Piñeros Miguel A, Cançado Geraldo M A, Kochian Leon V
United States Department of Agriculture-Agricultural Research Service, Cornell University, Ithaca, New York 14853-2901, USA.
Plant Physiol. 2008 Aug;147(4):2131-46. doi: 10.1104/pp.108.119636. Epub 2008 Jun 11.
Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding dicarboxylic and tricarboxylic acids that chelate and immobilize Al(3+) at the root surface, thus preventing it from entering root cells. Several novel genes that encode membrane transporters from the ALMT and MATE families recently were cloned and implicated in mediating the organic acid transport underlying this Al tolerance response. Given our limited understanding of the functional properties of ALMTs, in this study a detailed characterization of the transport properties of TaALMT1 (formerly named ALMT1) from wheat (Triticum aestivum) expressed in Xenopus laevis oocytes was conducted. The electrophysiological findings are as follows. Although the activity of TaALMT1 is highly dependent on the presence of extracellular Al(3+) (K(m1/2) of approximately 5 microm Al(3+) activity), TaALMT1 is functionally active and can mediate ion transport in the absence of extracellular Al(3+). The lack of change in the reversal potential (E(rev)) upon exposure to Al(3+) suggests that the "enhancement" of TaALMT1 malate transport by Al is not due to alteration in the transporter's selectivity properties but is solely due to increases in its anion permeability. The consistent shift in the direction of the E(rev) as the intracellular malate activity increases indicates that TaALMT1 is selective for the transport of malate over other anions. The estimated permeability ratio between malate and chloride varied between 1 and 30. However, the complex behavior of the E(rev) as the extracellular Cl(-) activity was varied indicates that this estimate can only be used as a general guide to understanding the relative affinity of TaALMT1 for malate, representing only an approximation of those expected under physiologically relevant ionic conditions. TaALMT1 can also mediate a large anion influx (i.e. outward currents). TaALMT1 is permeable not only to malate but also to other physiologically relevant anions such as Cl(-), NO(3)(-), and SO(4)(2-) (to a lesser degree).
许多植物物种通过分泌二羧酸和三羧酸来避免铝（Al）的植物毒性作用，这些酸在根表面螯合并固定Al(3+)，从而防止其进入根细胞。最近克隆了几个编码来自ALMT和MATE家族膜转运蛋白的新基因，这些基因与介导这种耐铝反应的有机酸转运有关。鉴于我们对ALMT功能特性的了解有限，在本研究中，对在非洲爪蟾卵母细胞中表达的小麦（Triticum aestivum）TaALMT1（以前称为ALMT1）的转运特性进行了详细表征。电生理结果如下。虽然TaALMT1的活性高度依赖于细胞外Al(3+)的存在（Al(3+)活性的K(m1/2)约为5微摩尔），但TaALMT1在功能上是活跃的，并且在没有细胞外Al(3+)的情况下可以介导离子转运。暴露于Al(3+)时反转电位（E(rev)）没有变化，这表明Al对TaALMT1苹果酸转运的“增强”不是由于转运体选择性特性的改变，而仅仅是由于其阴离子通透性的增加。随着细胞内苹果酸活性的增加，E(rev)方向的一致变化表明TaALMT1对苹果酸的转运比对其他阴离子具有选择性。苹果酸与氯离子之间的估计通透率在1到30之间变化。然而，随着细胞外Cl(-)活性的变化，E(rev)的复杂行为表明，这个估计值只能作为理解TaALMT1对苹果酸相对亲和力的一般指导，仅代表生理相关离子条件下预期值的近似值。TaALMT1还可以介导大量阴离子内流（即外向电流）。TaALMT1不仅对苹果酸通透，而且对其他生理相关阴离子如Cl(-)、NO(3)(-)和SO(4)(2-)（程度较小）也通透。
