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 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.     

Effect of boron on the expression of aluminium toxicity in Phaseolus vulgaris

The interaction of boron (B) and aluminium (Al) was investigated in 5-day-old seedlings of soybean cv. Maple Arrow. Al treatment inhibited root elongation and callose formation in root tips particularly after 4-h Al treatment. After 10 and 24 h, both parameters indicated increasing recovery from Al stress. B deficiency aggravated Al toxicity compared with B sufficiency. B deficiency did lead to an increase in unmethylated pectin in the first 3 mm of the root tip. This increase in potential binding sites is reflected in generally higher Al contents in root tips of B-deficient plants. A fractionated extraction of Al from the root tips showed that citrate-exchangeable and non-exchangeable Al steeply increased up to 4 h, but then decreased after 10- and 24-h Al treatment faster in B-sufficient than in B-deficient plants. This decrease of Al contents can be explained by an Al-enhanced release of citrate from the root tips after 10-h Al treatment. However, the citrate exudation rate was the same (after 10 h) or even lower (after 24 h) in B-sufficient plants and thus cannot explain the faster decrease in Al contents of the root tips compared with the B-deficient plants. We, therefore, propose that under B deficiency, Al is more strongly bound by the pectic network of the cell wall of the root tips, which delays or prevents the recovery from initial Al stress through exudation of citrate, and thus explains the greater Al sensitivity of B-deficient common bean roots.     

Cell-wall pectin and its degree of methylation in the maize root-apex: significance for genotypic differences in aluminium resistance

Cell-wall (CW) pectin content and its degree of methylation in root apices of selected maize cultivars were studied in relation to genotypic Al resistance. Maize cultivars differing in Al resistance were grown in nutrient solution treated with or without Al, and pectin content of the root tips was determined. Control plants did not differ in pectin content in the 5 mm root apex. Al treatment increased the pectin content of the root apex in all cultivars but more prominently in the Al-sensitive cultivars. Pectin and Al contents in 1 mm root sections decreased from the apex to the 3–4 mm zone. Pectin contents of the apical root sections were consistently higher although significantly different only in the 1–2 mm zone in the Al-sensitive cv Lixis. Al contents in most root sections were significantly higher in cv Lixis than in Al-resistant cv ATP-Y. Localization of pectins by immunofluorescence revealed that Al-sensitive cv. Lixis has a higher proportion of low-methylated pectin and thus a higher negativity of the cell wall than Al-resistant cv ATP-Y. This is in agreement with the higher Al content and Al sensitivity of cv Lixis. It is concluded that differences in CW pectin and its degree of methylation contribute to genotypic differences in Al resistance in maize in addition to the release of organic acid anions previously reported.     

Differential Al resistance and citrate secretion in the tap and basal roots of common bean seedlings

The common bean root system is composed of several types of root (e.g. tap, basal, and lateral roots), whose physiological functions may be of great difference. However, we do not know if the root system of common bean differs in organic acid secretion and thus aluminium (Al) resistance. In the present study, the tap and basal roots of three common bean genotypes (i.e. G19842, SQ12 and BAT881) from different origins were compared for their citrate secretion and Al resistance. Grown in a simple solution containing 30 µM Al³⁺ for 24 h, genotype G19842 maintained 75% relative tap root length [RTRL = (tap root length with Al)/(tap root length without Al)], 48% relative basal root length [RBRL = (basal root length with Al)/ (basal root length without Al)], genotype SQ12 maintained 62% RTRL and 57% RBRL, while BAT881 only maintained 31% RTRL and 19% RBRL, indicating differential sensitivity of bean genotypes and root types to Al stress. The amounts of Al‐induced citrate secretion by the tap/basal roots were 9.8/5.1, 8.2/5.9 and 5.4/4.1 nmol cm^?2 FR (fresh root) [12 h]^?1 for G19842, SQ12 and BAT881, respectively, indicating that both bean genotypes and root types differ in organic acid secretion. In G19842, the root surface area was 25% higher in tap root apex than that in basal root apex, and the amounts of citrate secretion per unit surface area and per root apex were 29 and 62% higher in tap root apex than those in basal root apex, respectively, suggesting that the higher citrate secretion in the tap root apex could be attributed to the larger surface area and the higher secretion activity. Stronger inhibition of Al‐induced citrate secretion in the basal than tap roots by anthracene‐9‐carboxylic acid, an inhibitor of anion channel and K‐252a, a broad range inhibitor of protein kinase may also imply the differences in the activities of anion channels and K‐252a‐sensitive protein kinases on the plasma membrane between the tap and basal roots, resulting in differential citrate secretion. We propose that the higher Al resistance in the tap root than in basal roots might be attributed to both greater number and higher activity of the anion channels in the former, thus allowing more citrate secretion in this root type.     

