Role of dynamics of intracellular calcium in aluminium-toxicity syndrome

Zinc-Dependent Protection of Tobacco and Rice Cells From Aluminum-Induced Superoxide-Mediated Cytotoxicity

Al(3+) toxicity in growing plants is considered as one of the major factors limiting the production of crops on acidic soils worldwide. In the last 15 years, it has been proposed that Al(3+) toxicity are mediated with distortion of the cellular signaling mechanisms such as calcium signaling pathways, and production of cytotoxic reactive oxygen species (ROS) causing oxidative damages. On the other hand, zinc is normally present in plants at high concentrations and its deficiency is one of the most widespread micronutrient deficiencies in plants. Earlier studies suggested that lack of zinc often results in ROS-mediated oxidative damage to plant cells. Previously, inhibitory action of Zn(2+) against lanthanide-induced superoxide generation in tobacco cells have been reported, suggesting that Zn(2+) interferes with the cation-induced ROS production via stimulation of NADPH oxidase. In the present study, the effect of Zn(2+) on Al(3+)-induced superoxide generation in the cell suspension cultures of tobacco (Nicotiana tabacum L., cell-line, BY-2) and rice (Oryza sativa L., cv. Nipponbare), was examined. The Zn(2+)-dependent inhibition of the Al(3+)-induced oxidative burst was observed in both model cells selected from the monocots and dicots (rice and tobacco), suggesting that this phenomenon (Al(3+)/Zn(2+) interaction) can be preserved in higher plants. Subsequently induced cell death in tobacco cells was analyzed by lethal cell staining with Evans blue. Obtained results indicated that presence of Zn(2+) at physiological concentrations can protect the cells by preventing the Al(3+)-induced superoxide generation and cell death. Furthermore, the regulation of the Ca(2+) signaling, i.e., change in the cytosolic Ca(2+) ion concentration, and the cross-talks among the elements which participate in the pathway were further explored.

Вплив Cu2+ та кислотності середовища на вміст цитоплазматичного кальцію та перекисне окиснення ліпідів у коренях озимої пшениці

Досліджено вплив іонів Cu2+ на внутрішньоклітинний гомеостаз Са2+ та перекисне окиснення ліпідів в умовах різної кислотності середовища. Показано, що зниження рН середовища зумовлює підвищення [Ca2+]cyt і може бути відгуком клітин кореня на кислотний стрес. Додавання міді до середовища для пророщування насіння ймовірно підвищує неселективну проникність мембрани та спричинює її деполяризацію. Це може активувати Са-канали, що активуються за умов деполяризації та відповідають за надходження кальцію до цитоплазми з апопласту. Підвищення рівня МДА свідчить про наявність окисного стресу, який може активувати гіперполяризаційні Са-канали, що також впливають на [Ca2+]cyt. Таким чином, зміни інтенсивності Са2+-залежних метаболічних процесів (регуляція поділу клітин та ріст кореня) можуть бути наслідком токсичного впливу іонів Cu2+ на рослини озимої пшениці.  

Opportunities and challenges in the subsoil: pathways to deeper rooted crops

Greater exploitation of subsoil resources by annual crops would afford multiple benefits, including greater water and N acquisition in most agroecosystems, and greater sequestration of atmospheric C. Constraints to root growth in the subsoil include soil acidity (an edaphic stress complex consisting of toxic levels of Al, inadequate levels of P and Ca, and often toxic levels of Mn), soil compaction, hypoxia, and suboptimal temperature. Multiple root phenes under genetic control are associated with adaptation to these constraints, opening up the possibility of breeding annual crops with root traits improving subsoil exploration. Adaptation to Al toxicity, hypoxia, and P deficiency are intensively researched, adaptation to soil hardness and suboptimal temperature less so, and adaptations to Ca deficiency and Mn toxicity are poorly understood. The utility of specific phene states may vary among soil taxa and management scenarios, interactions which in general are poorly understood. These traits and issues merit research because of their potential value in developing more productive, sustainable, benign, and resilient agricultural systems.

Non-specific phospholipase C4 mediates response to aluminum toxicity in Arabidopsis thaliana

Aluminum ions (Al) have been recognized as a major toxic factor for crop production in acidic soils. The first indication of the Al toxicity in plants is the cessation of root growth, but the mechanism of root growth inhibition is largely unknown. Here we examined the impact of Al on the expression, activity, and function of the non-specific phospholipase C4 (NPC4), a plasma membrane-bound isoform of NPC, a member of the plant phospholipase family, in Arabidopsis thaliana. We observed a lower expression of NPC4 using β-glucuronidase assay and a decreased formation of labeled diacylglycerol, product of NPC activity, using fluorescently labeled phosphatidylcholine as a phospholipase substrate in Arabidopsis WT seedlings treated with AlCl3 for 2 h. The effect on in situ NPC activity persisted for longer Al treatment periods (8, 14 h). Interestingly, in seedlings overexpressing NPC4, the Al-mediated NPC-inhibiting effect was alleviated at 14 h. However, in vitro activity and localization of NPC4 were not affected by Al, thus excluding direct inhibition by Al ions or possible translocation of NPC4 as the mechanisms involved in NPC-inhibiting effect. Furthermore, the growth of tobacco pollen tubes rapidly arrested by Al was partially rescued by the overexpression of AtNPC4 while Arabidopsis npc4 knockout lines were found to be more sensitive to Al stress during long-term exposure of Al at low phosphate conditions. Our observations suggest that NPC4 plays a role in both early and long-term responses to Al stress.

