Sorption of Aluminum to Plasma Membrane Vesicles Isolated from Roots of Scout 66 and Atlas 66 Cultivars of Wheat

To further elucidate the mechanisms of differential genotypic tolerance to Al, plasma membrane (PM) vesicles were isolated from whole roots, root tips, and tipless roots of Al3+-sensitive and Al3+-tolerant cultivars (cv) of wheat (Triticum aestivum L. cv Scout 66 and cv Atlas 66, respectively). Vesicles from cv Scout root tips sorbed more Al than vesicles prepared from any other source. The intrinsic surface-charge density of vesicles isolated from cv Scout was 26% more negative than vesicles from cv Atlas (-37.2 versus -29.5 millicoulombs m-2). Growth experiments indicated that cv Scout is slightly more sensitive to La3+ than is cv Atlas, that the cultivars are equally sensitive to H+, and that cv Atlas is slightly more sensitive to SeO42-. The difference in sensitivity to Al3+ was very large; for a 50% inhibition, a 16-fold greater activity of Al3+ was required for cv Atlas. Using a newly developed Gouy-Chapman-Stern model for ion sorption to the PM together with growth-response curves, we estimate that the difference in surface-charge density can account for the slightly greater sensitivity of cv Scout to cationic toxicants and the slightly greater sensitivity of cv Atlas to anionic toxicants. According to our estimates the differences in PM surface negativity and Al sorptive capacity probably account for some of the difference in sensitivity to Al3+, but the greater part of the difference probably arises from other tolerance mechanisms expressed in cv Atlas root tips that reduce the amount of Al3+ that can reach the PM.     

Differential effects of aluminium on osmotic potential and sugar accumulation in the root cells of Al-resistant and Al-sensitive wheat

The changes in osmotic potential and the concentration of osmotic solutes in the cell sap of the root tips exposed to Al were examined in two cultivars of wheat ( Triticum aestivum ) differing in Al resistance. Root elongation was less influenced by an 8-h exposure to 20 μ M or 50 μ M Al in Al-resistant cv. Atlas 66 than in Al-sensitive cv. Scout 66. After Al treatment the osmotic potential of the root cells was decreased in Atlas 66 but increased in Scout 66 indicating that the Al treatment osmotically stimulated the driving force for water uptake in Atlas 66 but suppressed it in Scout 66. Al increased the concentration of soluble sugars, the major osmotic solute in the root cells in Atlas 66, but decreased it in Scout 66. Al at both low (5 μ M ) and high (50 μ M ) concentrations, also increased the concentration of soluble sugars in the Al-resistant genotype ET8 but a high Al concentration decreased it in Al-sensitive genotype ES8. Enzymatic analyses and thin-layer chromatography revealed that soluble sugars in the root cells of both Atlas 66 and Scout 66 mainly consisted of monosaccharides such as glucose, fructose and a small amount of sucrose. These results suggest that the accumulation of soluble sugars in Al-resistant wheat Atlas 66 keeps the osmotic potential in the root cells low and thus, enables the root cells to take up water and to elongate against the pressure produced by cell wall rigidification under Al stress.     

Changes in cell-wall properties of wheat (Triticum aestivum) roots during aluminum-induced growth inhibition

The effects of aluminum (Al) on root elongation, the mechanical extensibility of the cell wall, and the amount of cell-wall polysaccharides in the roots of Al-resistant (Atlas 66) and Al-sensitive (Scout 66) cultivars of wheat ( Triticum aestivum L.) were examined. Exposure to 10 μ M AlCl₃ for 6 h inhibited root elongation in Scout 66 but not in Atlas 66. It also decreased the mechanical extensibility of the cell wall in the roots of both cultivars, but prominently only in the roots of Scout 66. The amount of hemicellulose in the 10-mm region of root apex of Scout 66 was increased by the exposure to Al, especially in the apical regions. Al did not influence the neutral sugar composition of either pectin or hemicellulose in Scout 66 roots. However, Al increased the weight-average molecular mass of hemicellulosic polysaccharides and the amounts of wall-bound ferulic and diferulic acids in Scout 66 roots. These findings suggest that Al modifies the metabolism of cell-wall components and thus makes the cell wall thick and rigid, thereby inhibiting the growth of wheat roots.     

