Malate Efflux Research Articles

Restricted O2 consumption in pea roots induced by hexanoic acid is linked to depletion of Krebs cycle substrates.

Plant roots are exposed to hypoxia in waterlogged soils, and they are further challenged by specific phytotoxins produced by microorganisms in such conditions. One such toxin is hexanoic acid (HxA), which, at toxic levels, causes a strong decline in root O2 consumption. However, the mechanism underlying this process is still unknown. We treated pea (Pisum sativum L.) roots with 20 mM HxA at pH 5.0 and 6.0 for a short time (1 h) and measured leakage of key electrolytes such as metal cations, malate, citrate and nonstructural carbohydrates (NSC). After treatment, mitochondria were isolated to assess their functionality evaluated as electrical potential and O2 consumption rate. HxA treatment resulted in root tissue extrusion of K+ , malate, citrate and NSC, but only the leakage of the organic acids and NSC increased at pH 5.0, concomitantly with the inhibition of O2 consumption. The activity of mitochondria isolated from treated roots was almost unaffected, showing just a slight decrease in oxygen consumption after treatment at pH 5.0. Similar results were obtained by treating the pea roots with another organic acid with a short carbon chain, that is, butyric acid. Based on these results, we propose a model in which HxA, in its undissociated form prevalent at acidic pH, stimulates the efflux of citrate, malate and NSC, which would, in turn, cause starvation of mitochondrial respiratory substrates of the Krebs cycle and a consequent decline in O2 consumption. Cation extrusion would be a compensatory mechanism in order to restore plasma membrane potential.

Multiple ALMT subunits combine to form functional anion channels: A case study for rice ALMT7

The Aluminum Activated Malate Transporter (ALMT) family members are anion channels that play important roles in organic acid transport, stress resistance, growth, development, fertilization and GABA responses. The rice malate permeable OsALMT7 influences panicle development and grain yield. A truncated OsALMT7 mutant, panicle apical abortion1 (paab1) lacking at least 2 transmembrane helices, mediates reduced malate efflux resulting in yield reducing. Here, we further investigated the contribution of OsALMT7 transmembrane helices to channel activity, using heterologous expression in Xenopus laevis oocytes. We further found that OsALMT7 formed as a homomer by co-expressing OsALMT7 and paab1 proteins in oocytes and detecting the physical interaction between two OsALMT7, and between OsALMT7 and paab1 mutant protein. Further study proved that not just OsALMT7, mutants of TaALMT1 inhibit wild-type TaALMT1 channel, indicating that ALMTs might perform channel function as homomers. Our discovery brings a light for ion channel structure and homomultimer regulation understanding for ALMT anion channels and potential for crop grain yield and stress response improvement in the context of the essential role of ALMTs in these plant processes.

Aluminum induced malate transporter (ALMT1) is regulating the Aluminum stress tolerance responses of mungbean seedlings

Under acidic soil and Aluminum (Al) stress conditions, the crop plants are facing several growth defects. Particularly significant decrease in root growth, nutrient uptake and low yield are the very common problems. Besides, several organic acid transporters are involved in Al sensing, transport and detoxification mechanisms. In this study, Al stress tolerance of mungbean plants was studied under different Al stress conditions. The results showed that the mungbean plants are severely affected by Al stress and released significant amount of malate in hydroponics media when compared with control plants. Based on this study and expression analysis of Al stress responsive genes, further, we developed ALMT1 overexpressors and ALMT1-RNAi transgenic plants of mungbean to analyse the acid soil tolerance. Later, these transgenic mungbean plants were challenged with different Al concentrations to check their stress responses. The ALMT1-RNAi lines showed almost no root growth variations in hydroponics media when compared to wild-type (WT) plants under normal growth conditions but displayed significant decrease in root length on exposure to Al. Further, when AtALMT1 was overexpressed, even in absence of Al ions, there was approximately 12% increase in length of primary roots when compared with WT. Malate efflux was almost doubled in the AtALMT1 overexpressed plant lines whereas KD lines showed around 36% drop in malate efflux under Al stress conditions. Therefore, based on these observations, we concluded that VrALMT1 has a significant role on Al stress tolerance responses in mungbean plants.

