Target sites of aluminum phytotoxicity

The primary phytotoxic effect of aluminum (Al) is confined to the root apex. It is a matter of debate whether the primary injury of Al toxicity is apoplastic or symplastic. This review paper summarizes our current understanding of the spatial and metabolic sites of Al phytotoxicity. At tissue level, the meristematic, distal transition, and apical elongation zones of the root apex are most sensitive to Al. At cellular and molecular level, many cell components are implicated in Al toxicity including DNA in nucleus, numerous cytoplastic compounds, the plasma membrane, and the cell wall. Although it is difficult to distinguish the primary targets from the secondary effects so far, understanding of the target sites of Al toxicity is helpful for elucidating the mechanisms by which Al exerts its deleterious effects on root growth.This work was partly supported by fund from the Huoyingdong Foundation, Education Ministry of China and Natural Science Foundation of China (Contact No. 30170548).     

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

The mechanism of aluminium-induced inhibition of root elongation is still not well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize. Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm. The most Al-sensitive apical root zone appeared to be the 1–2 mm segment. Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies. Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days. NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al. This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex. On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum. This revised version was published online in June 2006 with corrections to the Cover Date.     

Intracellular distribution and binding state of aluminum in root apices of two common bean (Phaseolus vulgaris) genotypes in relation to Al toxicity

The role of the intracellular distribution and binding state of aluminum (Al) in Al toxicity, using Al exchange and Al fractionation methodologies, were studied in two common bean ( Phaseolus vulgaris L.) genotypes differing in Al resistance. These two genotypes are characterized by a similar initial period (4 h) of Al sensitivity followed by a contrasting recovery period (8–24 h). A higher initial Al accumulation in Quimbaya (Al resistant) in the 5-mm root apex compared with VAX-1 (Al sensitive) could be related to its higher content of unmethylated pectin and thus higher negative charge of the cell walls (CWs). The binding state and cellular distribution of Al in the root apices revealed that the root elongation rate was significantly negatively correlated with the free apoplastic and the stable-bound, not citrate-exchangeable CW Al representing the most important Al fraction in the root apex (80%), but not with the symplastic and the labile-bound, citrate-exchangeable CW Al. It is postulated that the induced and sustained recovery from the initial Al stress in the Al-resistant genotype Quimbaya requires reducing the stable-bound Al in the apoplast thus allowing cell elongation and division to resume.     

Toxicities of soluble Al, Cu, and La include ruptures to rhizodermal and root cortical cells of cowpea

Elevated levels of many metals are toxic to plant roots, but their modes of action are not well understood. We investigated the toxicities of aluminium (Al), copper (Cu), and lanthanum (La) in solution on the growth and external morphology of 3-d-old cowpea (Vigna unguiculata L.) roots for periods of up to 48 h. Root elongation rate decreased by 50% at ca. 30 μM Al, 0.3 μM Cu, or 2.0 μM La, accompanied by a decrease in the distance from the root tip to the proximal lateral root. Kinks developed in some roots 2.0 ± 0.4 mm from the root apex on exposure to Al or La (but not Cu). Light and scanning electron microscopy showed that soluble Al, Cu, or La caused similar transverse ruptures to develop > 1 mm from the root apex through the breaking and separation of the rhizodermis and outer cortex from inner-layers. The metals differed, however, in the range in concentration at which they had this effect; developing in solutions containing 54 to‑600 μM Al, but only from 0.85 to 1.8 μM Cu or 2.0 to 5.5 μM La. These findings suggest that Al, Cu, and La bind to the walls of cells, causing increased cell wall rigidity and eventual cell rupturing of the rhizodermis and outer cortex in the elongating zone. We propose that this is a major toxic effect of Al, and that Cu and La also have additional toxic effects.     

Spatial aluminium sensitivity of root apices of two common bean (Phaseolus vulgaris L.) genotypes with contrasting aluminium resistance

