Recent progress in the research of external Al detoxification in higher plants: a minireview

Aluminum (Al) is highly toxic to plant growth. The toxicity is characterized by rapid inhibition of root elongation. However, some plant species and cultivars have evolved some mechanisms for detoxifying Al both internally and externally. In this review, the recent progress made in the research of external detoxification of Al is described. Accumulating evidence has shown that organic acids play an important role in the detoxification of Al. Some plant species and cultivars respond to Al by secreting citrate, malate or oxalate from the roots. Recently, the anion channel of malate and citrate in the plasma membrane has been characterized and a gene encoding the malate channel has been cloned. The metabolism of organic acids seems to be poorly correlated with the Al-induced secretion of organic acid anions. A number of QTLs (quantitative trait loci) for Al resistance have been identified in rice, Arabidopsis, and other species. Transgenic plants with enhanced resistance to Al have also been reported, but introduction of multiple genes may be required to gain high Al resistance in future.     

A wheat gene encoding an aluminum-activated malate transporter

The major constraint to plant growth in acid soils is the presence of toxic aluminum (Al) cations, which inhibit root elongation. The enhanced Al tolerance exhibited by some cultivars of wheat is associated with the Al-dependent efflux of malate from root apices. Malate forms a stable complex with Al that is harmless to plants and, therefore, this efflux of malate forms the basis of a hypothesis to explain Al tolerance in wheat. Here, we report on the cloning of a wheat gene, ALMT1 (aluminum-activated malate transporter), that co-segregates with Al tolerance in F2 and F3 populations derived from crosses between near-isogenic wheat lines that differ in Al tolerance. The ALMT1 gene encodes a membrane protein, which is constitutively expressed in the root apices of the Al-tolerant line at greater levels than in the near-isogenic but Al-sensitive line. Heterologous expression of ALMT1 in Xenopus oocytes, rice and cultured tobacco cells conferred an Al-activated malate efflux. Additionally, ALMT1 increased the tolerance of tobacco cells to Al treatment. These findings demonstrate that ALMT1 encodes an Al-activated malate transporter that is capable of conferring Al tolerance to plant cells.     

Overexpression of MsALMT1, from the Aluminum-Sensitive Medicago sativa, Enhances Malate Exudation and Aluminum Resistance in Tobacco

Aluminum (Al) toxicity is a major limiting factor for plant growth and crop production in acidic soils. Al-induced organic acid (OA) exudation plays an important role in plant Al resistance. The exudation of OAs is mediated by membrane-localized OA transporters. In our previous study, a gene encoding the Al-induced malate transporter (MsALMT1) was identified in the roots of the Al-sensitive plant Medicago sativa L. cv. Yumu no. 1 (YM1). To further validate the function of MsALMT1, transgenic plants that overexpressed MsALMT1 under the control of the CaMV 35S (35S) promoter were generated. This transgenic tobacco showed an enhanced capacity for malate efflux and better Al resistance than wild type (WT) plants after exposure to 30 μM Al for 24 h. The Al content in the transgenic plant roots decreased to 40–52 % of that in WT plant roots. These results demonstrate that MsALMT1 is an Al-resistant gene in YM1 and encodes a malate transporter, the overexpression of which effectively enhances the Al resistance of transgenic tobacco plants.     

Strategies to isolate transporters that facilitate organic anion efflux from plant roots

The efflux of organic anions from roots plays an important role in plant nutrition. The release of simple carboxylic anions such as citrate, malate and oxalate have been implicated in mechanisms of aluminium (Al) tolerance and improved acquisition of soil phosphorus. These metabolites are likely to cross cell membranes as multivalent anions and recent evidence indicates that anion-permeable channels facilitate this flow in the Al-dependent efflux of malate and citrate from wheat and maize, respectively. However, the genes encoding these anion channels, or any other protein that facilitates the release of citrate, malate or oxalate have not been isolated. This is an obstacle for the application of biotechnology to combat Al toxicity and to improve P-acquisition efficiency in plants. We discuss several strategies aimed at isolating genes that facilitate organic anion release from plant roots.     

