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 - 5). On these acid soils, aluminum (Al) toxicity is the primary factor limiting agricultural productivity, as toxic Al results in damaged and stunted plant root systems, ultimately resulting in a reduction of crop yields. Given that a large proportion of the acid soils are found in the tropics/subtropics regions where many developing countries are located, Al toxicity limits agricultural productivity in the very areas where food security is most tenuous. Because of the importance of this problem to agriculture worldwide, there is considerable interest and research effort by researchers at universities, government agencies, and international agriculture organizations in identifying genes that provide tolerance to Al toxicity in order to improve crop Al tolerance via molecular breeding and biotechnology. The release of organic acid from the root apex in response to Al-stress constitutes a widespread Al-tolerance mechanism by which plant roots are able to ameliorate the toxic levels of Al surrounding the growing root. Several genes encoding for membrane proteins involved in the transport and the release of these organic acids from root cells have been identified and cloned. However, very little is known regarding the functional characteristics of these membrane transport proteins. In this study, we used molecular and biophysical approaches to clone and characterize the functional properties of ZmALMT2, an organic acid membrane transport protein, isolated from root cells from maize. We have correlated the functional properties of this protein with its potential physiological role in the whole plant response to Al toxicity. The importance of these findings is that they increase our understanding of the functional properties of the proteins potentially involved in mediating the Al-tolerance response in crops, thus providing a platform for future improvements of Al tolerance in cereals via molecular breeding approaches.
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.