Aluminum resistance in maize cannot be solely explained by root organic acid exudation. A comparative physiological study

Root apical aluminum (Al) exclusion via Al-activated root citrate exudation is widely accepted as the main Al-resistance mechanism operating in maize (Zea mays) roots. Nonetheless, the correlation between Al resistance and this Al-exclusion mechanism has not been tested beyond a very small number of Al-resistant and Al-sensitive maize lines. In this study, we conducted a comparative study of the physiology of Al resistance using six different maize genotypes that capture the range of maize Al resistance and differ significantly in their genetic background (three Brazilian and three North American genotypes). In these maize lines, we were able to establish a clear correlation between root tip Al exclusion (based on root Al content) and Al resistance. Both Al-resistant genotypes and three of the four Al-sensitive lines exhibited a significant Al-activated citrate exudation, with no evidence for Al activation of root malate or phosphate release. There was a lack of correlation between differential Al resistance and root citrate exudation for the six maize genotypes; in fact, one of the Al-sensitive lines, Mo17, had the largest Al-activated citrate exudation of all of the maize lines. Our results indicate that although root organic acid release may play a role in maize Al resistance, it is clearly not the only or the main resistance mechanism operating in these maize roots. A number of other potential Al-resistance mechanisms were investigated, including release of other Al-chelating ligands, Al-induced alkalinization of rhizosphere pH, changes in internal levels of Al-chelating compounds in the root, and Al translocation to the shoot. However, we were unsuccessful in identifying additional Al-resistance mechanisms in maize. It is likely that a purely physiological approach may not be sufficient to identify these novel Al-resistance mechanisms in maize and this will require an interdisciplinary approach integrating genetic, molecular, and physiological investigations.     

Interaction between iron plaque and root border cells ameliorates aluminum toxicity of Oryza sativa differing in aluminum tolerance

Root border cells (RBCs), which are generated during plant growth and surround the root cap, and iron plaque (IP), ubiquitously formed on the root surfaces of rice, are known to alleviate aluminum (Al) toxicity. To verify the interactive effects of IP and RBCs on ameliorating Al toxicity, two rice cultivars differing in Al resistance were used to compare Al tolerance between cultivars. Additionally, root elongation, Al uptake and RBCs viability were measured as indicators of the effects of Al. The amounts of DCB-extractable Fe and Al on the root surfaces were much higher in the presence of IP than the absence. IP presence significantly decreased Al-induced inhibition of root elongation and Al contents in roots and root tips. The removal of RBCs from the root tips caused a more severe inhibition of root elongation and a higher Al accumulation in rice roots and root tips. Furthermore, root growth inhibition and Al contents in roots and root tips were significantly lower in roots with a combination of IP and RBCs than in roots with IP or RBCs only. The formation of IP on the root surface maintained higher RBCs viability and depressed mucilage exudation in an Al-tolerant rice cultivar. The results suggest that both IP and RBCs ameliorate Al toxicity, and IP has a greater capacity for Al resistance. The combination of IP and RBCs exhibited a synergistic effect associated with Al resistance.     

Aluminum stress response in rice: effects on membrane lipid composition and expression of lipid biosynthesis genes

The presence of aluminum (Al) in acidic soils is a major abiotic stress limiting the production of cultivated plants. Cell membranes are the main targets of environmental stresses and there is growing evidence for the involvement of membrane lipids in plant adaptation. The aim of this study was to evaluate the mid-long effects of Al on membrane lipid content and composition in the roots and shoots of rice plants grown under hydroponic conditions. Four rice cultivars were compared: two acknowledged as Al-resistant (Koshihikari) and Al-sensitive (Kasalath), respectively, and two Vietnamese cultivars, OM6073 and OM1490. Al treatment inhibited root and shoot growth in the sensitive cultivars and the observed changes in root and shoot lipid and fatty acid composition revealed patterns associated with Al sensitivity: larger decreases in lipid content and decreases in fatty acid unsaturation. In the roots, phospholipids, and particularly phosphatidylcholine (PC), decreased dramatically in the susceptible cultivars whereas the amount of lipid classes remained unchanged in the tolerant ones. In the shoots, the glycolipids monogalactosyldiacylglycerol and digalactosyldiacylglycerol as well as PC were mostly affected by Al treatment in the susceptible varieties. mRNA accumulation corresponding to genes coding for galactolipid synthases, enzymes of the PC and phosphatidylethanolamine biosynthetic pathways and fatty acid desaturases correlated well with changes in lipid contents in roots and partly explained lipid changes in leaves. The results suggested that the capacity to maintain the proper functioning of some lipid biosynthetic activities and hence the stability of lipid composition may help the rice plant to withstand Al stress.     

