Magnesium enhances aluminum-induced citrate secretion in rice bean roots (Vigna umbellata) by restoring plasma membrane H+-ATPase activity

Nitric oxide-mediated regulation of macronutrients in plants

In plant physiology, nitric oxide (NO) is a widely used signaling molecule. It is a free radical and an important component of the N-cycle. NO is produced endogenously inside plant cells, where it participates in multiple functions and provides protection against several abiotic and biotic stresses. NO and its interplay with macronutrients had remarkable effects on plant growth and development, the signaling pathway, and defense mechanisms. Its chemical properties, synthetic pathways, physiological effects, antioxidant action, signal transduction, and regulation of transporter genes and proteins have been studied. NO emerges as a key regulator under macronutrient deficiency. In plants, NO also affects reactive oxygen species (ROS), reactive nitrogen species (RNS), and post-translational modifications (PTMs). The function of NO and its significant control in the functions and adjustments of macronutrients under macronutrient deficit were summed up in this review. NO regulate functions of macronutrients and associated signaling events involved with macronutrient transporters in different plants.

Revalorization of degraded maguey pulquero substrate for Lycopersicon esculentum germination

The bagasse of Agave salmiana (maguey pulquero) is a residue generated during the exploitation of the plant to obtain pulque, inulin, honey, etc. Due to its chemical composition, it can be used for the cultivation of fungi of the Pleurotus genus and the subsequent use of the degraded material "degraded substrate (DS)" as a support for the germination of vegetables. The objective of the study was to characterize the bagasse of maguey pulquero biodegraded by Pleurotus djamor as a new perspective in its value chain, and subsequent use for the germination of Lycopersicon esculetum (tomato). The DS was recovered at 60 d from the P. djamor culture, characterized physicochemically and the conformation of the plant tissue was observed by scanning electron microscopy. The DS showed a decrease in protein (4.8-3.3 %) content and fibrous fraction (54-36 %), but dry matter digestibility increased from 47 to 71 %; in addition, changes in mineral composition were observed, mainly in calcium concentration (6 %). Due to its composition, it is possible to revalue DS in the germination of L. esculetum to reduce the use of peat moss (commercial peat). The results show that up to 25 % of maguey DS mixed with 75 % peat moss can be used (25:75), reaching a germination percentage of 85 % and increasing the seedling emergence speed index from 0.96 - 1.25. Concluding that it is possible to implement a circular strategy in which agave bagasse is used for mushroom cultivation and the subsequent recovery of the spent substrate for tomato germination.

Exploring freeze-injury mechanism through ion-specific analysis of leachate from reversibly versus irreversibly injured spinach (Spinacia oleracea L.) leaves

The present study analyzed four cations (K+, Ca2+, Mg2+, Fe2+) in leachate from freeze-injured spinach (Spinacia oleracea L. 'Reflect') leaves exposed for four freezing-durations (FDs) (0.5, 3.0, 5.5, 10.5 h) at -4.8 °C. Comparison of electrolyte leakage from right-after-thaw with that after 6-d recovery revealed that injury at 0.5 or 3 h FDs was recoverable but irreversible at 5.5 or 10.5 h FDs. Data suggests leakage of K+, the most abundant cation in leachate, can serve as a proxy for total electrolyte-leakage in determining plant freezing-tolerance and an ionic marker discerning moderate vs. severe injury. Quantitative correspondence between Ca2+- and K+-leakage supports earlier proposition that leaked K+ induces loss of membrane-Ca2+, which, in turn, promotes further K+-leakage due to weakened membrane. Reduced/undetectable Fe2+ in leachate at longer FDs suggests activation of Fenton reaction converting soluble Fe2+ into insoluble Fe3+. Enhanced Mg2+-leakage at greater freeze-injury suggests structural/functional impairment of chlorophyll/chloroplast complex.

Magnesium fertilization reduces high-temperature damages during anthesis in spring wheat (Triticum aestivum L.) by affecting pollen viability and seed weight

Terminal heat during reproductive stages of wheat (Triticum aestivum L.) limits the productivity of the crop. Magnesium (Mg) is an essential macronutrient that is involved in many physiological and biochemical processes to affect photosynthesis and seed weight. The present study comparatively evaluated Mg applied to soil (80 kg MgSO4·7H2O ha-1) and to plant foliage (4% w/v) in improving wheat performance under terminal heat. Wheat crop was grown in two sets of treatments until the booting stage, and then one set of plants was shifted to a glasshouse (±5 °C) at the booting stage to grow until maturity in comparison to control plants kept under ambient warehouse condition. Heat stress reduced the pollen viability while foliar- and soil-applied Mg improved it by 3% and 6% under heat stress, respectively, compared to the control without Mg treatment. The 100-seed weight, spike length, and biological yield reduced by 39%, 19%, and 50% under heat stress; however, foliar and soil application increased 100-seed weight by 45% and 40%, spike length by 8% and 5%, and biological yield by 35% and 25% under heat stress, respectively. Soil Mg showed maximum SPAD chlorophyll values; however, response was statistically similar to that of foliar Mg as compared to the control without Mg supply. Membrane stability decreased (4%) due to heat stress while foliar and soil treatments improved membrane stability by 8% and 5% compared to that of the control, respectively. Thus, Mg application through soil or plant foliage can be an effective way to reduce negative impacts of terminal heat in wheat by improving pollen viability at anthesis and 100-seed weight that was attributed to increased chlorophyll contents during anthesis.

