ALUMINUM RESISTANCE IN THE ARABIDOPSIS MUTANT ALR-104 IS CAUSED BY ANALUMINUM-INDUCED INCREASE IN RHIZOSPHERE PH

A mechanism that confers increased Al resistance in the Arabidopsis th aliana mutant alr-104 was investigated. A modified vibrating microelec trode system was used to measure H+ fluxes generated along the surface of small Arabidopsis roots. In the absence of Al, no differences in r oot H+ fluxes between wild type and alr-104 were detected. However, Al exposure induced a 2-fold increase in net H+ influx in alr-104 locali zed to the root tip. The increased flux raised the root surface pH of alr-104 by 0.15 unit. A root growth assay was used to assess the Al re sistance of alr-104 and wild type in a strongly pH-buffered nutrient s olution. Increasing the nutrient solution pH from 4.4 to 4.5 significa ntly increased Al resistance in wild type, which is consistent with th e idea that the increased net H+ influx can account for greater Al res istance in alr-104. Differences in Al resistance between wild type and alr-104 disappeared when roots were grown in pH-buffered medium, sugg esting that Al resistance in alr-104 is mediated only by pH changes in the rhizosphere. This mutant provides the first evidence, to our know ledge, for an Al-resistance mechanism based on an Al-induced increase in root surface pH.