Functional characterization of plasma membrane-localized organic acid transporter (CsALMT1) involved in aluminum tolerance in <Emphasis Type="Italic">Camelina sativa</Emphasis> L.

Aluminum (Al) toxicity is a major limiting factor that inhibits root elongation and decreases crop production in acidic soils. The symptoms of inhibited root growth include a reduced uptake of nutrients because the roots become stubby and brittle. The release of organic anions from roots can protect a plant from Al toxicity. The mechanism relies on the efflux of organic anions, such as malate or citrate, which protect roots by chelating the Al³⁺. In this study, homologs of TaALMT1, a Camelina gene that encodes an aluminum-activated malate transporter, were investigated. The expression of this gene was induced by Al in the root, but not in the shoots. Using green fluorescent protein (GFP) fusion constructs and Western-blot analysis, we observed that CsALMT1 was localized in the plasma membrane. Also, to determine the degree to which Al tolerance was affected by malate secretion in Camelina root, we generated CsALMT1 overexpressing plants. CsALMT1 overexpressing transgenic plants showed a higher root elongation rate than the wild-type plant. Damaged cell staining analysis by hematoxylin under 25 µM Al treatment for 2, 4, and 6 h showed a pattern of less damage in CsALMT1 transgenic plants than in wild-type plant, especially in the root elongation zone. Furthermore, the rate of increase of secretion of organic acid in overexpressed plants after Al treatment was higher than that in the wild-type plant. In addition, in the Al-specific dye morin staining on root protoplast under 50 µM Al treatment, less Al accumulation was observed in the CsALMT1 transgenic plants than in the wild-type plant. The Al contents in the roots of the transgenic plants were at a lower level than those in the wild-type plant. These results show that the overexpression of CsALMT1 improves Al tolerance by increasing the release of malate from the root to the soil and, thereby, detoxifies the Al³⁺.     

Wheat genotypes differing in aluminum tolerance differ in their growth response to CO2 enrichment in acid soils

Aluminum (Al) toxicity is a major factor limiting plant growth in acid soils. Elevated atmospheric CO₂ [CO₂] enhances plant growth. However, there is no report on the effect of elevated [CO₂] on growth of plant genotypes differing in Al tolerance grown in acid soils. We investigated the effect of short‐term elevated [CO₂] on growth of Al‐tolerant (ET8) and Al‐sensitive (ES8) wheat plants and malate exudation from root apices by growing them in acid soils under ambient [CO₂] and elevated [CO₂] using open‐top chambers. Exposure of ET8 plants to elevated [CO₂] enhanced root biomass only. In contrast, shoot biomass of ES8 was enhanced by elevated [CO₂]. Given that exudation of malate to detoxify apoplastic Al is a mechanism for Al tolerance in wheat plants, ET8 plants exuded greater amounts of malate from root apices than ES8 plants under both ambient and elevated [CO₂]. These results indicate that elevated [CO₂] has no effect on malate exudation in both ET8 and ES8 plants. These novel findings have important implications for our understanding how plants respond to elevated [CO₂] grown in unfavorable edaphic conditions in general and in acid soils in particular.     

酸性土壤上植物应对铝胁迫的过程与机制

铝胁迫是酸性土壤上影响作物产量最重要的因素之一.目前,全球土壤酸化程度进一步加剧了铝胁迫.植物可通过将铝离子与有机酸螯合储藏于液泡和从根系中排出铝毒.排出铝毒主要通过苹果酸转运蛋白ALMT和柠檬酸转运蛋白MATE的跨膜运输来实现.编码ABC转运蛋白和锌指转录因子的基因与植物抗铝胁迫有关.这些抗铝毒基因的鉴别使得通过转基因和分子标记辅助育种等生物技术来提高农作物的抗铝毒能力成为可能.最后提出了植物抗铝胁迫研究中需要解决的关键问题及今后的研究方向.     