Harnessing the power of exogenous factors to enhance plant resistance to aluminum toxicity; a critical review

Aluminum (Al) is the most prevalent element in the earth crust and is toxic to plants in acidic soils. However, plants can address Al toxicity through external exclusion (which prevents Al from entering roots) and internal detoxification (which counterbalances the toxic-Al absorbed by roots). Nowadays, certain categories of exogenously added regulatory factors (EARF), such as nutritional elements, organic acids, amino acids, phytohormones, or biochar, etc. play a critical role in reducing the bioavailability/toxicity of Al in plants. Numerous studies suggest that regulating factors against Al toxicity mediate the expression of Al-responsive genes and transcription factors, thereby regulating the secretion of organic acids, alkalizing rhizosphere pH, modulating cell wall (CW) modifications, improving antioxidant defense systems, and promoting the compartmentalization of non-toxic Al within intracellular. This review primarily discusses recent and older published papers to demonstrate the basic concepts of Al phytotoxicity. Furthermore, we provide a comprehensive explanation of the crucial roles of EARF-induced responses against Al toxicity in plants. This information may serve as a foundation for improving plant resistance to Al and enhancing the growth of susceptible species in acidic soils. And this review holds significant theoretical significance for EARF to improve the quality of acidic soils cultivated land, increase crop yield and quality, and ensure food security.

Magnesium alleviates aluminum-induced growth inhibition by enhancing antioxidant enzyme activity and carbon-nitrogen metabolism in apple seedlings

Previous studies have determined that magnesium (Mg) in appropriate concentrations prevents plants from suffering from abiotic stress. To better understand the mechanism of Mg alleviation of aluminum (Al) stress in apple, we investigated the effect of Mg on plant growth, photosynthetic fluorescence, antioxidant system, and carbon (C) and nitrogen (N) metabolism of apple seedlings under Al toxicity (1.5 mmol/L) via a hydroponic experiment. Al stress induced the production of reactive oxygen in the leaves and roots and reduced the total dry weight (DW) by 52.37 % after 20 days of treatment relative to plants grown without Al, due to hindered photosynthesis and alterations in C and N metabolism. By contrast, total DW decreased by only 11.07 % in the Mg-treated plants under Al stress. Supplementation with 3.0 mmol/L Mg in the Al treatment decreased Al accumulation in the apple plants and reduced Al-induced oxidative damage by enhancing the activity of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and reducing the production of H₂O₂ and malondialdehyde (MDA). Under Al stress, the Mg-treated plants showed a 46.17 % higher photosynthetic rate than the non-treated plants. Supplementation with Mg significantly increased the sucrose content by increasing sucrose synthase (SS) and sucrose-phosphate synthase (SPS) activities. Moreover, Mg facilitated the transport of ¹³C-carbohydrates from the leaves to roots. Regarding N metabolism, the nitrate reductase (NR), glutamine synthase (GS), and glutamate synthase (GOGAT) activities in the roots and leaves of the Mg-treated plants were significantly higher than those of the non-treated plants under Al stress. Compared with the non-treated plants under Al stress, the Mg-treated plants exhibited a significantly high level of NO₃^- and soluble protein content in the leaves, roots, and stems, but a low level of free amino acids. Furthermore, Mg significantly improved nitrogen accumulation and enhanced the transport of ¹⁵N from the roots to leaves. Overall, our results revealed that Mg alleviates Al-induced growth inhibition by enhancing antioxidant capacity and C-N metabolism in apple seedlings.

Impact of the Arbuscular Mycorrhizal Fungus Funneliformis mosseae on the Physiological and Defence Responses of Canna indica to Copper Oxide Nanoparticles Stress

Copper oxide nanoparticles (nano-CuO) are recognized as an emerging pollutant. Arbuscular mycorrhizal fungi (AMF) can mitigate the adverse impacts of various pollutants on host plants. However, AMF's mechanism for alleviating nano-CuO phytotoxicity remains unclear. The goal of this study was to evaluate how AMF inoculations affect the physiological features of Canna indica seedlings exposed to nano-CuO stress. Compared with the non-AMF inoculated treatment, AMF inoculations noticeably improved plant biomass, mycorrhizal colonization, leaf chlorophyll contents, and the photosynthetic parameters of C. indica under nano-CuO treatments. Moreover, AMF inoculation was able to significantly mitigate nano-CuO stress by enhancing antioxidant enzyme activities and decreasing ROS levels in the leaves and roots of C. indica, thus increasing the expression of genes involved in the antioxidant response. In addition, AMF inoculation reduced the level of Cu in seedlings and was associated with an increased expression of Cu transport genes and metallothionein genes. Furthermore, AMF inoculations increased the expression levels of organic acid metabolism-associated genes while facilitating organic acid secretion, thus reducing the accumulation of Cu. The data demonstrate that AMF-plant symbiosis is a feasible biocontrol approach to remediate nano-CuO pollution.

Tea plant roots respond to aluminum-induced mineral nutrient imbalances by transcriptional regulation of multiple cation and anion transporters

BACKGROUND

Tea is one of the most popular non-alcoholic beverages in the world for its flavors and numerous health benefits. The tea tree (Camellia sinensis L.) is a well-known aluminum (Al) hyperaccumulator. However, it is not fully understood how tea plants have adapted to tolerate high concentrations of Al, which causes an imbalance of mineral nutrition in the roots.

RESULTS

Here, we combined ionomic and transcriptomic profiling alongside biochemical characterization, to probe the changes of metal nutrients and Al responsive genes in tea roots grown under increasing concentrations of Al. It was found that a low level of Al (~ 0.4 mM) maintains proper nutrient balance, whereas a higher Al concentration (2.5 mM) compromised tea plants by altering micro- and macro-nutrient accumulation into roots, including a decrease in calcium (Ca), manganese (Mn), and magnesium (Mg) and an increase in iron (Fe), which corresponded with oxidative stress, cellular damage, and retarded root growth. Transcriptome analysis revealed more than 1000 transporter genes that were significantly changed in expression upon Al exposure compared to control (no Al) treatments. These included transporters related to Ca and Fe uptake and translocation, while genes required for N, P, and S nutrition in roots did not significantly alter. Transporters related to organic acid secretion, together with other putative Al-tolerance genes also significantly changed in response to Al. Two of these transporters, CsALMT1 and CsALS8, were functionally tested by yeast heterologous expression and confirmed to provide Al tolerance.

