Al Accumulation In Roots Research Articles

Two Half-Size ATP-Binding Cassette Transporters Are Implicated in Aluminum Tolerance in Soybean.

Aluminum (Al) toxicity severely restricts plant production in acidic soils. ATP-binding cassette (ABC) transporters participate in plant tolerance to various environmental stresses. However, ABC transporters implicated in soybean Al tolerance are still rare. Here, we functionally characterized two half-size ABC transporters (GmABCB48 and GmABCB52) in soybean. Expression analysis showed that GmABCB48 and GmABCB52 were induced only in the roots, especially in the root tips. Both GmABCB48 and GmABCB52 were localized at the plasma membrane. Overexpression of GmABCB48 or GmABCB52 in Arabidopsis reduced Al accumulation in roots and enhanced Al tolerance. However, expression of GmABCB48 or GmABCB52 in yeast cells did not affect Al uptake. Furthermore, transgenic lines expressing GmABCB48 or GmABCB52 had lower Al content in root cell walls than wild-type plants under Al stress. Further investigation showed that the Al content in cell wall fractions (pectin and hemicellulose 1) of transgenic lines was significantly lower than that of wild-type plants, which was coincident with the changes of pectin and hemicellulose 1 content under Al exposure. These results indicate that GmABCB48 and GmABCB52 confer Al tolerance by regulating the cell wall polysaccharides metabolism to reduce Al accumulation in roots.

γ-Aminobutyric acid (GABA) priming alleviates acid-aluminum toxicity to roots of creeping bentgrass via enhancements in antioxidant defense and organic metabolites remodeling.

γ-Aminobutyric acid alleviates acid-aluminum toxicity to roots associated with enhanced antioxidant metabolismas well asaccumulation and transportation of citric and malic acids. Aluminum (Al) toxicity has become the main limiting factor for crop growth and development in acidic soils and is further being aggravated worldwide due to continuous industrial pollution. The current study was designed to examine effects of GABA priming on alleviating acid-Al toxicity in terms of root growth, antioxidant defense, citrate and malate metabolisms, and extensive metabolites remodeling in roots under acidic conditions. Thirty-seven-day-old creeping bentgrass (Agrostis stolonifera) plants were used as test materials. Roots priming with or without 0.5mM GABA for 3days were cultivated in standard nutrient solution for 15days as control or subjected to nutrient solution containing 5mM AlCl3·6H2O for 15days as acid-Al stress treatment. Roots were sampled for determinations of root characteristics, physiological andbiochemical parameters, and metabolomics. GABA priming significantly alleviated acid-Al-induced root growth inhibition and oxidative damage, despite it promoted the accumulation of Al in roots. Analysis of metabolomics showed that GABA priming significantly increased accumulations of organic acids, amino acids, carbohydrates, and other metabolites in roots under acid-Al stress. In addition, GABA priming also significantly up-regulated key genes related to accumulation and transportation ofmalic and citric acids in roots under acid-Al stress. GABA-regulated metabolites participated in tricarboxylic acid cycle, GABA shunt, antioxidant defense system, and lipid metabolism, which played positive roles in reactive oxygen species scavenging, energy conversion, osmotic adjustment, and Al ion chelation in roots.

SbXTH7, acting downstream of transcription factor SbHY5, regulates aluminum tolerance by modulating cell wall hemicellulose content and aluminum accumulation in sweet sorghum [Sorghum bicolor (L.)]

