Ward J. M., Schroeder J. I.
Department of Biology and Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093-0116.
Plant Cell. 1994 May;6(5):669-683. doi: 10.1105/tpc.6.5.669.
Stomatal closing requires the efflux of K+ from the large vacuolar organelle into the cytosol and across the plasma membrane of guard cells. More than 90% of the K+ released from guard cells during stomatal closure originates from the guard cell vacuole. However, the corresponding molecular mechanisms for the release of K+ from guard cell vacuoles have remained unknown. Rises in the cytoplasmic Ca2+ concentration have been shown to trigger ion efflux from guard cells, resulting in stomatal closure. Here, we report a novel type of largely voltage-independent K+-selective ion channel in the vacuolar membrane of guard cells that is activated by physiological increases in the cytoplasmic Ca2+ concentration. These vacuolar K+ (VK) channels had a single channel conductance of 70 pS with 100 mM KCI on both sides of the membrane and were highly selective for K+ over NH4+ and Rb+. Na+, Li+, and Cs+ were not measurably permeant. The Ca2+, voltage, and pH dependences, high selectivity for K+, and high density of VK channels in the vacuolar membrane of guard cells suggest a central role for these K+ channels in the initiation and control of K+ release from the vacuole to the cytoplasm required for stomatal closure. The activation of K+-selective VK channels can shift the vacuolar membrane to more positive potentials on the cytoplasmic side, sufficient to activate previously described slow vacuolar cation channels (SV-type). Analysis of the ionic selectivity of SV channels demonstrated a Ca2+ over K+ selectivity (permeability ratio for Ca2+ to K+ of ~3:1) of these channels in broad bean guard cells and red beet vacuoles, suggesting that SV channels play an important role in Ca2+-induced Ca2+ release from the vacuole during stomatal closure. A model is presented suggesting that the interaction of VK and SV channel activities is crucial in regulating vacuolar K+ and Ca2+ release during stomatal closure. Furthermore, the possibility that the ubiquitous SV channels may represent a general mechanism for Ca2+-induced Ca2+ release from higher plant vacuoles is discussed.
气孔关闭需要钾离子从大型液泡细胞器流出进入细胞质溶胶，并穿过保卫细胞的质膜。在气孔关闭过程中，从保卫细胞释放的钾离子超过90% 源自保卫细胞液泡。然而，钾离子从保卫细胞液泡释放的相应分子机制仍不清楚。细胞质钙离子浓度的升高已被证明会触发保卫细胞的离子外流，导致气孔关闭。在此，我们报道了保卫细胞液泡膜中一种新型的、很大程度上不依赖电压的钾离子选择性离子通道，它由细胞质钙离子浓度的生理性升高激活。这些液泡钾离子（VK）通道在膜两侧均有100 mM KCl时的单通道电导为70 pS，对钾离子的选择性远高于铵离子和铷离子。钠离子、锂离子和铯离子不可测量地通透。保卫细胞液泡膜中VK通道的钙离子、电压和pH依赖性、对钾离子的高选择性以及高密度表明这些钾离子通道在气孔关闭所需的钾离子从液泡释放到细胞质的起始和控制中起核心作用。钾离子选择性VK通道的激活可使液泡膜在细胞质一侧的电位变得更正，足以激活先前描述的慢液泡阳离子通道（SV型）。对SV通道离子选择性的分析表明，蚕豆保卫细胞和红甜菜液泡中的这些通道对钙离子的选择性高于钾离子（钙离子与钾离子的渗透率比约为3:1），这表明SV通道在气孔关闭期间钙离子诱导的钙离子从液泡释放中起重要作用。本文提出了一个模型，表明VK和SV通道活性的相互作用在调节气孔关闭期间液泡钾离子和钙离子释放中至关重要。此外，还讨论了普遍存在的SV通道可能代表高等植物液泡中钙离子诱导的钙离子释放的一般机制的可能性。