Pattern of aluminum-induced secretion of organic acids differs between rye and wheat

Al-Induced secretion of organic acids from the roots has been considered as a mechanism of Al tolerance, but the processes leading to the secretion of organic acids are still unknown. In this study, the secretion pattern and alteration in the metabolism of organic acids under Al stress were examined in rye (Secale cereale L. cv King) and wheat (Triticum aestivum L. cv Atlas 66). Al induced rapid secretion of malate in the wheat, but a lag (6 and 10 h for malic and citric acids, respectively) between the exposure to Al and the secretion of organic acids was observed in the rye. The activities of isocitrate dehydrogenase, phosphoenolpyruvate carboxylase, and malate dehydrogenase were not affected by Al in either plant. The activity of citrate synthase was increased by the exposure to Al in the rye, but not in the wheat. The secretion of malate was not suppressed at low temperature in the wheat, but that of citrate was stopped in the rye. The Al-induced secretion of citrate from roots of the rye was inhibited by the inhibitors of a citrate carrier, which transports citrate from the mitochondria to the cytoplasm. All of these results suggest that alteration in the metabolism of organic acids is involved in the Al-induced secretion of organic acids in rye, but only activation of an anion channel seems to be responsible for the rapid secretion of malate in the wheat.     

The BnALMT1 and BnALMT2 genes from rape encode aluminum-activated malate transporters that enhance the aluminum resistance of plant cells

The release of organic anions from roots can protect plants from aluminum (Al) toxicity and help them overcome phosphorus (P) deficiency. Our previous findings showed that Al treatment induced malate and citrate efflux from rape (Brassica napus) roots, and that P deficiency did not induce the efflux. Since this response is similar to the malate efflux from wheat (Triticum aestivum) that is controlled by the TaALMT1 gene, we investigated whether homologs of TaALMT1 are present in rape and whether they are involved in the release of organic anions. We isolated two TaALMT1 homologs from rape designated BnALMT1 and BnALMT2 (B. napus Al-activated malate transporter). The expression of these genes was induced in roots, but not shoots, by Al treatment but P deficiency had no effect. Several other cations (lanthanum, ytterbium, and erbium) also increased BnALMT1 and BnALMT2 expression in the roots. The function of the BnALMT1 and BnALMT2 proteins was investigated by heterologous expression in cultured tobacco (Nicotiana tabacum) cells and in Xenopus laevis oocytes. Both transfection systems showed an enhanced capacity for malate efflux but not citrate efflux, when exposed to Al. Smaller malate fluxes were also activated by ytterbium and erbium treatment. Transgenic tobacco cells grew significantly better than control cells following an 18 h treatment with Al, indicating that the expression of BnALMT1 and BnALMT2 increased the resistance of these plant cells to Al stress. This report demonstrates that homologs of the TaALMT1 gene from wheat perform similar functions in other species.     

Changes induced by aluminum stress in the organic acid content of maize seedlings: The crucial role of exogenous citrate in enhancing seedling growth

We have studied the effect of Al on growth and morphology of maize seedlings (Zea mays L.), the changes in organic acid content as well as the role of application of exogenous citrate in enhancing the Al tolerance. Al treatment induced inhibition of root growth, causing morphological symptoms of Al toxicity. Al decreased significantly the malate content in roots compared to control plants. However, the citrate and total organic acids did not show any change, indicating that one mechanism underlying plant defense may involve the maintenance a normal levels of organic acids in roots. The succinate content increased in roots at 1000 μmol L⁻¹ Al, while that of lactate decreased. However, 500 and 1000 μmol L⁻¹ Al significantly increased the total organic acid in shoots, due to an increase in the succinate and malate contents. By contrast, the citrate and lactate levels decreased at 250 and 500 μmol L⁻¹ Al. To investigate the role of citrate in enhancing the plant growth, citrate was supplied to nutrient medium containing 500 μmol L⁻¹ Al at different Al:Citrate ratios (1:1, 1:2 and 1:3). The addition of citrate in the nutrient solution resulted in an alleviation of Al toxicity, with the maximal effect obtained at Al:Citrate ratio of 1:2. These data provide evidence that in maize, the organic acids, mainly citrate play an important role in enabling the plant to tolerate elevated exposure to Al concentration.     

