Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2002
Publication Date: N/A
Citation: Interpretive Summary: Aluminum (Al) toxicity is a primary limitation to crop productivity on acid soils, which comprise a significant fraction of the world's arable lands and also include many areas of the US. The development of Al-tolerant genotypes will contribute greatly to increased crop productivity on acid soils, and future agricultural expansion onto acidic soils will require that Al tolerance be a central trait for crop improvement programs. An understanding of the genetics of Al tolerance in a crop will improve the efficiency of crop improvement programs, but in no crop is this question fully resolved. Therefore, this study was undertaken to further our understanding of the genetics of Al tolerance in wheat, and the physiological mode of action of genes encoding the trait. We compared physiological parameters associated with Al tolerance in the Al-tolerant variety Atlas 66 and Al-tolerant near-isogenic lines (NILs) of the cultivars Century and Chisholm, into which a single Al tolerance gene from Atlas 66 was transferred by backcrossing (Century-T and Chisholm-T). Century-T and Chisholm-T were not as Al-tolerant as Atlas 66, indicating that more than one Al tolerance gene is present in Atlas 66. Additional results suggest that the Al tolerance genes in Atlas 66 confer Al tolerance by additively increasing the release of malate from root tips to exclude Al from entering this region of the root, rather than by coding for different mechanisms of Al tolerance. This is the first concrete demonstration that differences in the expression of a major mechanism of Al tolerance among wheat cultivars are due in part to natural variation at more than one genetic locus in this crop.
Technical Abstract: Both genetic & physiological lines of evidence suggest the presence of more than one gene for aluminum (Al) tolerance in the wheat (Triticum aestivum L.) cultivar 'Atlas 66'. To investigate whether a direct relationship can be made between these two forms of evidence, the physiology of Al tolerance was studied in Atlas 66, two Al sensitive wheat varieties ('Century' and 'Chisholm'), and two Al-tolerant near-isogenic lines (NILs) of Century and Chisholm that harbor a single Al tolerance gene from Atlas 66 ('Century-T' and 'Chisholm-T'). Determination of relative root growth in low salt hydroponic media containing 10, 20, or 30 uM Al revealed that Century-T and Chisholm-T exhibited significantly less Al tolerance than Atlas 66, but were considerably more tolerant than their Al sensitive parents. The physiological basis of this incomplete transfer of Al tolerance was subsequently examined. The reduced Al tolerance of Century-T and Chisholm-T Trelative to Atlas 66 was associated with a reduced ability to exclude Al from the root apex, and with a lower rate of Al-induced malate release from the root apex. In contrast to previous results, there was little evidence supporting a role for root phosphate exudation in the Al tolerance of Atlas 66. Taken together, these data suggest that the high degree of Al tolerance in Atlas 66 wheat is due to the action of two or more genes, and that these genes confer Al tolerance by acting in concert to enhance malate efflux in response to Al exposure. As malate metabolism, compartmentation and transport in plant cells involves a number of pathways, enzymes, and other proteins, genetic variation for any of these processes could ostensibly contribute to the phenotypic differences in Al tolerance found in Atlas 66 and other wheat varieties.