Lignin deposition induced by aluminum in wheat (Triticum aestivum) roots

We investigated the relation between the toxic effect of aluminum (Al) on root growth and the lignin deposition in wheat ( Triticum aestivum L. cvs Atlas 66 and Scout 66). In the Al-tolerant cultivar Atlas 66, control treatment without AlCl₃ at pH 4.75, cell length increased dramatically in the portion of the root that was 0.6 to 3.2 mm from the root cap junction (approximately 1.0 to 3.6 mm from the root tip). However, treatment with 20 μ M AlCl₃ for 24 and 48 h completely inhibited root elongation and markedly decreased the length and increased the diameter of the cells in the same portion of the root. Moreover, marked deposition of lignin was observed in the cells that corresponded to the portion 1.5 to 4.5 mm from the root tip in Atlas 66 roots treated with 20 μ M AlCl₃, while no deposition of lignin was detected in control roots. Treatment with 5 μ M AlCl₃ slightly inhibited root growth and there was no deposition of lignin in the root. On the other hand, in roots of the Al-sensitive cultivar Scout 66, treatment with 5 μ M AlCl₃ completely inhibited root growth and markedly induced deposition of lignin. These results suggest that lignification in the elongating region coincided with the extent of inhibition of root growth by Al in two wheat cultivars that differed in their sensitivity to Al.     

Aluminum Effects on Calcium (45Ca2+) Translocation in Aluminum-Tolerant and Aluminum-Sensitive Wheat (Triticum aestivum L.) Cultivars (Differential Responses of the Root Apex versus Mature Root Regions)

The influence of Al exposure on long-distance Ca2+ translocation from specific root zones (root apex or mature root) to the shoot was studied in intact seedlings of winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). Seedlings were grown in 100 [mu]M CaCl2 solution (pH 4.5) for 3 d. Subsequently, a divided chamber technique using 45Ca2+-labeled solutions (100 [mu]M CaCl2 with or without 5 or 20 [mu]M AlCl3, pH 4.5) was used to study Ca2+ translocation from either the terminal 5 to 10 mm of the root or a 10-mm region of intact root approximately 50 mm behind the root apex. The Al concentrations used, which were toxic to Scout 66, caused a significant inhibition of Ca2+ translocation from the apical region of Scout 66 roots. The same Al exposures had a much smaller effect on root apical Ca2+ translocation in Atlas 66. When a 10-mm region of the mature root was exposed to 45Ca2+, smaller genotypic differences in the Al effects effects on Ca2+ translocation were observed, because the degree of Al-induced inhibition of Ca2+ translocation was less than that at the root apex. Exposure of the root apex to Al inhibited root elongation by 70 to 99% in Scout 66 but had a lesser effect (less than 40% inhibition) in Atlas 66. When a mature root region was exposed to Al, root elongation was not significantly affected in either cultivar. These results demonstrate that genotypic differences in Al-induced inhibition of Ca2+ translocation and root growth are localized primarily in the root apex. The pattern of Ca2+ translocation within the intact root was mainly basipetal, with most of the absorbed Ca2+ translocated toward the shoot. A small amount of acropetal Ca2+ translocation from the mature root regions to the apex was also observed, which accounted for less than 5% of the total Ca2+ translocation within the entire root. Because Ca2+ translocation toward the root apex is limited, most of the Ca2+ needed for normal cellular function in the apex must be absorbed from the external solution. Thus, continuous Al disruption of Ca2+ absorption into cells of the root apex could alter Ca2+ nutrition and homeostasis in these cells and could play a pivotal role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars.     

