Shachar-Hill Bruria, Hill Adrian E, Powell Janet, Skepper Jeremy N, Shachar-Hill Yair
Multi-Imaging Centre, Cambridge University, Cambridge, UK.
J Exp Bot. 2013 Nov;64(16):5195-205. doi: 10.1093/jxb/ert311. Epub 2013 Oct 5.
The growing pollen tube is central to plant reproduction and is a long-standing model for cellular tip growth in biology. Rapid osmotically driven growth is maintained under variable conditions, which requires osmosensing and regulation. This study explores the mechanism of water entry and the potential role of osmosensory regulation in maintaining pollen growth. The osmotic permeability of the plasmalemma of Lilium pollen tubes was measured from plasmolysis rates to be 1.32±0.31×10(-3) cm s(-1). Mercuric ions reduce this permeability by 65%. Simulations using an osmotic model of pollen tube growth predict that an osmosensor at the cell membrane controls pectin deposition at the cell tip; inhibiting the sensor is predicted to cause tip bursting due to cell wall thinning. It was found that adding mercury to growing pollen tubes caused such a bursting of the tips. The model indicates that lowering the osmotic permeability per se does not lead to bursting but rather to thickening of the tip. The time course of induced bursting showed no time lag and was independent of mercury concentration, compatible with a surface site of action. The submaximal bursting response to intermediate mercuric ion concentration was independent of the concentration of calcium ions, showing that bursting is not due to a competitive inhibition of calcium binding or entry. Bursting with the same time course was also shown by cells growing on potassium-free media, indicating that potassium channels (implicated in mechanosensing) are not involved in the bursting response. The possible involvement of mercury-sensitive water channels as osmosensors and current knowledge of these in pollen cells are discussed.
生长中的花粉管是植物繁殖的核心，也是生物学中细胞顶端生长的长期模型。在可变条件下维持快速的渗透驱动生长，这需要渗透感知和调节。本研究探讨了水分进入的机制以及渗透感知调节在维持花粉生长中的潜在作用。通过质壁分离率测定百合花粉管质膜的渗透渗透率为1.32±0.31×10(-3) cm s(-1)。汞离子使这种渗透率降低65%。使用花粉管生长的渗透模型进行的模拟预测，细胞膜上的渗透传感器控制着细胞顶端的果胶沉积；预计抑制该传感器会由于细胞壁变薄而导致顶端破裂。研究发现，向生长中的花粉管中添加汞会导致顶端出现这种破裂。该模型表明，降低渗透渗透率本身不会导致破裂，而是会导致顶端增厚。诱导破裂的时间进程没有时间滞后，且与汞浓度无关，这与作用于表面位点一致。对中等汞离子浓度的次最大破裂反应与钙离子浓度无关，表明破裂不是由于对钙结合或进入的竞争性抑制。在无钾培养基上生长的细胞也表现出相同时间进程的破裂，这表明钾通道（与机械感知有关）不参与破裂反应。讨论了汞敏感水通道作为渗透传感器的可能参与情况以及目前对花粉细胞中这些通道的了解。