Novel properties of the wheat aluminum tolerance organic acid transporter (TaALMT1) revealed by electrophysiological characterization in Xenopus Oocytes: functional and structural implications

Many plant species avoid the phytotoxic effects of aluminum (Al) by exuding dicarboxylic and tricarboxylic acids that chelate and immobilize Al(3+) at the root surface, thus preventing it from entering root cells. Several novel genes that encode membrane transporters from the ALMT and MATE families recently were cloned and implicated in mediating the organic acid transport underlying this Al tolerance response. Given our limited understanding of the functional properties of ALMTs, in this study a detailed characterization of the transport properties of TaALMT1 (formerly named ALMT1) from wheat (Triticum aestivum) expressed in Xenopus laevis oocytes was conducted. The electrophysiological findings are as follows. Although the activity of TaALMT1 is highly dependent on the presence of extracellular Al(3+) (K(m1/2) of approximately 5 microm Al(3+) activity), TaALMT1 is functionally active and can mediate ion transport in the absence of extracellular Al(3+). The lack of change in the reversal potential (E(rev)) upon exposure to Al(3+) suggests that the "enhancement" of TaALMT1 malate transport by Al is not due to alteration in the transporter's selectivity properties but is solely due to increases in its anion permeability. The consistent shift in the direction of the E(rev) as the intracellular malate activity increases indicates that TaALMT1 is selective for the transport of malate over other anions. The estimated permeability ratio between malate and chloride varied between 1 and 30. However, the complex behavior of the E(rev) as the extracellular Cl(-) activity was varied indicates that this estimate can only be used as a general guide to understanding the relative affinity of TaALMT1 for malate, representing only an approximation of those expected under physiologically relevant ionic conditions. TaALMT1 can also mediate a large anion influx (i.e. outward currents). TaALMT1 is permeable not only to malate but also to other physiologically relevant anions such as Cl(-), NO(3)(-), and SO(4)(2-) (to a lesser degree).     

Overexpression of Malate Dehydrogenase in Transgenic Alfalfa Enhances Organic Acid Synthesis and Confers Tolerance to Aluminum

Al toxicity is a severe impediment to production of many crops in acid soil. Toxicity can be reduced through lime application to raise soil pH, however this amendment does not remedy subsoil acidity, and liming may not always be practical or cost-effective. Addition of organic acids to plant nutrient solutions alleviates phytotoxic Al effects, presumably by chelating Al and rendering it less toxic. In an effort to increase organic acid secretion and thereby enhance Al tolerance in alfalfa (Medicago sativa), we produced transgenic plants using nodule-enhanced forms of malate dehydrogenase and phosphoenolpyruvate carboxylase cDNAs under the control of the constitutive cauliflower mosaic virus 35S promoter. We report that a 1.6-fold increase in malate dehydrogenase enzyme specific activity in root tips of selected transgenic alfalfa led to a 4.2-fold increase in root concentration as well as a 7.1-fold increase in root exudation of citrate, oxalate, malate, succinate, and acetate compared with untransformed control alfalfa plants. Overexpression of phosphoenolpyruvate carboxylase enzyme specific activity in transgenic alfalfa did not result in increased root exudation of organic acids. The degree of Al tolerance by transformed plants in hydroponic solutions and in naturally acid soil corresponded with their patterns of organic acid exudation and supports the concept that enhancing organic acid synthesis in plants may be an effective strategy to cope with soil acidity and Al toxicity.     