The Distal Part of the Transition Zone Is the Most Aluminum-Sensitive Apical Root Zone of Maize

For a better understanding of Al inhibition of root elongation, knowledge of the morphological and functional organization of the root apex is a prerequisite. We developed a polyvinyl chloride-block technique to supply Al (90 μm monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured during a period of 5 h. After Al treatment, callose (5 h) and Al (1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root apex. Only applying Al to the three apical 1-mm root zones inhibited root elongation after 1 h. The order of sensitivity was 1 to 2 > 0 to 1 > 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was found, independently of whether Al was applied to individual apical root zones or uniformly to the whole-root apex. We conclude from these results that the distal part of the transition zone of the root apex, where the cells are undergoing a preparatory phase for rapid elongation (F. Baluška, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112: 3–4), is the primary target of Al in this Al-sensitive maize cultivar.     

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.     

Differential aluminum tolerance in soybean: An evaluation of the role of organic acids

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al³⁺ activities up to 4.7 μ M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r²=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.     

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.     

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.     

Morphological and Structural Responses of Plant Roots to Aluminium at Organ, Tissue, and Cellular Levels

Toxic effects of aluminium are primarily root-related. This review deals with growth, morphological, and ultrastructural responses of root to aluminium, their diversity along the root axis, and in the root tissues. The cell elongation seems to be most sensitive and responsible for early inhibition of root elongation. Longer Al treatment is required to reduce cell division or to interfere with nucleic acids in the root apex. Alterations of root morphology include root thickening, disturbances of root peripheral tissues, and initiation of lateral roots closer to the root tip. Ultrastructure alterations depend strongly on position of the cells with respect to the Al source, and on their developmental stage. Cell elongation and cell ultrastructure including organisation of cytoskeleton are most sensitive within the distal part of the transition zone of the root apex. This correlates with the rate of uptake and accumulation of Al along the root apex. Recognising the diverse responses and the most sensitive sites within the root apex can help in elucidating the mechanism(s) of Al effects on plants.     

Oxidative stress is associated with aluminum toxicity recovery in apex of pea root

Aims

Although many studies on the mechanism of Al toxicity and tolerance have been conducted independently, events occurring during the recovery process from Al injury is limited. This study was to investigate Al toxicity recovery mechanism focusing in morphological and physiological aspect.

Methods

We investigated the mechanisms underlying Al toxicity recovery in terms of oxidative stress using the pea root apex as a model system.

Results

The accumulation of reactive oxygen species was remarkably high in the root under continued Al treatment but decreased in the recovering root. The superoxide anion exuded in the presence of nicotinamide adenine dinucleotide phosphate (NADPH) showed a similar tendency with respect to the accumulation of reactive oxygen species. A similar pattern of lignin content and superoxide dismutase activity was observed among the treatments, while the increased peroxidation in the root under continued Al treatment did not decline with recovery treatment. A longitudinal section of the root under continued Al treatment showed the accumulation of superoxide anion, lignin and peroxide (H₂O₂) at the epidermal and outer cortex region where the Al induced injuries, including ruptures, are detected.

Conclusions

Oxidative stress is associated with the mechanism of Al toxicity recovery. The recovery process might include the elongation of the central cylinder as a consequence of the oxidative stress-induced formation of the zonal region (ZR). The results further suggest a plausible role for the ZR in the programmed cell death-like function involved in Al toxicity recovery.     

Protecting Cell Walls from Binding Aluminum by Organic Acids Contributes to Aluminum Resistance

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major AI resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify AI in the growth medium is frequently questioned. The genotypes of Al-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate, respectively. In the present study we found that at a 35% inhibition of root elongation, the AI activities in the solution were 10, 20, and 50 μM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol/cm2 per 12 h, respectively, for the above three plant species. When exogenous organic acids were added to ameliorate Al toxicity, twofold and eightfold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of AI adsorbed to the cell walls all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect the cell walls from binding AI. These findings suggest that protection of cell walls from binding Al by organic acids may contribute significantly to AI resistance.     

