Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/13/2003
Publication Date: 6/1/2003
Citation: Hoekenga, O., Vision, T.J., Shaff, J.E., Monforte, A.J., Howell, S.H., Kochian, L.V. 2003. Identification and characterization of aluminum tolerance loci in arabidopsis (landsberg erecta x columbia) by quantitative trait locus mapping. A physiologically simple but genetically complex trait. Plant Physiology. 132:936-948. 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. There is considerable genetic variation in tolerance to Al between different plant species and genotypes, yet the molecular, genetic, and physiological basis for Al tolerance is still poorly understood. Thus, we need a more complete understanding of the genetic and mechanistic basis for Al tolerance if we are going to be able to develop more Al tolerant crop plants for improved cultivation on acid soils. A strategy we are taking is to carry out research that ultimately is aimed at isolating genes conferring Al tolerance. In this paper, we investigated the genetics of Al tolerance in the model plant species, Arabidopsis thaliana. We identified two discrete regions of the Arabidopsis genome that contain different Al tolerance genes. Both of these genes participate in the same physiological mechanism of tolerance involving the release of the organic acid malate from roots, which binds and detoxifies Al before it can enter the plant. This work has set the stage for fine scale molecular mapping of these regions of the Arabidopsis genome to clone the genes involved in Al tolerance. Once these genes are cloned, this will open up the identification of similar genes in crop plant species, which in turn will be used to increase Al tolerance and generate crops better suited for cultivation on acid soils throughout the world.
Technical Abstract: Aluminum (Al) toxicity, which is caused by the solubilization of Al3+ in acid soils resulting in inhibition of root growth and nutrient/water acquisition, is a serious limitation to crop production, as up to one-half of the world's potentially arable land is acidic. To date, however, no Al tolerance genes have yet been cloned. The physiological mechanisms of tolerance are somewhat better understood; the major documented mechanism involves the Al-activated release of Al-binding organic acids from the root tip, preventing uptake into the primary site of toxicity. In this study a quantitative trait loci analysis of Al tolerance in Arabidopsis thaliana was conducted which also correlated Al tolerance QTL with physiological mechanisms of tolerance. The analysis identified two major loci, which explain approximately 40% of the variance in Al tolerance observed among recombinant inbred lines derived from Landsberg erecta (sensitive) and Columbia (tolerant). We characterized the mechanism by which tolerance is achieved, and found that the two QTL cosegregate with an Al-activated release of malate from Arabidopsis roots. Although only two of the QTL have been identified, malate release explains nearly all (95%) of the variation in Al tolerance in this population. Al tolerance in Ler x Col is more complex genetically than physiologically, in that a number of genes underlie a single physiological mechanism involving root malate release. These findings have set the stage for the subsequent cloning of the genes responsible for the Al tolerance QTL, and a genomics-based cloning strategy and initial progress on this are also discussed.