Aluminum (Al) toxicity is considered a critical abiotic stress, limiting crop yields in acid soil. Sweet sorghum (Sorghum bicolor L.) is a promising non-food crop for the manufacturing of industrial products such as sugar, alcohol and biofuel. It mainly grows in Al toxicity widespread tropics and subtropics. Previous studies have demonstrated that LONG HYPOCOTYL5 of sweet sorghum confers transgenic Arabidopsis Al tolerance by modulating the expressions of some typical Al-responsive genes. Here, RNA sequencing analysis revealed that SbHY5 is involved in regulating cell components and cell wall modification, especially the xyloglucan metabolic process (GO: 0010411). Furthermore, all 34 xyloglucan endotransglucosylases/hydrolases (SbXTHs) in sweet sorghum were subjected to bioinformatics analysis and named according to their chromosomal location. Multiple experimental analyses demonstrated that SbHY5 could directly bind to SbXTH7 promoter and downregulate its expression in response to Al toxicity. SbXTH7 expression in root apices and elongation zone significantly downregulated after Al treatment. In addition, SbXTH7 overexpression reduced Arabidopsis Al tolerance by increasing cell wall hemicellulose content and enhancing Al accumulation in roots. In conclusion, we showed the presence of a SbHY5-SbXTH7 axis-mediated hemicellulose-dependent regulatory pathway underlying plant Al tolerance.

Characterization of SgALMT genes reveals the function of SgALMT2 in conferring aluminum tolerance in Stylosanthes guianensis through the mediation of malate exudation

Aluminum (Al) toxicity is the major constraint on plant growth and productivity in acidic soils. An adaptive mechanism to enhance Al tolerance in plants is mediated malate exudation from roots through the involvement of ALMT (Al-activated malate transporter) channels. The underlying Al tolerance mechanisms of stylo (Stylosanthes guianensis), an important tropical legume that exhibits superior Al tolerance, remain largely unknown, and knowledge of the potential contribution of ALMT genes to Al detoxification in stylo is limited. In this study, stylo root growth was inhibited by Al toxicity, accompanied by increases in malate and citrate exudation from roots. A total of 11 ALMT genes were subsequently identified in the stylo genome and named SgALMT1 to SgALMT11. Diverse responses to metal stresses were observed for these SgALMT genes in stylo roots. Among them, the expressions of 6 out of the 11 SgALMTs were upregulated by Al toxicity. SgALMT2, a root-specific and Al-activated gene, was selected for functional characterization. Subcellular localization analysis revealed that the SgALMT2 protein is localized to the plasma membrane. The function of SgALMT2 in mediating malate release was confirmed by analysis of the malate exudation rate from transgenic composite stylo plants overexpressing SgALMT2. Furthermore, overexpression of SgALMT2 led to increased root growth in transgenic stylo plants treated with Al through decreased Al accumulation in roots. Taken together, the results of this study suggest that malate secretion mediated by SgALMT2 contributes to the ability of stylo to cope with Al toxicity.

Higher Aluminum Tolerance of Lespedeza bicolor Relative to Lespedeza cuneata Is Associated with Saccharide Components of Root Tips

Aluminum (Al) toxicity is the primary factor limiting agricultural productivity in acid soils. The cell wall is mainly composed of saccharides and the first barrier for aluminum (Al) to enter plant root cells, but it is unknown whether and how root saccharide components are involved in regulating the Al tolerance of Lespedeza that is well adapted to acid soils. Here, we used Al-tolerant Lespedeza bicolor and Al-sensitive Lespedeza cuneata to examine the association of root saccharide components with Lespedeza Al tolerance through analyzing the saccharide changes of roots exposed to Al toxicity. Al-sensitive Lespedeza accumulated more Al and pectin but less hemicellulose in the root cell walls than Al-tolerant Lespedeza. Al treatment decreased the amounts of total sugar secreted from and within the roots of only Al-tolerant Lespedeza. Al treatment decreased the content of root monosaccharides including glucose and mannose in both Lespedeza species, but increased the xylose contents in only Al-sensitive Lespedeza. Taken together, less cell wall pectin rather than hemicellulose is responsible for less root Al accumulation, and Al-decreased root saccharide contents may enhance root organic-acid secretion to chelate toxic Al, both of which contribute to Lespedeza Al tolerance.

Effects of polygalacturonase overexpression on pectin distribution in the elongation zones of roots under aluminium stress.