Aluminum effects on calcium fluxes at the root apex of aluminum-tolerant and aluminum-sensitive wheat cultivars

The role of Ca²⁺ transport in the mechanism of Al toxicity was investigated, using a Ca²⁺-selective microelectrode system to study Al effects on root apical Ca²⁺ fluxes in two wheat (Triticum aestivum L.) cultivars: Al-tolerant Atlas 66 and Al-sensitive Scout 66. Intact 3-day-old low-salt-grown (100 micromolar CaCl₂, pH 4.5) wheat seedlings were used, and it was found that both cultivars maintained similar rates of net Ca²⁺ uptake in the absence of Al. Addition of Al concentrations that were toxic to Scout (5-20 micromolar AlCl₃) immediately and dramatically inhibited Ca²⁺ uptake in Scout, whereas Ca²⁺ transport in Atlas was relatively unaffected. The Al-induced inhibition of Ca²⁺ uptake in Scout 66 was rapidly reversed following removal of Al from the solution bathing the roots. Similar studies with morphologically intact root cell wall preparations indicated that the Al effects did not involve Al-Ca interactions in the cell wall. These results suggest that Al inhibits Ca²⁺ influx across the root plasmalemma, possibly via blockage of calcium channels. The differential effect of Al on Ca²⁺ transport in Al-sensitive Scout and Al-tolerant Atlas suggests that Al blockage of Ca²⁺ channels could play a role in the cellular mechanism of Al toxicity in higher plants.     

Aluminum-induced deposition of (1,3)-β-glucans (callose) inTriticum aestivum L.

Aluminum (Al)-induced damage to leaves and roots of two Al-resistant (cv. Atlas 66, experimental line PT741) and two Al-sensitive (cv. Scout 66, cv. Katepwa) lines ofTriticum aestivum L. was estimated using the deposition of (1, 3)--glucans (callose) as a marker for injury. Two-day-old seedlings were grown for forty hours in nutrient solutions with or without added Al, and callose deposition was quantified by spectrofluorometry (0–1000 µM Al) and localized by fluorescence microscopy (0 and 400 µM Al). Results suggested that Al caused little damage to leaves. No callose was observed in leaves with up to 400 µM Al treatment. In contrast, root callose concentration increased with Al treatment, especially in the Al-sensitive lines. At 400 µM Al, root callose concentration of Al-sensitive Scout 66 was nearly four-fold that of Al-resistant Atlas 66. After Al treatment, large callose deposits were observed in the root cap, epidermis and outer cortex of root tips of Scout 66, but not Atlas 66. The identity of callose was confirmed by a reduced fluorescence in Al-treated roots: firstly, after adding an inhibitor of callose synthesis (2-deoxy-D-glucose) to the nutrient solution, and secondly, after incubating root sections with the callosedegrading enzyme -D-glucoside glucohydrolase [EC 3.2.1.21]. Root callose deposition may be a good marker for Al-induced injury due to its early detection by spectrofluorometry and its close association with stress perception.Abbreviations DDG 2-deoxy-D-glucose - PAS periodic acid - Schiffs reagent - PE pachyman equivalents     

Aluminum Interactions with Voltage-Dependent Calcium Transport in Plasma Membrane Vesicles Isolated from Roots of Aluminum-Sensitive and -Resistant Wheat Cultivars

