Aluminum-Resistant Arabidopsis Mutants That Exhibit Altered Patterns of Aluminum Accumulation and Organic Acid Release from Roots1

Abstract

Al-resistant (alr) mutants of Arabidopsis thaliana were isolated and characterized to gain a better understanding of the genetic and physiological mechanisms of Al resistance. alr mutants were identified on the basis of enhanced root growth in the presence of levels of Al that strongly inhibited root growth in wild-type seedlings. Genetic analysis of the alr mutants showed that Al resistance was semidominant, and chromosome mapping of the mutants with microsatellite and random amplified polymorphic DNA markers indicated that the mutants mapped to two sites 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 accumulation 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 alr mutants accumulated lower levels of Al in the root tips compared with wild type. The possibility that the mutants released Alchelating organic acids was examined. The mutants that mapped together on chromosome 1 released greater amounts of citrate or malate (as well as pyruvate) compared with wild type, suggesting that Al exclusion from roots of these alr mutants results from enhanced organic acid exudation. Roots of alr-104, on the other hand, did not exhibit increased release of malate or citrate, but did alkalinize the rhizosphere to a greater extent than wild-type roots. A detailed examination of Al resistance in this mutant is described in an accompanying paper (J. Al toxicity is a global problem that limits crop productivity on acidic soils. Al is the most abundant metal in the earth's crust, and in acidic soils (pH Ͻ 5.5) the phytotoxic species Al 3ϩ is solubilized to levels that inhibit root growth and crop yield (Kochian, 1995). Large areas of the world contain acidic soils (Ͼ30% of the arable land), so Al toxicity is a very important worldwide agricultural problem (Von Uexkull and Mutert, 1995). Despite the agronomic importance of this problem, little is known about fundamental mechanisms of Al toxicity and resistance. It has been well documented that many plant species exhibit significant genetic variability in their ability to resist Al toxicity (Delhaize and Ryan, 1995; Kochian, 1995, and refs. therein). Although it is clear that certain plant genotypes have evolved mechanisms that confer Al resistance, the cellular and molecular basis for Al resistance is still poorly understood.DegenhardtThere are two strategies that plants can use to deal with Al toxicity: exclusion from the root apex or development of the ability to tolerate Al once it enters the plant symplasm (Delhaize and Ryan, 1995;Kochian, 1995). Because of the complex interactions between Al and the plant, it is very likely that there are a number of different mechanisms that plants use to confer Al resistance. This is supported by genetic studies of Al resistance, which have shown it to be a dominant, multigenic trait controlled by one or a few major...

Fluorescence Resonance Energy Transfer-Sensitized Emission of Yellow Cameleon 3.60 Reveals Root Zone-Specific Calcium Signatures in Arabidopsis in Response to Aluminum and Other Trivalent Cations

“…This notion is further supported by the fact that alr104 and als3-1 mutants, which were previously shown to be resistant and sensitive to Al 3+ , respectively (Larsen et al, 1998(Larsen et al, , 2005, displayed the same [Ca 2+ ] cyt responses to all three trivalent cations as wild-type seedlings (Fig. 9).…”

“…9). The als3-1 mutant, which is disrupted in a gene that encodes a phloem-localized ATP-binding cassette-like transporter, was hypersensitive to Al 3+ but not to La 3+ (Larsen et al, 1998), indicating that the trivalent ion-induced [Ca 2+ ] cyt rise in Arabidopsis root cells is unrelated to the Al 3+ response mechanism involving the ALS3 gene. To the best of our knowledge, the ALR104 gene has not yet been cloned, but given the similar nature of the Al 3+ -induced [Ca 2+ ] cyt response between the wild type and alr104, these early [Ca 2+ ] cyt responses might be independent of ALR104.…”

“…Here (Larsen et al, 1998) and the Al 3+ -sensitive mutant als3-1 (Larsen et al, 2005), indicating that the early [Ca 2+ ] cyt increases we report here may not be tightly linked to mechanisms of Al 3+ toxicity but rather to a general trivalent cation response. Our study further shows that FRETsensitized emission imaging of Arabidopsis roots expressing YC3.60 provides a robust method for documenting [Ca 2+ ] cyt signatures in different root developmental zones that should be useful for future studies on Ca 2+ signaling mechanisms in plants.…”

“…NPPB alone did not evoke any [Ca 2+ ] cyt changes in the root transition zone (Fig. 8G Larsen et al, 1998) and one Al 3+ -sensitive mutant (als3-1; Larsen et al, 2005) with the YC3.60 reporter and evaluated the [Ca 2+ ] cyt response after treatment with Al 3+ , La 3+ , or Gd 3+ . We again focused our imaging on the region of the root that exhibited the biphasic [Ca 2+ ] cyt response, since this pattern of [Ca 2+ ] cyt change appears to be the most distinguishing and common feature of the trivalent cation response.…”

Parâmetros morfológicos para estabelecer tolerância diferencial à toxicidade de alumínio em cultivares de arroz

“…Embora as bases celulares e moleculares das diferenças em tolerância à toxicidade do Al ainda não estejam esclarecidas (Larsen et al, 1998), sabe-se que a região do ápice radicular é o alvo primário da ação tóxica do alumínio. A toxicidade manifesta-se pela redução da taxa de elongação radicular após uma breve exposição das raízes às soluções contendo Al (Ryan et al, 1993).…”