The roots of many plant species contain large amounts of pectin and it contributes to the formation of the rhizosphere. In the present study, the relationship between the root-tip pectin content and aluminium (Al) tolerance in wild-type (WT) and demethylesterified pectin degradation enzyme gene overexpressor (OsPG2-FOX) rice lines was compared. OsPG2-FOX rice showed reduced pectin content in roots, even under control conditions; Al treatment reduced root elongation and the pectin content in the root elongation zone. Wild-type rice showed more pectin accumulation in the root elongation zone after Al treatment. Relative to WT rice, OsPG2-FOX rice showed more Al accumulation in the root elongation zone. These results indicate that the amount of pectin influences Al tolerance and that the distribution of pectin in the root elongation zone inhibits Al accumulation in rice roots. Pectin accumulation in cell walls in the root elongation zone may play a role in protecting rice plants from the Al-induced inhibition of root elongation by regulating pectin distribution.

Silicon Reduces Aluminum-Induced Suberization by Inhibiting the Uptake and Transport of Aluminum in Rice Roots and Consequently Promotes Root Growth.

Silicon (Si) can alleviate aluminum (Al) toxicity in rice (Oryza sativa L.), but the mechanisms underlying this beneficial effect have not been elucidated, especially under long-term Al stress. Here, the effects of Al and Si on the suberization and development of rice roots were investigated. The results show that, as the Al exposure time increased, the roots accumulated more Al, and Al enhanced the deposition of suberin in roots, both of which ultimately inhibited root growth and nutrient absorption. However, Si restricted the apoplastic and symplastic pathways of Al in roots by inhibiting the uptake and transport of Al, thereby reducing the accumulation of Al in roots. Meanwhile, the Si-induced drop in Al concentration reduced the suberization of roots caused by Al through down-regulating the expression of genes related to suberin synthesis and then promoted the development of roots (such as longer and more adventitious roots and lateral roots). Moreover, Si also increased nutrient uptake by Al-stressed roots and thence promoted the growth of rice. Overall, these results indicate that Si reduced Al-induced suberization of roots by inhibiting the uptake and transport of Al in roots, thereby amending root growth and ultimately alleviating Al stress in rice. Our study further clarified the toxicity mechanism of Al in rice and the role of Si in reducing Al content and restoring root development under Al stress.

Overexpression of the Aldehyde Dehydrogenase Gene ZmALDH Confers Aluminum Tolerance in Arabidopsis thaliana.

Aluminum (Al) toxicity is the main factor limiting plant growth and the yield of cereal crops in acidic soils. Al-induced oxidative stress could lead to the excessive accumulation of reactive oxygen species (ROS) and aldehydes in plants. Aldehyde dehydrogenase (ALDH) genes, which play an important role in detoxification of aldehydes when exposed to abiotic stress, have been identified in most species. However, little is known about the function of this gene family in the response to Al stress. Here, we identified an ALDH gene in maize, ZmALDH, involved in protection against Al-induced oxidative stress. Al stress up-regulated ZmALDH expression in both the roots and leaves. The expression of ZmALDH only responded to Al toxicity but not to other stresses including low pH and other metals. The heterologous overexpression of ZmALDH in Arabidopsis increased Al tolerance by promoting the ascorbate-glutathione cycle, increasing the transcript levels of antioxidant enzyme genes as well as the activities of their products, reducing MDA, and increasing free proline synthesis. The overexpression of ZmALDH also reduced Al accumulation in roots. Taken together, these findings suggest that ZmALDH participates in Al-induced oxidative stress and Al accumulation in roots, conferring Al tolerance in transgenic Arabidopsis.

Response of Vallisneria natans to aluminum phytotoxicity and their synergistic effect on nitrogen, phosphorus change in sediments

Increasing aluminum (Al) use and its effects on aquatic systems have been a global issue, however the Al impacts on submerged plants and their ecological functions were poorly understood. Aquatic simulation experiments were performed to study Al-toxicity on the germination and seedling morphological and physiological characteristics of Vallisneria natans, and investigate their synergistic effect on nitrogen (N), phosphorus (P) change and microbial community in sediment. The seeds germination characteristics, growth and physiological parameters of seedlings, including root activity, were significantly affected by alum treatments and the inhibition levels increased with Al3+ concentration. The Al accumulation in roots and leaves were significantly different. Al3+ concentration above 0.3 mg/L showed toxic to V. natans. TN, TP, IP, Fe/Al-P contents in sediments varied markedly under co-existence of Al and V. natans. Additionally, the relative abundance of sediment microbial community related to N, P cycle was effected. Results concluded that the increasing aquatic Al-concentration inhibits growth and propagation of submerged plants and the ecological restoration effect, and exerts synergistic effect with submerged plants on N, P components in sediments. Such findings were helpful for Al ecological evaluation, and were instructive for the submerged plants restoration in shallow eutrophic lakes with Al input.