The role of Al interactions with root-cell plasma membrane (PM) Ca2+ channels in Al toxicity and resistance was studied. The experimental approach involved the imposition of a transmembrane electrical potential (via K+ diffusion) in right-side-out PM vesicles derived from roots of two wheat (Triticum aestivum L.) cultivars (Al-sensitive Scout 66 and Al-resistant Atlas 66). We previously used this technique to characterize a voltage-dependent Ca2+ channel in the wheat root PM (J.W. Huang, D.L. Grunes, L.V. Kochian [1994] Proc Natl Acad Sci USA 91: 3473-3477). We found that Al3+ effectively blocked this PM Ca2+ channel; however, Al3+ blocked this Ca2+ channel equally well in both the Al-sensitive and -resistant cultivars. It was found that the differential genotypic sensitivity of this Ca2+ transport system to Al in intact roots versus isolated PM vesicles was due to Al-induced malate exudation localized to the root apex in Al-resistant Atlas but not in Al-sensitive Scout. Because malate can effectively chelate Al3+ in the rhizosphere and exclude it from the root apex, the differential sensitivity of Ca2+ influx to Al in intact roots of Al-resistant versus Al-sensitive wheat cultivars is probably due to the maintenance of lower Al3+ activities in the root apical rhizosphere of the resistant cultivar.     

Aluminum Partitioning in Intact Roots of Aluminum-Tolerant and Aluminum-Sensitive Wheat (Triticum aestivum L.) Cultivars

Aluminum (Al) partitioning in intact roots of wheat (Triticum aestivum L.) cultivars that differ in sensitivity to Al was investigated. Roots of intact seedlings were exposed to Al for up to 24 hours and distribution of Al was assessed visually by hematoxylin staining or by direct measurement of concentration of Al by atomic absorption spectrophotometry or ion chromatography. Major differences in Al accumulation between Al-tolerant (Atlas 66) and Al-sensitive (Tam 105) cultivars were found in the growing regions 0 to 2 and 2 to 5 millimeters from the root apex. Al content was 9 to 13 times greater in the 0 to 2 millimeters root tips of cv Tam 105 than in the tips of cv Atlas 66 when exposed to 50 micromolar Al for 19 to 24 hours. The oxidative phosphorylation inhibitor carbonyl cyanide m-chlorophenylhydrazone and the protein synthesis inhibitor cycloheximide increased Al uptake by intact root tips of cv Atlas 66. Also, loss of Al from the roots of both cultivars was measured after the roots were “pulsed” with 50 micromolar Al for 2 hours and then placed in an Al-free nutrient solution for 6 hours. The 0 to 2 millimeter root tips of cv Tam 105 lost 30% of the absorbed Al, whereas the tips of cv Atlas 66 lost 60%. In light of these results, we conclude that the differential Al sensitivity in wheat correlates with the concentration of Al in the root meristems. The data support the hypothesis that part of the mechanism for Al tolerance in wheat is based on a metabolism-dependent exclusion of Al from the sensitive meristems.     

Aluminum targets elongating cells by reducing cell wall extensibility in wheat roots

Phytotoxicity of aluminum is characterized by a rapid inhibition of root elongation at micromolar concentrations, however, the mechanisms primarily responsible for this response are not well understood. We investigated the effect of Al on the viscosity and elasticity parameters of root cell wall by a creep-extension analysis in two cultivars of wheat (Triticum aestivum L.) differing in Al resistance. The root elongation and both viscous and elastic extensibility of cell wall of the root apices were hardly affected by the exposure to 10 microM Al in an Al-resistant cultivar, Atlas 66. However, similar exposure rapidly inhibited root elongation in an Al-sensitive cultivar, Scout 66 and this was associated with a time-dependent accumulation of Al in the root tissues with more than 77% residing in the cell wall. Al caused a significant decrease in both the viscous and elastic extensibility of cell wall of the root apices of Scout 66. The "break load" of the root apex of Scout 66 was also decreased by Al. However, neither the viscosity nor elasticity of the cell wall was affected by in vitro Al treatment. Furthermore, pre-treatment of seedlings with Al in conditions where root elongation was slow (i.e. low temperature) did not affect the subsequent elongation of roots in a 0 Al treatment at room temperature. These results suggest that the Al-dependent changes in the cell wall viscosity and elasticity are involved in the inhibition of root growth. Furthermore, for Al to reduce cell wall extensibility it must interact with the cell walls of actively elongating cells.     

