We investigated the role of organic acids i n conferring AI tolerance in near-isogenic wheat (Trificum aestivum 1.) lines differing i n AI tolerance at the AI tolerance locus (Altl). Addition of AI to nutrient solutions stimulated excretion of malic and succinic acids from roots of wheat seedlings, and AI-tolerant genotypes excreted 5-to 10-fold more malic acid than AI-sensitive genotypes. Malic acid excretion was detectable after 15 min of exposure to 200 p~ AI, and the amount excreted increased linearly over 24 h. The amount of malic acid excreted was dependent on the externa1 AI concentration, and excretion was stimulated by as little as 10 p~ AI. Malic acid added to nutrient solutions was able to protect Alsensitive seedlings from normally phytotoxic AI concentrations. Root apices (terminal 3-5 mm of root) were the primary source of the malic acid excreted. Root apices of AI-tolerant and AI-sensitive seedlings contained similar amounts of malic acid before and after a 2-h exposure to 200 p~ AI. During this treatment, AI-tolerant seedlings excreted about four times the total amount of malic acid initially present within root apices, indicating that continua1 synthesis of malic acid was occurring. Malic acid excretion was specifically stimulated by AI, and neither La, Fe, nor the absence of Pi was able to elicit this response. There was a consistent correlation of AI tolerance with high rates of malic acid excretion stimulated by AI in a population of seedlings segregating for AI tolerance. These data are consistent with the hypothesis that the Altl locus in wheat encodes an AI tolerance mechanism based on AI-stimulated excretion of malic acid.

Aluminium (A1) stimulates the efflux of malate from the apices of wheat (Triticum aestivum L.) roots (Delhaize et al. 1993, Plant Physiol. 103, 695-702). The response was five to tenfold higher in Al-tolerant seedlings than Al-sensitive seedlings and the capacity for Al-stimulated malate efflux was found to co-segregate with A1 tolerance in a pair of near-isogenic wheat lines differing in Al-tolerance at a single dominant locus. We have investigated this response further using excised root apices. Half-maximal efflux of malate from apices of A1-tolerant seedlings was measured with 30 gM A1 in 0.2raM CaC12, pH4.2, while saturating rates of 2.0 nmol. apex -~. h 1 occurred with concentrations above 100 gM A1. The stimulation of malate efflux by A1 was accompanied by an increase in K + efflux which appeared to account for electroneutrality. The greater stimulation of malate efflux from Al-tolerant apices compared to A1-sensitive apices could not be explained by differences in the activities of phosphoenolpyruvate carboxylase or NAD-malate dehydrogenase. Several other polyvalent cations, including gallium, indium and the tridecamer Al13, failed to elicit malate efflux. Aluminium-stimulated efflux of malate was correlated with the measured concentration of total monomeric AI present, and with the predicted concentrations of A13 + and A1OH 2 + ions in the solution. Several antagonists of anion channels inhibited Al-stimulated efflux of malate with the following order of effectiveness: niflumic acid ~NPPB > IAA-94 ~ A-9-C>ethacrynic acid. Lanthanum, chlorate, perchlorate, zinc and ~-cyano-4-hydroxycinnamic acid inhibited malate release by less than 30% at 100 gM while 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) had no Abbreviations: A-9-C = anthracene-9-carboxylic acid; DIDS = 4,4'-diisothiocyanatostilbene-2,2'-disulphonate; ET3, ET8, ES3, and ES8 = Al-tolerant and Al-sensitive lines of wheat; IAA-94 = [6, 7-effect. These results suggest that the AI 3+ cation stimulates malate efflux via anion channels in apical cells of Al-tolerant wheat roots.

We have characterized a novel mutation of Arabidopsis fhaliana at a locus designated pho2. pho2 mutants accumulated up to 3-fold more total P in leaves, mostly as inorganic phosphate (Pi), than wild-type seedlings. In addition, we isolated a mutant (locus desig-

We investigated the uptake and distribution of AI in root apices of near-isogenic wheat (Triticum aesfivum L.) lines differing in AI tolerance at a single locus (Altl: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 p~ AI, and the roots were subsequently stained with hematoxylin, a compound that binds AI in vitro to form a colored complex. Root apices of Alsensitive genotypes stained after short exposures to AI (10 min and 1 h), whereas apices of AI-tolerant seedlings showed less intense staining after equivalent exposures. Differential staining preceded differences observed in either root elongation or total AI concentrations of root apices (terminal 2-3 mm of root). After 4 h of exposure to 100 p~ AI in nutrient solution, AI-sensitive genotypes accumulated more total AI in root apices than AI-tolerant genotypes, and the differences became more marked with time. Analysis of freeze-dried root apices by x-ray microanalysis showed that AI entered root apices of AI-sensitive plants and accumulated in the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to AI (24 h) resulted in a distribution of AI coinciding with the absence of K. Quantitation of AI in the cortical layer showed that sensitive apices accumulated 5-to 10-fold more AI than tolerant apices exposed to AI solutions for equivalent times. These data are consistent with the hypothesis that Altl encodes a mechanism that excludes AI from root apices.A1 toxicity is one of the major factors that limit plant growth in many acid soils (Wright, 1989). The primary effect of A1 is to inhibit root growth in Al-sensitive genotypes with subsequent effects on nutrient and water uptake (Foy, 1983). Root elongation is affected within hours of A1 exposure (Wallace et al., 1982), and, as in many plant species, tlie primary site of A1 toxicity in wheat (Triticum aestivum L.) appears to be the root apex (Bennet and Breen, 1991). have shown that in wheat and maize, root elongation is inhibited only when apices are exposed to Al, whereas selectively exposing the remainder of the root does not inhibit elongation. Hematoxylin, a stain for Al, stains root apices of Al-sensitive wheat genotypes more intensely than root apices of Al-tolerant genotypes, but the remainder of the root shows the same degree of staining in different genotypes (Polle et al., 1978;Wallace et al., 1982), indicating that tolerance might be a property of the root apex.Differential uptake of A1 into roots could account for differences in tolerance between genotypes, but conflicting results have been reported regarding differences in A1 uptake in roots of different wheat genotypes. Some of these conflicting results appear to be due to the size of the root portion analyzed and the time of exposure to Al. Recently RincÓn and Gonzales (1992) showed that an Al-sensitive wheat cultivar accumulated more A1 in its root apices (2 mm terminus of root) than an Al-tolerant cultivar, which is consistent with the above discus...