CONCLUSION

This study shows that tea plant roots respond to high Al-induced mineral nutrient imbalances by transcriptional regulation of both cation and anion transporters, and therefore provides new insights into Al tolerance mechanism of tea plants. The altered transporter gene expression profiles partly explain the imbalanced metal ion accumulation that occurred in the Al-stressed roots, while increases to organic acid and Al tolerance gene expression partly explains the ability of tea plants to be able to grow in high Al containing soils. The improved transcriptomic understanding of Al exposure gained here has highlighted potential gene targets for breeding or genetic engineering approaches to develop safer tea products.

Genetic aluminium resistance coupled with foliar magnesium application enhances wheat growth in acidic soil

BACKGROUND

Soil acidity causes an increase in the solubility of toxic aluminium (Al), inhibiting root growth and limiting plant access to soil water and nutrients. Toxicity of Al decreases growth and development more in Al-sensitive than Al-resistant wheat (Triticum aestivum L.) genotypes. Applying magnesium (Mg) may alleviate Al toxicity in acidic soil. It is unclear if growing Al-resistant wheat, coupled with Mg application, will enhance wheat growth and physiological responses in acidic soil. Wheat was grown in the reconstituted acidic soil profile in a three-factor glasshouse experiment comprising foliar Mg at [Mg] = 0 and 200 mg L^-1 (using magnesium sulfate), wheat genotypes ES8 (Al sensitive) and ET8 (Al resistant), and four soil treatments [non-amended soil (pH_{0.1 M CaCl2}    4.0); 100% of the recommended lime dose in subsoil to raise pH to 6.0; or 20 mg Mg kg^-1 soil to the whole soil profile (as magnesium sulfate or magnesium chloride)].

RESULTS

Applying foliar Mg to Al-resistant wheat significantly increased shoot dry weight (24%), subsoil root dry weight (12%), subsoil coarse (>0.2 mm in diameter) root length (12%), net photosynthetic rate (28%), and total leaf area (10%) compared with plants not treated with foliar Mg. Combining foliar Mg application with subsoil liming increased the subsoil total root length by 1.3-fold compared with zero foliar Mg. Without foliar Mg, subsoil liming and soil Mg amendment significantly (by 10%) enhanced shoot growth and total leaf area compared with non-amended soil.

CONCLUSION

Growing an Al-resistant genotype coupled with applying Mg foliarly and to soil improved wheat growth in an acidic soil. © 2021 Society of Chemical Industry.

The role of solute transporters in aluminum toxicity and tolerance

The prevalence of aluminum ions (Al³⁺ ) under acidic soil conditions inhibits primary root elongation and hinders plant growth and productivity. Al³⁺ alters the membrane potential, displaces critical ions in the apoplast and disrupts intracellular ionic concentrations by targeting membrane-localized solute transporters. Here, we provide an overview of how Al³⁺ affects the activities of several solute transporters especially in the root. High Al³⁺ level impairs the functions of potassium (K⁺ ), calcium (Ca²⁺ ), magnesium (Mg²⁺ ), nitrate (NO₃ ^- ) and ammonium (NH₄ ⁺ ) transporters. We further discuss the role of some key transporters in mediating Al tolerance either by exclusion or sequestration. Anion channels responsible for organic acid efflux modulate the sensitivity to Al³⁺ . The ALUMINUM ACTIVATED MALATE TRANSPORTER (ALMT) and MULTIDRUG AND TOXIC COMPOUND EXTRUSION (MATE) family of transporters exude malate and citrate, respectively, to the rhizosphere to alleviate Al toxicity by Al exclusion. The ABC transporters, aquaporins and H⁺ -ATPases perform vacuolar sequestration of Al³⁺ , leading to aluminum tolerance in plants. Targeting these solute transporters in crop plants can help generating aluminum-tolerant crops in future.

Auxin Is Involved in Magnesium-Mediated Photoprotection in Photosystems of Alfalfa Seedlings Under Aluminum Stress

The objective of this study was to investigate the effects of Mg and IAA on the photosystems of Al-stressed alfalfa (Medicago sativa L.). Alfalfa seedlings with or without apical buds were exposed to solutions fully mixed with 0 or 100 μM AlCl₃ and 0 or 50 μM MgCl₂ followed by foliar spray with water or IAA. Results from seedlings with apical buds showed that application of Mg and IAA either alone or combine greatly alleviated the Al-induced damage on photosystems. The values of photosynthetic rate (Pn), effective quantum yields [Y(I) and Y(II)] and electron transfer rates (ETRI and ETRII), proton motive force (pmf), cyclic electron flow (CEF), proton efflux rate (g_H ⁺), and activities of ATP synthase and PM H⁺-ATPase significantly increased, and proton gradient (ΔpH _pmf ) between lumen and stroma decreased under Al stress. After removing apical buds of seedlings, the Y(I), Y(II), ETRI, ETRII, pmf, and g_H ⁺ under exogenous spraying IAA significantly increased, and ΔpH _pmf significantly decreased in Mg addition than Al treatment alone, but they were no significant difference under none spraying IAA. The interaction of Mg and IAA directly increased quantum yields and electron transfer rates, and decreased O₂ ^- accumulation in Al-stressed seedlings with or without apical buds. These results suggest that IAA involves in Mg alleviation of Al-induced photosystem damage via increasing pmf and PM H⁺-ATPase activity, and decreasing ΔpH _pmf .