Possible role of root border cells in detection and avoidance of aluminum toxicity

Root border cells are living cells that surround root apices of most plant species and are involved in production of root exudates. We tested predictions of the hypothesis that they participate in detection and avoidance of aluminum (Al) toxicity by comparing responses of two snapbean (Phaseolus vulgaris) cultivars (cv Dade and cv Romano) known to differ in Al resistance at the whole-root level. Root border cells of these cultivars were killed by excess Al in agarose gels or in simple salt solutions. Percent viability of Al-sensitive cv Romano border cells exposed in situ for 96 h to 200 microM total Al in an agarose gel was significantly less than that of cv Dade border cells; similarly, relative viability of harvested cv Romano border cells was significantly less than that of cv Dade cells after 24 h in 25 microM total Al in a simple salt solution. These results indicate that Al-resistance mechanisms that operate at the level of whole roots also operate at the cellular level in border cells. Al induced a thicker mucilage layer around detached border cells of both cultivars. Cultivar Dade border cells produced a thicker mucilage layer in response to 25 microM Al compared with that of cv Romano cells after 8 h of treatment and this phenomenon preceded that of observed cultivar differences in relative cell viability. Release of an Al-binding mucilage by border cells could play a role in protecting root tips from Al-induced cellular damage.     

Aluminum tolerance of wheat does not induce changes in dominant bacterial community composition or abundance in an acidic soil

Aims

Aluminum-tolerant wheat plants often produce more root exudates such as malate and phosphate than aluminum-sensitive ones under aluminum (Al) stress, which provides environmental differences for microorganism growth in their rhizosphere soils. This study investigated whether soil bacterial community composition and abundance can be affected by wheat plants with different Al tolerance.

Methods

Two wheat varieties, Atlas 66 (Al-tolerant) and Scout 66 (Al-sensitive), were grown for 60 days in acidic soils amended with or without CaCO₃. Plant growth, soil pH, exchangeable Al content, bacterial community composition and abundance were investigated.

Results

Atlas 66 showed better growth and lower rhizosphere soil pH than Scout 66 irrespective of CaCO₃ amendment or not, while there was no significant difference in the exchangeable Al content of rhizosphere soil between the two wheat lines. The dominant bacterial community composition and abundance in rhizosphere soils did not differ between Atlas 66 and Scout 66, although the bacterial abundance in rhizosphere soil of both wheat lines was significantly higher than that in bulk soil. Sphingobacteriales, Clostridiales, Burkholderiales and Acidobacteriales were the dominant bacteria phylotypes.

Conclusions

The difference in wheat Al tolerance does not induce the changes in the dominant bacterial community composition or abundance in the rhizosphere soils.     

Aluminum effects on the kinetics of calcium uptake into cells of the wheat root apex