Nitric Oxide Participates in Aluminum-Stress-Induced Pollen Tube Growth Inhibition in Tea (Camelliasinensis) by Regulating CsALMTs.

Nitric oxide (NO), as a signal molecule, is involved in the mediation of heavy-metal-stress-induced physiological responses in plants. In this study, we investigated the effect of NO on Camellia sinensis pollen tubes exposed to aluminum (Al) stress. Exogenous application of the NO donor decreased the pollen germination rate and pollen tube length and increased the malondialdehyde (MDA) content and antioxidant enzyme activities under Al stress. Simultaneously, the NO donor effectively increased NO content in pollen tube of C. sinensis under Al stress and could aggravate the damage of Al3+ to C. sinensis pollen tubes by promoting the uptake of Al3+. In addition, application of the NO-specific scavenger significantly alleviated stress damage in C. sinensis pollen tube under Al stress. Moreover, 18 CsALMT members from a key Al-transporting gene family were identified, which could be divided into four subclasses. Pearson correlation analysis showed the expression level of CsALMT8 showed significant positive correlation with the Al3+ concentration gradient and NO levels, but a significant negative correlation with pollen germination rate and pollen tube length. The expression level of CsALMT5 was negatively correlated with the Al3+ concentration gradient and NO level, and positively correlated with pollen germination rate and pollen tube length. The expression level of CsALMT17 showed a significant negative correlation with Al3+ concentration and NO content in pollen tubes, but significant positive correlation with pollen germination rate and pollen tube length. In conclusion, a complex signal network regulated by NO-mediated CsALMTs revealed that CsALMT8 was regulated by environmental Al3+ and NO to assist Al3+ entry into pollen tubes; CsALMT5 might be influenced by the Al3+ signal, stimulate malate efflux in vacuoles and chelate with Al3+ to detoxify Al in C. sinensis pollen tube.

Elevated CO2 improves phosphorus nutrition and growth of citrate-secreting wheat when grown under adequate phosphorus supply on an Al3+ -toxic soil.

Understanding how climate change affects the phosphorus (P) nutrition of crops grown on acid soils is important in optimizing the management of P, and to secure future food production on these soils. This study assessed the impact of elevated CO2 (eCO2 ) on the P nutrition of wheat (Triticum aestivum) grown on Al3+ -toxic and P-deficient soils or in hydroponics. The aluminium-resistant near-isogenic wheat lines EGA-Burke (malate efflux only) and EGA-Burke TaMATE1B (malate and citrate efflux) were grown under ambient (400 μmol mol-1 ) and elevated CO2 (800 μmol mol-1 ) in growth chambers for 4-6 weeks. Elevated CO2 enhanced shoot growth and total P uptake of both lines at P rates >250 mg kg-1 , which was associated with improved root biomass allocation and thus increased root growth, but these effects were not apparent at lower P rates. Elevated CO2 decreased specific P uptake (P uptake per unit root length) at P supply >250 mg kg-1 , but did not significantly affect external or internal P requirements. This effect on the specific P uptake was less for EGA-Burke TaMATE1B than for EGA-Burke, possibly due to the increased citrate efflux and decreased Al concentration in root tips of EGA-Burke TaMATE1B. Compared to EGA-Burke, citrate-exuding EGA-Burke TaMATE1B had greater shoot P concentration and greater specific P uptake. Elevated CO2 improved root growth, and thus total P uptake and plant production of both lines when high P alleviated Al3+ toxicity and improved P nutrition in acid soils. The decreased P uptake efficiency under eCO2 was less for EGA-Burke TaMATE1B than EGA-Burke. © 2022 Society of Chemical Industry.