Magnesium reduces cadmium accumulation by decreasing the nitrate reductase-mediated nitric oxide production in Panax notoginseng roots

Panax notoginseng is a traditional medicinal herb in China. However, the high capacity of its roots to accumulate cadmium (Cd) poses a potential risk to human health. Our previous study showed that nitrate reductase (NR)-dependent nitric oxide (NO) production promoted Cd accumulation in P. notoginseng root cell walls. In this study, the role of Mg in the regulation of NO production and Cd accumulation in P. notoginseng roots was characterized. Exposure of P. notoginseng roots to increasing concentrations of Cd resulted in a linear increase in NO production. The application of 2 mM Mg for 24 h significantly alleviated Cd-induced NO production and Cd accumulation in roots, which coincided with a significant decrease in the NR activity. Western analysis suggested that Mg increased the interaction between the 14-3-3 protein and NR, which might have been a reason for the Mg-mediated decrease in NR activity and NO production under Cd stress. These results suggested that Mg-mediated alleviation of Cd-induced NO production and Cd accumulation is achieved by enhancement of the interaction between the 14-3-3 protein and NR in P. notoginseng roots.

Importance of Mineral Nutrition for Mitigating Aluminum Toxicity in Plants on Acidic Soils: Current Status and Opportunities

Aluminum (Al) toxicity is one of the major limitations that inhibit plant growth and development in acidic soils. In acidic soils (pH < 5.0), phototoxic-aluminum (Al3+) rapidly inhibits root growth, and subsequently affects water and nutrient uptake in plants. This review updates the existing knowledge concerning the role of mineral nutrition for alleviating Al toxicity in plants to acid soils. Here, we explored phosphorus (P) is more beneficial in plants under P-deficient, and Al toxic conditions. Exogenous P addition increased root respiration, plant growth, chlorophyll content, and dry matter yield. Calcium (Ca) amendment (liming) is effective for correcting soil acidity, and for alleviating Al toxicity. Magnesium (Mg) is able to prevent Al migration through the cytosolic plasma membrane in root tips. Sulfur (S) is recognized as a versatile element that alleviates several metals toxicity including Al. Moreover, silicon (Si), and other components such as industrial byproducts, hormones, organic acids, polyamines, biofertilizers, and biochars played promising roles for mitigating Al toxicity in plants. Furthermore, this review provides a comprehensive understanding of several new methods and low-cost effective strategies relevant to the exogenous application of mineral nutrition on Al toxicity mitigation. This information would be effective for further improvement of crop plants in acid soils.

Developmental Responses of Root Hairs to Mg Deficiency

Magnesium (Mg), an essential element for plants is easily leached in acidic and sandy soils. Magnesium deficiency induces the initiation and elongation of root hairs, which allows the plant roots to acquire more Mg under Mg-limited conditions. However, the signals involved in the regulatory cascade leading to the induction of root hair development under Mg deficiency are largely unknown to date. Recent studies have revealed that many chemical signal molecules such as ethylene, nitric oxide, auxin, reactive oxygen, and calcium regulate the root hair development induced owing to Mg deficiency. This mini-review intends to briefly discuss the role of these chemical signals in the induction of root hair development under Mg-deficient conditions.

Auxin Acts Downstream of Ethylene and Nitric Oxide to Regulate Magnesium Deficiency-Induced Root Hair Development in Arabidopsis thaliana

This study examines the association of auxin with ethylene and nitric oxide (NO) in regulating the magnesium (Mg) deficiency-induced root hair development in Arabidopsis thaliana. With Mg deficiency, both ethylene and NO promoted the elevation of root auxin levels in roots by inducing the expression of AUXIN-RESISTANT1 (AUX1), PIN-FORMED 1 (PIN1) and PIN2 transporters. In turn, auxin stimulated ethylene and NO production by activating the activities of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO), ACC synthase (ACS), nitrate reductase (NR) and NO synthase-like (NOS-L). These processes constituted an NO/ethylene-auxin feedback loop. Interestingly, however, the roles of ethylene and NO in regulating Mg deficiency-induced root hair development required the action of auxin, but not vice versa. In summary, these results suggest that Mg deficiency induces a positive interaction between the accumulation of auxin and ethylene/NO in roots, with auxin acting downstream of ethylene and NO signals to regulate Mg deficiency-induced root hair morphogenesis.

Metabolomics Reveals Distinct Carbon and Nitrogen Metabolic Responses to Magnesium Deficiency in Leaves and Roots of Soybean [Glycine max (Linn.) Merr.]

Magnesium (Mg) deficiency, a widespread yet overlooked problem in agriculture, has been reported to retard plant growth and development, through affecting key metabolic pathways. However, the metabolic responses of plant to Mg deficiency is still not fully understood. Here we report a metabolomic study to evaluate the metabolic responses to Mg deficiency in soybean leaves and roots. Hydroponic grown soybean were exposed to Mg starvation for 4 and 8 days, respectively. Metabolic changes in the first mature trifoliolate leaves and roots were quantified by conducting GC-TOF-MS based metabolomic analysis. Principal component analysis (PCA) showed that Mg deficient plants became distinguishable from controls at 4 days after stress (DAS) at metabolic level, and were clearly discriminated at 8 DAS. Mg deficiency could cause large metabolite alterations on carbon and nitrogen metabolism. At 8 DAS, carbon allocation from shoot to root is decreased by Mg deficiency. Remarkably, most amino acids (such as phenylalanine, asparagine, leucine, isoleucine, glycine, glutamine, and serine) showed pronounced accumulation in the leaves, while most organic acids (including pyruvic acid, citric acid, 2-keto-glutaric acid, succinic acid, fumaric acid, and malic acid) were significantly decreased in the roots. Our study shows that the carbon and nitrogen metabolic responses are distinct in leaves and roots under Mg deficiency.

The Role of the Plasma Membrane H+-ATPase in Plant Responses to Aluminum Toxicity

Aluminum (Al) toxicity is a key factor limiting plant growth and crop production on acid soils. Increasing the plant Al-detoxification capacity and/or breeding Al-resistant cultivars are a cost-effective strategy to support crop growth on acidic soils. The plasma membrane H+-ATPase plays a central role in all plant physiological processes. Changes in the activity of the plasma membrane H+-ATPase through regulating the expression and phosphorylation of this enzyme are also involved in many plant responses to Al toxicity. The plasma membrane H+-ATPase mediated H+ influx may be associated with the maintenance of cytosolic pH and the plasma membrane gradients as well as Al-induced citrate efflux mediated by a H+-ATPase-coupled MATE co-transport system. In particular, modulating the activity of plasma membrane H+-ATPase through application of its activators (e.g., magnesium or IAA) or using transgenics has effectively enhanced plant resistance to Al stress in several species. In this review, we critically assess the available knowledge on the role of the plasma membrane H+-ATPase in plant responses to Al stress, incorporating physiological and molecular aspects.

