Submitted to: Plant Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/18/2007
Publication Date: 1/8/2008
Citation: Pineros, M., Cancado, G., Maron, L., Lyi, S., Menossi, M., Kochian, L.V. 2008. Not all ALMT1-type transporters mediate aluminum-activated organic acid responses: The case of AmALMT1-an anion selective transporter. Plant Journal. 53:352-367. 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 which was a wheat protein that transports organic acid anions out of the root; these organic acids bind toxic Al ions and keep them out of the root. In this paper, we studied a close homolog of this gene in a related cereal, maize. We found this gene did encode a transporter; however, it is not involved in maize Al tolerance. Unlike its wheat counterpart, it is not expressed to higher levels in tolerant versus sensitive maize lines and is expressed primarily in the shoot, not the root. Also, its ion transport is not activated by Al like its wheat counterpart. Finally, this transporter mediates the efflux of inorganic mineral anions such as nitrate and phosphate. These findings are helping us determine for these new gene families that harbor Al tolerance genes what makes a gene a good candidate for Al tolerance. These findings will also help us identify candidate Al tolerance genes and the protein domains that confer the properties associated with Al tolerance, such as Al activation, expression in root tips of tolerant lines, and mediation of organic acid efflux.
Technical Abstract: The toxic effects of aluminum (Al) on crop root systems constitute a major agricultural problem in many areas of the world. Root exudation of Al-chelating molecules such as organic acids has been shown to be an important mechanism of plant Al resistance. Differences observed in the root physiology for two maize genotypes contrasting in Al tolerance revealed an association between rates of Al-activated root organic acid release and Al tolerance. Using these genotypes we cloned ZmALMT1, a maize gene homologous to the wheat ALMT1 and Arabidopsis AtALMT1, recently described as encoding functional, Al-activated transporters that play a role in tolerance by mediating Al-activated organic acid exudation in roots. The ZmALMT1 cDNA encodes a 451-amino acid protein containing 6 transmembrane helices. Transient expression of a ZmALMT1::GFP chimera confirmed that the protein is targeted to the plant cell plasma membrane. We addressed whether ZmALMT1 might underlie the Al-resistance response (i.e. Al-activated citrate exudation) observed in the roots of the Al tolerant genotypes. The physiological, gene expression, and functional data from this study confirmed that ZmALMT1 is a plasma membrane transporter that mediates selective anion efflux and influx. However, the gene expression data as well as biophysical transport characteristics obtained from Xenopus oocytes expressing ZmALMT1 indicate that this transporter is implicated in the selective transport of anions involved in mineral nutrition and ion homeostasis processes, rather than mediating a specific Al-activated root citrate exudation response.