Phosphorus relieves aluminum toxicity in oil tea seedlings by regulating the metabolic profiling in the roots

Oil tea (Camellia oleifera Abel.) is an important edible oil tree mainly grown in acidic soils, whose growth and yield can be severely limited due to soil aluminum (Al) toxicity and phosphorus (P) deficiency. In this study, we investigated the physiological and metabolic responses of oil tea to Al and P treatment for an 8-week duration. Al reduced root length, root volume, and plant biomass, while P addition alleviated the effects of Al toxicity. P addition increased P content and reduced Al accumulation in roots. The profiles of 58 metabolites were significantly changed in roots of oil tea seedlings. Al toxicity increased various amino acids, but decreased many kinds of organic acids and carbohydrates. Interestingly, P addition reduced the amino acids accumulation which were induced by Al toxicity, while only a few organic acids changed under P supply. Most carbohydrates, including sucrose and glucose, significantly increased with P addition under Al toxicity. Results indicated that Al toxicity increased the accumulation of amino acids and reduced the accumulation of organic acids and carbohydrates, while the addition of P promoted root growth by alleviating the inhibition of protein synthesis and increasing carbohydrates content. However, P addition did not increase the organic acids content in roots.

A Defective Vacuolar Proton Pump Enhances Aluminum Tolerance by Reducing Vacuole Sequestration of Organic Acids.

Plants cope with aluminum (Al) toxicity by secreting organic acids (OAs) into the apoplastic space, which is driven by proton (H+) pumps. Here, we show that mutation of vacuolar H+-translocating adenosine triphosphatase (H+-ATPase) subunit a2 (VHA-a2) and VHA-a3 of the vacuolar H+-ATPase enhances Al resistance in Arabidopsis (Arabidopsis thaliana). vha-a2 vha-a3 mutant plants displayed less Al sensitivity with less Al accumulation in roots compared to wild-type plants when grown under excessive Al3+ Interestingly, in response to Al3+ exposure, plants showed decreased vacuolar H+ pump activity and reduced expression of VHA-a2 and VHA-a3, which were accompanied by increased plasma membrane H+ pump (PM H+-ATPase) activity. Genetic analysis of plants with altered PM H+-ATPase activity established a correlation between Al-induced increase in PM H+-ATPase activity and enhanced Al resistance in vha-a2 vha-a3 plants. We determined that external OAs, such as malate and citrate whose secretion is driven by PM H+-ATPase, increased with PM H+-ATPase activity upon Al stress. On the other hand, elevated secretion of malate and citrate in vha-a2 vha-a3 root exudates appeared to be independent of OAs metabolism and tolerance of phosphate starvation but was likely related to impaired vacuolar sequestration. These results suggest that coordination of vacuolar H+-ATPase and PM H+-ATPase dictates the distribution of OAs into either the vacuolar lumen or the apoplastic space that, in turn, determines Al tolerance capacity in plants.

Aluminium toxicity and phosphate deficiency activates antioxidant systems and up-regulates expression of phosphate transporters gene in ryegrass (Lolium perenne L.) plants

