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Published on June 11, 2008; 10.1104/pp.108.119636

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Received March 24, 2008
Accepted June 1, 2008

Novel properties of the wheat aluminum tolerance organic acid transporter (TaALMT1) revealed by electrophysiological characterization in Xenopus oocytes: Functional and structural implications

Miguel A. Pineros *, Geraldo M. A. Cancado , and Leon V. Kochian *

U. S. Plant, Soil, and Nutrition Laboratory, U. S. Department of Agriculture-Agricultural Research Service, Cornell University, Ithaca, New York, USA 14853-2901; Laboratorio de Genomica Funcional, Centro de Biologia Molecular e Engenharia Genetica, Universidade Estadual de Campinas, Campinas, Sao Paulo, Brazil 13083-970; 3Current address: Empresa de Pesquisa Agropecuaria de Minas Gerais (EPAMIG), Belo Horizonte, Brazil 31170-000

* Corresponding author; email: map25{at}cornell.edu.

Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding di- and tricarboxylic acids which 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 have recently been cloned and implicated in mediating the organic acid transport underling this Al tolerance response. Given our limited understanding of the functional properties of ALMTs, in the present study a detailed characterization of the transport properties of TaALMT1 (formerly named ALMT1 from wheat) expressed in Xenopus oocytes was conducted. The electrophysiological findings indicate that: Although the activity of TaALMT1 is highly dependent on the presence of extracellular Al3+ (Km1/2 of approx. 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, Erev, 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 solely due to increases in its anion permeability. The consistent shift in the direction of the reversal potential, Erev, 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 (Pmal2-/PCl-) varied between 1 and 30. However, the complex behavior of the reversal potential (Erev) as the extracellular Cl- activity was varied indicates 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 not only permeable to malate, but also to other physiologically relevant anions such as Cl-, NO3- and SO42- (to a lesser degree).




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P. R. Ryan, H. Raman, S. Gupta, W. J. Horst, and E. Delhaize
A Second Mechanism for Aluminum Resistance in Wheat Relies on the Constitutive Efflux of Citrate from Roots
Plant Physiology, January 1, 2009; 149(1): 340 - 351.
[Abstract] [Full Text] [PDF]


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