Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/1998
Publication Date: N/A
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. There is considerable genetic variation in sensitivity to Al between different plant species and genotypes, yet the genetic and molecular basis for Al tolerance is still poorly understood. We need a more complete understanding of the molecular genetics of Al toxicity and tolerance if we are going to be able to develop more Al resistant crop plants. In order to increase our understanding of this topic, we have screened mutagenized populations of Arabidopsis thaliana (a model plant system for molecular and genetic research) and isolated 5 Al tolerant mutants. Four of these mutants are related and the mutation maps to a location on chromosome 1. The other mutant is quite different and maps to chromosome 4. In this study, we have characterized the Al tolerant mutant that maps to chromosome 4. Using a unique vibrating micro-pH electrode system, it was shown that Al tolerance was due to an Al-induced increase in soil solution pH mediated by the root tip. This response was much larger than what is seen in wild type plants. This increase in pH will cause the concentration of the toxic Al3+ cation to drop, and thus reduce Al toxicity. Thus, we have identified an important new Al tolerance mechanism, and this information should be very useful in developing more Al tolerant crop species.
Technical Abstract: A mechanism that confers increased aluminum (Al) resistance in an Arabidopsis thaliana mutant, alr-104, was investigated. A modified vibrating microelectrode system was used to measure proton fluxes generated along the surface of small Arabidopsis roots. In the absence of Al, no differences in root H+ fluxes between wildtype and alr-104 were detected. However, Al exposure induced a two-fold increase in net proton influx in alr-104 localized to the root tip. The increased flux raised the root surface pH of alr-104 by 0.15 units. A root growth assay was used to assess the Al resistance of alr-104 and wild type in a strongly pH buffered nutrient solution. Increasing the nutrient solution pH from 4.4 to 4.5 significantly increased Al resistance in wildtype, consistent with the idea that the increased net proton influx can account for greater Al resistance in alr-104. Differences in Al resistance between wild type and alr-104 disappeared when roots were grown in pH buffered media, suggesting that the Al resistance in alr-104 is mediated only by pH changes in the rhizosphere. This mutant provides the first evidence for an Al-resistance mechanism based on Al-induced increase in root surface pH.