Cation amelioration of aluminum toxicity in wheat

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practical interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing 0.4 millimolar CaCl₂ at pH 4.3 variously supplemented with AlCl₃ and additional amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 {Al³⁺}/({Al³⁺} + K_m + α{C}^β) where {Al³⁺} is the activity of Al³⁺ expressed in micromolar, {C} is the activity of the added cation expressed in millimolar, and K_m (= 1.2 micromolar) is the {Al³⁺} required for 50% inhibition in the absence of added salt. For Ca²⁺, Mg²⁺, and Na⁺ the values of α were 2.4, 1.6, and 0.011, respectively, and the values for β were 1.5, 1.5, and 1.8, respectively. With regard to relative ameliorative effectiveness, Ca²⁺ > Mg²⁺ ≈ Sr²⁺ K⁺ ≈ Na⁺. Other cations were tested, but La³⁺, Sc³⁺, Li⁺, Rb⁺, and Cs⁺ were toxic at potentially ameliorative levels. The salt amelioration is not solely attributable to reductions in {Al³⁺} caused by increases in ionic strength. Competition between the cation and Al for external binding sites may account for most of the amelioration.     

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.     

Proteome analysis of roots of wheat seedlings under aluminum stress

The root apex is considered the first sites of aluminum (Al) toxicity and the reduction in root biomass leads to poor uptake of water and nutrients. Aluminum is considered the most limiting factor for plant productivity in acidic soils. Aluminum is a light metal that makes up 7 % of the earth’s scab dissolving ionic forms. The inhibition of root growth is recognized as the primary effect of Al toxicity. Seeds of wheat cv. Keumkang were germinated on petridish for 5 days and then transferred hydroponic apparatus which was treated without or with 100 and 150 μM AlCl₃ for 5 days. The length of roots, shoots and fresh weight of wheat seedlings were decreased under aluminum stress. The concentration of K⁺, Mg²⁺ and Ca²⁺ were decreased, whereas Al³⁺ and P₂O₅ ^? concentration was increased under aluminum stress. Using confocal microscopy, the fluorescence intensity of aluminum increased with morin staining. A proteome analysis was performed to identify proteins, which are responsible to aluminum stress in wheat roots. Proteins were extracted from roots and separated by 2-DE. A total of 47 protein spots were changed under Al stress. Nineteen proteins were significantly increased such as sadenosylmethionine, oxalate oxidase, malate dehydrogenase, cysteine synthase, ascorbate peroxidase and/or, 28 protein spots were significantly decreased such as heat shock protein 70, O-methytransferase 4, enolase, and amylogenin. Our results highlight the importance and identification of stress and defense responsive proteins with morphological and physiological state under Al stress.     

Five genes induced by aluminum in wheat (Triticum aestivum L.) roots.

Five different cDNAs (termed wali1 to wali5 for Wheat Aluminum Induced) whose expression was induced by Al stress have been isolated from the root tips of Al-treated wheat (Triticum aestivum L.) plants. Four of these genes were induced 24 to 96 h after Al treatment, and their expression is reduced when the Al is removed. Each of these four genes was induced by inhibitory levels of Al in two wheat cultivars--Warigal, an Al-sensitive cultivar, and Waalt, an Al-tolerant cultivar. The fifth gene (wali2) showed a complex bimodal pattern of induction and was induced by Al only in the sensitive cultivar. Comparison of the nucleotide sequences of these clones to those in the sequence data bases showed that wali4 is homologous to phenylalanine ammonia-lyase and wali1 is homologous to a group of plant proteins that are cysteine-rich and have homology to metallothioneins. wali2 encodes a novel protein with a repeating motif of cysteine amino acids. The remaining two wali clones (wali3 and wali5) encode related, cysteine-rich proteins that show no significant homology to any known sequences.     

