Physiological aspects of aluminium tolerance associated with the long arm of chromosome 2D of the wheat (Triticum aestivum L.) genome

Aluminum (Al) uptake in roots of wheat nearisogenic lines having differing tolerances to aluminium toxicity was studied using roots and root segments immersed in a nutrient solution at a controlled pH and temperature. At low Al concentrations a mechanism preventing root tips from accumulating too much Al was observed in an Al-tolerant isoline and a BH1146 euploid. This mechanism was more efficient when divalent cations of calcium or magnesium were present in the nutrient medium. Al accumulation steadily increased in root tips of the Al-sensitive wheat isoline during all 24 h of incubation, and the presence of divalent cations in the medium even increased Al concentration in root tissue. However, at higher Al concentrations in the medium the mechanism preventing the root tips of Al-tolerant genotypes from accumulating too much Al was not observed, and in effect Al concentration in root tips of both Al-tolerant and Al-sensitive isolines increased. It is concluded that genetical factors are located on the long arm of chromosome 2D from the BH1146 euploid that control the mechanism preventing root apical meristems from accumulating too much Al at low Al concentrations in the medium. However, there must be other genetical factors also located on this chromosome segment that control Al detoxication in root tips of Al-tolerant lines at higher external Al concentrations.     

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.     

Effect of Enhanced Calcium Supply on Aluminum Toxicity in Relation to Cell Wall Properties in the Root Apex of Two Wheat Cultivars Differing in Aluminum Resistance

The present study was conducted to investigate the effects of enhanced Ca supply on Al toxicity in relation to cell wall properties in two wheat (Triticum aestivum L.) cultivars differing in Al resistance. Seedlings of Al-tolerant Inia66 and Al-sensitive Kalyansona cultivars were grown in complete nutrient solutions for 4 days then subjected to treatment solutions containing Al (0, 50 μM) and Ca (500, 2500 μM) at pH 4.5 for 24 h. Root elongation was affected greatly by Al treatment in the Al-sensitive cultivar and a significant improvement in root growth was observed with enhanced Ca supply during Al stress. Pectin and hemicellulose contents in the root cell walls increased with Al stress, and this increase was more conspicuous in the Al-sensitive cultivar. The molecular mass of hemicellulosic polysaccharides increased with Al treatment in the Al-sensitive cultivar and decreased with enhanced Ca supply. The increase in the molecular mass of hemicellulosic polysaccharides was attributed to increased content of glucose, arabinose and xylose in neutral sugars. Enhanced Ca supply slightly decreased the content of these components with Al stress. Aluminum treatment increased the contents of ferulic and p-coumaric acid, especially in the Al-sensitive cultivar, by increasing peroxidase (POD, EC 1.11.1.7) and phenylalanine ammonia lyase (PAL, EC 4.3.1.5) activity, whereas enhanced Ca supply during Al stress decreased the content of these components by decreasing POD and PAL activity. These results suggest that the increased molecular mass of hemicellulosic polysaccharides and phenolic compounds in the Al-sensitive cultivar with Al stress might have inhibited root elongation associated with cell wall stiffening related to cross-linking among cell-wall polymers and lignin. Enhanced Ca supply might maintain the normal synthesis of these materials even with Al stress.     

Responses of Al-tolerant Dayton and Al-sensitive Kearney barley cultivars to calcium and magnesium during al stress

Summary Two barley cultivars differing in Al tolerance, Kearney (Al-sensitive) and Dayton (Al-tolerant) were exposed to Al stress with varied Ca and Mg concentrations in the nutrient solution. Increase in calcium and magnesium supply protected root meristems and root growth from Al toxicity more effectively in the Al-tolerant cultivar than in the Al-sensitive one. Lateral roots were much more sensitive to Al than adventitious roots. Exposure to 0.33 mM Al with low concentrations of Ca (1.3 mM) and Mg (0.3 mM) caused damage to root tips in both cultivars. Increasing the Ca concentration to 4.3 and 6.3 mM prevented root tip damage in Dayton but not in Kearney. In the Al-tolerant cultivar Dayton, however, the root tips regenerated even at the low Ca concentration of 1.3 mM, whereas 6.3 mM Ca was necessary for this to occur in Kearney. This difference was due to the fact that Dayton's root meristem cells were more resistant to damage. Magnesium responses also varied between the two cultivars. At the lowest Ca concentration an increase in Mg to 6.3 mM permitted regeneration of damaged Kearney root tips and completely prevented any damage in Dayton. It is to be assumed that the different responses of the two cultivars are due to differences in plasma membrane properties.     