The effects of aluminum on the concentration-dependent kinetics of Ca²⁺ uptake were studied in two winter wheat (Triticum aestivum L.) cultivars, Al-tolerant Atlas 66 and Al-sensitive Scout 66. Seedlings were grown in 100 M CaCl₂ solution (pH 4.5) for 3 d. Subsequently, net Ca²⁺ fluxes in intact roots were measured using a highly sensitive technique, employing a vibrating Ca²⁺-selective microelectrode. The kinetics of Ca²⁺ uptake into cells of the root apex, for external Ca²⁺ concentrations from 20 to 300 M, were found to be quite similar for both cultivars in the absence of external Al; Ca²⁺ transport could be described by Michaelis-Menten kinetics. When roots were exposed to solutions containing levels of Al that were toxic to Al-sensitive Scout 66 but not to Atlas 66 (5 to 20 M total Al), a strong correlation was observed between Al toxicity and Al-induced inhibition of Ca²⁺ absorption by root apices. For Scout 66, exposure to Al immediately and dramatically inhibited Ca²⁺ uptake over the entire Ca²⁺ concentration range used for these experiments. Kinetic analyses of the Al-Ca interactions in Scout 66 roots were consistent with competitive inhibition of Ca²⁺ uptake by Al. For example, exposure of Scout 66 roots to increasing Al levels (from 0 to 10 M) caused the K _m for Ca²⁺ uptake to increase with each rise in Al concentration, from approx. 100 M in the absence of Al to approx. 300 M in the presence of 10 M Al, while having no effect on the V _max. The same Al exposures had little effect on the kinetics of Ca²⁺ uptake into roots of Atlas 66. The results of this study indicate that Al disruption of Ca²⁺ transport at the root apex may play an important role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars, and that differential Al tolerance may be associated with the ability of Ca²⁺-transport systems in cells of the root apex to resist disruption by potentially toxic levels of Al in the soil solution.We would like to thank Dr. Lionel F. Jaffe, Director of the National Vibrating Probe Facility, Marine Biological Laboratory, Woods Hole, Mass., USA, for making his calcium-selective vibrating-mi-croelectrode system available for a portion of this work. The research presented here was supported in part by USDA/NRI Competitive Grant number 91-37100-6630 to Leon Kochian. Contribution from the USDA-ARS, U.S. Plant, Soil and Nutrition Laboratory, Cornell University, Ithaca, N.Y. This research was part of the program of the Center for Root-Soil Research, Cornell University, Ithaca, N.Y. Department of Soil, Crop and Atmosphere Science, paper No. 1741.     

Continuous secretion of organic acids is related to aluminium resistance during relatively long‐term exposure to aluminium stress

Secretion of organic acid has been suggested to be one of the mechanisms for Al resistance in short‐term experiments. In the present study, relatively long‐term response of roots to Al stress was investigated in terms of organic acid secretion. Eight plant cultivars belonging to 5 species that exhibited differential sensitivity to Al were used. Ten days of intermittent exposure to Al (one day in 0.5 m M CaCl₂ containing 50 µ M AlCl₃ at pH 4.5, alternating with one day in nutrient solution without Al) inhibited root growth by 65% in an Al‐sensitive cultivar of wheat ( Triticum aestivum L. Scout 66) and by 25‐50% in two cultivars of oilseed rape ( Brassica napus L. 94008 and H166), two cultivars of oat ( Avena sativa L. Tochiyutaka and Heoats), and an Al‐tolerant cultivar of wheat (Atlas 66). However, root growth was hardly affected by the same treatment in buckwheat ( Fagopyrum esculentum Moench Jianxi) and radish ( Raphanus sativus L. Guangxi). Organic acids were monitored during the first 6 h of each day of Al treatment, and both the kind and amount of organic acids secreted were found to differ among different species. Roots of buckwheat secreted oxalic acid, those of wheat exuded malic acid, while those of rapeseed, oats, and radish secreted both citric and malic acids. Three different patterns in response to relatively long‐term treatment of Al were found in terms of total amount of organic acids secreted: (1) the amount secreted was very low during the treatment (wheat cv. Scout 66, oat), (2) the amount gradually decreased with duration of treatment (wheat cv. Atlas 66, oilseed rape), and (3) the amount maintained at a high level during the whole period of Al treatment (buckwheat and radish). Combined with the results of growth inhibition, it is suggested that the continuous secretion of organic acids at a high level is related to high Al resistance.     