Quality and flavour stability of coffee substitute prepared by extrusion of wheat germ and chicory roots

Summary. A mixture of roasted chicory roots and wheat germ (1:1 w/w) was subjected to extrusion processing for preparation of coffee substitute. Comparative studies concerning sensory characteristics and headspace volatiles were carried out between genuine coffee and a freshly prepared coffee substitute. The sensory evaluation revealed similarities between the two samples. The comparative odour profile analysis showed that the sweetish/caramel-like note scored higher in our coffee substitute sample than in real coffee, whereas the other odour quality attributes showed an opposite trend. The high quality of the fresh coffee substitute was correlated to the presence of volatiles that are responsible for the fresh coffee aroma, such as: 2-methylbutanal, 3-methylbutanal, 2-methylfuran and 2,3-butanedione in high concentration. Storage of coffee substitute samples revealed a noticeable decrease in concentration of the Strecker aldehydes and diketones and a remarkable increase in phenolic compounds, whereas pyrazine and furan derivatives showed no linear changes during storage. The ratio of 2,3-butanedione/2-methylfuran (B/M) was used as an indicator for aging of coffee substitute samples. The variation in this ratio (B/M) during storage for 6 months was consistent with that of the odour profile analysis. Authors’ address: Prof. Dr. Hoda H. M. Fadel, Chemistry of Flavour and Aroma Department, National Research Centre, Al-Behos St., Dokki, Cairo, Egypt     

Malate-permeable channels and cation channels activated by aluminum in the apical cells of wheat roots

Aluminum (Al(3+))-dependent efflux of malate from root apices is a mechanism for Al(3+) tolerance in wheat (Triticum aestivum). The malate anions protect the sensitive root tips by chelating the toxic Al(3+) cations in the rhizosphere to form non-toxic complexes. Activation of malate-permeable channels in the plasma membrane could be critical in regulating this malate efflux. We examined this by investigating Al(3+)-activated channels in protoplasts from root apices of near-isogenic wheat differing in Al(3+) tolerance at a single locus. Using whole-cell patch clamp we found that Al(3+) stimulated an electrical current carried by anion efflux across the plasma membrane in the Al(3+)-tolerant (ET8) and Al(3+)-sensitive (ES8) genotypes. This current occurred more frequently, had a greater current density, and remained active for longer in ET8 protoplasts than for ES8 protoplasts. The Al(3+)-activated current exhibited higher permeability to malate(2-) than to Cl(-) (P(mal)/P(Cl) > or = 2.6) and was inhibited by anion channel antagonists, niflumate and diphenylamine-2-carboxylic acid. In ET8, but not ES8, protoplasts an outward-rectifying K(+) current was activated in the presence of Al(3+) when cAMP was included in the pipette solution. These findings provide evidence that the difference in Al(3+)-induced malate efflux between Al(3+)-tolerant and Al(3+)-sensitive genotypes lies in the differing capacity for Al(3+) to activate malate permeable channels and cation channels for sustained malate release.     

Protein carbonylation in barley seedling roots caused by aluminum and proton toxicity is suppressed by salicylic acid

Proton (H⁺) and aluminum (Al³⁺) toxicities are major factors limiting crop production on acid soils. To study whether salicylic acid (SA) is functional in alleviating protein damage caused by H⁺ and Al³⁺ toxicities, an investigation of the antioxidant defense response regulated by SA was carried out on barley (Hordeum vulgare L.) seedlings under H⁺, Al³⁺, and combined stresses. It was found that the relative root elongation of seedlings, which grew in the solutions supplemented with SA, was significantly higher than that of seedlings without SA treatment after 24-h treatments with H⁺, Al³⁺, and combined stresses. The lesser amount of carbonylated proteins with molecular weights ranging from 14.4 to 97 kD, was accumulated in seedlings treated with SA than that in the seedlings without SA treatment. The higher activities of antioxidant enzymes and lesser content of MDA were observed in seedlings treated with SA compared with the seedlings without SA treatment. Moreover, the nitroblue tetrazolium staining of roots showed that ROS accumulation was decreased by SA treatments. This study suggested that SA could alleviate cell damage caused by H⁺ and Al³⁺ toxicities on acid soils by both activating antioxidant defense responses and reducing the contents of carbonylated proteins caused by ROS in barley seedlings.     