Aluminum Tolerance in Wheat (Triticum aestivum L.) (II. Aluminum-Stimulated Excretion of Malic Acid from Root Apices)

We investigated the role of organic acids in conferring Al tolerance in near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at the Al tolerance locus (Alt1). Addition of Al to nutrient solutions stimulated excretion of malic and succinic acids from roots of wheat seedlings, and Al-tolerant genotypes excreted 5- to 10-fold more malic acid than Al-sensitive genotypes. Malic acid excretion was detectable after 15 min of exposure to 200 [mu]M Al, and the amount excreted increased linearly over 24 h. The amount of malic acid excreted was dependent on the external Al concentration, and excretion was stimulated by as little as 10 [mu]M Al. Malic acid added to nutrient solutions was able to protect Al-sensitive seedlings from normally phytotoxic Al concentrations. Root apices (terminal 3-5 mm of root) were the primary source of the malic acid excreted. Root apices of Al-tolerant and Al-sensitive seedlings contained similar amounts of malic acid before and after a 2-h exposure to 200 [mu]M Al. During this treatment, Al-tolerant seedlings excreted about four times the total amount of malic acid initially present within root apices, indicating that continual synthesis of malic acid was occurring. Malic acid excretion was specifically stimulated by Al, and neither La, Fe, nor the absence of Pi was able to elicit this response. There was a consistent correlation of Al tolerance with high rates of malic acid excretion stimulated by Al in a population of seedlings segregating for Al tolerance. These data are consistent with the hypothesis that the Alt1 locus in wheat encodes an Al tolerance mechanism based on Al-stimulated excretion of malic acid.     

Aluminum resistance in wheat (Triticum aestivum) is associated with rapid, Al-induced changes in activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in root apices

We have investigated the effect of aluminum (Al) on the activity of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) isolated from 5-mm root apices of 4-day-old wheat ( Triticum aestivum ) cultivars differing in resistance to Al. Rapid increases in G6PDH and 6PGDH activities were observed in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 μ M Al, while no change in the activity of either enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa) during a 24-h exposure to Al. The Al-induced increases in enzyme activities observed in the Al-resistant PT741 appear to reflect an induction of protein synthesis since the increases were completely abolished by 1 m M cycloheximide. No differences in G6PDH and 6PGDH activities were observed between the Al-sensitive and the Al-resistant genotypes when Al was supplied in vitro. Under these conditions, an increase in Al concentration from 0 to 1.4 m M caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings. Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm root apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, accumulation in the Al-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a slowly-exchanging compartment as well as a decrease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 μ M Al. These results suggest that rapid induction of G6PDH and 6PGDH in the Al-resistant line PT741 by Al may play a role in the mechanism of Al resistance, possibly by regulation of the pentose phosphate pathway.     

Aluminum resistance in wheat (Triticum aestivum) is associated with rapid, Al-induced changes in activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in root apices

We have investigated the effect of aluminum (Al) on the activity of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) isolated from 5-mm root apices of 4-day-old wheat ( Triticum aestivum ) cultivars differing in resistance to Al. Rapid increases in G6PDH and 6PGDH activities were observed in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 μ M Al, while no change in the activity of either enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa) during a 24-h exposure to Al. The Al-induced increases in enzyme activities observed in the Al-resistant PT741 appear to reflect an induction of protein synthesis since the increases were completely abolished by 1 m M cycloheximide. No differences in G6PDH and 6PGDH activities were observed between the Al-sensitive and the Al-resistant genotypes when Al was supplied in vitro. Under these conditions, an increase in Al concentration from 0 to 1.4 m M caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings. Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm root apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, accumulation in the Al-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a slowly-exchanging compartment as well as a decrease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 μ M Al. These results suggest that rapid induction of G6PDH and 6PGDH in the Al-resistant line PT741 by Al may play a role in the mechanism of Al resistance, possibly by regulation of the pentose phosphate pathway.     

