Institute of Plant Biology, University of Zurich, Zurich, Switzerland.
Plant J. 2011 Jul;67(2):247-57. doi: 10.1111/j.1365-313X.2011.04587.x. Epub 2011 Apr 26.
Gas exchange in plants is controlled by guard cells, specialized cells acting as turgor pressure-driven valves. Malate is one of the major anions accumulated inside the vacuole during stomatal opening counteracting the positive charge of potassium. AtALMT6, a member of the aluminum-activated malate transporter family, is expressed in guard cells of leaves and stems as well as in flower organs of Arabidopsis thaliana. An AtALMT6-GFP fusion protein was targeted to the vacuolar membrane both in transient and stable expression systems. Patch-clamp experiments on vacuoles isolated from AtALMT6-GFP over-expressing Arabidopsis plants revealed large inward-rectifying malate currents only in the presence of micromolar cytosolic calcium concentrations. Further analyses showed that vacuolar pH and cytosolic malate regulate the threshold of activation of AtALMT6-mediated currents. The interplay of these two factors determines the AtALMT6 function as a malate influx or efflux channel depending on the tonoplast potential. Guard cell vacuoles isolated from Atalmt6 knock-out plants displayed reduced malate currents compared with wild-type vacuoles. This reduction, however, was not accompanied by phenotypic differences in the stomatal movements in knock-out plants, probably because of functional redundancy of malate transporters in guard cell vacuoles.
植物的气体交换受保卫细胞控制，保卫细胞是作为膨压驱动的阀门的特化细胞。苹果酸是气孔开放时积累在液泡内的主要阴离子之一，可抵消钾离子的正电荷。拟南芥的铝激活苹果酸转运蛋白家族（AtALMT6）成员在叶片和茎的保卫细胞以及花器官中表达。AtALMT6-GFP 融合蛋白在瞬时和稳定表达系统中均靶向液泡膜。在过表达 AtALMT6-GFP 的拟南芥植物中分离的液泡进行的膜片钳实验表明，仅在存在微摩尔细胞溶质钙离子浓度的情况下，才会出现大的内向整流苹果酸电流。进一步的分析表明，液泡 pH 值和细胞溶质苹果酸调节 AtALMT6 介导电流的激活阈值。这两个因素的相互作用决定了 AtALMT6 作为苹果酸流入或流出通道的功能，这取决于液泡膜电位。与野生型液泡相比，从 Atalmt6 敲除植物中分离的保卫细胞液泡显示出减少的苹果酸电流。然而，这种减少并没有伴随着敲除植物气孔运动的表型差异，这可能是由于保卫细胞液泡中苹果酸转运蛋白的功能冗余。