四种常见杂草根系及根边缘细胞对铝胁迫的响应

以2种禾本科杂草(升马唐、稗草)和2种菊科杂草(旱莲草、野茼蒿)为实验材料,通过砂培法研究不同科属杂草根部对铝胁迫的响应.结果表明:4种杂草根边缘细胞活性均随着铝胁迫浓度和时间呈显著下降的趋势,但禾本科杂草根系边缘细胞的活性高于菊科杂草,且活性的降低幅度较小;4种杂草根相对伸长率均随铝浓度和处理时间的增加呈递减趋势,但铝对旱莲草和野茼蒿根生长的抑制程度要明显高于升马唐和稗草;根系的铝含量、游离脯氨酸含量、MDA 含量和质膜透性均随铝处理浓度和处理时间的增加而增大,且在高铝浓度(1000 mg · L~(-1))时达到最大值,但升马唐和稗草根系的铝含量、游离脯氨酸含量、MDA含量和质膜透性均显著低于旱莲草和野茼蒿,且随着铝浓度的增加,禾本科杂草根系的游离脯氨酸含量及MDA含量的变化没有达到显著水平(P＞0.05).由此说明,铝毒对杂草造成的伤害随着浓度增加和时间延长而加重;升马唐和稗草的根系通过较高的根边缘细胞活性和根相对伸长率及较低的铝含量、游离脯氨酸含量、MDA含量和质膜透性来增加其对铝的耐性;2种禾本科杂草(升马唐、稗草)的耐铝性高于2种菊科杂草(旱莲草、野茼蒿).     

Proton efflux from corn roots induced by tripropyltin

Tripropyltin restores medium acidification by washed corn root tissue in which electrogenic H⁺ efflux has been blocked by ATPase inhibitors or injury. However, the restored H⁺ efflux is not electrogenic and will not drive K⁺ influx, and, by itself, tripropyltin is inhibitory to K⁺ influx. Tripropyltin elicits a 5-fold increase in endogenous chloride efflux, and Cl⁻/OH⁻ exchange can, thus, account for the observed acidification of the medium. This explanation cannot be applied equally to the acidification produced by the K⁺/H⁺ exchanging ionophore nigericin.     

Nitric oxide reduces aluminum toxicity by preventing oxidative stress in the roots of Cassia tora L

Nitric oxide (NO) as a key signaling molecule has been involved in mediation of various biotic and abiotic stress-induced physiological responses in plants. In the present study, we investigated the effect of NO on Cassia tora L. plants exposed to aluminum (Al). Plants pre-treated for 12 h with 0.4 mM sodium nitroprusside (SNP), an NO donor, and subsequently exposed to 10 microM Al treatment for 24 h exhibited significantly greater root elongation as compared with the plants without SNP treatment. The NO-promoted root elongation was correlated with a decrease in Al accumulation in root apexes. Furthermore, oxidative stress associated with Al treatment increased lipid peroxidation and reactive oxygen species, and the activation of lipoxygenase and antioxidant enzymes was reduced by NO. Such effects were confirmed by the histochemical staining for the detection of peroxidation of lipids and loss of membrane integrity in roots. The ameliorating effect of NO was specific, because the NO scavenger cPTIO [2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide] completely reversed the effect of NO on root growth in the presence of Al. These results indicate that NO plays an important role in protecting the plant against Al-induced oxidative stress.     