Melatonin alleviates aluminium toxicity through modulating antioxidative enzymes and enhancing organic acid anion exudation in soybean

Aluminium (Al) toxicity is a major chemical constraint limiting plant growth and production on acidic soils. Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous molecule that plays crucial roles in plant growth and stress tolerance. However, there is no knowledge regarding whether melatonin is involved in plant responses to Al stress. Here, we show that optimal concentrations of melatonin could effectively ameliorate Al-induced phytotoxicity in soybean (Glycine max L.). The concentration of melatonin in roots was significantly increased by the 50μM Al treatment. Such an increase in endogenous melatonin coincided with the upregulation of the gene encoding acetyltransferase NSI-like (nuclear shuttle protein-interacting) in soybean roots. Supplementation with low concentrations of melatonin (0.1 and 1μM) conferred Al resistance as evident in partial alleviation of root growth inhibition and decreased H2O2 production: in contrast, high concentrations of melatonin (100 and 200μM) had an opposite effect and even decreased root growth in Al-exposed seedlings. Mitigation of Al stress by the 1μM melatonin root treatment was associated with enhanced activities of the antioxidant enzymes and increased exudation of malate and citrate. In conclusion, melatonin might play a critical role in soybean resistance to Al toxicity.

Functional dissection and transport mechanism of magnesium in plants

Magnesium (Mg) is the second most abundant cation in plants, and, as such, is involved in numerous physiological and biochemical processes, including photosynthesis, enzyme activation, and synthesis of nucleic acids and proteins. Due to its relatively small ionic radius and large hydrated radius, Mg binds weakly to soil and root surfaces, and thereby is easily leached from soil. Mg deficiency not only affects crop productivity and quality, but also contributes to numerous chronic human diseases. Therefore, Mg nutrition in plants is an important issue in nutrition and food security. To acquire and maintain high concentrations of Mg, plants have evolved highly-efficient systems for Mg uptake, storage and translocation. Advances in the understanding of fundamental principles of Mg nutrition and physiology are required in order to improve Mg nutrient management, Mg stress diagnosis, and genetic marker assisted breeding efforts. The aims of this review are to highlight physiological and molecular mechanisms underlying Mg biological functions and to summarize recent developments in the elucidation of Mg transport systems in plants.

Auxin enhances aluminium-induced citrate exudation through upregulation of GmMATE and activation of the plasma membrane H+-ATPase in soybean roots

Background and Aims Aluminium (Al) toxicity is a limiting factor for plant growth and crop production in acidic soils. Citrate exudation and activation of the plasma membrane H+-ATPase are involved in soybean responses to Al stress. Auxin has crucial functions in plant growth and stress responses. However, little is known about possible interactions between auxin and citrate exudation under Al stress. In this study, we elucidated the regulatory roles of IAA in Al-induced citrate exudation in soybean roots. Methods We measured IAA content, Al concentration, citrate exudation, plasma membrane H+-ATPase activity, expression of the relevant genes and phosphorylation of the plasma membrane H+-ATPase by integrating physiological characterization and molecular analysis using hydroponically grown soybean. Key Results The concentration of IAA was increased by 25 and 50 μm Al, but decreased to the control level at 200 μm Al. External addition of 50 μm IAA to the root medium containing 25, 50 or 200 μm Al decreased root Al concentration and stimulated Al-induced citrate exudation and the plasma membrane H+-ATPase activity. Reverse transcription-PCR analysis showed that exogenous IAA enhanced the expression of citrate exudation transporter (GmMATE) but not the plasma membrane H+-ATPase gene. The western blot results suggested that IAA enhanced phosphorylation of the plasma membrane H+-ATPase under Al stress. Conclusions Auxin enhanced Al-induced citrate exudation through upregulation of GmMATE and an increase in phosphorylation of the plasma membrane H+-ATPase in soybean roots.

A potential contribution of the less negatively charged cell wall to the high aluminum tolerance of Rhodotorula taiwanensis RS1

Rhodotorula taiwanensis RS1 (Rt) is a high-aluminum (Al)-tolerant yeast that can survive Al at concentrations up to 200 mM. In this study, we compared Rt with an Al-sensitive congeneric strain, R. mucilaginosa AKU 4812 (Rm) and Al sensitive mutant 1 (alsm1) of Rt, to explore the Al tolerance mechanisms of Rt. The growth of Rm was completely inhibited by 1 mM Al, but that of Rt was not inhibited until Al concentration was more than 70 mM. The growth of alsm1 was inhibited much more by 70 mM and 100 mM Al than that of Rt. Compared with Rm cells, Rt cells accumulated less Al in the cell wall and cytoplasm. A time-course analysis showed that Al was absorbed by Rm cells much more rapidly than by Rt cells when exposed to the same Al concentration. Meanwhile, the Al content of alsm1 was higher than that of Rt. Although the cell wall of Rt was thicker than that of alsm1 and Rm under control and 0.1 mM Al, that of Rt was thinner than that of alsm1 under 70 mM Al despite that their cell walls were thickened. The alcian blue adsorption was lower and cell wall zeta-potential was higher in Rt and alsm1 than in Rm, indicating a less negative charge of cell wall of Rt and alsm1 than that of Rm. Taken together, the less negatively charged cell wall of Rt may restrict the adsorption of cationic Al in cells, potentially contributing to its high Al tolerance. Copyright © 2016 John Wiley & Sons, Ltd.