Pectin enhances rice (Oryza sativa) root phosphorus remobilization

Plants growing in phosphorus (P)-deficient conditions can either increase their exploration of the environment (hence increasing P uptake) or can solubilize and reutilize P from established tissue sources. However, it is currently unclear if P stored in root cell wall can be reutilized. The present study shows that culture of the rice cultivars 'Nipponbare' (Nip) and 'Kasalath' (Kas) in P-deficient conditions results in progressive reductions in root soluble inorganic phosphate (Pi). However, Nip consistently maintains a higher level of soluble Pi and lower relative cell wall P content than does Kas, indicating that more cell wall P is released in Nip than in Kas. P-deficient Nip has a greater pectin and hemicellulose 1 (HC1) content than does P-deficient Kas, consistent with the significant positive relationship between pectin and root-soluble Pi levels amongst multiple rice cultivars. These observations suggest that increased soluble Pi might result from increased pectin content during P starvation. In vitro experiments showed that pectin releases Pi from insoluble FePO4. Furthermore, an Arabidopsis thaliana mutant with reduced pectin levels (qua1-2), has less root soluble Pi and is more sensitive to P deficiency than the wild type (WT) Col-0, whereas NaCl-treated WT plants exhibit both an increased root pectin content and an elevated soluble Pi content during P-starvation. These observations indicate that pectin can facilitate the remobilization of P deposited in the cell wall. This is a previously unknown mechanism for the reutilization of P in P-starved plants.

Hydrogen sulfide alleviates the aluminum-induced changes in Brassica napus as revealed by physiochemical and ultrastructural study of plant

In the present study, ameliorating role of hydrogen sulfide (H2S) in oilseed rape (Brassica napus L.) was studied with or without application of H2S donor sodium hydrosulfide (NaHS) (0.3 mM) in hydroponic conditions under three levels (0, 0.1 and 0.3 mM) of aluminum (Al). Results showed that addition of H2S significantly improved the plant growth, photosynthetic gas exchange, and nutrients concentration in the leaves and roots of B. napus plants under Al stress. Exogenously applied H2S significantly lowered the Al concentration in different plant parts, and reduced the production of malondialdehyde and reactive oxygen species by improving antioxidant enzyme activities in the leaves and roots under Al stress. Moreover, the present study indicated that exogenously applied H2S improved the cell structure and displayed clean mesophyll and root tip cells. The chloroplast with well-developed thylakoid membranes could be observed in the micrographs. Under the combined application of H2S and Al, a number of modifications could be observed in root tip cell, such as mitochondria, endoplasmic reticulum, and golgi bodies. Thus, it can be concluded that exogenous application of H2S under Al stress improved the plant growth, photosynthetic parameters, elements concentration, and biochemical and ultrastructural changes in leaves and roots of B. napus.

Aluminum ions alter the function of non-specific phospholipase C through the changes in plasma membrane physical properties

The first indication of the aluminum (Al) toxicity in plants growing in acidic soils is the cessation of root growth, but the detailed mechanism of Al effect is unknown. Here we examined the impact of Al stress on the activity of non-specific phospholipase C (NPC) in the connection with the processes related to the plasma membrane using fluorescently labeled phosphatidylcholine. We observed a rapid and significant decrease of labeled diacylglycerol (DAG), product of NPC activity, in Arabidopsis seedlings treated with AlCl₃. Interestingly, an application of the membrane fluidizer, benzyl alcohol, restored the level of DAG during Al treatment. Our observations suggest that the activity of NPC is affected by Al-induced changes in plasma membrane physical properties.

A reversible fluorescent-colorimetric imino-pyridyl bis-Schiff base sensor for expeditious detection of Al3+ and HSO3− in aqueous media

A reversible fluorescent-colorimetric imino-pyridyl bis-Schiff base receptor (N¹E,N⁴E)-N¹,N⁴-bis(pyridine-4-ylmethylene)benzene-1,4-diamine (L) for easy, convenient, rapid and sensitive detection of both Al³⁺ and HSO₃⁻ in aqueous medium has been developed.

Magnesium alleviates plant toxicity of aluminium and heavy metals

Magnesium (Mg) is an essential nutrient that can alleviate soilborne toxicity of many ions. This review paper critically assesses the literature on interactions and mechanisms influencing Mg alleviation of aluminium (Al) and heavy metal toxicity. Hydrated radii of Mg2+ and Al3+ are similar; therefore, these two ions compete for binding to ion transporters and other important biological molecules. In monocotyledonous species such as rice and wheat, millimolar concentrations of Mg alleviate Al toxicity, mainly by decreasing Al saturation and activity at cell wall and plasma membrane binding sites. In dicotyledonous legume species such as soybean (Glycine max), rice bean (Vigna umbellata) and broad bean (Vicia faba), micromolar concentrations of Mg may enhance biosynthesis of organic ligands and thus underpin alleviation of Al toxicity. Resistance to Al may be enhanced by increased expression of the genes coding for Mg transporters, as well as by upregulation of activity of Mg-transport proteins; intracellular Mg2+ activity may thus be increased under Al stress, which may increase the activity of H+-ATPases. In Vicia faba, Mg-related enhancement in the activity of plasma membrane H+-ATPase under Al stress was found to be due to post-translational modification (increased phosphorylation of the penultimate threonine as well as association with regulatory 14-3-3 proteins), resulting in increased resistance to Al stress. Magnesium can alleviate heavy metal stress by decreasing negative electrical potential and thus metal ion activities at the plasma membrane surface (physico-chemical competition), by enhancing activities of enzymes involved in biosynthesis of organic ligands, and by increasing vacuolar sequestration of heavy metals via increasing H+-pumping activity at the tonoplast. Future work should concentrate on characterising the role of intracellular Mg2+ homeostasis and Mg transporters in alleviating metal stress as well as in transcriptional, translational and post-translational regulation of H+-pumps and enzymes involved in biosynthesis and exudation of organic ligands.