Dual effects of transgenic Brassica napus overexpressing CS gene on tolerances to aluminum toxicity and phosphorus deficiency

Background and aims

Low phosphorus (P) bioavailability and aluminum (Al) toxicity are two major constraints to plant growth in acid soil. To improve the tolerance of Brassica napus to Al toxicity and P deficiency, we generated transgenic canola (Brassica napus cv Westar) lines overexpressing a Pseudomonas aeruginosa citrate synthase (CS) gene and then investigated the effects of CS gene overexpressing in canola on enhancing tolerance to the two constraints.

Methods

The vector construction and plant transformation, molecular identification, estimation of extracellular and cellular citrate and malate concentrations, enzyme activity and gene expression analyse and Al tolerance and P acquisition assays were conducted using both hydroponics and soil culturing in the study.

Results

Both the root citrate and malate concentrations and their exudations in the two transgenic lines significantly increased compared with wild type (WT) following exposure to Al. These increases may be attributed to higher activities of the CS, malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC) enzymes in the TCA cycle and the expression of BnALMT and BnMATE in the transgenic plants following Al exposure. The primary root elongation and prolonged Al treatment (10 days) experiments revealed that the transgenic lines displayed enhanced levels of Al tolerance. In addition, they showed enhanced citrate and malate exudation when grown in P-deficient conditions. Moreover, the enzyme activities of the transgenic lines were significantly higher compared with WT in response to P-deficient stress. The soil culture experiment showed that the transgenic lines possessed improved P uptake from the soil and accumulated more P in their shoots and seeds when FePO₄ was used as the sole P source.

Conclusions

These results indicate that the overexpression of the CS gene in B. napus not only leads to increased citrate synthesis and exudation but also changes malate metabolism, which confers improved tolerances to Al toxicity and P deficiency in the transgenic plants. These findings provide further insight into the dual effects of CS gene overexpression on Al toxicity and P deficiency in plants.     

A comparative study on the aluminium- and copper-induced organic acid exudation from wheat roots

It is well established that aluminium (Al) and some heavy metals can elicit organic acid exudation from a range of species. In the present research we found that copper (Cu) can also induce organic acid exudation from the roots of wheat, rye, triticale, maize and soybean. Using intact wheat plants, we made a comparative study of Al- and Cu- induced organic acid exudation. In 5-day-old wheat seedlings, severe Cu stress (40 µ M CuCl₂) mainly induced the exudation of malate and citrate, and Al-tolerant genotypes could release significantly greater amounts of malate than Al-sensitive genotypes. The time course of the exudation of malate and citrate from the roots of 5-day-old seedlings of wheat (cv. Atlas) in 200 µ M AlCl₃ was similar to that in 40 µ M CuCl₂. In older wheat plants (15-day-old), moderate Cu stress (12 µ M CuCl₂) induced the exudation of large amounts of citrate and addition of Al or La sharply reduced Cu-induced citrate exudation, while Cu or La did not affect Al-induced malate efflux. When half of the root system of Atlas wheat was immersed in Al- or Cu-containing solution and the remaining half in Al- or Cu-free solution, organic acids were only exuded into the solution containing Al or Cu. This suggests that no long distance signal transport is involved in organic acid exudation induced by Al or Cu, and that direct contact of Al or Cu with plant roots is a prerequisite for the induction of organic acid exudation. The anion-channel inhibitor niflumic acid (NIF) significantly stimulated the exudation of both citrate and malate from 5-day-old wheat seedlings under severe Al or Cu stress. Our results suggest that Cu-induced organic acid efflux may be a common response, which may play a role in alleviating Cu toxicity in plants.     