Limitation of seedling growth by potassium and magnesium supply for two ectomycorrhizal tree species of a Central African rain forest and its implication for their recruitment

In the ectomycorrhizal caesalpiniaceous groves of southern Korup National Park, the dominant tree species, Microberlinia bisulcata, displays very poor in situ recruitment compared with its codominant, Tetraberlinia bifoliolata. The reported ex situ experiment tested whether availabilities of soil potassium and magnesium play a role. Seedlings of the two species received applications of K and Mg fertilizer in potted native soil in a local shade house, and their responses in terms of growth and nutrient concentrations were recorded over 2 years. Amended soil concentrations were also determined. Microberlinia responded strongly and positively in its growth to Mg, but less to K; Tetraberlinia responded weakly to both. Added Mg led to strongly increased Mg concentration for Microberlinia while added K changed that concentration only slightly; Tetraberlinia strongly increased its concentration of K with added K, but only somewhat its Mg concentration with added Mg. Additions of Mg and K had small but important antagonistic effects. Microberlinia is Mg-demanding and apparently Mg-limited in Korup soil; Tetraberlinia, whilst K-demanding, appeared not to be K-limited (for growth). Added K enhanced plant P concentrations of both species. Extra applied Mg may also be alleviating soil aluminum toxicity, and hence improving growth indirectly and especially to the benefit of Microberlinia. Mg appears to be essential for Microberlinia seedling growth and its low soil availability in grove soils at Korup may be an important contributing factor to its poor recruitment. Microberlinia is highly shade-intolerant and strongly light-responding, whilst Tetraberlinia is more shade-tolerant and moderately light-responding, which affords an interesting contrast with respect to their differing responses to Mg supply. The study revealed novel aspects of functional traits and likely niche-partitioning among ectomycorrhizal caesalps in African rain forests. Identifying the direct and interacting indirect effects of essential elements on tropical tree seedling growth presents a considerable challenge due the complex nexus of causes involved.

Phosphorylation and Interaction with the 14-3-3 Protein of the Plasma Membrane H+-ATPase are Involved in the Regulation of Magnesium-Mediated Increases in Aluminum-Induced Citrate Exudation in Broad Bean (Vicia faba. L)

Several studies have shown that external application of micromolar magnesium (Mg) can increase the resistance of legumes to aluminum (Al) stress by enhancing Al-induced citrate exudation. However, the exact mechanism underlying this regulation remains unknown. In this study, the physiological and molecular mechanisms by which Mg enhances Al-induced citrate exudation to alleviate Al toxicity were investigated in broad bean. Micromolar concentrations of Mg that alleviated Al toxicity paralleled the stimulation of Al-induced citrate exudation and increased the activity of the plasma membrane (PM) H(+)-ATPase. Northern blot analysis shows that a putative MATE-like gene (multidrug and toxic compound extrusion) was induced after treatment with Al for 4, 8 and 12 h, whereas the mRNA abundance of the MATE-like gene showed no significant difference between Al plus Mg and Al-only treatments during the entire treatment period. Real-time reverse transcription-PCR (RT-PCR) and Western blot analyses suggest that the transcription and translation of the PM H(+)-ATPase were induced by Al but not by Mg. In contrast, immunoprecipitation suggests that Mg enhanced the phosphorylation levels of VHA2 and its interaction with the vf14-3-3b protein under Al stress. Taken together, our results suggest that micromolar concentrations of Mg can alleviate the Al rhizotoxicity by increasing PM H(+)-ATPase activity and Al-induced citrate exudation in YD roots. This enhancement is likely to be attributable to Al-induced increases in the expression of the MATE-like gene and vha2 and Mg-induced changes in the phosphorylation levels of VHA2, thus changing its interaction with the vf14-3-3b protein.

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.

An update on magnesium homeostasis mechanisms in plants

Worldwide, nearly two-thirds of the population do not consume the recommended amount of magnesium (Mg) in their diet. Furthermore, low Mg status (hypomagnesaemia) is known to contribute to a number of human chronic disease conditions. Because the principal dietary Mg source is of plant origin, agronomic and genetic biofortification strategies are aimed at improving nutritional Mg content in food crops to overcome this mineral deficiency in humans. This update incorporates the contributions of annotated permeases involved in Mg uptake, storage and recycling with a schematic model of Mg movement at the organ and cellular levels in the model species Arabidopsis thaliana. Furthermore, approaches using mutagenesis or natural ionomic variation to identify loci involved in Mg homeostasis in roots, leaves and seeds will be summarised. A brief overview will be presented on how Arabidopsis research can help to develop strategies for biofortification of crops.

The secretion of organic acids is also regulated by factors other than aluminum

As a result of natural processes and human activities, aluminum (Al) toxicity is recognized as a major limiting factor for plant productivity, and the secretion of organic acids facilitated by channel proteins is one of the most important Al resistance mechanisms in plants. The objective of this study was to evaluate the effects of several types of stress, including herbicide (imazethapyr (IM) and diclofop-methyl (DM)), heavy metal (Al and Cu), salt stress (NaCl), and proton stress (HCl), on the release of organic acids in rice. The results showed that 0.05 mg/L IM, 0.1 mg/L DM, 4680 mg/L NaCl, 0.5 mg/L CuSO4, and 18 mg/L AlCl3 significantly inhibited rice root elongation and the root fresh weight. In contrast, no significant inhibitory effects on rice growth were found with HCl (pH = 4.5). Similar to the effect of AlCl3 on organic acid induction, treatment with IM, DM, NaCl, and CuSO4 also induced the synthesis of endogenous citric acid and oxalic acid but decreased endogenous malic acid synthesis in the seedlings, though only citric acid was released into the environment after these treatments. We also analyzed the transcripts of three citrate channel proteins and found they were up-regulated by NaCl, CuSO4, and AlCl3 but not by IM or DM. This study clarified that organic acid secretion in plants might be a common phenomenon when plants are exposed to environmental stress other than Al toxicity.