Plant Adaptation to Acid Soils: The Molecular Basis for Crop Aluminum Resistance

Aluminum (Al) toxicity in acid soils is a significant limitation to crop production worldwide, as approximately 50% of the world's potentially arable soil is acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring resistance to Al toxicity has been a focus of intense research interest in the decade since the last article on crop acid soil tolerance was published in this journal. An impressive amount of progress has been made during that time that has greatly increased our understanding of the diversity of Al resistance genes and mechanisms, how resistance gene expression is regulated and triggered by Al and Al-induced signals, and how the proteins encoded by these genes function and are regulated. This review examines the state of our understanding of the physiological, genetic, and molecular bases for crop Al tolerance, looking at the novel Al resistance genes and mechanisms that have been identified over the past ten years. Additionally, it examines how the integration of molecular and genetic analyses of crop Al resistance is starting to be exploited for the improvement of crop plants grown on acid soils via both molecular-assisted breeding and biotechnology approaches.

"Microscopic evidences of heavy metals distribution and anatomic alterations in breaching-leaves of Cupressus lindleyi growing around mining wastes"

In this article a study of the distribution of heavy metals in Cupressus lindleyi breaching-leaves was done in Taxco, Guerrero. At the same, heavy metals micro-localization was conducted in the breaching-leaves to understand the structural changes provoked by mining waste on plants. The most abundant contaminants in soils, tailings and different plant organs (roots, stems, and leaves) were Zn, Mn, and Pb. Nevertheless, As was more accumulated in the stem and breaching-leaves. The translocation factor and the bio-concentration factor were less than 1. The structural changes observed were the great accumulation of starch grains and phenolic compounds in the palisade parenchyma, changes in the hypodermis cell wall and necrotic zones in the palisade parenchyma. The distribution of heavy metals in breaching-leaves tissues was homogeneous in most of the elements. These results showed that C. lindleyi is a species that can be employed in phytostabilization of contaminated zones with mining waste because it is a native plant that does not require a lot of conditions for its development.

Ethylene negatively regulates aluminium-induced malate efflux from wheat roots and tobacco cells transformed with TaALMT1

An important mechanism for Al(3+) tolerance in wheat is exudation of malate anions from the root apex through activation of malate-permeable TaALMT1 channels. Here, the effect of ethylene on Al(3+)-activated efflux of malate was investigated using Al(3+)-tolerant wheat genotype ET8, which has high expression of TaALMT1. Exposure of ET8 plants to Al(3+) enhanced ethylene evolution in root apices. Treatment with the ethylene synthesis precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and ethylene gas suppressed Al(3+)-induced malate efflux from root apices, whereas the intracellular malate concentrations in roots were not affected. Malate efflux from root apices was enhanced in the presence of Al(3+) by two antagonists of ethylene biosynthesis, aminoethoxyvinylglycine (AVG) and 2-aminoisobutyric acid (AIB). An increase in Al accumulation in root apices was observed when treated with ACC, whereas AVG and AIB suppressed Al accumulation in root apices. Al(3+)-induced inhibition of root elongation was ameliorated by pretreatment with AIB. In addition, ethylene donor (Ethrel) also inhibited Al(3+)-induced malate efflux from tobacco cells transformed with TaALMT1. ACC and the anion-channel blocker niflumate had a similar and non-additive effect on Al-induced malate efflux from root apices. Treatment of ET8 plants with ACC enhanced expression of TaALMT1, suggesting that the inhibitory effect of ethylene on Al-induced malate efflux is unlikely to occur at the transcriptional level. These findings indicate that ethylene may behave as a negative regulator of Al(3+)-induced malate efflux by targeting TaALMT1-mediated malate efflux by an unknown mechanism.

Nitrate reductase-mediated early nitric oxide burst alleviates oxidative damage induced by aluminum through enhancement of antioxidant defenses in roots of wheat (Triticum aestivum)

• Nitric oxide (NO) is an important signaling molecule involved in the physiological processes of plants. The role of NO release in the tolerance strategies of roots of wheat (Triticum aestivum) under aluminum (Al) stress was investigated using two genotypes with different Al resistances. • An early NO burst at 3 h was observed in the root tips of the Al-tolerant genotype Jian-864, whereas the Al-sensitive genotype Yang-5 showed no NO accumulation at 3 h but an extremely high NO concentration after 12 h. Stimulating NO production at 3 h in the root tips of Yang-5 with the NO donor relieved Al-induced root inhibition and callose production, as well as oxidative damage and ROS accumulation, while elimination of the early NO burst by NO scavenger aggravated root inhibition in Jian-864. • Synthesis of early NO in roots of Jian-864 was mediated through nitrate reductase (NR) but not through NO synthase. Elevated antioxidant enzyme activities were induced by Al stress in both wheat genotypes and significantly enhanced by NO donor, but suppressed by NO scavenger or NR inhibitor. • These results suggest that an NR-mediated early NO burst plays an important role in Al resistance of wheat through modulating enhanced antioxidant defense to adapt to Al stress.

The role of aluminum sensing and signaling in plant aluminum resistance

As researchers have gained a better understanding in recent years into the physiological, molecular, and genetic basis of how plants deal with aluminum (Al) toxicity in acid soils prevalent in the tropics and sub-tropics, it has become clear that an important component of these responses is the triggering and regulation of cellular pathways and processes by Al. In this review of plant Al signaling, we begin by summarizing the understanding of physiological mechanisms of Al resistance, which first led researchers to realize that Al stress induces gene expression and modifies protein function during the activation of Al resistance responses. Subsequently, an overview of Al resistance genes and their function provides verification that Al induction of gene expression plays a major role in Al resistance in many plant species. More recent research into the mechanistic basis for Al-induced transcriptional activation of resistance genes has led to the identification of several transcription factors as well as cis-elements in the promoters of Al resistance genes that play a role in greater Al-induced gene expression as well as higher constitutive expression of resistance genes in some plant species. Finally, the post-transcriptional and translational regulation of Al resistance proteins is addressed, where recent research has shown that Al can both directly bind to and alter activity of certain organic acid transporters, and also influence Al resistance proteins indirectly, via protein phosphorylation.