Aluminum resistance in two cultivars of Zea mays L.: Root exudation of organic acids and influence of phosphorus nutrition

Root exudation of organic acids as Al-chelating compounds and P nutrition have been suggested to play a major role in Al-resistance in higher plants. Effects of Al exposure on maize plant growth, and organic acid root content and root exudation under various levels of P nutrition were examined. Sikuani, a Colombian maize cultivar tolerant to acid soils with high Al saturation, and Corso, a Swiss cultivar, were grown in sterile hydroponic conditions for 21 days. Al-caused inhibition of root growth was lower in Sikuani than in Corso. Al effect on plant growth was decreased with increasing P content in roots. Al content in roots increased with increasing P content and was higher in Sikuani than in Corso. When exposed to Al, the contents in root apices as well as the root exudation of citric and malic acids in Corso and citric, malic and succinic acids in Sikuani increased, and were higher in Sikuani than in Corso. Increased PEP carboxylase (PEPC) activity in root apices after Al exposure partially explained the variations of organic acid content in the roots. These Al-induced changes in PEPC activity, organic acid content and exudation were reduced in plants supplied with higher P concentrations during the 21 days prior to treatment. Increased secretion of organic acids after exposure to Al appeared to be specific to Al and was not totally explained by increased root content in organic acids.     

Response of rice to Al stress and identification of quantitative trait Loci for Al tolerance

Rice (Oryza sativa L.) shows the highest tolerance to Al toxicity among small-grain cereal crops, however, the mechanisms and genetics responsible for its high Al tolerance are not yet well understood. We investigated the response of rice to Al stress using the japonica variety Koshihikari in comparison to the indica variety Kasalath. Koshihikari showed higher tolerance at various Al concentrations than Kasalath. The Al content in root apexes was less in Koshihikari than in Kasalath, suggesting that exclusion mechanisms rather than internal detoxification are acting in Koshihikari. Al-induced secretion of citrate was observed in both Koshihikari and Kasalath, however, it is unlikely to be the mechanism for Al tolerance because there was no significant difference in the amount of citrate secreted between Koshihikari and Kasalath. Quantitative trait loci (QTLs) for Al tolerance were mapped in a population of 183 backcross inbred lines (BILs) derived from a cross of Koshihikari and Kasalath. Three putative QTLs controlling Al tolerance were detected on chromosomes 1, 2 and 6. Kasalath QTL alleles on chromosome 1 and 2 reduced Al tolerance but increased tolerance on chromosome 6. The three QTLs explained about 27% of the phenotypic variation in Al tolerance. The existence of QTLs for Al tolerance was confirmed in substitution lines for corresponding chromosomal segments.     

Response of rice (Oryza sativa) with root surface iron plaque under aluminium stress

BACKGROUND AND AIMS: Rice (Oryza sativa) is an aquatic plant with a characteristic of forming iron plaque on its root surfaces. It is considered to be the most Al-tolerant species among the cereal crops. The objective of this study was to determine the effects of root surface iron plaque on Al translocation, accumulation and the change of physiological responses under Al stress in rice in the presence of iron plaque. METHODS: The japonica variety rice, Koshihikari, was used in this study and was grown hydroponically in a growth chamber. Iron plaque was induced by exposing the rice roots to 30 mg L(-1) ferrous iron either as Fe(II)-EDTA in nutrient solution (6 d, Method I) or as FeSO(4) in water solution (12 h, Method II). Organic acid in root exudates was retained in the anion-exchange resin and eluted with 2 m HCl, then analysed by high-performance liquid chromatography (HPLC) after proper pre-treatment. Fe and Al in iron plaque were extracted with DCB (dithionite-citrate-bicarbonate) solution. KEY RESULTS AND CONCLUSIONS: Both methods (I and II) could induce the formation of iron plaque on rice root surfaces. The amounts of DCB-extractable Fe and Al on root surfaces were much higher in the presence of iron plaque than in the absence of iron plaque. Al contents in root tips were significantly decreased with iron plaque; translocation of Al from roots to shoots was significantly reduced with iron plaque. Al-induced secretion of citrate was observed and iron plaque could greatly depress this citrate secretion. These results suggested that iron plaque on rice root surfaces can be a sink to sequester Al onto the root surfaces and Fe ions can pre-saturate Al-binding sites in root tips, which protects the rice root tips from suffering Al stress to a certain extent.     

