Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/17/2008
Publication Date: 8/1/2008
Citation: Pineros, M., Kochian, L.V. 2008. Novel properties of the wheat aluminum tolerance organic acid transporter (TaALMT1) revealed by electrophysiological characterization in Xenopus oocytes: Functional and structural implications. Plant Physiology. 147:2131-2146. Interpretive Summary: Large areas of land within the U.S. and over 40% of the world’s lands are acidic, where aluminum (Al) toxicity is the primary factor limiting crop production via inhibition of root growth. The physiological and molecular basis for Al tolerance is still poorly understood. Thus, we need a more complete understanding of the mechanisms and associated genes underlying Al tolerance if we are going to be able to develop more Al tolerant crop plants for improved cultivation on acid soils. Previously our group was involved in the cloning of the first Al tolerance gene, TaALMT1, which encodes a wheat protein that transports the organic acid anion malate out of the root; the extruded malate then binds toxic Al ions and keep them out of the root. In this paper, we investigated the transport properties of TaALMT1 via an electrophysiological analysis of the currents mediated by TaALMT1. It was found that counter to previous reports, TaALMT1 can transport malate in the absence of Al; Al exposure enhances this baseline malate transport. It was also found that TaALMT1 can transport important mineral anions such as nitrate, sulfate, and chloride, although less effectively than it transports malate These findings are helping us determine for newly identified Al tolerance genes that are organic acid transporters what transport properties make them effective in Al tolerance. We will integrate this information with computer-based analysis of protein structure to identify the protein domains that confer the properties associated with Al tolerance, such as Al activation and selective transport of Al-binding organic acids.
Technical Abstract: Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding organic acid anions which chelate Al3+ and prevent its entry into the root. 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, a detailed characterization of the transport properties of TaALMT1 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+, 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. Furthermore, 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 indicate this estimate can only be used as a general guide to understanding the relative affinity of TaALMT1 for malate. TaALMT1 can also mediate a large anion influx (i.e. outward currents). These findings indicate that TaALMT1 is not only permeable to malate, but also to other physiologically relevant anions such as Cl-, NO3- and SO42-.