Lemtiri-Chlieh Fouad, MacRobbie Enid A C, Webb Alex A R, Manison Nick F, Brownlee Colin, Skepper Jeremy N, Chen Jian, Prestwich Glenn D, Brearley Charles A
Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK.
Proc Natl Acad Sci U S A. 2003 Aug 19;100(17):10091-5. doi: 10.1073/pnas.1133289100. Epub 2003 Aug 11.
myo-Inositol hexakisphosphate (InsP6) is the most abundant inositol phosphate in cells, yet it remains the most enigmatic of this class of signaling molecule. InsP6 plays a role in the processes by which the drought stress hormone abscisic acid (ABA) induces stomatal closure, conserving water and ensuring plant survival. Previous work has shown that InsP6 levels in guard cells are elevated in response to ABA, and InsP6 inactivates the plasma membrane inward K+ conductance (IK,in) in a cytosolic calcium-dependent manner. The use of laser-scanning confocal microscopy in dye-loaded patch-clamped guard cell protoplasts shows that release of InsP6 from a caged precursor mobilizes calcium. Measurement of calcium (barium) currents ICa in patch-clamped protoplasts in whole cell mode shows that InsP6 has no effect on the calcium-permeable channels in the plasma membrane activated by ABA. The InsP6-mediated inhibition of IK,in can also be observed in the absence of external calcium. Thus the InsP6-induced increase in cytoplasmic calcium does not result from calcium influx but must arise from InsP6-triggered release of calcium from endomembrane stores. Measurements of vacuolar currents in patch-clamped isolated vacuoles in whole-vacuole mode showed that InsP6 activates both the fast and slow conductances of the guard cell vacuole. These data define InsP6 as an endomembrane-acting calcium-release signal in guard cells; the vacuole may contribute to InsP6-triggered Ca2+ release, but other endomembranes may also be involved.
肌醇六磷酸（InsP6）是细胞中含量最丰富的肌醇磷酸，但它仍是这类信号分子中最神秘的一种。InsP6在干旱胁迫激素脱落酸（ABA）诱导气孔关闭、保存水分并确保植物存活的过程中发挥作用。先前的研究表明，保卫细胞中的InsP6水平会因ABA而升高，并且InsP6会以胞质钙依赖的方式使质膜内向钾离子电导（IK,in）失活。在染料加载的膜片钳制保卫细胞原生质体中使用激光扫描共聚焦显微镜显示，笼形前体释放的InsP6会动员钙。在全细胞模式下对膜片钳制原生质体中的钙（钡）电流ICa进行测量表明，InsP6对ABA激活的质膜钙通透通道没有影响。在没有外部钙的情况下也能观察到InsP6对IK,in的介导抑制作用。因此，InsP6诱导的细胞质钙增加并非由钙内流引起，而必定是由InsP6触发的内膜钙库释放所致。在全液泡模式下对膜片钳制的分离液泡中的液泡电流进行测量表明，InsP6激活了保卫细胞液泡的快速和慢速电导。这些数据将InsP6定义为保卫细胞中一种作用于内膜的钙释放信号；液泡可能参与了InsP6触发的Ca2+释放，但其他内膜也可能涉及其中。