Al3+ selective coumarin based reversible chemosensor: application in living cell imaging and as integrated molecular logic gate

The new coumarin based fluorescent ‘turn-on’ chemosensor (H₂L) efficiently detects Al³⁺ over other metal ions. It is efficient in detecting Al³⁺ in the intracellular region of HeLa cells and also exhibits an INHIBIT logic gate with Al³⁺ and EDTA as chemical inputs.

Molecular characterisation of a calmodulin gene, VcCaM1, that is differentially expressed under aluminium stress in highbush blueberry

Calmodulin (CaM), a small acidic protein, is one of the best characterised Ca(2+) sensors in eukaryotes. This Ca(2+) -regulated protein plays a critical role in decoding and transducing environmental stress signals by activating specific targets. Many environmental stresses elicit changes in intracellular Ca(2+) activity that could initiate adaptive responses under adverse conditions. We report the first molecular cloning and characterisation of a calmodulin gene, VcCaM1 (Vaccinium corymbosum Calmodulin 1), in the woody shrub, highbush blueberry. VcCaM1 was first identified as VCAL19, a gene induced by aluminium stress in V. corymbosum L. A full-length cDNA of VcCaM1 containing a 766-bp open reading frame (ORF) encoding 149 amino acids was cloned from root RNA. The sequence encodes four Ca(2+) -binding motifs (EF-hands) and shows high similarity (99%) with the isoform CaM 201 of Daucus carota. Expression analyses showed that following Al treatment, VcCaM1 message level decreased in roots of Brigitta, an Al-resistant cultivar, and after 48 h, was lower than in Bluegold, an Al-sensitive cultivar. VcCAM1 message also decreased in leaves of both cultivars within 2 h of treatment. Message levels in leaves then increased by 24 h to control levels in Brigitta, but not in Bluegold, but then decreased again by 48 h. In conclusion, VcCaM1 does not appear to be directly involved in Al resistance, but may be involved in improved plant performance under Al toxicity conditions through regulation of Ca(2+) homeostasis and antioxidant systems in leaves.

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.

The role of arbuscular mycorrhizas in decreasing aluminium phytotoxicity in acidic soils: a review

Soil acidity is an impediment to agricultural production on a significant portion of arable land worldwide. Low productivity of these soils is mainly due to nutrient limitation and the presence of high levels of aluminium (Al), which causes deleterious effects on plant physiology and growth. In response to acidic soil stress, plants have evolved various mechanisms to tolerate high concentrations of Al in the soil solution. These strategies for Al detoxification include mechanisms that reduce the activity of Al3+ and its toxicity, either externally through exudation of Al-chelating compounds such as organic acids into the rhizosphere or internally through the accumulation of Al-organic acid complexes sequestered within plant cells. Additionally, root colonization by symbiotic arbuscular mycorrhizal (AM) fungi increases plant resistance to acidity and phytotoxic levels of Al in the soil environment. In this review, the role of the AM symbiosis in increasing the Al resistance of plants in natural and agricultural ecosystems under phytotoxic conditions of Al is discussed. Mechanisms of Al resistance induced by AM fungi in host plants and variation in resistance among AM fungi that contribute to detoxifying Al in the rhizosphere environment are considered with respect to altering Al bioavailability.

Cellular responses of two Latin-American cultivars of Lotus corniculatus to low pH and Al stress

Toxic effects of acidic root medium and aluminium were evaluated in two forage cultivars of Lotus corniculatus differing in their tolerance to Al stress. The structural response of most of the root cells exposed to low pH without Al3+ differed markedly from that induced by the combined stress. Conspicuous alteration of the nucleus was present only at low pH 4.0 and disintegration of the cytoplasmic components was more drastic than in the roots exposed to acidic solution containing Al3+. Cells exposed to low pH without Al, did not produce wall thickenings. Severely damaged cytoplasm and localized death in some cortical cells or groups of cells contrasting with almost intact cells exposed to Al3+ stress were found. In this respect, a strong correlation between the occurrence of cell wall thickenings and a better preserved structure of the cytoplasm was observed. The frequency of cell damage in the more tolerant cultivar UFRGS was generally lower, significantly more cortical cells capable of maintaining their resting membrane potential were present than in the sensitive INIA Draco. The difference in their tolerance is related rather to the exudation of citrate and oxalate that was higher in UFRGS than to the accumulation of tannins, which increased after Al treatment in both cultivars.

Loss of membrane fluidity and endocytosis inhibition are involved in rapid aluminum-induced root growth cessation in Arabidopsis thaliana

Aluminum (Al) toxicity is the main limiting factor in crop production on acid soils. The main symptom of Al toxicity is a rapid inhibition of root growth, but the mechanism of root growth cessation remains unclear. Here we examined the earliest changes in the plasma membrane and processes related to the membrane in the Arabidopsis thaliana root tip cells of roots grown in a hydropony. Al suppressed root growth within 2 min, inhibited endocytosis within 10 min of exposure and stabilized cortical microtubules within the first 30 min. Spectrofluorometric measurements of the plasma membrane isolated from Arabidopsis plants and labeled with the fluorescent probe laurdan showed that Al induced a reduction in membrane fluidity. Application of the membrane fluidizer, benzyl alcohol, restored partially membrane fluidity and also partially restored root growth during first 30 min of Al treatment. We concluded that Al-induced loss of membrane fluidity and endocytosis inhibition occurred very early during Al toxicity in plant roots and could be the earliest targets of Al treatment.

The plant non-specific phospholipase C gene family. Novel competitors in lipid signalling

Non-specific phospholipases C (NPCs) were discovered as a novel type of plant phospholipid-cleaving enzyme homologous to bacterial phosphatidylcholine-specific phospholipases C and responsible for lipid conversion during phosphate-limiting conditions. The six-gene family was established in Arabidopsis, and growing evidence suggests the involvement of two articles NPCs in biotic and abiotic stress responses as well as phytohormone actions. In addition, the diacylglycerol produced via NPCs is postulated to participate in membrane remodelling, general lipid metabolism and cross-talk with other phospholipid signalling systems in plants. This review summarises information concerning this new plant protein family and focusses on its sequence analysis, biochemical properties, cellular and tissue distribution and physiological functions. Possible modes of action are also discussed.