Soil acidity, associated with aluminium (Al) toxicity and low phosphorus (P) availability, is considered the most important problem for agricultural production. Even though the Al-P interaction has been widely investigated, the impact of P-nutrition on Al-toxicity still remains controversial and poorly understood. To elucidate further insights into the underlying mechanisms of this interaction in ryegrass (Lolium perenne L.), P uptake, antioxidant responses and the gene expression of phosphate transporters were determined. Two ryegrass cultivars with different Al resistances, the Al-tolerant Nui cultivar and the Al-sensitive Expo cultivar were hydroponically grown under low (16 μM) and optimal (100 μM) P doses for 16 days. After P treatments, plants were exposed to Al doses (0 and 200 μM) under acidic conditions (pH 4.8) for 24 h. Al and P accumulation were higher in the roots of Nui than that of Expo. Moreover, lower Al accumulation was found in shoots of Nui independent of P supplies. Oxidative stress induced by Al-toxicity and P-deficiency was more severe in the Al-sensitive Expo. Expression levels of L. perenne phosphate transporters were higher in Nui than they were in Expo. While LpPHT1 expression was up-regulated by P deficiency and Al toxicity in both cultivars, LpPHT4 expression only increased in the Al-tolerant cultivar. This report shows that the higher Al-tolerance of Nui can be attributed to a greater antioxidant system under both P conditions. The observation of higher P and Al accumulation in roots of Nui might indicate that the Al-tolerance of Nui is a consequence of Al immobilization by P mediated by the high expression of phosphate transporters.

Salicylic acid reduces the accumulation of aluminum in Panax notoginsen root cell wall pectin via the NO signaling pathway

Salicylic acid (SA) and nitric oxide (NO) are key signal molecules involved in the reduction of Al accumulation in many plants. The purpose of this study was to investigate whether they could reduce the root content of Al and/or its binding to the root cell wall (CW) fractions in Panax notoginseng, and to investigate the corresponding regulatory mechanisms. Effects of Al treatments on the endogenous SA and NO contents of P. notoginseng were detected. Exogenous SA and paclobutrazol (PAC, a SA synthesis inhibitor) treatments were conducted to test the effects on endogenous NO content; exogenous sodium nitroprusside (SNP, a NO doner) and 2-phenyl-4,4,5,5-tetramethyl- imidazoline-1-oxyl-3-oxyde (cPTIO, a NO synthesis inhibitor) treatments were conducted to test the effects on endogenous SA content. Pectin methyltransferase (PMT) and pectin methylesterase (PME) activity and the gene expression, pectin methylesterification degree (PMD), pectin content, and the accumulation of Al in roots were also studied under the treatments described above. Al stress induced a significant increase of the activity and expression of phenylalanine ammonia-lyase (PAL), and promoted the significant increase of endogenous SA content of P. notoginseng roots. The Al promoted accumulation of endogenous NO could be enhanced by exogenous SA treatment, but was reduced by PAC. Accumulation of endogenous NO that promoted by Al could be enhanced by exogenous SA treatment, but was reduced by PAC treatment. However, SNP or cPTIO treatments had no significant effect on the Al-induced endogenous SA content of P. notoginseng. This showed that NO acted downstream of SA signaling of P. notoginseng under Al stress. Al stress can increase the activity and genes expression of PME in the roots of P. notoginseng, reduce PMD, increase CW pectin content, and thus, the binding capacity of CW pectin to Al was enhanced in P. notoginseng roots. Exogenous SA or SNP both reduced the Al-binding capacity of root CW pectin by decreasing the pectin content, and increased the PMD by inhibiting the genes expression and activity of PME. The effects of PAC or cPTIO on the above-mentioned indicators under Al stress were opposite to that of exogenous SA or SNP treatments. The Al stress induced accumulation of pectin and the reduction of PMD in the root CW of P. notoginseng could be reversed by the treatments of exogenous SA or SNP, then the Al contents in root CW pectin were reduced. Al stress activated the endogenous SA and NO signaling pathways in P. notoginseng, and NO acted downstream of SA. It showed that the Al-activated NO-SA signaling pathway contributed in the reduction of Al binding to the root under Al stress.

Aluminum effects on photosynthesis, reactive oxygen species and methylglyoxal detoxification in two Citrus species differing in aluminum tolerance.

