Cheffings C M, Pantoja O, Ashcroft F M, Smith J A
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford 0X13RB, UK.
J Exp Bot. 1997 Mar;48 Spec No:623-31. doi: 10.1093/jxb/48.Special_Issue.623.
Malate is a ubiquitous vacuolar anion in terrestrial plants that plays an important role in carbon metabolism and ionic homeostasis. In plants showing crassulacean acid metabolism (CAM), malate is accumulated as a central intermediary in the process of photosynthetic carbon assimilation, and it is also one of the major charge-balancing anions present in the vacuole. During the CAM cycle, malic acid produced as a result of dark CO(2) fixation accumulates in the vacuole at night (2 H(+) per malate), and is remobilized from the vacuole in the following light period. CAM plants thus provide a good model for studying both the mechanism and control of malate transport across the tonoplast. Thermodynamic considerations suggest that malate(2-) (the anionic species transported out of the cytosol) is passively distributed across the tonoplast. Malic acid accumulation could thus be explained by malate(2-) transport into the vacuole occurring electrophoretically in response to the transmembrane electrical potential difference established by the tonoplast H(+)-ATPase and/or H(+)-PPase. Recent studies using the patch-clamp technique have provided evidence for the existence of a vacuolar malate-selective anion channel (VMAL) in both CAM species and C(3) species. The VMAL current has a number of distinctive properties that include strong rectification (opening only at cytosolicside negative membrane potentials that would favour malate uptake into the vacuole), lack of Ca(2+) dependence, and slow activation kinetics. The kinetics of VMAL activation can be resolved into three components, consisting of an instantaneous current and two slower components with voltage-independent time constants of 0.76 s and 5.3 s in Kalanchoë daigremontiana. These characteristics suggest that the VMAL channel represents the major pathway for malate transport into the vacuole, although the strong rectification of the channel means there may be a separate, still-to-be-identified, transport mechanism for malate efflux.
苹果酸是陆生植物中普遍存在的液泡阴离子，在碳代谢和离子稳态中发挥着重要作用。在表现出景天酸代谢（CAM）的植物中，苹果酸作为光合碳同化过程中的核心中间体积累，并且它也是液泡中存在的主要电荷平衡阴离子之一。在CAM循环中，夜间由于暗CO₂固定产生的苹果酸在液泡中积累（每分子苹果酸有2个H⁺），并在随后的光照期从液泡中重新转运出来。因此，CAM植物为研究苹果酸跨液泡膜运输的机制和调控提供了一个很好的模型。热力学考虑表明，苹果酸根离子（从细胞质中转运出的阴离子形式）被动地分布在液泡膜两侧。因此，苹果酸的积累可以解释为苹果酸根离子响应液泡膜H⁺-ATP酶和/或H⁺-焦磷酸酶建立的跨膜电势差，以电泳方式转运到液泡中。最近使用膜片钳技术的研究已经证明，在CAM植物和C₃植物中都存在液泡苹果酸选择性阴离子通道（VMAL）。VMAL电流具有许多独特的特性，包括强整流性（仅在有利于苹果酸进入液泡的细胞质侧负膜电位时开放）、不依赖Ca²⁺以及缓慢的激活动力学。在落地生根中，VMAL激活的动力学可以分解为三个组分，包括一个瞬时电流和两个较慢的组分，其电压无关的时间常数分别为0.76秒和5.3秒。这些特征表明，VMAL通道代表了苹果酸转运到液泡中的主要途径，尽管该通道的强整流性意味着可能存在一个单独的、尚未确定的苹果酸外流转运机制。