Aluminium effects on growth, nutrient net uptake and transport in 3 rice (Oryza sativa) cultivars with different sensitivity to aluminium

Varietal differences in net nutrient uptake rate and transport efficiency in the presence of aluminium have seldom been investigated in rice. Therefore, effects of Al on growth, uptake and transport of macronutrients (K, P, Ca, and Mg) and micronutrients (Fe, Zn, Cu, and Mn) were evaluated in 3 rice cultivars (BG35, DA14 and IR45) with different Al sensitivity. The plants were grown in nutrient solution at pH 4.1. An initial growth was completed in the time interval 1 to 5 days immediately before the addition of Al. The final growth period with Al (0, 140, 280 or 560 μ M ) was completed on day 26. With Al, a comparatively high P accumulation occurred in shoots and roots of the Al tolerant cultivar BG35. In contrast, the Al sensitive cultivar IR45 maintained a relatively high Ca accumulation during the Al treatment. A reduced total net uptake rate of P and Ca by IR45 in the time period 5 to 26 days was due to both a reduced root fresh weight and a reduced net uptake rate per g fresh weight of root. Moreover, net Ca transport to the shoots higher than net uptake rate in DA14 and IR45 at > 140 μ M Al during the test period suggests restricted Ca uptake by the roots in combination with a continuous net loss of Ca from the roots to the shoots as time proceeds. In the case of Mg and Mn, there was a general reduction of net uptake rates, irrespective of Al sensitivity of cultivars. With Al treatment, comparatively high accumulation of Fe, Zn and Cu occurred in the roots of IR45, concomitant with a high net Zn and Cu uptake rate. It is concluded that differences in Al sensitivity among rice cultivars BG35, DA14 and IR45 are not primarily linked to the depressed internal Mg or Mn status of the plants but rather to changes in the uptake and distribution of Ca and P.     

Assessment of Allelopathic Potential of Root Exudate of Rice Seedlings

To determine the allelopathic potential of root exudate from early developmental stage of rice (Oryza sativa L), 6-d-old seedlings of eight cultivars were grown with 3-d-old alfalfa (Medicago sativa L.), cress (Lepidium sativum L.) or lettuce (Lactuca sativa L.) seedlings in Petri dishes under controlled condition. All rice cultivars (cv. Norin 8, Kamenoo, Nipponbare, Kinuhikari, Koshihikari, Sasanishiki, Yukihikari and Hinohikari) inhibited growth of roots, shoots and fresh mass of alfalfa, cress and lettuce seedlings. Effectiveness of cv. Koshihikari was the greatest and more than 60% inhibition was recorded in all bioassays, followed by that of cv. Norin 8 of which effectiveness was more than 40%.     

Mobilization and Acquisition of Sparingly Soluble P-Sources by Brassica Cultivars under P-Starved Environment Ⅱ. Rhizospheric pH changes, Redesigned Root Architecture and Pi-Uptake Kinetics

Non-mycorrhizal Brassica does not produce specialized root structures such as cluster or dauciform roots but is an effective user of P compared with other crops. In addition to P-uptake, utilization and remobilization activity, acquisition of orthophosphate (Pi) from extracellular sparingly P-sources or unavailable bound P-forms can be enhanced by biochemical rescue mechanisms such copious H~+-efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H~+ ATPase and anion channels triggered by P-starvation. To visualize the dissolution of sparingly soluble Ca-phosphate (Ca-P), newly formed Ca-P was suspended in agar containing other essential nutrients. With NH_4~+ applied as the N source, the precipitate dissolved in the root vicinity can be ascribed to rhizosphere acidification, whereas no dissolution occurred with nitrate nutrition. To observe in situ rhizospheric pH changes, images were recorded after embedding the roots in agar containing bromocresol purple as a pH indicator. P-tolerant cultivar showed a greater decrease in pH than the sensitive cultivar in the culture media (the appearance of typical patterns of various colors of pH indicator in the root vicinity), and at stress P-level this acidification was more prominent. In experiment 2, low P-tolerant class-Ⅰ cultivars (Oscar and Con-Ⅱ) showed a greater decrease in solution media pH than low P-sensitive class-Ⅱ (Gold Rush and RL-18) cultivars, and P-contents of the cultivars was inversely related to decrease in culture media pH. To elucidate P-stressinduced remodeling and redesigning in a root architectural system, cultivars were grown in rhizoboxes in experiment 3.The elongation rates of primary roots increased as P-supply increased, but the elongation rates of the branched zones of primary roots decreased. The length of the lateral roots and topological index values increased when cultivars were exposed to a P-stress environment. To elucidate Pi-uptake kinetics, parameters related to P influx: maximal transport rate (V_(max)), the Michaelis-Menten constant (K_m), and the external concentration when net uptake is zero (C_(min)) were tested in experiment 4. Lower K_m and C_(min) values were better indicative of the P-uptake ability of the class-Ⅰ cultivars, evidencing their adaptability to P-starved environmental cues. In experiment 5, class-Ⅰ cultivars exuded two- to threefold more carboxylates than class-Ⅱ cultivars under the P-stress environment. The amount and types of carboxylates exuded from the roots of P-starved plants differed from those of plants grown under P-sufficient conditions. Nevertheless, the exudation rate of both class-Ⅰ and class-Ⅱ cultivars decreased with time, and the highest exudation rate was found after the first 4 h of carboxylates collection. Higher P uptake by class-Ⅰ cultivars was significantly related to the drop in root medium pH, which can be ascribed to H~+-efflux from the roots supplied with sparingly soluble rock-P and Ca_3(PO_4)_2. These classical rescue strategies provided the basis of P-solubilization and acquisition from sparingly soluble P-sources by Brassica cultivars to thrive in a typically stressful environment.     