水稻和小麦根尖细胞壁多糖的铝积累能力比较

采用水培法比较4种禾本科植物水稻（Oryza sativa L.）、玉米（Zea mays L.）、高粱（Sorghum bicolor（L.）Moench）和小麦（Triticum aestivum L.）8个基因型的抗铝（Al）能力,并对他们在Al积累后细胞壁的多糖组分进行分析。结果显示,在5~200 μmol/L Al处理下,水稻抗Al能力较强,而小麦抗Al能力较弱。在50 μmol/L Al处理下,小麦根尖的果胶和半纤维素1含量的增幅明显高于水稻。水稻基因型‘日本晴’与‘浙辐802’的细胞壁Al含量分别占根尖总Al含量的78.7%和91.6%;小麦基因型‘扬麦18’与‘扬麦16’Al含量分别占根尖总Al含量的64.9%和72.1%。Al吸附-解吸实验结果显示,小麦根尖细胞壁上Al的吸附量高于水稻。研究结果表明,细胞壁是Al积累的主要部位,对Al敏感的水稻和小麦基因型细胞壁中的Al主要分布在果胶中;而对Al耐性较强的水稻和小麦基因型细胞壁中的Al主要分布在半纤维素1中。     

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.     

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.     

Aluminium distribution in soybean root tip for a short time Al treatment

Using the lumogallion staining method which we developed (Kataoka et al. 1997a), Al movement in soybean (Glycine max. (L.)Merr. cv. Tsurunoko) root tips treated for a short time was studied. We have indicated that the majority of Al accumulated in the root was found between 0 and 2 mm from the root apex within 2 h (Kataoka et al. 1997a, b). In the study presented here two-day seedlings of the soybean were treated with 50 μmol/L AlCl₃ (pH 4.4), including 0.2 mmol/L CaCl₂, for 1 h, and Al accumulation in the root sections at both 1 and 2 mm apart from root apex was analyzed by a confocal laser microscopy. Although the early indicators, callose induction and the decrease of growth recovery, were not observed in the root when treated for 15 min, a trace amount of Al was already incorporated into the nucleus of cells and the middle tissue of the root. The non-toxic level of Al was more rapidly absorbed than previously thought. The initial increase of callose accumulation and the reduction of the growth recovery were found after 30 min. Therefore, the difference between Al accumulation profiles of 15 and 30 min was analyzed to find out what triggered a toxic Al effect. Increase of Al accumulation in whole root tissue was observed in the root sections, at both 1 and 2 mm from the root apex, and the greatest amount of Al was found in the cytoplasm of the outer cortex, 1 mm away from the root apex. These results are consistent with the fact that Al exclusion from root tip cells is an important mechanism of Al tolerance.     

Wheat genotypes show contrasting abilities to recover from anoxia in spite of similar anoxic carbohydrate metabolism

Physiological and metabolic responses to anoxia and reaeration were compared for 4–7-day-old seedlings of 11 genotypes of wheat (Triticum aestivum) with reputed differences in waterlogging tolerance. Genotypes differed in seminal root elongation, and recovery of root tissue K⁺ concentration, during reaeration following 72 h anoxia. Post-anoxic recovery ranged from complete (100% retention of seminal root elongation potential) to almost nil (death of all seminal root apices and inability to recover K⁺ concentration). The anoxia tolerance ranking of the genotypes based on these parameters corresponded with that of their reputed waterlogging tolerance, but with some exceptions. However, the differences in anoxia tolerance of the seedlings could not be explained by differences in capacity for ethanol production. A decreased ability to utilise seed starch reserves under anoxia, due to inadequate levels of -amylase activity at the time anoxia was imposed, was apparent in all genotypes.     

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.     

边缘细胞对荞麦根尖铝毒的防护效应和对细胞壁多糖的影响

以2个荞麦(Fygopyrum esculentum Moench)基因型‘江西荞麦’(耐性)和‘内蒙荞麦’(敏感)为材料,采用悬空培养(保持边缘细胞附着于根尖和去除根尖边缘细胞),研究边缘细胞对根尖铝毒的防护效应以及对细胞壁多糖组分的影响。结果表明,铝毒抑制荞麦根系伸长,导致根尖Al积累。去除边缘细胞的根伸长抑制率和根尖Al含量高于保留边缘细胞的根。去除边缘细胞使江西荞麦和内蒙荞麦根尖的酸性磷酸酶(APA)活性显著升高,前者在铝毒下增幅更大。同时,铝毒胁迫下去除边缘细胞的根尖果胶甲酯酶(PME)活性和细胞壁果胶、半纤维素1、半纤维素2含量显著高于保留边缘细胞的酶活性和细胞壁多糖含量。表明边缘细胞对荞麦根尖的防护效应,与其阻止Al的吸收,降低根尖细胞壁多糖含量及提高酸性磷酸酶活性有关,以此缓解Al对根伸长的抑制。