Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2008
Publication Date: 2/20/2008
Citation: Kobayashi, Y., Hoekenga, O., Ito, H., Nakashima, M., Saito, S., Shaff, J., Maron, L., Pineros, M., Kochian, L.V., Koyama, H. 2008. Characterization of AtALMT1 expression in aluminum inducible malate release and its role for rhizotoxic stress in Arabidopsis. Plant Physiology. 145:843-852. Interpretive Summary: Large areas of land within the U.S. and over 40% of the world’s arable lands are acidic. In these acid soils, aluminum (Al) toxicity is the primary factor limiting crop production via Al-induced 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 underlying Al tolerance if we are going to be able to develop more Al tolerant crop plants for improved cultivation on acid soils. In this paper, we used molecular, genetic, and physiological methods to study Al tolerance in the model plant species, Arabidopsis. The investigation is a continuation of previous work showing that a gene that is a close relative of a recently identified wheat Al tolerance gene, ALMT1, is important for Arabidopsis Al tolerance. This protein, called AtALMT1, is an Al-activated transport protein that pumps the organic acid, malate, out of root cells into the soil where it binds and detoxifies Al ions. In the current study we showed that expression of the gene is induced by Al and not by other toxic metal ions such as cadmium, sodium, and copper, and the time course for induction of AtALMT1 expression closely follows the time course for Al-activation of root malate release. We also showed that the gene is expressed in very localized regions of the root, which presumably is a strategy to minimize carbon loss by the plant while protecting the root from Al. The information obtained from this study will help us in developing more Al tolerant crops for agriculture on acid soils that are widespread both in the US and also in developing countries.
Technical Abstract: Malate transporters play a critical role in aluminum (Al) tolerance responses for some plant species, such as Arabidopsis (Arabidopsis thaliana). Here we further characterize AtALMT1, an Arabidopsis malate transporter, to clarify its specific role in malate release and Al stress responses. Malate excretion from the roots of accession Columbia (Col-4) was sharply induced by Al, which is concomitant with the induction of AtALMT1 gene expression. The malate release was specific for Al among rhizotoxic stressors, namely Cadmium (Cd), Copper (Cu), Erbium (Er), Lanthanum (La), Sodium (Na) and low pH, which accounts for the specific sensitivity of a null mutant to Al stress. Al specific malate excretion can be explained by a combined regulation of AtALMT1 expression and activation of AtALMT1 protein, which is specific for Al. Although low pH treatment slightly induced gene expression, other treatments did not. In addition, malate excretion in Al activated seedlings was rapidly stopped by removing Al from the solution. Other rhizotoxic stressors were not effective in maintaining malate release. Protein kinase and phosphatase inhibitor studies indicated that reversible phosphorylation was important for the transcriptional and post-translational regulation of AtALMT1. AtALMT1 promoter-'-glucuronidase (GUS) fusion lines revealed that AtALMT1 has restricted expression within the root, such that unnecessary carbon loss is likely minimized. Lastly, a natural nonsense mutation allele of AtALMT1 was identified, from the Al-hypersensitive natural accession Warschau-1 (Wa-1).