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 M Al³⁺, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al³⁺ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.Abbreviations SA3 South American 3, an Al-tolerant maize cultivar - SA5 South American 5, an Al-sensitive maize cultivar The authors would like to express their appreciation to Drs. John Thompson, Ross Welch and Mr. Stephen Schaefer for their training and guidance in the use of the chromatography systems. This work was supported by a Swiss National Science Foundation Fellowship to Didier Pellet, and U.S. Department of Agriculture/National Research Initiative Competitive Grant 93-37100-8874 to Leon Kochian. We would also like to thank Drs. S. Pandey and E. Ceballos from the CIMMYT Regional office at CIAT Cali, Colombia for providing seed for the maize varieties and inbred line.     

Growth and cell wall properties of two wheat cultivars differing in their sensitivity to aluminum stress

The present study was conducted to investigate the cell wall properties in two wheat (Triticum aestivum L.) cultivars differing in their sensitivity to Al stress. Seedlings of Al-resistant, Inia66 and Al-sensitive, Kalyansona cultivars were grown in complete nutrient solutions for 4 days and then subjected to treatment solutions containing Al (0, 50 microM) in a 0.5 mM CaCl(2) solution at pH 4.5 for 24 h. Root elongation was inhibited greatly by the Al treatment in the Al-sensitive cultivar compared to the Al-resistant cultivar. The Al-resistant cultivar accumulated less amount of Al in the root apex than in the Al-sensitive cultivar. The contents of pectin and hemicellulose in roots were increased with Al stress, and this increase was more conspicuous in the Al-sensitive cultivar. The molecular mass of hemicellulosic polysaccharides was increased by the Al treatment in the Al-sensitive cultivar. The increase in the content of hemicellulose was attributed to increase in the contents of glucose, arabinose and xylose in neutral sugars. Aluminum treatment increased the contents of ferulic acid and p-coumaric acid especially in the Al-sensitive cultivar by increasing the activity of phenylalanine ammonia lyase (PAL, EC 4.3.1.5). Aluminum treatment markedly decreased the beta-glucanase activity in the Al-sensitive cultivar, but did not exert any effect in the Al-resistant cultivar. These results suggest that the modulation of the activity of beta-glucanase with Al stress may be involved in part in the alteration of the molecular mass of hemicellulosic polysaccharides in the Al-sensitive cultivar. The increase in the molecular mass of hemicellulosic polysaccharides and ferulic acid synthesis in the Al-sensitive cultivar with Al stress may induce the mechanical rigidity of the cell wall and inhibit the elongation of wheat roots.     

Comparative aspects of some bacterial dehydrogenases and transhydrogenases

Ragland, T. E. (Brandeis University, Waltham, Mass.), T. Kawasaki, and J. M. Lowenstein. Comparative aspects of some bacterial dehydrogenases and transhydrogenases. J. Bacteriol. 91:236-244. 1966.-Twenty-eight diverse bacterial species were surveyed for the activities and coenzyme specificities of four enzymes: isocitrate dehydrogenase (ICDH), glucose-6-phosphate dehydrogenase (G-6-PDH), 6-phosphogluconate dehydrogenase (6-PGDH), and reduced nicotinamide adenine dinucleotide phosphate-nicotinamide adenine dinucleotide (NAD) transhydrogenase (TH). Most of the species that exhibited a nicotinamide adenine dinucleotide phosphate (NADP)-linked ICDH also showed significant TH activity, but there were several which did not. Only one of the organisms tested, Xanthomonas pruni, had an ICDH active with both NAD and NADP; it was devoid of TH activity. Acetobacter suboxydans, which lacks ICDH altogether, also had no TH. Some of the species examined had G-6-PDH or 6-PGDH (or both) of dual coenzyme specificity, but there was no apparent relation between these findings and the presence or absence of TH. The TH reaction was assayed by use of analogues of NAD as acceptors. The bacteria could be divided into two groups on the basis of TH specificity, one group reacting at a much faster rate with the 3-acetylpyridine analogue of NAD than with the thionicotinamide analogue, whereas the converse was true for the other group. A few organisms showed no marked specificity for either analogue. This division of specificity can be related to the currently accepted taxonomic classification of the organisms, although a few apparent anomalies were found.     

Effect of pH and nitrogen source on aluminium tolerance of rye (Secale cereale L.) and yellow lupin (Lupinus luteus L.)