Differences in physiological features associated with aluminum tolerance in Tibetan wild and cultivated barleys

Aluminum (Al) toxicity is a major limiting factor for plant production in acid soils. Wild barley germplasm is a treasure trove of useful genes and offers rich sources of genetic variation for crop improvement. Al-stress-hydroponic-experiments were performed, and the physiochemical characteristic of two contrasting Tibetan wild barley genotypes (Al-resistant XZ16 and Al-sensitive XZ61) and Al-resistant cv. Dayton were compared. Ultrastructure of chloroplasts and root cells in XZ16 was less injured than that in Dayton and XZ61. Moreover, XZ16 secreted significantly more malate besides citrate and exhibited less Al uptake and distribution than both of XZ61 and Dayton in response to Al stress, simultaneously maintained higher H⁺-, Ca²⁺Mg²⁺- and total-ATPase activities over XZ61. The protein synthesis inhibitor cycloheximide reduced citrate secretion from XZ16, but not from Dayton. In Tibetan wild barley, our findings highlight the significant correlations between Al tolerance, ATPase activity and citrate secretion, providing some insights into the physiological basis for Al-detoxification.

Low-pH and aluminum resistance in arabidopsis correlates with high cytosolic magnesium content and increased magnesium uptake by plant roots

Low-pH stress and Al(3+) toxicity affect root growth in acid soils. It was hypothesized that the capacity of genotypes to maintain Mg(2+) uptake in acidic environments may contribute to low-pH and Al resistance, but explicit evidence is lacking. In this work, an Al-resistant alr104 mutant and two Al-sensitive mutants (als5 and als3) of Arabidopsis thaliana were compared with the wild type (Col-0) for Mg(2+) uptake and intracellular Mg(2+) concentration under low-pH and combined low-pH/Al stresses. Magnesium accumulation in roots was measured in long-term (7 d) experiments. The Mg(2+) fluxes were measured using ion-sensitive microelectrodes at the distal elongation and the mature root zones in short-term (0-60 min) experiments. Intracellular Mg(2+) concentrations were measured in intact root cells at the distal elongation zone using magnesium-specific fluorescent dye and fluorescent lifetime imaging (FLIM) analysis. Under low-pH stress, Arabidopsis mutants als5 and alr104 maintained a higher Mg concentration in roots, and had greater Mg(2+) influx than the wild type and the als3 mutant. Under combined low-pH/Al treatment, Al-resistant genotypes (wild type and alr104) maintained a higher Mg(2+) accumulation, and had a higher Mg(2+) influx and higher intracellular Mg(2+) concentration than Al-sensitive genotypes (als3 and als5). Overall, these results show that increased Mg(2+) uptake correlates with an enhanced capacity of Arabidopsis genotypes to cope with low-pH and combined low-pH/Al stresses.

Roles of organic acid anion secretion in aluminium tolerance of higher plants

Approximately 30% of the world's total land area and over 50% of the world's potential arable lands are acidic. Furthermore, the acidity of the soils is gradually increasing as a result of the environmental problems including some farming practices and acid rain. At mildly acidic or neutral soils, aluminium (Al) occurs primarily as insoluble deposits and is essentially biologically inactive. However, in many acidic soils throughout the tropics and subtropics, Al toxicity is a major factor limiting crop productivity. The Al-induced secretion of organic acid (OA) anions, mainly citrate, oxalate, and malate, from roots is the best documented mechanism of Al tolerance in higher plants. Increasing evidence shows that the Al-induced secretion of OA anions may be related to the following several factors, including (a) anion channels or transporters, (b) internal concentrations of OA anions in plant tissues, (d) temperature, (e) root plasma membrane (PM) H(+)-ATPase, (f) magnesium (Mg), and (e) phosphorus (P). Genetically modified plants and cells with higher Al tolerance by overexpressing genes for the secretion and the biosynthesis of OA anions have been obtained. In addition, some aspects needed to be further studied are also discussed.

Up-regulation of a magnesium transporter gene OsMGT1 is required for conferring aluminum tolerance in rice

Magnesium (Mg)-mediated alleviation of aluminum (Al) toxicity has been observed in a number of plant species, but the mechanisms underlying the alleviation are still poorly understood. When a putative rice (Oryza sativa) Mg transporter gene, Oryza sativa MAGNESIUM TRANSPORTER1 (OsMGT1), was knocked out, the tolerance to Al, but not to cadmium and lanthanum, was decreased. However, this inhibition could be rescued by addition of 10 μm Mg, but not by the same concentration of barium or strontium. OsMGT1 was expressed in both the roots and shoots in the absence of Al, but the expression only in the roots was rapidly up-regulated by Al. Furthermore, the expression did not respond to low pH and other metals including cadmium and lanthanum, and was regulated by an Al-responsive transcription factor, AL RESISTANCE TRANSCRIPTION FACTOR1. An investigation of subcellular localization showed that OsMGT1 was localized to the plasma membrane. A short-term (30 min) uptake experiment with stable isotope (25)Mg showed that knockout of OsMGT1 resulted in decreased Mg uptake, but that the uptake in the wild type was enhanced by Al. Mg concentration in the cell sap of the root tips was also increased in the wild-type rice, but not in the knockout lines in the presence of Al. A microarray analysis showed that transcripts of genes related to stress were more up- and down-regulated in the knockout lines. Taken together, our results indicate that OsMGT1 is a transporter for Mg uptake in the roots and that up-regulation of this gene is required for conferring Al tolerance in rice by increasing Mg concentration in the cell.