Several lanthanides activate malate efflux from roots of aluminium‐tolerant wheat

Many elements of the lanthanide series exist as trivalent cations in solution below pH 6. The present study was carried out to investigate whether lanthanides could stimulate malate efflux from wheat (Triticum aestivum L.) roots, as has been found for trivalent aluminium (Al) cations. Excised root apices treated with 100 µm of each of seven different lanthanide elements (lanthanum, praseodymium, europium, gadolinium, terbium, erbium, and ytterbium) stimulated malate efflux, with five‐ to fifty‐fold more malate being released from an Al‐tolerant wheat line than from a near‐isogenic Al‐sensitive line. As erbium stimulated the greatest malate efflux of the lanthanides tested, this response was characterized further. The characteristics of the erbium‐activated efflux were similar to the Al‐activated efflux described previously suggesting that both of these ions activate the same transport mechanism. The capacity for erbium‐activated malate efflux cosegregated with Al tolerance in wheat seedlings derived from a cross between Al‐sensitive and Al‐tolerant near‐isogenic lines. This is the first study to identify cations, other than Al, which can activate malate release from wheat roots. It also provides additional evidence that malate efflux from root apices is the primary mechanism for Al tolerance in wheat.     

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

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

Maize ZmALMT2 is a root anion transporter that mediates constitutive root malate efflux

Root efflux of organic acid anions underlies a major mechanism of plant aluminium (Al) tolerance on acid soils. This efflux is mediated by transporters of the Al-activated malate transporter (ALMT) or the multi-drug and toxin extrusion (MATE) families. ZmALMT2 was previously suggested to be involved in Al tolerance based on joint association-linkage mapping for maize Al tolerance. In the current study, we functionally characterized ZmALMT2 by heterologously expressing it in Xenopus laevis oocytes and transgenic Arabidopsis. In oocytes, ZmALMT2 mediated an Al-independent electrogenic transport product of organic and inorganic anion efflux. Ectopic overexpression of ZmALMT2 in an Al-hypersensitive Arabidopsis KO/KD line lacking the Al tolerance genes, AtALMT1 and AtMATE, resulted in Al-independent constitutive root malate efflux which partially restored the Al tolerance phenotype. The lack of correlation between ZmALMT2 expression and Al tolerance (e.g., expression not localized to the root tip, not up-regulated by Al, and higher in sensitive versus tolerance maize lines) also led us to question ZmALMT2's role in Al tolerance. The functional properties of the ZmALMT2 transporter presented here, along with the gene expression data, suggest that ZmALMT2 is not involved in maize Al tolerance but, rather, may play a role in mineral nutrient acquisition and transport.     

Molecular and physiological strategies to increase aluminum resistance in plants

Aluminum (Al) toxicity is a primary limitation to plant growth on acid soils. Root meristems are the first site for toxic Al accumulation, and therefore inhibition of root elongation is the most evident physiological manifestation of Al toxicity. Plants may resist Al toxicity by avoidance (Al exclusion) and/or tolerance mechanisms (detoxification of Al inside the cells). The Al exclusion involves the exudation of organic acid anions from the root apices, whereas tolerance mechanisms comprise internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals. One of the most important advances in understanding the molecular events associated with the Al exclusion mechanism was the identification of the ALMT1 gene (Al-activated malate transporter) in Triticum aestivum root cells, which codes for a plasma membrane anion channel that allows efflux of organic acid anions, such as malate, citrate or oxalate. On the other hand, the scavenging of free radicals is dependent on the expression of genes involved in antioxidant defenses, such as peroxidases (e.g. in Arabidopsis thaliana and Nicotiana tabacum), catalases (e.g. in Capsicum annuum), and the gene WMnSOD1 from T. aestivum. However, other recent findings show that reactive oxygen species (ROS) induced stress may be due to acidic (low pH) conditions rather than to Al stress. In this review, we summarize recent findings regarding molecular and physiological mechanisms of Al toxicity and resistance in higher plants. Advances have been made in understanding some of the underlying strategies that plants use to cope with Al toxicity. Furthermore, we discuss the physiological and molecular responses to Al toxicity, including genes involved in Al resistance that have been identified and characterized in several plant species. The better understanding of these strategies and mechanisms is essential for improving plant performance in acidic, Al-toxic soils.