The initial response of plants to aluminium (Al) is an inhibition of root elongation. In the present study, short and medium-term effects of Al treatment (20 muM) on root growth and Al accumulation of two common bean (Phaseolus vulgaris L.) genotypes, VAX-1 (Al-sensitive) and Quimbaya (Al-resistant), were studied. Root elongation of both genotypes was severely inhibited during the first 3-4 h of Al treatment. Thereafter, both genotypes showed gradual recovery. However, this recovery continued in genotype Quimbaya until the root elongation rate reached the level of the control (without Al) while the genotype VAX-1 was increasingly damaged by Al after 12 h of Al treatment. Short-term Al treatment (90 microM Al) to different zones of the root apex using agarose blocks corroborated the importance of the transition zone (TZ, 1-2 mm) as a main target of Al. However, Al applied to the elongation zone (EZ) also contributed to the overall inhibition of root elongation. Enhanced inhibition of root elongation during the initial 4 h of Al treatment was related to high Al accumulation in root apices in both genotypes (Quimbaya>VAX-1). Recovery from Al stress was reflected by decreasing Al contents especially in the TZ, but also in the EZ. After 24 h of Al treatment the high Al resistance of Quimbaya was reflected by much lower Al contents in the entire root apex. The results confirmed that genotypic differences in Al resistance in common bean are built up during medium-term exposure of the roots to Al. For this acquisition of Al resistance, the activation and maintenance of an Al exclusion mechanism, especially in the TZ but also in the EZ, appears to be decisive.     

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.     

Genotypical differences in aluminum resistance of maize are expressed in the distal part of the transition zone. Is reduced basipetal auxin flow involved in inhibition of root elongation by aluminum?

Short-term Al treatment (90 microM Al at pH 4.5 for 1 h) of the distal transition zone (DTZ; 1-2 mm from the root tip), which does not contribute significantly to root elongation, inhibited root elongation in the main elongation zone (EZ; 2.5-5 mm from the root tip) to the same extent as treatment of the entire maize (Zea mays) root apex. Application of Al to the EZ had no effect on root elongation. Higher genotypical resistance to Al applied to the entire root apex, and specifically to the DTZ, was expressed by less inhibition of root elongation, Al accumulation, and Al-induced callose formation, primarily in the DTZ. A characteristic pH profile along the surface of the root apex with a maximum of pH 5.3 in the DTZ was demonstrated. Al application induced a substantial flattening of the pH profile moreso in the Al-sensitive than in the Al-resistant cultivar. Application of indole-3-acetic acid to the EZ but not to the meristematic zone significantly alleviated the inhibition of root elongation induced by the application of Al to the DTZ. Basipetal transport of exogenously applied [(3)H]indole-3-acetic acid to the meristematic zone was significantly inhibited by Al application to the DTZ in the Al-sensitive maize cv Lixis. Our results provide evidence that the primary mechanisms of genotypical differences in Al resistance are located within the DTZ, and suggest a signaling pathway in the root apex mediating the Al signal between the DTZ and the EZ through basipetal auxin transport.     

Role of phytohormones in aluminium rhizotoxicity

Elevated concentrations of soluble aluminium (Al) reduce root growth in acid soils, but much remains unknown regarding the toxicity of this Al as well as the mechanisms by which plants respond. This review examines changes in phytohormones in Al‐stressed plants. Al often results in a rapid ‘burst’ of ethylene in root apical tissues within 15–30 min, with this regulating an increase in auxin. This production of ethylene and auxin seems to be a component of a plant‐response to toxic Al, resulting in cell wall modification or regulation of organic acid release. There is also evidence of a role of auxin in the expression of Al toxicity itself, with Al decreasing basipetal transport of auxin, thereby potentially decreasing wall loosening as required for elongation. Increasingly, changes in abscisic acid in root apices also seem to be involved in plant‐responses to toxic Al. Changes in cytokinins, gibberellins and jasmonates following exposure to Al are also examined, although little information is available. Finally, although not a phytohormone, concentrations of nitric oxide change rapidly in Al‐exposed tissues. The information presented in this review will assist in focusing future research efforts in examining the importance of phytohormones in plant tissues exposed to toxic levels of 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.     

Effect of aluminium on lipid peroxidation, superoxide dismutase, catalase, and peroxidase activities in root tips of soybean (Glycine max)

Inhibition of root elongation and modification of membrane properties are sensitive responses of plants to aluminium. The present paper reports on the effect of AI on lipid peroxidation and activities of enzymes related to production of activated oxygen species. Soybean seedlings (Glycine max L. cv. Sito) were precultured in solution culture for 3–5 days and then treated for 1–72 h with Al (AICI₃) concentrations ranging from 10 to 75 μM at a constant pH of 4.1. In response to Al supply, lipid peroxidation in the root tips (< 2 cm) was enhanced only after longer durations of treatment. Aluminium-dependent increase in lipid peroxidation was intensified by Fe²⁺ (FeSO₄). A close relationship existed between lipid peroxidation and inhibition of root-elongation rate induced by Al and/or Fe toxicity and/or Ca deficiency. Besides enhancement of lipid peroxidation in the crude extracts of root tips due to Al, the activities of superoxide dismutase (EC 1.15.1.1) and peroxidase (EC 1.11.1.7) increased, whereas catalase (EC 1.11.1.6) activity decreased. This indicates a greater generation of oxygen free radicals and related tissue damage. The results suggest that lipid peroxidation is part of the overall expression of Al toxicity in roots and that enhanced lipid peroxidation by oxygen free radicals is a consequence of primary effects of Al on membrane structure.     

