cap.Results from this research will extend the understanding of the gravity sensing machinery used by plants. In particular, these investigations will help identify molecular candidates for the initial elements of the plant gravity sensing system of the root.