Abstract
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).
Highlights
Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding dicarboxylic and tricarboxylic acids that chelate and immobilize Al31 at the root surface, preventing it from entering root cells
The complex behavior of the Erev as the extracellular Cl2 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
The initial electrophysiological characterization of TaALMT1 expressed in Xenopus oocytes was performed under identical extracellular ionic conditions (i.e. ND88 solution) to those originally reported by Sasaki and coworkers (2004)
Summary
Empresa de Pesquisa Agropecuaria de Minas Gerais, Belo Horizonte, Brazil 31170–000. Plant Physiology, August 2008, Vol 147, pp. 2131–2146, www.plantphysiol.org Ó 2008 American Society of Plant Biologists Downloaded from on October 5, 2020 - Published by www.plantphysiol.org. Physiological approaches to study these processes, which led to the discovery and characterization of plasma membrane anion channel-mediated currents, which were activated by extracellular Al31 These studies performed in protoplasts isolated from root tips of Al-resistant wheat (Ryan et al, 1997; Zhang et al, 2001) and maize (Zea mays; Kollmeier et al, 2001; Pineros and Kochian, 2001; Pineros et al, 2002) showed remarkable similarities between the transport and activation properties of these channels and the Alactivated malate and citrate exudation responses reported in intact wheat and maize roots, suggesting that these types of transporters are likely to mediate the Al-activated organic acid release observed at the whole root level. This study greatly expands the preliminary characterization of TaALMT1 from wheat first reported by Sasaki et al (2004) by thoroughly characterizing its biophysical properties in Xenopus oocytes and by beginning to provide an understanding of the relationship between the structure and the function of this transporter
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