Cell wall polysaccharides are specifically involved in the exclusion of aluminum from the rice root apex

Rice (Oryza sativa) is the most aluminum (Al)-resistant crop species among the small-grain cereals, but the mechanisms responsible for this trait are still unclear. Using two rice cultivars differing in Al resistance, rice sp. japonica 'Nipponbare' (an Al-resistant cultivar) and rice sp. indica 'Zhefu802' (an Al-sensitive cultivar), it was found that Al content in the root apex (0-10 mm) was significantly lower in Al-resistant 'Nipponbare' than in sensitive 'Zhefu802', with more of the Al localized to cell walls in 'Zhefu802', indicating that an Al exclusion mechanism is operating in 'Nipponbare'. However, neither organic acid efflux nor changes in rhizosphere pH appear to be responsible for the Al exclusion. Interestingly, cell wall polysaccharides (pectin, hemicellulose 1, and hemicellulose 2) in the root apex were found to be significantly higher in 'Zhefu802' than in 'Nipponbare' in the absence of Al, and Al exposure increased root apex hemicellulose content more significantly in 'Zhefu802'. Root tip cell wall pectin methylesterase (PME) activity was constitutively higher in 'Zhefu802' than in 'Nipponbare', although Al treatment resulted in increased PME activity in both cultivars. Immunolocalization of pectins showed a higher proportion of demethylated pectins in 'Zhefu802', indicating a higher proportion of free pectic acid residues in the cell walls of 'Zhefu802' root tips. Al adsorption and desorption kinetics of root tip cell walls also indicated that more Al was adsorbed and bound Al was retained more tightly in 'Zhefu802', which was consistent with Al content, PME activity, and pectin demethylesterification results. These responses were specific to Al compared with other metals (CdCl(2), LaCl(3), and CuCl(2)), and the ability of the cell wall to adsorb these metals was also not related to levels of cell wall pectins. All of these results suggest that cell wall polysaccharides may play an important role in excluding Al specifically from the rice root apex.     

Evaluating the role of root citrate exudation as a mechanism of aluminium resistance in maize genotypes

Organic anion exudation by roots as a mechanism of aluminium (Al) resistance has been intensively studied lately. In the present study, we evaluated qualitative and quantitative aspects of root exudation of organic anions in maize genotypes of distinct sensitivity to Al in response to Al exposure. Maize seedlings were grown axenically in nutrient solution and root exudates were collected along the whole seminal root axis for a short period (4 h) using a divided-root-chamber technique. In root exudates collected from 10-mm long root apices, citrate accounted for 67% of the total organic anions found, followed by malate (29%), trans-aconitate (3%), fumarate (<1%), and cis-aconitate (1%). Rates of citrate exudation from root apices of two genotypes with differential resistance to Al were consistently higher in the Al resistant one, differing by a factor of 1.7 – 3.0 across a range of external Al concentrations. Furthermore, relative Al resistance of eight maize genotypes correlated significantly well with their citrate exudation rate measured at 40 M Al. Higher exudation rates were accompanied by a less inhibited root elongation. The exudation of citrate along the longitudinal axis of fully developed seminal roots showed a particular pattern: citrate was exuded mainly in the regions of root apices, either belonging to the main root or to the lateral roots in the most basal part of the main root. The involvement of citrate in a mechanism of Al resistance is evaluated in terms of protection of the root from the effects of excess Al on root elongation and on nutrient uptake along a root axis showing distinct sites of citrate exudation.     

Root elongation rate is correlated with the length of the bare root apex of maize and lupin roots despite contrasting responses of root growth to compact and dry soils

Background

We investigated interacting effects of matric potential and soil strength on root elongation of maize and lupin, and relations between root elongation rates and the length of bare (hairless) root apex.

