The release of organic anions from roots can protect plants from aluminum (Al) toxicity and help them overcome phosphorus (P) deficiency. Our previous findings showed that Al treatment induced malate and citrate efflux from rape (Brassica napus) roots, and that P deficiency did not induce the efflux. Since this response is similar to the malate efflux from wheat (Triticum aestivum) that is controlled by the TaALMT1 gene, we investigated whether homologs of TaALMT1 are present in rape and whether they are involved in the release of organic anions. We isolated two TaALMT1 homologs from rape designated BnALMT1 and BnALMT2 (B. napus Al-activated malate transporter). The expression of these genes was induced in roots, but not shoots, by Al treatment but P deficiency had no effect. Several other cations (lanthanum, ytterbium, and erbium) also increased BnALMT1 and BnALMT2 expression in the roots. The function of the BnALMT1 and BnALMT2 proteins was investigated by heterologous expression in cultured tobacco (Nicotiana tabacum) cells and in Xenopus laevis oocytes. Both transfection systems showed an enhanced capacity for malate efflux but not citrate efflux, when exposed to Al. Smaller malate fluxes were also activated by ytterbium and erbium treatment. Transgenic tobacco cells grew significantly better than control cells following an 18 h treatment with Al, indicating that the expression of BnALMT1 and BnALMT2 increased the resistance of these plant cells to Al stress. This report demonstrates that homologs of the TaALMT1 gene from wheat perform similar functions in other species.Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils. When the soil pH falls below 5.0 the soluble Al in the soil solution exists predominantly as the toxic trivalent cation Al 31 that can inhibit root growth in many species at micromolar concentrations . Some species have developed mechanisms to overcome Al-related stresses by either excluding Al from the root cells (resistance mechanisms) or by increasing their tolerance to Al once these cations have been absorbed by the roots (tolerance mechanisms). Exclusion mechanisms might involve the exudation of Al-chelating ligands, the binding of Al within the cell wall or mucilage, plant-induced pH changes in the rhizosphere that reduce the local concentration of Al 31 relative to other hydrolysis products, the selective permeability of the plasma membrane, or perhaps the efflux of Al itself from the root cells. Tolerance mechanisms might include the chelation of Al in the cytosol, the sequestration of Al in the vacuole or other organelles, or modifications to metabolism that allow cellular function to continue normally in the presence of Al (Foy et al., 1978;Taylor, 1991;Delhaize and Ryan, 1995;Horst, 1995;Kochian, 1995;Ma et al., 2001;Matsumoto, 2002).The mechanism of Al resistance that has been observed most commonly in monocotyledonous and dicotyledonous species involves the release of organic anions such as malate, citrate, and oxalate from roots (Miyasaka et a...

The aluminum (Al)-induced secretion of citrate has been regarded as an important mechanism for Al resistance in soybean (Glycine max). However, the mechanism of how Al induces citrate secretion remains unclear. In this study, we investigated the regulatory role of plasma membrane H 1 -ATPase activity and expression were higher in an Al-resistant cultivar than in an Al-sensitive cultivar. Al activated the threonine-oriented phosphorylation of plasma membrane H 1 -ATPase in a dose-and time-dependent manner. Taken together, our results demonstrated that up-regulation of plasma membrane H 1 -ATPase activity was associated with the secretion of citrate from soybean roots.

SUMMARYTriticum aestivum aluminum-activated malate transporter (TaALMT1) is the founding member of a unique gene family of anion transporters (ALMTs) that mediate the efflux of organic acids. A small sub-group of root-localized ALMTs, including TaALMT1, is physiologically associated with in planta aluminum (Al) resistance. TaALMT1 exhibits significant enhancement of transport activity in response to extracellular Al. In this study, we integrated structure-function analyses of structurally altered TaALMT1 proteins expressed in Xenopus oocytes with phylogenic analyses of the ALMT family. Our aim is to re-examine the role of protein domains in terms of their potential involvement in the Al-dependent enhancement (i.e. Al-responsiveness) of TaALMT1 transport activity, as well as the roles of all its 43 negatively charged amino acid residues. Our results indicate that the N-domain, which is predicted to form the conductive pathway, mediates ion transport even in the absence of the C-domain. However, segments in both domains are involved in Al 3+ sensing.We identified two regions, one at the N-terminus and a hydrophobic region at the C-terminus, that jointly contribute to the Al-response phenotype. Interestingly, the characteristic motif at the N-terminus appears to be specific for Al-responsive ALMTs. Our study highlights the need to include a comprehensive phylogenetic analysis when drawing inferences from structure-function analyses, as a significant proportion of the functional changes observed for TaALMT1 are most likely the result of alterations in the overall structural integrity of ALMT family proteins rather than modifications of specific sites involved in Al 3+ sensing.