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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Novel Properties of the Wheat Aluminum Tolerance Organic Acid Transporter (TaALMT1) Revealed by Electrophysiological Characterization in Xenopus Oocytes: Functional and Structural Implications^1,[OA]

United States Plant, Soil, and Nutrition Laboratory, United States Department of Agriculture-Agricultural Research Service, Cornell University, Ithaca, New York 14853–2901 (M.A.P., L.V.K.); and Laboratório de Genômica Funcional, Centro de Biologia Molecular e Engenharia Genética, Universidade Estadual de Campinas, Campinas, São Paulo, Brazil 13083–970 (G.M.A.C.)

Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding dicarboxylic and tricarboxylic acids that chelate and immobilize Al³⁺ 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 Al³⁺ (K_m1/2 of approximately 5 µM Al³⁺ activity), TaALMT1 is functionally active and can mediate ion transport in the absence of extracellular Al³⁺. The lack of change in the reversal potential (E_rev) upon exposure to Al³⁺ 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^–, NO₃^–, and SO₄^2– (to a lesser degree).

² Present address: Empresa de Pesquisa Agropecuária de Minas Gerais, Belo Horizonte, Brazil 31170–000.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Leon V. Kochian (lvk1{at}cornell.edu).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.119636

^* Corresponding author; e-mail lvk1{at}cornell.edu.

Received March 24, 2008; accepted June 1, 2008; published June 11, 2008.

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