Aluminium Toxicity Targets in Plants

Aluminium (Al) is the third most abundant metallic element in soil but becomes available to plants only when the soil pH drops below 5.5. At those conditions, plants present several signals of Al toxicity. As reported by literature, major consequences of Al exposure are the decrease of plant production and the inhibition of root growth. The root growth inhibition may be directly/indirectly responsible for the loss of plant production. In this paper the most remarkable symptoms of Al toxicity in plants and the latest findings in this area are addressed. Root growth inhibition, ROS production, alterations on root cell wall and plasma membrane, nutrient unbalances, callose accumulation, and disturbance of cytoplasmic Ca2+ homeostasis, among other signals of Al toxicity are discussed, and, when possible, the behavior of Al-tolerant versus Al-sensitive genotypes under Al is compared.

Zonal responses of sensitive vs. tolerant wheat roots during Al exposure and recovery

Aluminium (Al) irreversibly inhibits root growth in sensitive, but not in some tolerant genotypes. To better understand tolerance mechanisms, seedlings from tolerant ('Barbela 7/72' line) and sensitive ('Anahuac') Triticum aestivum L. genotypes were exposed to AlCl(3) 185 μM for: (a) 24 h followed by 48 h without Al (recovery); (b) 72 h of continuous exposure. Three root zones were analyzed (meristematic (MZ), elongation (EZ) and hairy (HZ)) for callose deposition, reserves (starch and lipids) accumulation, endodermis differentiation and tissue architecture. Putative Al-induced genotoxic or cytostatic/mytogenic effects were assessed by flow cytometry in root apices. Tolerant plants accumulated less Al, presented less root damage and a less generalized callose distribution than sensitive ones. Starch and lipid reserves remained constant in tolerant roots but drastically decreased in sensitive ones. Al induced different profiles of endodermis differentiation: differentiation was promoted in EZ and HZ, respectively, in sensitive and tolerant genotypes. No ploidy changes or clastogenicity were observed. However, differences in cell cycle blockage profiles were detected, being less severe in tolerant roots. After Al removal, only the 'Barbela 7/72' line reversed Al-induced effects to values closer to the control, mostly with respect to callose deposition and cell cycle progression. We demonstrate for the first time that: (a) cell cycle progression is differently regulated by Al-tolerant and Al-sensitive genotypes; (b) Al induces callose deposition >3 cm above root apex (in HZ); (c) callose deposition is a transient Al-induced effect in tolerant plants; and (d) in HZ, endodermis differentiation is also stimulated only in tolerant plants, probably functioning in tolerant genotypes as a protective mechanism in addition to callose.

Nitric oxide exacerbates Al-induced inhibition of root elongation in rice bean by affecting cell wall and plasma membrane properties

Aluminum (Al) toxicity is one of the most widespread problems for crop production on acid soils, and nitric oxide (NO) is a key signaling molecule involved in the mediation of various biotic and abiotic stresses in plants. Here we found that exogenous application of the NO donor sodium nitroprusside (SNP) exacerbated the inhibition of Al-induced root growth in rice bean [Vigna umbellata (Thunb.) Ohwi & Ohashi 'Jiangnan', Fabaceae]. This was accompanied by an increased accumulation of Al in the root apex. However, Al treatments had no effect on endogenous NO concentrations in root apices. These results indicate that a change in NO concentration is not the cause of Al-induced root growth inhibition and the adverse effect of SNP on Al-induced root growth inhibition should result from increased Al accumulation. Al could significantly induce citrate efflux but SNP had no effects on citrate efflux either in the absence or presence of Al. On the other hand, SNP pretreatment significantly increased Al-induced malondialdehyde accumulation and Evans Blue staining, indicating an intensification of the disruption of plasma membrane integrity. Furthermore, SNP pretreatment also caused greater induction of pectin methylesterase activity by Al, which could be the cause of the increased Al accumulation. Taken together, it is concluded that NO exacerbates Al-induced root growth inhibition by affecting cell wall and plasma membrane properties.

Identification of aluminum-responsive microRNAs in Medicago truncatula by genome-wide high-throughput sequencing

MicroRNAs (miRNAs) play important roles in response of plants to biotic and abiotic stresses. Aluminum (Al) toxicity is a major factor limiting plant growth in acidic soils. However, there has been limited report on the involvement of miRNAs in response of plants to toxic Al(3+). To identify Al(3+)-responsive miRNAs at whole-genome level, high-throughput sequencing technology was used to sequence libraries constructed from root apices of the model legume plant Medicago truncatula treated with and without Al(3+). High-throughput sequencing of the control and two Al(3+)-treated libraries led to generation of 17.1, 14.1 and 17.4 M primary reads, respectively. We identified 326 known miRNAs and 21 new miRNAs. Among the miRNAs, expression of 23 miRNAs was responsive to Al(3+), and the majority of Al(3+)-responsive mRNAs was down-regulated. We further classified the Al(3+)-responsive miRNAs into three groups based on their expression patterns: rapid-responsive, late-responsive and sustained-responsive miRNAs. The majority of Al(3+)-responsive miRNAs belonged to the 'rapid-responsive' category, i.e. they were responsive to short-term, but not long-term Al(3+) treatment. The Al(3+)-responsive miRNAs were also verified by quantitative real-time PCR. The potential targets of the 21 new miRNAs were predicted to be involved in diverse cellular processes in plants, and their potential roles in Al(3+)-induced inhibition of root growth were discussed. These findings provide valuable information for functional characterization of miRNAs in Al(3+) toxicity and tolerance.