Soluble aluminium (Al) is a major factor limiting plant growth in acid mineral soils. Aluminium concentrations in soil solutions are mainly determined by soil pH. However, pH also affects the ratio between activities of protons and cationic Al species and the equilibrium between mono-and polynuclear hydroxy-Al species. The phytotoxicity of these species is not yet clear. The objective of the present study was to clarify the role of minor changes of pH in the rhizosphere on Al phytotoxicity in two Al-tolerant plant species by direct control of the pH in the nutrient solution (4.1, 4.3, 4.5) and in addition by varying the pH in the root apoplast using either nitrate or ammonium as N source. The plants were grown in solution culture at constant external pH. Whereas the Al-sensitive plant species barley and horse bean were damaged at very low Al supplies (1.85 μM and 9.3 μM respectively), 222 μM had to be applied to rye and yellw lupin for a comparable inhibition of root elongation. Yellow lupin was initially severely inhibited in root growth by Al, but then gradually recovered from this ‘Al shock’ within 3 days. In contrast to lupin, rye was hardly affected by Al initially, and it took about 16 h until maximum inhibition of root elongation. In the presence of nitrate, raising the pH from 4.1 to 4.5 aggravated root-growth depression by Al in rye and lupin. Whereas rye roots were severely damaged by ammonium especially at low pH, lupin was rather indifferent to the N source. Aluminium toxicity was less severe in presence of ammonium compared to nitrate N. This effect was less clear with rye at lower pH, because of it's higher proton sensitivity compared to lupin. Less Al injury at lower pH and in presence of ammonium was related to lower Al concentrations in the 1 cm root tips. The results are compatible with data showing high phytotoxicity of mononuclear and polynuclear hydroxy-Al species. However, they could also be interpreted in the light of proton amelioration of Al toxicity owing to competition for Al-sensitive binding sites in the root apoplast.     

Aluminium tolerance in triticale,wheat and rye as measured by root growth characteristics and aluminium concentration

Summary Screening large populations of plant species for Al tolerance requires simple and rapid tests. In this study, root characteristics of 12 cultivars of triticale (X Triticosecale, Witt Mack), wheat (Triticum aestivum L.), and rye (Secale cereale L.) were measured in nutrient solution with 0 or 6 ppm Al added. Aluminum injury to roots of triticale and wheat was characterized by decreases in root length, increases in the number of roots, and in Al-sensitive Redcoat and Arthur wheats by decrease in root weight. Root length and number of roots were correlated in triticale (r=−0.73^*) and in wheat (r=−0.85^*). Root length was also correlated with root weight in wheat (r=0.65^*); there was no relationship between the number of roots and weight. Differences in Al tolerance of cultivars of the three species were greater when the solution was adjusted to pH 4.8 only on the first day of the experiment than when pH was maintained at pH 4.8 throughout the growing period. Triticale and rye cultivars low in ability to increase solution pH gradually overcame Al toxicity by increasing the nutrient solution pH between 12 and 22 days. Aluminum sensitive triticale and wheat accumulated more Al in roots than tolerant cultivars when the solution pH was not adjusted daily; but no differences in Al accumulation were obtained between wheat cultivars at constant pH value. This study indicated that root length and number of roots can be reliably used for screening triticales for Al tolerance within 12 days of exposure to Al. Root length, Al concentration, and dry weight after 22 days of Al treatment were also reliable criteria for evaluating differential Al tolerances among triticale cultivars.     

Aluminum Tolerance in Wheat (Triticum aestivum L.) (I. Uptake and Distribution of Aluminum in Root Apices)

We investigated the uptake and distribution of Al in root apices of near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at a single locus (Alt1: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 [mu]M Al, and the roots were subsequently stained with hematoxylin, a compound that binds Al in vitro to form a colored complex. Root apices of Al-sensitive genotypes stained after short exposures to Al (10 min and 1 h), whereas apices of Al-tolerant seedlings showed less intense staining after equivalent exposures. Differential staining preceded differences observed in either root elongation or total Al concentrations of root apices (terminal 2-3 mm of root). After 4 h of exposure to 100 [mu]M Al in nutrient solution, Al-sensitive genotypes accumulated more total Al in root apices than Al-tolerant genotypes, and the differences became more marked with time. Analysis of freeze-dried root apices by x-ray microanalysis showed that Al entered root apices of Al-sensitive plants and accumulated in the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to Al (24 h) resulted in a distribution of Al coinciding with the absence of K. Quantitation of Al in the cortical layer showed that sensitive apices accumulated 5- to 10-fold more Al than tolerant apices exposed to Al solutions for equivalent times. These data are consistent with the hypothesis that Alt1 encodes a mechanism that excludes Al from root apices.     