Short-chain aldehydes increase aluminum retention and sensitivity by enhancing cell wall polysaccharide contents and pectin demethylation in wheat seedlings

Upon environmental stimuli, aldehydes are generated downstream of reactive oxygen species and thereby contribute to severe cell damage. In this study, using two wheat genotypes differing in aluminum (Al) tolerance, we investigated the effects of lipid peroxidation-derived aldehydes on cell wall composition and subsequent Al-binding capacities. The spatial accumulation of Al along wheat roots was found to the generation of reactive aldehydes, which are highly localized to the apical regions of roots. Elimination of aldehydes by carnosine significantly reduced Al contents in root tips, with a concomitant alleviation of root growth inhibition. In contrast, root growth and Al accumulation were exacerbated by application of the short-chain aldehyde (E)−2-hexenal. We further confirmed that cell wall binding capacity, rather than malate efflux or pH alteration strategies, is associated with the aldehyde-induced accumulation of Al. Scavenging of lipid-derived aldehydes reduced Al accumulation in the pectin and hemicellulose 1 (HC1) fractions of root cell walls, whereas exposure to (E)−2-hexenal promoted a further accumulation of Al, particularly in the cell wall HC1 fraction of the Al-sensitive genotype. Different strategies were introduced by pectin and HC1 to accumulate Al in response to aldehydes in wheat roots. Accumulation in pectin is based on a reduction of methylation levels in response to elevated pectin methylesterase activity and gene expression, whereas that in HC1 is associated with an increase in polysaccharide contents. These findings indicate that aldehydes exacerbate Al phytotoxicity by enhancing Al retention in cell wall polysaccharides.

EAR APICAL DEGENERATION1 regulates maize ear development by maintaining malate supply for apical inflorescence.

Ear length (EL) is a key trait that contributes greatly to grain yield in maize (Zea mays). While numerous quantitative trait loci for EL have been identified, few causal genes have been studied in detail. Here we report the characterization of ear apical degeneration1 (ead1) exhibiting strikingly shorter ears and the map-based cloning of the casual gene EAD1. EAD1 is preferentially expressed in the xylem of immature ears and encodes an aluminum-activated malate transporter localizing to the plasma membrane. We show that EAD1 is a malate efflux transporter and loss of EAD1 leads to lower malate contents in the apical part of developing inflorescences. Exogenous injections of malate rescued the shortened ears of ead1. These results demonstrate that EAD1 plays essential roles in regulating maize ear development by delivering malate through xylem vessels to the apical part of the immature ear. Overexpression of EAD1 led to greater EL and kernel number per row and the EAD1 genotype showed a positive association with EL in two different genetic segregating populations. Our work elucidates the critical role of EAD1 in malate-mediated female inflorescence development and provides a promising genetic resource for enhancing maize grain yield.

Structural basis of ALMT1-mediated aluminum resistance in Arabidopsis.

The plant aluminum (Al)-activated malate transporter ALMT1 mediates the efflux of malate to chelate the Al in acidic soils and underlies the plant Al resistance. Here we present cryo-electron microscopy (cryo-EM) structures of Arabidopsis thaliana ALMT1 (AtALMT1) in the apo, malate-bound, and Al-bound states at neutral and/or acidic pH at up to 3.0 Å resolution. The AtALMT1 dimer assembles an anion channel and each subunit contains six transmembrane helices (TMs) and six cytosolic α-helices. Two pairs of Arg residues are located in the center of the channel pore and contribute to malate recognition. Al binds at the extracellular side of AtALMT1 and induces conformational changes of the TM1-2 loop and the TM5-6 loop, resulting in the opening of the extracellular gate. These structures, along with electrophysiological measurements, molecular dynamic simulations, and mutagenesis study in Arabidopsis, elucidate the structural basis for Al-activated malate transport by ALMT1.

γ-Aminobutyrate (GABA) Regulated Plant Defense: Mechanisms and Opportunities.

Global climate change and associated adverse abiotic and biotic stress conditions affect plant growth and development, and agricultural sustainability in general. Abiotic and biotic stresses reduce respiration and associated energy generation in mitochondria, resulting in the elevated production of reactive oxygen species (ROS), which are employed to transmit cellular signaling information in response to the changing conditions. Excessive ROS accumulation can contribute to cell damage and death. Production of the non-protein amino acid γ-aminobutyrate (GABA) is also stimulated, resulting in partial restoration of respiratory processes and energy production. Accumulated GABA can bind directly to the aluminum-activated malate transporter and the guard cell outward rectifying K+ channel, thereby improving drought and hypoxia tolerance, respectively. Genetic manipulation of GABA metabolism and receptors, respectively, reveal positive relationships between GABA levels and abiotic/biotic stress tolerance, and between malate efflux from the root and heavy metal tolerance. The application of exogenous GABA is associated with lower ROS levels, enhanced membrane stability, changes in the levels of non-enzymatic and enzymatic antioxidants, and crosstalk among phytohormones. Exogenous GABA may be an effective and sustainable tolerance strategy against multiple stresses under field conditions.