How does an Al-hyperaccumulator plant respond to a natural field gradient of soil phytoavailable Al?

The physiological ability of plants to cope with Al-toxicity has attracted considerable attention. In this study we used an endemic Al-hyperaccumulator plant, Plantago almogravensis, which is the only known representative of the Plantaginaceae with this trait growing under a field gradient of Al, to understand the root and shoot patterns of Al accumulation and tolerance in its natural environment. We analysed phytoavailable elements in the soil and their accumulation in the plant. For the first time under field conditions, the accumulation pattern of an Al-hyperaccumulator showed a saturation curve with a maximum accumulation capacity being reached (ca. 3.0 mg g(-1)). The Al toxicity was not associated with the expected reduction in the Ca and Mg uptake by the plant. Iron was accumulated in a more linear pattern. The magnitude and the proportion of the elements found in the apoplastic fraction of the root, compared to the soil and plant internal fractions, suggested that the control of uptake occurs at the rhizospheric level. Unlike the majority of the Al-hyperaccumulator plants that are found in tropical humid areas, this plant is described from a sub-arid Mediterranean climate, subject to drought conditions which give it a unique status that deserves to be studied further.

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.

Determination of stress responses induced by aluminum in maize (Zea mays)

To assess the alternative responses to aluminum toxicity, maize (Zea mays L. cv Karadeniz yıldızı) roots were exposed to different concentrations of AlCl3 (150, 300 and 450 μM). Aluminum reduced the root elongation by 39.6% in 150 μM, 44.1% in 300 μM, 50.1% in 450 μM AlCl3 after 96 h period. To correlate the root elongation with the alternative stress responses including aluminum accumulation, lipid peroxidation, mitotic abnormalities, reduction of starch content, intracellular Ca2+ accumulation, callose formation, lignin deposition and peroxidase activity, cytochemical and biochemical tests were performed. The results indicated that aluminum accumulation and lipid peroxidation were observed more densely on the root cap and the outer cortex cells. In addition to morphological deformations, cytochemical analysis displayed cellular deformations. Furthermore, mitotic abnormalities were observed such as c-mitosis, micronuclei, bi- and trinucleated cells in aluminum treated root tips. Aluminum treatment induced starch reduction, callose formation, lignin accumulation and intracellular Ca2+ increase. Moreover, the peroxidase activity increased significantly by 3, 4.4 and 7.7 times higher than in that of control after 96 h, respectively. In conclusion, aluminum is significantly stressful in maize culminating in morphological and cellular alterations.

Aluminium-induced alteration of ion homeostasis in root tip vacuoles of two maize varieties differing in Al tolerance

Root elongation is a primary target of Al toxicity in plants. The objective of this study was to see whether Al-induced disturbance of ion homeostasis is related to the inhibition of root elongation. For this purpose, root growth rate, free cytoplasmic calcium (Ca²+) and vacuolar content of phosphate (P(i)), potassium (K+), nitrate (NO₃⁻) and malate, as well as malate and citrate exudation and nitrate reductase activity were analysed in tips of two Zea mays L. varieties differing in Al resistance. Aluminium treatment affected root growth and cytoplasmic Ca²+ in the Al sensitive variety Bakero, but not in the Al tolerant variety Sikuani. However, both varieties suffered Al-induced decrease of vacuolar K+, and phosphate concentrations. Vacuolar malate concentrations were more affected by Al in Bakero than in Sikuani. Vacuolar nitrate concentrations increased upon Al exposure in both varieties. Only in Sikuani rhizosphere, pH slightly increased upon Al exposure. Our data are consistent with the hypothesis that disturbance of Ca²+ homeostasis is an early event in the Al toxicity syndrome. However, Al-induced alterations of the root tip homeostasis of major ions seem unrelated to Al-induced inhibition of root elongation.

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.

Proteomic Analysis of Soybean Roots under Aluminum Stress

Toxic levels of aluminum (Al) in acid soils inhibit root growth and cause substantial reduction in yields of Al-sensitive crops. Aluminum-tolerant cultivars detoxify Al through multiple mechanisms that are currently not well understood at genetic and molecular levels. To enhance our understanding of the molecular mechanisms involved in soybean Al tolerance and toxicity, we conducted proteomic analysis of soybean roots under Al stress using a tandem combination of 2-D-DIGE, mass spectrometry, and bioinformatics tools and Al-tolerant (PI 416937) and Al-sensitive (Young) soybean genotypes at 6, 51 or 72 h of Al treatment. Comparison of the protein profile changes revealed that aluminum induced Al tolerance related proteins and enzymes in Al-tolerant PI 416937 but evoked proteins related to general stress response in Al-sensitive Young. Specifically, Al upregulated: malate dehydrogenase, enolase, malate oxidoreductase, and pyruvate dehydrogenase, in PI 416937 but not in Young. These enzymes contribute to increased synthesis of citrate, a key organic acid involved in Al detoxification. We postulate that simultaneous transgenic overexpression of several of these enzymes would be a robust genetic engineering strategy for developing Al-tolerant crops.

Pretreatment with H(2) O(2) alleviates aluminum-induced oxidative stress in wheat seedlings

Hydrogen peroxide (H(2) O(2) ) is a key reactive oxygen species (ROS) in signal transduction pathways leading to activation of plant defenses against biotic and abiotic stresses. In this study, we investigated the effects of H(2) O(2) pretreatment on aluminum (Al) induced antioxidant responses in root tips of two wheat (Triticum aestivum L.) genotypes, Yangmai-5 (Al-sensitive) and Jian-864 (Al-tolerant). Al increased accumulation of H(2) O(2) and O(2) (•-) leading to more predominant lipid peroxidation, programmed cell death and root elongation inhibition in Yangmai-5 than in Jian-864. However, H(2) O(2) pretreatment alleviated Al-induced deleterious effects in both genotypes. Under Al stress, H(2) O(2) pretreatment increased the activities of superoxide dismutase, catalase, peroxidase, ascorbate peroxidase and monodehydroascorbate reductase, glutathione reductase and glutathione peroxidase as well as the levels of ascorbate and glutathione more significantly in Yangmai-5 than in Jian-864. Furthermore, H(2) O(2) pretreatment also increased the total antioxidant capacity evaluated as the 2, 2-diphenyl-1-picrylhydrazyl-radical scavenging activity and the ferric reducing/antioxidant power more significantly in Yangmai-5 than in Jian-864. Therefore, we conclude that H(2) O(2) pretreatment improves wheat Al acclimation during subsequent Al exposure by enhancing the antioxidant defense capacity, which prevents ROS accumulation, and that the enhancement is greater in the Al-sensitive genotype than in the Al-tolerant genotype.

