ARS | Publication request: Maize ZmALMT2 is a root anion transporter that mediates constitutive root malate efflux

Submitted to: Plant, Cell & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 2011
Publication Date: July 2, 2012
Citation: Ligaba, A., Maron, L., Shaff, J.E., Kochian, L.V., Pineros, M. 2012. Maize ZmALMT2 is a root anion transporter that mediates constitutive root malate efflux. Plant, Cell & Environment. 35(7):1365-3040.

Interpretive Summary: Over 20% of the US land area and approximately 50% of the world’s arable lands are acidic (pH Technical Abstract: Aluminum (Al) toxicity is a primary limitation to crop productivity on acid soils throughout the plant. Root efflux of organic acid anions constitutes a mechanism by which plants cope with toxic aluminum (Al) ions on acid soils. In this study, we have characterized ZmALMT2 (a member of aluminum-activated malate transporters family), heterologously expressing it in Xenopus laevis oocytes and transgenic Arabidopsis plants to gain understanding on its functional characteristics and the potential physiological responses it may underlie. Expression of ZmALMT2 in X. oocytes resulted in large inward currents (anion efflux), which significantly increased with increasing intracellular malate and citrate concentration. Further electrophysiological characterization revealed that ZmALMT2 not only mediates the selective transport of these organic anions, but is also permeable to inorganic anions (e.g. Cl- and NO3-). The anion channel nature of ZmALMT2 is also supported by its sensitivity to the anion channel blocker, niflumic acid. Extracellular Al3+ had no effect on ZmALMT2 transport activity. Ion substitution experiments suggest an intrinsic regulatory mechanism by which the concentration and the nature of the anion surrounding the extracellular and intracellular faces of the transport protein modulate the magnitude of its transport activity. Expression of ZmALMT2::YFP chimeras indicated this transporter is localized to the plasma membrane of plant cells. Over-expression of ZmALMT2 in an Arabidopsis double KO line lacking the two organic acid efflux transporters (AtALMT1 and AtMATE) which underlie the modest Al-tolerance observed in wild type Arabidopsis resulted in partial reconstitution of Al-tolerance. Roots from transgenic Arabidopsis plants showed large constitutive malate and citrate efflux rates, relative to those observed in wild type. However, although the changes in ZmALMT2 expression in roots of two maize cultivars differing in their degree of Al-resistance was poorly correlated with the phenotypic differences among the two cultivars, the findings in this study suggest that ZmALMT2 could potentially underlie the constitutive organic acid reported in maize roots, implicated in providing a basal level of Al-resistance in maize. Additionally, ZmALMT2 may also play a role in the influx and efflux of anions across the plasma membrane of several distinct cells types involved in anion homeostasis, and acquisition and translocation mineral nutrients.