Aluminium tolerance is achieved by exudation of citric acid from roots of soybean (Glycine max)

Fourteen soybean ( Glycine max [L.] Merr.) cultivars were analysed and found to differ considerably in aluminium (Al) resistance. The cultivars Suzunari (Al-resistant) and Shishio (Al-sensitive) were selected for further analysis of physiological mechanisms of Al-resistance. The relative root growth of Shishio was 48% compared to 76% for Suzunari in response to 15 μ M Al (24 h). Aluminium accumulation and Al-induced callose formation in root apices were 50 and 25% of that in Suzunari, respectively. Al inhibited both Suzunari and Shishio during the first 6 h of exposure. However, the root growth inhibition was further increased in Shishio but not in Suzunari, suggesting an Al-induced Al-resistant mechanism operating in Suzunari. Organic acid analysis in root exudates of both cultivars revealed that they specifically exuded citrate in response to Al. However, the citrate exudation rate was significantly higher in Suzunari during the 6 h/24 h Al treatment, which was 52/330 compared to Shishio's 26/118 (nmol [g root fresh weight]⁻¹ [6 h]⁻¹), respectively. This Al-induced citric acid exudation was found to be specific for Al, as several other metals failed to induce citrate exudation in both cultivars. Fourteen days of P deficiency did not elicit citrate excretion in both cultivars, while application of Al to P-deficient plants rapidly induced citrate exudation in both cultivars, confirming the specificity of the response of these soybean cultivars to Al. To our knowledge, this is the first report demonstrating an Al-exclusion mechanism in soybean cultivars, which is conferred by enhanced and specific Al-induced exudation of citrate.     

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.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

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.     

Effect of phosphorus supply to the surface roots of wheat on root extension and rhizosphere chemistry in an acidic subsoil

Seedlings of two cultivars of wheat (Triticum aestivum L.) differing in tolerance to aluminium (Al) were grown using a split-root sand/soil culture technique. Each culture tube was divided horizontally into a surface (0–150 mm) compartment and a subsurface (150–250 mm) compartment separated by a root-permeable paraffin wax barrier. Thus phosphorus (P) supplied to surface roots could not percolate or diffuse into the soil in the subsurface compartment. The soil in the subsurface compartment was divided into ‘rhizosphere’ and ‘non-rhizosphere’ zones using a porous (5 μm) membrane. Root growth of both cultivars into the subsurface zone was enhanced by increased P supply to surface roots, but did not conform to known relationships between root growth and soil pH, extractable-Al, or pH, Al or P concentrations in soil solution. Concentrations of Al in soil solution in the rhizosphere were greater than those in solution in the bulk soil. Concentrations of Al reactive with pyrocatechol violet (30s-RRAI) in the rhizosphere soil solution were generally greater than those in non-rhizosphere soil. With the Al-sensitive cultivar, root dry weight and length increased as concentrations of RRAl in the rhizosphere soil solution increased. Increased concentrations of Al in rhizosphere soil solutions were not related to the presence of organic ligands in solution. The effect of P in promoting root penetration into the acidic subsurface stratum was not related to differential attainment of maturity by the plant shoots, but appeared to be related to the effect of P in enhancing the rate of root growth. Thus, suboptimal supply of P to the surface roots of a plant, even at levels sufficient to preclude development of nutritional (P) stress symptoms, may seriously reduce tolerance to Al, and hence diminish the ability of roots to penetrate into acidic subsoils.     

