ALUMINUM-RESISTANT ARABIDOPSIS MUTANTS THAT EXHIBIT ALTERED PATTERNS OF ALUMINUM ACCUMULATION AND ORGANIC-ACID RELEASE FROM ROOTS

Al-resistant (alr) mutants of Arabidopsis thaliana were isolated and c haracterized to gain a better understanding of the genetic and physiol ogical mechanisms of Al resistance, air mutants were identified on the basis of enhanced root growth in the presence of levels of Al that st rongly inhibited root growth in wild-type seedlings. Genetic analysis of the air mutants showed that Al resistance was semidominant, and chr omosome mapping of the mutants with microsatellite and random amplifie d polymorphic DNA markers indicated that the mutants mapped to two sit es in the Arabidopsis genome: one locus on chromosome 1 (alr-108, alr- 128, alr-131, and alr-139) and another on chromosome 4 (alr-104). Al a ccumulation in roots of mutant seedlings was studied by staining with the fluorescent Al-indicator dye morin and quantified via inductively coupled argon plasma mass spectrometry. It was found that the air muta nts accumulated lower levels of Al in the root tips compared with wild type. The possibility that the mutants released Alchelating organic a cids was examined. The mutants that mapped together on chromosome 1 re leased greater amounts of citrate or malate (as well as pyruvate) comp ared with wild type, suggesting that Al exclusion from roots of these air mutants results from enhanced organic acid exudation. Roots of alr -104, on the other hand, did not exhibit increased release of malate o r citrate, but did alkalinize the rhizosphere to a greater extent than wild-type roots. A detailed examination of Al resistance in this muta nt is described in an accompanying paper (J. Degenhardt, P.B. Larsen, S.H. Howell, L.V. Kochian [1998] Plant Physiol 117: 19-27).