Mechanisms of magnesium amelioration of aluminum toxicity in soybean at the gene expression level

Micromolar concentration of magnesium (Mg) in culture solution is known to ameliorate aluminum (Al) toxicity in soybean and other leguminous species. To advance the understanding of this phenomenon at the level of gene expression in soybean, we undertook a comparative transcriptome analysis using DNA microarrays and Al-tolerant and Al-sensitive genotypes treated with Al ions alone or Al plus Mg ions. We observed a more rapid alteration of transcription for Al-tolerant than Al-sensitive soybean after introduction of Mg into Al-containing medium, but at 72 h, far more genes were altered (both upregulated and downregulated) in the Al-sensitive line, reflecting the known greater saving effect of Mg for Al-sensitive than Al-tolerant lines. Mg appears to ameliorate Al toxicity in the sensitive genotype by the dual mechanisms of (i) specifically increasing the expression level of several genes that are upregulated in the Al-treated, Al-tolerant genotype in the absence of Mg and (ii) possibly saving energy by decreasing expression of most genes relative to expression under Al stress. Mg-mediated reduction in gene expression also appears to be an important mechanism of Mg protection of the Al-tolerant genotype.

Aluminium ions inhibit the formation of diacylglycerol generated by phosphatidylcholine-hydrolysing phospholipase C in tobacco cells

• Aluminium ions (Al) have been recognized as a major toxic factor for crop production in acidic soils. This study aimed to assess the impact of Al on the activity of phosphatidylcholine-hydrolysing phospholipase C (PC-PLC), a new member of the plant phospholipase family. • We labelled the tobacco cell line BY-2 and pollen tubes with a fluorescent derivative of phosphatidylcholine and assayed for patterns of fluorescently labelled products. Growth of pollen tubes was analysed. • We observed a significant decrease of labelled diacylglycerol (DAG) in cells treated with AlCl(3). Investigation of possible metabolic pathways that control DAG generation and consumption during the response to Al showed that DAG originated from the reaction catalysed by PC-PLC. The growth of pollen tubes was retarded in the presence of Al and this effect was accompanied by the decrease of labelled DAG similar to the case of the BY-2 cell line. The growth of pollen tubes arrested by Al was rescued by externally added DAG. • Our observation strongly supports the role of DAG generated by PC-PLC in the response of tobacco cells to Al.

Aluminum-dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low-pH and Al(3+) stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low-pH and Al(3+) toxicity. Low-pH or combined low-pH and Al(3+) stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H(+) concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low-pH stress facilitated H(+) influx into root tissues and caused cytoplasmic acidification; by contrast, combined low-pH/Al(3+) treatment either decreased H(+) influx in the distal elongation zone (DEZ) or induced H(+) efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low-pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low-pH/Al(3+) stress resulted in lower rhizosphere pH in both root zones compared with the low-pH treatment alone. Low-pH stress facilitated K(+) efflux; the presence of Al(3+) diminished K(+) efflux or favored K(+) influx into root tissues. In both zones, low-pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P<or= 0.05) when combined stresses (low-pH/100 microM Al(3+)) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 microM Al(3+) caused PM hyperpolarization. Thus, low pH and Al(3+) toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses.

The response of high and low polyamine-producing cell lines to aluminum and calcium stress

The diamine putrescine (Put) has been shown to accumulate in tree leaves in response to high Al and low Ca in the soil, leading to the suggestion that this response may provide a physiological advantage to leaf cells under conditions of Al stress. The increase in Put is reversed by Ca supplementation in the soil. Using two cell lines of poplar (Populus nigra x maximowiczii), one with constitutively high Put (resulting from transgenic expression of a mouse ornithine decarboxylase--called HP cells) and the other with low Put (control cells), we investigated the effects of reduced Ca (0.2-0.8 mM vs. 4 mM) and treatment with 0.1 mM Al on several biochemical parameters of cells. We found that in the presence of reduced Ca concentration, the HP cells were at a disadvantage as compared to control cells in that they showed greater reduction in mitochondrial activity and a reduction in the yield of cell mass. Upon addition of Al to the medium, the HP cells, however, showed a reversal of low-Ca effects. We conclude that due to increased ROS production in the HP cells, their tolerance to low Ca is compromised. Contrary to the expectation of deleterious effects, the HP cells showed an apparent advantage in the presence of Al in the medium, which could have come from reduced uptake of Al, enhanced extrusion of Al following its accumulation, and perhaps a reduction in Put catabolism as a result of a reduction in its biosynthesis.

The role of the root apoplast in aluminium-induced inhibition of root elongation and in aluminium resistance of plants: a review

BACKGROUND

Aluminium (Al) toxicity is the most important soil constraint for plant growth and development in acid soils. The mechanism of Al-induced inhibition of root elongation is still not well understood, and it is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic.

SCOPE

The present review focuses on the role of the apoplast in Al toxicity and resistance, summarizing evidence from our own experimental work and other evidence published since 1995.