CH4 emission with differences in atmospheric CO2 enrichment and rice cultivars in a Japanese paddy soil

A pot experiment was conducted to investigate CH₄ emissions from a sandy paddy soil as influenced by rice cultivars and atmospheric CO₂ elevation. The experiment with two CO₂ levels, 370 μL L⁻¹ (ambient) and 570 μL L⁻¹ (elevated), was performed in a climatron, located at the National Institute for Agro‐Environmental Sciences, Tsukuba, Japan. Four rice cultivars were tested in this experiment, including IR65598, IR72, Dular and Koshihikari. Tiller number, root length and grain yield were clearly larger under elevated CO₂ than under ambient CO₂. IR72 and Dular showed significantly higher tiller number, root length and grain yield than Koshihikari and IR65598. Average daily CH₄ fluxes under elevated CO₂ were significantly larger by 10.9–23.8% than those under ambient CO₂, and varied with the cultivars in the sequence Dular ≧ IR72>IR65598 ≧ Koshihikari. Dissolved organic C (DOC) content in the soil was obviously higher under elevated CO₂ than under ambient CO₂ and differed among the cultivars, in the sequence IR72>Dular>Koshihikari>IR65598. The differences in average daily CH₄ fluxes between CO₂ levels and among the cultivars were related to different root exudation as DOC content, root length and tiller number. This study indicated that Koshihikari should be a potential cultivar for mitigating CH₄ emission and simultaneously keeping stable grain yield, because this cultivar emitted lowest CH₄ emission and produced medium grain yield.     

Rice cultivar evaluation for phosphorus use efficiency

Phosphorus deficiency is one of the most growth-limiting factors in acid soils in various parts of the world. The objective of this study was to screen 25 rice cultivars (Oryza sativa L.) at low, medium, and high levels of soil P. Number of tillers, root length, plant height, root dry weight and shoot dry weight were related to tissue P concentrations, P uptake and P-use efficiency. Shoot weight was found to be the plant parameter most sensitive to P deficiency. Significant cultivar differences in P use efficiency were found. Phosphorus use efficiency was higher in roots than shoots and decreased with increasing levels of soil P. Positive correlations were found among growth parameters such as plant height, tillers, root and shoot weight, and P content of roots and shoots. These results indicate selection of rice cultivars for satisfactory performance under low P availability can be carried out using shoot and root dry weight as criteria.     

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

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

Identification of new sources of aluminum resistance in wheat

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

铝胁迫下胡枝子根尖胼胝质形成规律及影响因素

采用水培试验,研究了铝胁迫下两个胡枝子品种根尖产生胼胝质的变化规律及影响因素。结果表明,两个品种的根尖铝吸收量与胼胝质形成量呈正比例关系。品种间差异主要是在根尖0—0.5 cm处。敏感品种胼胝质形成量同铝吸收量的变化趋势相一致,而耐性品种则在铝处理6 h时出现一个高峰值后下降。去除铝胁迫后,耐性品种胼胝质形成量并不显著减少。与单独铝处理相比,阴离子通道抑制剂苯甲酰甲醛加铝处理对两个品种胼胝质形成无影响;尼氟灭酸加铝处理抑制敏感品种胼胝质的形成,对耐性品种无影响;蒽-9-羧酸加铝处理显著抑制两个品种的胼胝质形成。另外,抑制剂2-去氧-D-葡萄糖加铝共同处理与单独铝处理相比,敏感品种的胼胝质形成量显著降低,耐性品种无影响。甘露醇对两个品种胼胝质形成的影响无显著差别。镧处理下胼胝质的形成量是耐性品种显著高于敏感品种,铝、镧同时处理胼胝质的形成量最高。敏感品种胼胝质形成处理间无差别。总之,耐性品种在铝胁迫下胼胝质形成与有机酸分泌可能存在一定的协调关系;铝胁迫下胼胝质形成是敏感指标;在一定条件下,特别是有机酸分泌前胼胝质的形成可能具有一定抗性意义;铝诱导胼胝质的形成受多种外界因素(浓度、时间、有机酸分泌,渗透压等)的影响。     

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

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