Aluminium and calcium transport interactions in intact roots and root plasmalemma vesicles from aluminium-sensitive and tolerant wheat cultivars

Recent research from our laboratory indicates that aluminium (Al) and calcium (Ca) transport interactions may play an important role in the mechanisms of Al phytotoxicity. In this study, we investigated the effects of Al on Ca²⁺ transport in intact roots of winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). We used both a vibrating Ca²⁺-microelectrode technique and ⁴⁵Ca²⁺ to monitor Ca²⁺ influx in intact roots. Root apical Ca²⁺ uptake was immediately inhibited, when roots were exposed to Al levels that ultimately decreased root growth in Al-sensitive Scout 66. The Al-tolerant cultivar was able to resist this Al inhibition of Ca²⁺ uptake, and to resist Al inhibition of ⁴⁵Ca²⁺ translocation from roots to shoots. We also studied Ca²⁺ transport in right-side out plasmalemma vesicles isolated from roots of Al-sensitive and tolerant wheat cultivars. Calcium influx into the vesicles was mediated by a voltage-gated Ca²⁺ channel. Aluminium blocks the Ca²⁺ channel equally well in the plasmalemma vesicles isolated from Al-sensitive and Al-tolerant wheat roots. The results indicate that the differential response observed in intact roots is not due to differences in Ca²⁺ channels. The Al-tolerant wheat cultivar may have an ability to reduce Al³⁺ activity in the rhizosphere, thus reducing the Al-inhibition of Ca²⁺ influx.     

Multiple Aluminum-Resistance Mechanisms in Wheat (Roles of Root Apical Phosphate and Malate Exudation)

Although it is well known that aluminum (Al) resistance in wheat (Triticum aestivum) is multigenic, physiological evidence for multiple mechanisms of Al resistance has not yet been documented. The role of root apical phosphate and malate exudation in Al resistance was investigated in two wheat cultivars (Al-resistant Atlas and Al-sensitive Scout) and two near-isogenic lines (Al-resistant ET3 and Al-sensitive ES3). In Atlas Al resistance is multigenic, whereas in ET3 resistance is conditioned by the single Alt1 locus. Based on root- growth experiments, Atlas was found to be 3-fold more resistant in 20 [mu]M Al than ET3. Root-exudation experiments were conducted under sterile conditions; a large malate efflux localized to the root apex was observed only in Atlas and in ET3 and only in the presence of Al (5 and 20 [mu]M). Furthermore, the more Al-resistant Atlas exhibited a constitutive phosphate release localized to the root apex. As predicted from the formation constants for the Al-malate and Al-phosphate complexes, the addition of either ligand to the root bathing solution alleviated Al inhibition of root growth in Al-sensitive Scout. These results provide physiological evidence that Al resistance in Atlas is conditioned by at least two genes. In addition to the alt locus that controls Al-induced malate release from the root apex, other genetic loci appear to control constitutive phosphate release from the apex. We suggest that both exudation processes act in concert to enhance Al exclusion and Al resistance in Atlas.     

Mechanisms of Al‐induced iron chlorosis in wheat (Triticum aestivum). Al‐inhibited biosynthesis and secretion of phytosiderophore

Although Al‐induced iron chlorosis has been observed in many plants, the mechanisms responsible for this phenomenon are yet to be understood. We investigated the effect of Al on iron acquisition in a Strategy II plant, wheat ( Triticum aestivum L.) using both Al‐tolerant (Atlas 66) and ‐sensitive (Scout 66) cultivars. When iron was supplied as insoluble iron, ferric hydroxide, in the culture solution, both cultivars without Al treatment grew normally, while those with 100 µ M AlCl₃ developed chlorosis of the young leaves after 3 days of the treatment. A 21‐h treatment with 100 µ M AlCl₃ in 0.5 m M CaCl₂ solution (pH 4.5) decreased the amount of 2'‐deoxymugineic acid (DMA) secreted by Fe‐deficient Atlas 66 and Scout 66 plants by 85 and 90%, respectively. The amount of DMA secreted decreased with increasing external Al concentrations. Al treatment during the biosynthesis process caused the inhibition of that of DMA within 3 h. The secretion process was also found to be inhibited by Al, resulting in the biosynthesized DMA remaining in the roots. These results demonstrate the inhibition by Al of both biosynthesis and secretion of DMA attributed to Al‐induced iron chlorosis.     