Transcriptomic responses to aluminum stress in soybean roots

Aluminum (Al) toxicity is the primary limitation to crop production and plant growth in acid soils. Soybean has multiple mechanisms of Al resistance including the complexing and exclusion of Al in root apices by Al-induced citrate secretion. Microarray analysis is available for the identification of genes in soybean. In the present study, Affymetrix soybean genome array was used to identify the Al-induced differentially expressed genes in Al-resistant genotype Jiyu 70. With a cutoff of > 2.0-fold (p < 0.05) between non Al-treated and Al-treated root apices, 561 genes were upregulated and 78 genes were downregulated when roots were exposed to 30 μmol/L AlCl(3) for 4 h. Quantitative real-time PCR was used to test the microarray data. The analysis showed that nearly half of the Al-responsive genes were of unknown biological function. A higher proportion of genes related to transcription regulation and cell wall processes were observed in Al-induced up- and downregulated genes, respectively. Some genes homologous to the citrate transporter MATE family gene or C(2)H(2) family transcription factor gene, STOP1, were detected in our analysis. Some genes related to lignin deposition were upregulated, which might be related to Al-induced root elongation inhibition.

Aluminum regulates oxalate secretion and plasma membrane H+-ATPase activity independently in tomato roots

We demonstrated that aluminum (Al)-induced oxalate secretion and plasma membrane (PM) H(+)-ATPase activity in tomato (Lycopersicon esculentum 'Hezuo903') roots were poorly correlated. In addition, vanadate, an inhibitor of PM H(+)-ATPase, had no effect on Al-induced oxalate secretion, but significantly inhibited enzyme activity. An anion channel inhibitor phenylglyoxal inhibited oxalate secretion, but not PM H(+)-ATPase activity. Exposure of tomato roots to 10 μM LaCl(3) also stimulated PM H(+)-ATPase activity; however, La failed to induce oxalate secretion. Furthermore, Al-induced changes of PM H(+)-ATPase activity were not associated with oxalate secretion in two tomato cultivars differing in the ability to secrete oxalate under Al stress. These results indicate that Al independently regulates oxalate secretion and PM H(+)-ATPase activity in tomato roots. Analysis of expression levels of PM H(+)-ATPase genes by real-time reverse transcription-PCR and protein by Western blot and immunodetection revealed that the regulation of PM H(+)-ATPase in response to Al was subjected to transcriptional and posttranscriptional control. However, since neither transcriptional level of genes nor translational level of proteins directly relate to the enzyme activity, posttranslational modification of PM H(+)-ATPase under Al stress likely contributes to changes in activity of this protein.

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.

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.

Use of sugar cane vinasse to mitigate aluminum toxicity to Saccharomyces cerevisiae

Owing to its toxicity, aluminum (Al), which is one of the most abundant metals, inhibits the productivity of many cultures and affects the microbial metabolism. The aim of this work was to investigate the capacity of sugar cane vinasse to mitigate the adverse effects of Al on cell growth, viability, and budding, as the likely result of possible chelating action. For this purpose, Fleischmann's yeast (Saccharomyces cerevisiae) was used in growth tests performed in 125-mL Erlenmeyer flasks containing 30 mL of YED medium (5.0 g/L yeast extract plus 20 g/L glucose) supplemented with the selected amounts of either vinasse or Al in the form of AlCl(3) . H(2)O. Without vinasse, the addition of increasing levels of Al up to 54 mg/L reduced the specific growth rate by 18%, whereas no significant reduction was observed in its presence. The toxic effect of Al on S. cerevisiae growth and the mitigating effect of sugar cane vinasse were quantified by the exponential model of Ciftci et al. (Biotechnol Bioeng 25:2007-2023, 1983). The cell viability decreased from 97.7% at the start to 84.0% at the end of runs without vinasse and to 92.3% with vinasse. On the other hand, the cell budding increased from 7.62% at the start to 8.84% at the end of runs without vinasse and to 17.8% with vinasse. These results demonstrate the ability of this raw material to stimulate cell growth and mitigate the toxic effect of Al.

Response of representative cover crops to aluminum toxicity, phosphorus deprivation, and organic amendment

This study aimed to: (1) determine the effect of P depletion and presence of Al on root and shoot growth of representative cover crops, and on their nutrient uptake; (2) characterise the composition of root exudation under P and Al stress in nutrient solution; (3) evaluate the ability of aqueous extracts of composts in reducing Al phytotoxicity. Plants of cowpea (Vigna unguiculata subsp. unguiculata), black oat (Avena strigosa), and lablab (Lablab purpureous) were cultivated in different nutrient solution compositions and concentrations for 3 weeks. It was found that Al at concentration of 20 and 200 µmol/L increased citrate exudation at least 8 and 24 times, respectively, for cowpea and 18 and 36 times, respectively, for lablab, as compared with the blank. However, no release of organic acids occurred due to P deprivation, suggesting that citrate exudation was a specific response to excess Al. No response in organic acid release was observed for black oat under the stress of P deficiency or Al toxicity. Although the presence of Al in solution did not significantly affect chlorophyll content in leaves, it decreased root and shoot weight, as well as root length, surface area, volume, and number of tips. Organic extracts alleviated aluminum toxicity, improving plant growth and ameliorating plant nutrition status. Yard waste extract was more effective in enhancing plant growth than GreenEdge extract in plants under Al stress.

Syndrome of aluminum toxicity and diversity of aluminum resistance in higher plants

Aluminum (Al) is the most abundant metal in the earth's crust, while its soluble ionic form (Al(3+)) shows phytotoxicity, which is characterized by a rapid inhibition of root elongation. Aluminum targets multiple cellular sites by binding, resulting in disrupted structure and/or functions of the cell wall, plasma membrane, signal transduction pathway, and Ca homeostasis. On the other hand, some plant species have evolved mechanisms to cope with Al toxicity both externally and internally. The well-documented mechanisms for external detoxification of Al include the release of organic acid anions from roots and alkalination of the rhizosphere. Genes encoding transporters for Al-induced secretion of organic acid anions have been identified and characterized. Recent studies show that ABC transporters are involved in Al resistance. The internal detoxification of Al in Al-accumulating plants is achieved by the formation of nontoxic Al complexes with organic acids or other chelators and sequestration of these complexes in the vacuoles. In some plant species, Al shows beneficial effects on plant growth under particular conditions, although the exact mechanisms for these effects are unknown.