Citrus are mainly grown in low pH soils with high active aluminum (Al). 'Xuegan' (Citrus sinensis (L.) Osbeck) and 'Shatian pummelo' (Citrus grandis (L.) Osbeck) seedlings were fertilized for 18 weeks with nutrient solution containing either 0 mM (control) or 1 mM (Al toxicity) AlCl3·6H2O. Aluminum induced decreases of biomass, leaf photosynthesis, relative water content and total soluble protein levels, and increases of methylglyoxal levels only occurred in C. grandis roots and leaves. Besides, the Al-induced decreases of pigments and alterations of chlorophyll a fluorescence transients and fluorescence parameters were greater in C. grandis leaves than those in C. sinensis leaves. Aluminum-treated C. grandis had higher stem and leaf Al levels and similar root Al levels relative to Al-treated C. sinensis, but lower Al distribution in roots and Al uptake per plant. Aluminum toxicity decreased nitrogen, phosphorus, potassium, calcium, magnesium and sulfur uptake per plant in C. grandis and C. sinensis seedlings, with the exception of Al-treated C. sinensis seedlings exhibiting increased sulfur uptake per plant and unaltered magnesium uptake per plant. Under Al-stress, macroelement uptake per plant was higher in C. sinensis than that in C. grandis. Aluminum toxicity decreased the ratios of reduced glutathione/(reduced + oxidized glutathione) and of ascorbate/(ascorbate + dehydroascorbate) only in C. grandis roots and leaves. The activities of most antioxidant enzymes, sulfur metabolism-related enzymes and glyoxalases and the levels of S-containing compounds were higher in Al-treated C. sinensis roots and leaves than those in Al-treated C. grandis ones. Thus, C. sinensis displayed higher Al tolerance than C. grandis did. The higher Al tolerance of C. sinensis might involve: (i) more Al accumulation in roots and less transport of Al from roots to shoots; (ii) efficient maintenance of nutrient homeostasis; and (iii) efficient maintenance of redox homeostasis via detoxification systems of reactive oxygen species and methylglyoxal.

The interaction of salicylic acid and Ca2+ alleviates aluminum toxicity in soybean (Glycine max L.)

Both calcium ion (Ca2+) and salicylic acid (SA) influence various stress responses in plants. In acidic soils, aluminum (Al) toxicity adversely affects crop yield. In this study, we determined the influences of Ca2+ and SA on root elongation, Al accumulation, and citrate secretion in soybean plant. We also investigated the activity of antioxidative enzymes in Al-exposed soybean roots. Root elongation was severally inhibited when the roots were exposed to 30 μM Al. The Al-induced inhibition of root elongation was ameliorated by Ca2+ and SA but aggravated by Ca2+ channel inhibitor (VP), CaM antagonists (TFP), Ca2+ chelator (EGTA), and SA biosynthesis inhibitor (PAC). Furthermore, 1.0 mM CaCl2 and 10 μM SA reduced the accumulation of Al in roots, but their inhibitors stimulated the accumulation of Al in roots. Citrate secretion from these roots increased with the addition of either 1.0 mM CaCl2 or 10 μM SA but did not increase significantly when treated with higher Ca2+ concentration. Enzymatic analysis showed that Ca2+ and SA stimulated the activities of superoxidase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) in Al-treated roots. In addition, SA restored the inhibition of Ca2+ inhibitors on root elongation and Al content. Thus, both Ca2+ and SA contribute to Al tolerance in soybean. Furthermore, Ca2+ supplements rapidly increased Al-induced accumulation of free-SA or conjugated SA (SAG), while Ca2+ inhibitors delayed the accumulation of SA for more than 8 h. Within 4 h of treatment, SA increased cytosolic Ca2+ concentration in Al-treated roots, and upregulated the expression of four genes that possibly encode calmodulin-like (CML) proteins. These findings indicate that SA is involved in Ca2+-mediated signal transduction pathways in Al tolerance.