Methods

Root elongation rates and the length of bare root apex were determined for maize and lupin seedlings in sandy loam soil of various matric potentials (?0.01 to ?1.6 MPa) and bulk densities (0.9 to 1.5 Mg m^?3).

Results

Root elongation rates slowed with both decreasing matric potential and increasing penetrometer resistance. Root elongation of maize slowed to 10 % of the unimpeded rate when penetrometer resistance increased to 2 MPa, whereas lupin elongated at about 40 % of the unimpeded rate. Maize root elongation rate was more sensitive to changes in matric potential in loosely packed soil (penetrometer resistances <1 MPa) than lupin. Despite these differing responses, root elongation rate of both species was linearly correlated with length of the bare root apex (r² 0.69 to 0.97).

Conclusion

Maize root elongation was more sensitive to changes in matric potential and mechanical impedance than lupin. Robust linear relationships between elongation rate and length of bare apex suggest good potential for estimating root elongation rates for excavated roots.     

Spatial characteristics of aluminum uptake and translocation in roots of buckwheat (Fagopyrum esculentum)

The detoxification of aluminum (Al) in root tips of the Al accumulator buckwheat by exudation of oxalate leading to reduced Al uptake (Al resistance) is difficult to reconcile with the Al accumulation (Al tolerance). The objective of this study was to analyze resistance and tolerance mechanisms at the same time evaluating particularly possible stratification of Al uptake, Al transport and oxalate exudation along the root apex. The use of a minirhizotron made it possible to differentiate between spatial responses to Al along the root apex with regard to Al uptake and organic acid anion exudation, but also to measure at the same time Al and organic acid transport in the xylem. Al accumulates particularly in the 3‐mm root apex. The study showed that Al taken up by the 10‐mm root apex is rapidly transferred to the xylem which differentiates in the 10 to 15‐mm root zone as revealed by a microscopic study. Al induces the release of oxalate from the root apex but particularly from the subapical 6–20 mm root zone even when Al was applied only to the 5‐mm root apex suggesting a basipetal signal transduction. Citrate proved to be the most likely ligand for Al in the xylem because Al and citrate transport rates were positively correlated. In conclusion, the data presented show that the Al‐induced release of oxalate, and Al uptake as well as Al accumulation are spatially not separated in the root apex.     

Cell membrane integrity,callose accumulation,and root growth in aluminum-stressed sorghum seedlings

Aluminum stress usually reduces plant root growth due to the accumulation of Al in specific zones of the root apex. The objectives of this study were to determine the localization of Al in the root apex of Sorghum bicolor (L.) Moech. and its effects on membrane integrity, callose accumulation, and root growth in selected cultivars. Seedlings were grown in a nutrient solution containing 0, 27, or 39 μM Al³⁺ for 24, 48, and 120 h. The Al stress significantly reduced root growth, especially after 48 and 120 h of exposure. A higher Al accumulation, determined by fluorescence microscopy after staining with a Morin dye, occurred in the root extension zone of the sensitive cultivar than in the tolerant cultivar. The membrane damage and callose accumulation were also higher in the sensitive than resistant cultivar. It was concluded that the Al stress significantly reduced root growth through the accumulation of Al in the root extension zone, callose accumulation, and impairment of plasma membrane integrity.     

Zonal responses of sensitive vs. tolerant wheat roots during Al exposure and recovery

Aluminium (Al) irreversibly inhibits root growth in sensitive, but not in some tolerant genotypes. To better understand tolerance mechanisms, seedlings from tolerant ('Barbela 7/72' line) and sensitive ('Anahuac') Triticum aestivum L. genotypes were exposed to AlCl(3) 185 μM for: (a) 24 h followed by 48 h without Al (recovery); (b) 72 h of continuous exposure. Three root zones were analyzed (meristematic (MZ), elongation (EZ) and hairy (HZ)) for callose deposition, reserves (starch and lipids) accumulation, endodermis differentiation and tissue architecture. Putative Al-induced genotoxic or cytostatic/mytogenic effects were assessed by flow cytometry in root apices. Tolerant plants accumulated less Al, presented less root damage and a less generalized callose distribution than sensitive ones. Starch and lipid reserves remained constant in tolerant roots but drastically decreased in sensitive ones. Al induced different profiles of endodermis differentiation: differentiation was promoted in EZ and HZ, respectively, in sensitive and tolerant genotypes. No ploidy changes or clastogenicity were observed. However, differences in cell cycle blockage profiles were detected, being less severe in tolerant roots. After Al removal, only the 'Barbela 7/72' line reversed Al-induced effects to values closer to the control, mostly with respect to callose deposition and cell cycle progression. We demonstrate for the first time that: (a) cell cycle progression is differently regulated by Al-tolerant and Al-sensitive genotypes; (b) Al induces callose deposition >3 cm above root apex (in HZ); (c) callose deposition is a transient Al-induced effect in tolerant plants; and (d) in HZ, endodermis differentiation is also stimulated only in tolerant plants, probably functioning in tolerant genotypes as a protective mechanism in addition to callose.     