Early events responsible for aluminum toxicity symptoms in suspension-cultured tobacco cells

We investigated the aluminum (Al)-induced alterations in zeta potential, plasma membrane (PM) potential and intracellular calcium levels to elucidate their interaction with callose production induced by Al toxicity. A noninvasive confocal laser microscopy has been used to analyse the live tobacco (Nicotiana tabacum) cell events by means of fluorescent probes Fluo-3 acetoxymethyl ester (intracellular calcium) and DiBAC4 (PM potential) as well as to monitor callose accumulation. Log-phase cells showed no detectable changes in the PM potential during the first 30 min of Al treatment, but sustained large depolarization from 60 min onwards. Measurement of zeta potential confirmed the depolarization effect of Al, but the kinetics were different. The Al-treated cells showed a moderate increase in intracellular Ca2+ levels and callose production in 1 h, which coincided with the time course of PM depolarization. Compared with the Al treatment, cyclopiazonic acid, an inhibitor of endoplasmic reticulum Ca(2+)-ATPase, facilitated a higher increase in intracellular Ca2+ levels, but resulted in accumulation of only moderate levels of callose. Calcium channel modulators and Al induced similar levels of callose in the initial 1 h of treatment. Callose production induced by Al toxicity is dependent on both depolarization of the PM and an increase in intracellular Ca2+ levels.     

Ammonium under solution culture alleviates aluminum toxicity in rice and reduces aluminum accumulation in roots compared with nitrate

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH₄ ⁺ as its major inorganic nitrogen source. This study investigates the effects of NH₄ ⁺ and NO₃ ^? on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH₄ ⁺ preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH₄ ⁺ and NO₃ ^? supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH₄ ⁺-preferring than Yangdao6. NH₄ ⁺ alleviated Al toxicity in two cultivars compared with NO₃ ^?. Both NH₄ ⁺-Al supply and pretreatment with NH₄ ⁺ reduced Al accumulation in roots and root tips compared with NO₃ ^?. NH₄ ⁺ decreased but NO₃ ^? increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO₃ ^? concentration enhanced Al accumulation in root tips but increasing the NH₄ ⁺ concentration had the opposite effect. These results show NH₄ ⁺ alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO₃ ^?, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH₄ ⁺, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH₄ ⁺ is suggested.     

A protein similar to PR (pathogenesis-related) proteins is elicited by metal toxicity in wheat roots

An acidic, low molecular weight protein called TA1-18 (T for Triticum. Al for aluminium and 18 for its approximate molecular weight) is induced in wheat roots that are exposed to growth-inhibiting concentrations of Al. Enhanced biosynthesis of TA1-18 began during the period 3 to 6 h after exposure to Al, and reached a maximum after 9 to 12 h of treatment. A protein with the same molecular weight and pl was also elicited during toxicity associated with Cu and Cd, with calcium deprivation, and low (3. 5) pH, but not by heat shock. TA1-18 was formed in small amounts in triticale, but was not detected in rye during exposure to growth-inhibiting levels of Al. Amino acid sequencing of trypsin fragments of TA1-18 revealed strong homology to pathogenesis-related protein PR2 from parsley cultures, with which TA1-18 also shares similar molecular weight and pl. Aluminium toxicity appears to have features in common with pathogenesis such that similar proteins are formed in response to both types of stress.     

Oxidative stress triggered by aluminum in plant roots

Aluminum (Al) is a major growth-limiting factor for plants in acid soils. The primary site of Al accumulation and toxicity is the root meristem, and the inhibition of root elongation is the most sensitive response to Al. Al cannot catalyze redox reactions but triggers lipid peroxidation and reactive oxygen species (ROS) production in roots. Furthermore, Al causes respiration inhibition and ATP depletion. Comparative studies of Al toxicity in roots with that in cultured plant cells suggest that Al causes dysfunction and ROS production in mitochondria, and that ROS production, but not lipid peroxidation, seems to be a determining factor of root-elongation inhibition by Al.     

Proton extrusion and attendant transport phenomena in Candida utilis induced by ethanol

Summary Ion fluxes after ethanol addition to Candida utilis depend crucially on aeration (air versus oxygen). In O₂-aerated non-growing cells ethanol causes an H ⁺ / K ⁺ exchange and an extrusion of acetate and lactate accompanied mostly by K ⁺, and their subsequent reimportation together with H ⁺. Cells from continuous culture display generally stronger acidification and more marked K ⁺ movements than non-growing ones. Offprint requests to: A. Prell