Elevation of arginine decarboxylase-dependent putrescine production enhances aluminum tolerance by decreasing aluminum retention in root cell walls of wheat

Aluminum (Al) stress induces putrescine (Put) accumulation in several plants and this response is proposed to alleviate Al toxicity. However, the mechanisms underlying this alleviation remain largely unknown. Here, we show that exposure to Al clearly increases Put accumulation in the roots of wheat plants (Triticum aestivum L. ‘Xi Aimai-1’) and that this was accompanied by significant increase in the activity of arginine decarboxylase (ADC), a Put producing enzyme. Application of an ADC inhibitor (d-arginine) terminated the Al-induced Put accumulation, indicating that increased ADC activity may be responsible for the increase in Put accumulation in response to Al. The d-arginine treatment also increased the Al-induced accumulation of cell wall polysaccharides and the degree of pectin demethylation in wheat roots. Thus, it elevated Al retention in cell walls and exacerbated Al accumulation in roots, both of which aggravate Al toxicity in wheat plants. The opposite effects were true for exogenous Put application. These results suggest that ADC-dependent Put accumulation plays important roles in providing protection against Al toxicity in wheat plants through decreasing cell wall polysaccharides and increasing the degree of pectin methylation, thus decreasing Al retention in the cell walls.

Alleviation of aluminum toxicity by hydrogen sulfide is related to elevated ATPase, and suppressed aluminum uptake and oxidative stress in barley

Greenhouse hydroponic experiments were performed to evaluate potential role of H2S on Al toxicity in barley seedlings. Seedlings pretreated with 200μM NaHS as a donor of H2S for 24h and subsequently exposed to 100μM AlCl3 for 24h had significantly longer roots than those without NaHS. The promoted root elongation was correlated with a substantial decrease in Al-induced overproduction of lipid peroxidation, electrolyte leakage and Al accumulation in roots, and a marked increase in Al-induced depress activities of Na+K+-ATPase and H+-ATPase. The alleviating role of H2S on Al-induced toxicity was also found in a time- and dose-dependent experiment. Addition of 200 and 400μM NaHS to 100μM AlCl3 effectively alleviated Al-toxicity, markedly diminished Al-induced MDA accumulation, and increased chlorophyll content, net photosynthetic rate (Pn) and maximal photochemical efficiency (Fv/Fm) compared with Al alone. Exogenous H2S significantly elevated depressed CAT activities, and further improved root POD activity. Moreover, NaHS decreased Al accumulation, but elevated concentrations of S, P, Ca, Mg and Fe in plants. These data suggest that H2S-induced alleviation in Al toxicity is attributed to reduced Al uptake and MDA accumulation, improved uptake of P, Ca, Mg and Fe, and elevated ATPase and photosynthetic performance.

Aluminum resistance mechanisms in oat (Avena sativa L.)

Enhanced aluminum (Al) resistance has been observed in dicots over-expressing enzymes involved in organic acid synthesis; however, this approach for improving Al resistance has not been investigated in monocots. Among the cereals, oat (Avena sativa L.) is considered to be Al resistant, but the basis of resistance is not known. A hydroponic assay and hematoxylin staining for Al accumulation in roots were used to evaluate Al resistance in 15 oat cultivars. Malate and citrate release from roots was measured over a 24 h period. A malate dehydrogenase gene, neMDH, from alfalfa (Medicago sativa L.) was used to transform oat. Oat seedlings were highly resistant to Al, as a concentration of 325 μM AlK(SO4)2 was needed to cause a 50% decrease in root growth. Most oat cultivars tested are naturally resistant to high concentrations of Al and effectively excluded Al from roots. Al-dependent release of malate and Al-independent release of citrate was observed. Al resistance was enhanced in a transgenic oat line with the highest accumulation of neMDH protein. However, overall root growth of this line was reduced and expression of neMDH in transgenic oat did not enhance malate secretion. Release of malate from oat roots was associated with Al resistance, which suggests that malate plays a role in Al resistance of oat. Over-expression of alfalfa neMDH enhanced Al resistance in some lines but was not effective alone for crop improvement.