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum

Background and Aims

Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance.

Methods

Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared.

Key Results

Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar ‘Chuan’ was significantly lower than that in the most Al-sensitive cultivar ‘Liuku2’. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in ‘Chuan’ and ‘Liuku2’ in the absence of Al, but Al treatment resulted in increased PME activity in ‘Liuku2’ compared with ‘Chuan’. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in ‘Liuku2’.

Conclusions

Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.     

Aluminum activates a citrate-permeable anion channel in the aluminum-sensitive zone of the maize root apex. A comparison between an aluminum- sensitive and an aluminum-resistant cultivar

In search for the cellular and molecular basis for differences in aluminum (Al) resistance between maize (Zea mays) cultivars we applied the patch-clamp technique to protoplasts isolated from the apical root cortex of two maize cultivars differing in Al resistance. Measurements were performed on protoplasts from two apical root zones: The 1- to 2-mm zone (DTZ), described as most Al-sensitive, and the main elongation zone (3-5 mm), the site of Al-induced inhibition of cell elongation. Al stimulated citrate and malate efflux from intact root apices, revealing cultivar differences. In the elongation zone, anion channels were not observed in the absence and presence of Al. Preincubation of intact roots with 90 microM Al for 1 h induced a citrate- and malate-permeable, large conductance anion channel in 80% of the DTZ protoplasts from the resistant cultivar, but only 30% from the sensitive cultivar. When Al was applied to the protoplasts in the whole-cell configuration, anion currents were elicited within 10 min in the resistant cultivar only. La3+ was not able to replace or counteract with Al3+ in the activation of this channel. In the presence of the anion-channel blockers, niflumic acid and 4, 4'-dinitrostilbene-2, 2'disulfonic acid, anion currents as well as exudation rates were strongly inhibited. Application of cycloheximide did not affect the Al response, suggesting that the channel is activated through post-translational modifications. We propose that the Al-activated large anion channel described here contributes to enhanced genotypical Al resistance by facilitating the exudation of organic acid anions from the DTZ of the maize root apex.     

Aluminum resistance in common bean (Phaseolus vulgaris) involves induction and maintenance of citrate exudation from root apices

Two common bean (Phaseolus vulgaris L.) genotypes differing in aluminum (Al) resistance, Quimbaya (Al‐resistant) and VAX‐1 (Al‐sensitive) were grown in hydroponics for up to 25 h with or without Al, and several parameters related to the exudation of organic acids anions from the root apex were investigated. Al treatment enhanced the exudation of citrate from the root tips of both genotypes. However, its dynamic offers the most consistent relationship between Al‐induced inhibition of root elongation and Al accumulation in and exclusion from the root apices. Initially, in both genotypes the short‐term (4 h) Al‐injury period was characterized by the absence of citrate efflux independent of the citrate content of the root apices, and reduction of cytosolic turnover of citrate conferred by a reduced Nicotinamide adenine dinucleotide phosphate–isocitrate dehydrogenase (EC 1.1.1.42) activity. Transient recovery from initial Al stress (4–12 h) was found to be dependent mainly on the capacity to utilize internal citrate pools (Al‐resistant genotype Quimbaya) or enhanced citrate synthesis [increased activities of NAD‐malate dehydrogenase (EC 1.1.1.37) and ATP‐phosphofructokinase (EC 2.7.1.11) in Al‐sensitive VAX‐1]. Sustained recovery from Al stress through citrate exudation in genotype Quimbaya after 24 h Al treatment relied on restoring the internal citrate pool and the constitutive high activity of citrate synthase (CS) (EC 4.1.3.7) fuelled by high phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity. In the Al‐sensitive genotype VAX‐1 the citrate exudation and thus Al exclusion and root elongation could not be maintained coinciding with an exhaustion of the internal citrate pool and decreased CS activity.