The effect of aluminium on polypeptide pattern of cell wall proteins isolated from the roots of Al-sensitive and Al-resistant barley cultivars

Accumulation of some proteins isolated from the cell wall of roots of the Al-sensitive (Alfor) and the Al-resistant (Bavaria) barley cultivars were followed during treatment with different Al³⁺ concentrations, pH changes of the root medium, and several heavy metals (Cu²⁺, Cd²⁺, Co²⁺). SDS-PAGE analysis revealed an Al-induced accumulation of polypeptides with molecular mass of 14, and 16 kDa and a group of polypeptides around 27 kDa. The accumulation pattern of Al-induced polypeptides was very similar in both cultivars but in the Al-resistant Bavaria it was induced at lower Al concentration and earlier than it was in the Al-sensitive cultivar Alfor. Changes in pH values of root medium (pH 3.5–6.5) did not show any effect on the accumulation of Al-induced cell wall polypeptides either in Al-sensitive or in Al-tolerant barley cultivar. Heavy metals (Cu, Cd, and Co) at concentration of 10 μM resulted in similar accumulation of individual polypeptides as we found after Al treatment. In comparison to Al, quantitative differences in polypeptides accumulation induced by Cu, Cd and Co were less expressed that of Al treatment. More pronounced accumulation and earlier induction of individual cell wall polypeptides in roots of Al-resistant barley cultivar than in Al-sensitive, might indicate some possible role of these polypeptides in plant resistance to Al stress.     

Mechanisms of Aluminum Tolerance in Wheat : An Investigation of Genotypic Differences in Rhizosphere pH, K, and H Transport, and Root-Cell Membrane Potentials

Control of rhizosphere pH and exclusion of Al by the plasma membrane have been hypothesized as possible mechanisms for Al tolerance. To test primarily the rhizosphere pH hypothesis, wheat cultivars (Triticum aestivum L. `Atlas 66' and `Scout'), which differ in Al tolerance, were grown in either complete nutrient solution, or 0.6 millimolar CaSO₄, with and without Al at pH 4.50. A microelectrode system was used to simultaneously measure rhizosphere pH, K⁺, and H⁺ fluxes, and membrane potentials (E_m) along the root at various distances from the root apex. In complete nutrient solution, the rhizosphere pH associated with mature root cells (measured 10-40 millimeters from the root apex) of Al-tolerant `Atlas 66' was slightly higher than that of the bulk solution, whereas roots of Al-sensitive `Scout' caused a very small decrease in the rhizosphere pH. In CaSO₄ solution, no significant differences in rhizosphere pH were found between wheat cultivars, while differential Al tolerance was still observed, indicating that the rhizosphere pH associated with mature root tissue is not directly involved in the mechanism(s) of differential Al tolerance. In Al-tolerant `Atlas 66', growth in a CaSO<sub>4</sub> solution with 5 micromolar Al (pH 4.50) had little effect on net K<sup>+</sup> influx, H<sup>+</sup> efflux, and root-cell membrane potential measured in cells of mature root tissue (from 10-40 mm back from apex). However, in Al-sensitive `Scout', Al treatment caused a dramatic inhibition of K⁺ influx and both a moderate reduction of H⁺ efflux and depolarization of the membrane potential. These results demonstrate that increased Al tolerance in wheat is associated with the increased ability of the tolerant plant to maintain normal ion fluxes and membrane potentials across the plasmalemma of root cells in the presence of Al.     

Aluminum Toxicity in Roots : Correlation among Ionic Currents, Ion Fluxes, and Root Elongation in Aluminum-Sensitive and Aluminum-Tolerant Wheat Cultivars