Aluminum stress induces up-regulation of an efficient antioxidant system in the Al-tolerant maize line but not in the Al-sensitive line

The response of the antioxidant enzymes, superoxide dismutase (SOD; EC 1.15.1.1) and peroxidase (POD; EC 1.11.1.7), to Aluminum (Al) stress was studied in roots of two inbred lines of maize ( Zea mays L.) differing in their tolerance to Al. In addition, the production of malondialdehyde (MDA) was measured to evaluate the level of lipid peroxidation as well as the accumulation of proline (Pro) and carbohydrates under 72 h Al stress. Roots of Al (0, 120, 240, 360 and 480 μM, at pH 4.2) -treated plants were sampled at various times (12, 24, 48, 72 h) after commencement of stress. A major difference in the antioxidant enzymes between the two maize lines associated with Al tolerance was observed after 24 h of Al exposure. A gradual increase in the membrane lipid peroxidation in Al-stressed root of the susceptible maize line was accompanied by decreased activities of the antioxidant enzymes SOD and POD. In contrast, increased activities of the SOD and POD were found in Al-treated roots of the tolerant maize line, in which the level of membrane lipid peroxidation remained almost unchanged. After 72 h exposure to 480 μM Al the accumulation of Al in roots was almost from 90 times (tolerant) to 140 times (sensitive) than the control (without Al), while at the same time Al treatment resulted in 2.2 to 2.5-fold (at 240, 360 and 480 μM Al) increased Pro content in the roots of the tolerant line compared to 0 μM Al. Yet, 72 h exposure to 480 μM Al increased 1.7-fold the carbohydrate concentration in the roots of the Al tolerant maize line VA-22 while in the sensitive line A 4/67 remained almost unchanged. These data provide evidence of an internal mechanism of tolerance that increase the antioxidant system activity in order to limit cellular damages and possibly linked to the Al tolerance of the maize line VA-22. Analyses of the 12, 24, 48, and 72 h POD and SOD isoforms showed that in the Al-tolerant maize plants the anionic POD isoforms A 1, A 3 and A 4 and the SOD isoforms SOD 1 and SOD 2 were induced by increased Al-stress. It seemed that in the Al tolerant maize line, the anionic POD isoforms A 1, A 3 and A 4 and the SOD isoforms SOD 1 and SOD 2 were required for adaptation as the oxidant level increased by the increased Al stress. Our results suggest that Al toxicity may be mediated by oxidative stress and that the better protection of the Al tolerant maize roots from Al-induced oxidative damage results, at least partially, from the increased activity of their antioxidative system.

The role of organic acids in the short- and long-term aluminum tolerance in maize seedlings (Zea mays L.)

Aluminum (Al) affects numerous physiological processes in plants. However, Al tolerance mechanisms mediated by increased synthesis of organic acids (OAs) have been outlined recently. In this study, we examined the role of OAs in the short (1–8 h) and long-term (4 days) Al tolerance in maize seedlings. Exposure to Al stress for 4 days results in a rapid inhibition of root growth. Al induced morphological changes in the maize roots, especially at a higher solution of Al concentration (1,000 μM Al). The increase in Al accumulation in roots, including strongly elevated levels of Al accumulated in root cell walls suggests that Al tolerance in maize is mediated in part by higher accumulation of Al in the roots. The enhanced citrate exudation, which was only observed at 1,000 μM Al may lead to detoxification of Al by formation of OA–Al complexes in the root apoplast. This mechanism has been suggested to play a significant role in Al resistance response in maize. The short-term responses underlying internal detoxification via OA-chelators were also investigated. Succinate, malate, citrate and total root OA contents decreased markedly, 2 h after the Al exposure. At 4 and 8 h time points, OA contents increased or remained unchanged, except for that of malate which decreased. The level of OAs in shoots, on the other hand, showed alterations that were less pronounced in response to Al. Specifically, the citrate and total OA concentrations significantly increased at 4 h, but showed a pronounced decrease at the 8 h time point. Based on our findings, we propose that multiple responses, including Al exclusion by Al accumulation in root cells and citrate efflux, may contribute towards higher Al resistance in maize. The rapid OA changes in responses to short-term Al treatment may not be responsible for Al tolerance. However, increased OA synthesis observed in this study may be involved in diminishing the stress triggered by Al. The molecular aspects underlying Al resistance mechanism via Al-induced expression of the enzymes catalyzing OA synthesis and metabolism remain to be elucidated.