CONCLUSIONS

The binding of Al in the cell wall particularly to the pectic matrix and to the apoplastic face of the plasma membrane in the most Al-sensitive root zone of the root apex thus impairing apoplastic and symplastic cell functions is a major factor leading to Al-induced inhibition of root elongation. Although symplastic lesions of Al toxicity cannot be excluded, the protection of the root apoplast appears to be a prerequisite for Al resistance in both Al-tolerant and Al-accumulating plant species. In many plant species the release of organic acid anions complexing Al, thus protecting the root apoplast from Al binding, is a most important Al resistance mechanism. However, there is increasing physiological, biochemical and, most recently also, molecular evidence showing that the modification of the binding properties of the root apoplast contributes to Al resistance. A further in-depth characterization of the Al-induced apoplastic reaction in the most Al-sensitive zone of the root apex is urgently required, particularly to understand the Al resistance of the most Al-resistant plant species.

A possible mechanism and sequence of events that lead to the Al3+-induced [Ca2+]cyt transients and inhibition of root growth

Fluorescence resonance energy transfer (FRET)- sensitized emission imaging of Arabidopsis thaliana roots expressing the yellow cameleon 3.60 calcium (Ca(2+)) reporter showed that the concentration of calcium in the cytosol ([Ca(2+)]cyt) increased upon aluminum ion (Al(3+)) treatment in root cells from the transition zone within seconds. The Al(3+)-induced [Ca(2+)]cyt transients were biphasic and were modified by Ca(2+) channel blockers and by an antagonist of neuronal glutamate receptors, 2-amino-5-phosphonopentanoate (AP-5), and by the anion channel blocker, 5-nitro-2-(3'-phenylpropyl-amino) benzoate (NPPB). The [Ca(2+)]cyt transients were not uniquely associated with Al(3+) toxicity mechanisms since lanthanum (La(3+)) and gadolinium (Gd(3+)) also elicited [Ca(2+)]cyt transients that were similar to those induced by Al(3+). Here a testable model that describes a possible mechanism and sequence of events that lead to the Al(3+)-induced [Ca(2+)]cyt transients and inhibition of root growth is proposed. This model can be applied to study also the signal-response coupling of the trivalent ions La(3+) and Gd(3+).

Fluorescence resonance energy transfer-sensitized emission of yellow cameleon 3.60 reveals root zone-specific calcium signatures in Arabidopsis in response to aluminum and other trivalent cations

Fluorescence resonance energy transfer-sensitized emission of the yellow cameleon 3.60 was used to study the dynamics of cytoplasmic calcium ([Ca(2+)](cyt)) in different zones of living Arabidopsis (Arabidopsis thaliana) roots. Transient elevations of [Ca(2+)](cyt) were observed in response to glutamic acid (Glu), ATP, and aluminum (Al(3+)). Each chemical induced a [Ca(2+)](cyt) signature that differed among the three treatments in regard to the onset, duration, and shape of the response. Glu and ATP triggered patterns of [Ca(2+)](cyt) increases that were similar among the different root zones, whereas Al(3+) evoked [Ca(2+)](cyt) transients that had monophasic and biphasic shapes, most notably in the root transition zone. The Al(3+)-induced [Ca(2+)](cyt) increases generally started in the maturation zone and propagated toward the cap, while the earliest [Ca(2+)](cyt) response after Glu or ATP treatment occurred in an area that encompassed the meristem and elongation zone. The biphasic [Ca(2+)](cyt) signature resulting from Al(3+) treatment originated mostly from cortical cells located at 300 to 500 mu m from the root tip, which could be triggered in part through ligand-gated Glu receptors. Lanthanum and gadolinium, cations commonly used as Ca(2+) channel blockers, elicited [Ca(2+)](cyt) responses similar to those induced by Al(3+). The trivalent ion-induced [Ca(2+)](cyt) signatures in roots of an Al(3+)-resistant and an Al(3+)-sensitive mutant were similar to those of wild-type plants, indicating that the early [Ca(2+)](cyt) changes we report here may not be tightly linked to Al(3+) toxicity but rather to a general response to trivalent cations.

Aluminium-induced inhibition of root elongation in Arabidopsis is mediated by ethylene and auxin

Aluminium (Al) is phytotoxic when solubilized into Al(3+) in acidic soils. One of the earliest and distinct symptoms of Al(3+) toxicity is inhibition of root elongation. To decipher the mechanism by which Al(3+) inhibits root elongation, the role of ethylene and auxin in Al(3+)-induced inhibition of root elongation in Arabidopsis thaliana was investigated using the wild type and mutants defective in ethylene signalling (etr1-3 and ein2-1) and auxin polar transport (aux1-7 and pin2). Exposure of wild-type Arabidopsis to AlCl(3) led to a marked inhibition of root elongation, and elicited a rapid ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in root apices. Root elongation in etr1-3 and ein2-1 mutants was less inhibited by Al(3+) than that in wild-type plants. Ethylene synthesis inhibitors, Co(2+) and aminoethoxyvinylglycine (AVG), and an antagonist of ethylene perception (Ag(+)) abolished the Al(3+)-induced inhibition of root elongation. There was less inhibition of root elongation by Al(3+) in aux1-7 and pin2 mutants than in the wild type. The auxin polar transport inhibitor, naphthylphthalamic acid (NPA), substantially alleviated the Al(3+)-induced inhibition of root elongation. The Al(3+) and ethylene synthesis precursor aminocyclopropane carboxylic acid (ACC) increased auxin reporter DR5:GUS activity in roots. The Al(3+)-induced increase in DR5:GUS activity was reduced by AVG, while the Al(3+)-induced increase in EBS:GUS activity was not altered by NPA. Al(3+) and ACC increased transcripts of AUX1 and PIN2, and this effect was no longer observed in the presence of AVG and Co(2+). These findings indicate that Al(3+)-induced ethylene production is likely to act as a signal to alter auxin distribution in roots by disrupting AUX1- and PIN2-mediated auxin polar transport, leading to arrest of root elongation.