The inhibition of root growth by aluminum (Al) is well established, yet a unifying mechanism for Al toxicity remains unclear. The association between cell growth and endogenously generated ionic currents measured in many different systems, including plant roots, suggests that these currents may be directing growth. A vibrating voltage microelectrode system was used to measure the net ionic currents at the apex of wheat (Triticum aestivum L.) roots from Al-tolerant and Al-sensitive cultivars. We examined the relationship between these currents and Al-induced inhibition of root growth. In the Al-sensitive cultivar, Scout 66, 10 micromolar Al (pH 4.5) began to inhibit the net current and root elongation within 1 to 3 hours. These changes occurred concurrently in 75% of experiments. A significant correlation was found between current magnitude and the rate of root growth when data were pooled. No changes in either current magnitude or growth rate were observed in similar experiments using the Al-tolerant cultivar Atlas 66. Measurements with ion-selective microelectrodes suggested that H⁺ influx was responsible for most of the current at the apex, with smaller contributions from Ca²⁺ and Cl⁻ fluxes. In 50% of experiments, Al began to inhibit the net H⁺ influx in Scott 66 roots at the same time that growth was affected. However, in more than 25% of cases, Al-induced inhibition of growth rate occurred before any sustained decrease in the current or H⁺ flux. Although showing a correlation between growth and current or H⁺ fluxes, these data do not suggest a mechanistic association between these processes. We conclude that the inhibition of root growth by Al is not caused by the reduction in current or H⁺ influx at the root apex.     

铝胁迫对不同小麦SOD、CAT、POD活性和MDA含量的影响

方法:采用室内水培试验法,研究了不同浓度铝胁迫对耐性不同的几种基因型小麦叶片和根系内SOD、CAT、POD活性和MDA含量的影响。结果:表明铝胁迫条件下导致小麦叶片和根系的3种酶活性在一定范围内随胁迫强度的增加而上升,重度胁迫下会有所下降。这说明SOD、POD、CAT活性的提高与维持是植物耐铝胁迫的重要生理基础。另外,耐铝品种变化不显著,始终维持在比较稳定的活性水平,这可能与铝诱导的有机酸分泌有关,敏感性品种的酶活性在胁迫下会有所下降。而MDA含量在轻度胁迫下变化不明显,在重度胁迫下才会有明显变化,其含量的变化与小麦的耐铝性也有着密切的关系。     

Water Transport Properties of Roots and Root Cortical Cells in Proton- and Al-Stressed Maize Varieties

Root and root cell pressure-probe techniques were used to investigate the possible relationship between Al- or H+-induced alterations of the hydraulic conductivity of root cells (LPc) and whole-root water conductivity (LPr) in maize (Zea mays L.) plants. To distinguish between H+ and Al effects two varieties that differ in H+ and Al tolerance were assayed. Based on root elongation rates after 24 h in nutrient solution of pH 6.0, pH 4.5, or pH 4.5 plus 50 [mu]M Al, the variety Adour 250 was found to be H+-sensitive and Al-tolerant, whereas the variety BR 201 F was found to be H+-tolerant but Al-sensitive. No Al-induced decrease of root pressure and root cell turgor was observed in Al-sensitive BR 201 F, indicating that Al toxicity did not cause a general breakdown of membrane integrity and that ion pumping to the stele was maintained. Al reduced LPc more than LPr in Al-sensitive BR 201 F. Proton toxicity in Adour 250 affected LPr more than LPc. In this Al-tolerant variety LPc was increased by Al. Nevertheless, this positive effect on LPc did not render higher LPr values. In conclusion, there were no direct relationships between Al- or H+-induced decreases of LPr and the effects on LPc. To our knowledge, this is the first time that the influence of H+ and Al on root and root cell water relations has been directly measured by pressure-probe techniques.     

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.     

A gene encoding multidrug resistance (MDR)-like protein is induced by aluminum and inhibitors of calcium flux in wheat

A cDNA clone exclusively induced by aluminum (Al) was isolated from root apices of wheat (Triticum aestivum L.) by the differential display method. The predicted amino acid sequence exhibited homology to the multidrug resistance (MDR) proteins that is known as a member of the ATP-binding cassette (ABC) protein superfamily. Thus this gene was named TaMDR1 (Triticum aestivum MDR). TaMDR1 was induced as a function of Al concentration in the range from 5 to 50 microM, which is in the range of Al content in natural acid soil environment. The concentration required for the induction was lower in the Al-sensitive cultivar than in the Al-tolerant cultivar, indicating that the accumulation of TaMDR1 mRNA was associated with the events caused by Al toxicity rather than Al tolerance. TaMDR1 was significantly induced by the exposure to lanthanum, gadolinium and ruthenium red, which are known as inhibitors of calcium channels. Furthermore, decreasing of calcium ion in growth medium caused stimulation of the gene expression. These results suggested that the induction of TaMDR1 is caused by the breaking of calcium homeostasis which occurred at early stage of Al toxicity.