Possible Involvement of Al-Induced Electrical Signals in Al Tolerance in Wheat

The relationship between Al-induced depolarization of root-cell transmembrane electrical potentials (Em) and Al tolerance in wheat (Triticum aestivum L.) was investigated. Al exposure induced depolarizations of Em in the Al-tolerant wheat cultivars Atlas and ET3, but not in the Al-sensitive wheat cultivars Scout and ES3. The depolarizations of Em occured in root cap cells and as far back as 10 mm from the root tip. The depolarization was specific to Al3+; no depolarization was observed when roots were exposed to the rhizotoxic trivalent cation La3+. The Al-induced depolarization occurred in the presence of anion-channel antagonists that blocked the release of malate, indicating that the depolarization is not due to the electrogenic efflux of malate2-. K+-induced depolarizations in the root cap were of the same magnitude as Al-induced depolarizations, but did not trigger malate release, indicating that Al-induced depolarization of root cap cell membrane potentials is probably linked to, but is not sufficient to trigger, malate release.     

Response and tolerance of root border cells to aluminum toxicity in soybean seedlings

Root border cells (RBCs) and their secreted mucilage are suggested to participate in the resistance against toxic metal cations, including aluminum (Al), in the rhizosphere. However, the mechanisms by which the individual cell populations respond to Al and their role in Al resistance still remain unclear. In this research, the response and tolerance of RBCs to Al toxicity were investigated in the root tips of two soybean cultivars [Zhechun No. 2 (Al-tolerant cultivar) and Huachun No. 18 (Al-sensitive cultivar)]. Al inhibited root elongation and increased pectin methylesterase (PME) activity in the root tip. Removal of RBCs from the root tips resulted in a more severe inhibition of root elongation, especially in Huachun No. 18. Increasing Al levels and treatment time decreased the relative percent viability of RBCs in situ and in vitro in both soybean cultivars. Al application significantly increased mucilage layer thickness around the detached RBCs of both cultivars. Additionally, a significantly higher relative percent cell viability of attached and detached RBCs and thicker mucilage layers were observed in Zhechun No. 2. The higher viability of attached and detached RBCs, as well as the thickening of the mucilage layer in separated RBCs, suggest that RBCs play an important role in protecting root apices from Al toxicity.     

Identification of new sources of aluminum resistance in wheat

Aluminum (Al) toxicity is a major constraint for wheat production in acidic soils. An Al resistance gene on chromosome 4DL that traces to Brazilian wheat has been extensively studied, and can provide partial protection from Al damage. To identify potentially new sources of Al resistance, 590 wheat accessions, including elite wheat breeding lines from the United States and other American and European countries, landraces and commercial cultivars from East Asia, and synthetic wheat lines from CIMMYT, Mexico, were screened for Al resistance by measuring relative root elongation in culture with a nutrient solution containing Al, and by staining Al-stressed root tips with hematoxylin. Eighty-eight wheat accessions demonstrated at least moderate resistance to Al toxicity. Those selected lines were subjected to analysis of microsatellite markers linked to an Al resistance gene on 4DL and a gene marker for the Al-activated malate transporter (ALMT1) locus. Many of the selected Al-resistant accessions from East Asia did not have the Al-resistant marker alleles of ALMT1, although they showed Al resistance similar to the US Al-resistant cultivar, Atlas 66. Most of the cultivars derived from Jagger and Atlas 66 have the Al-resistant marker alleles of ALMT1. Cluster analysis separated the selected Al-resistant germplasm into two major clusters, labeled as Asian and American–European clusters. Potentially new germplasm of Al resistance different from those derived from Brazil were identified. Further investigation of Al resistance in those new germplasms may reveal alternative Al-resistance mechanisms in wheat. Electronic supplementary material The online version of this article (doi:contains supplementary material, which is available to authorized users. Responsible Editor: Thomas B. Kinraide.