How to resolve the enigma of diurnal malate remobilisation from the vacuole in plants with crassulacean acid metabolism?

Opening of stomata in plants with crassulacean acid metabolism (CAM) is mainly shifted to the night period when atmospheric CO2 is fixed by phosphoenolpyruvate carboxylase and stored as malic acid in the vacuole. As such, CAM plants ameliorate transpirational water losses and display substantially higher water-use efficiency compared with C3 and C4 plants. In the past decade significant technical advances have allowed an unprecedented exploration of genomes, transcriptomes, proteomes and metabolomes of CAM plants and efforts are ongoing to engineer the CAM pathway in C3 plants. Whilst research efforts have traditionally focused on nocturnal carboxylation, less information is known regarding the drivers behind diurnal malate remobilisation from the vacuole that liberates CO2 to be fixed by RuBisCo behind closed stomata. To shed more light on this process, we provide a stoichiometric analysis to identify potentially rate-limiting steps underpinning diurnal malate mobilisation and help direct future research efforts. Within this remit we address three key questions: Q1 Does light-dependent assimilation of CO2 via RuBisCo dictate the rate of malate mobilisation? Q2: Do the enzymes responsible for malate decarboxylation limit daytime mobilisation from the vacuole? Q3: Does malate efflux from the vacuole set the pace of decarboxylation?

Are Fatty Acids Gluconeogenic Precursors?

It is widely accepted that the tricarboxylic acid (TCA) cycle is a critical partner for gluconeogenesis (GNG) in hepatocytes. Although researchers in the 1950s showed, using radiolabeled long-chain fatty acids, that acetate derived from fatty acid β-oxidation contributes carbon to glucose, fatty acids are not included on lists of gluconeogenic precursors in many textbooks of biochemistry and nutritional biochemistry. Here, by following the flow of carbon atoms through the mitochondrial TCA cycle and into cytosolic GNG, it is shown that carbons in acetyl-CoA derived from fatty acid β-oxidation will be found in glucose. Specifically, it is evident that, after the condensation of acetyl-CoA and oxaloacetate (OAA) to make citrate at the start of the TCA cycle, the 2 carbons lost from the cycle as carbon dioxide come from OAA, not acetyl-CoA. Carbons from acetyl-CoA are retained as the cycle progresses toward malate, and when malate exits the mitochondrion for GNG, carbons that originated in acetyl-CoA and OAA are found to contribute equally to glucose. With influx of other critical precursors into the TCA cycle and efflux of malate into the cytosol for GNG, the TCA cycle is in balance. During fasting-induced GNG, there is a net gain of glucose in glucogenic cells; however, the fact that there is no net gain in the TCA cycle is irrelevant as far as precursors are concerned. Given the physiological importance of fat as a source of reserve energy, and knowing that some cell types rely on glucose as their primary supplier of energy, a role for fatty acids in glucose production aligns both with intuition and with evidence provided by a careful look at the biochemistry and older isotope studies. Hopefully, subsequent editions of textbooks will list fatty acids among the gluconeogenic precursors.

Jasmonate and aluminum crosstalk in tomato: Identification and expression analysis of WRKYs and ALMTs during JA/Al-regulated root growth

The WRKY transcription factors (TFs) are involved in aluminum (Al) stress and jasmonic acid (JA)-regulated resistance responses. WRKYs act as regulators of Al-activated malate transporter (ALMT) proteins (anion channels) by directly binding to their promoters and altering malate efflux, thereby regulating Al ion toxicity in plant roots. JA enhances Al-induced root growth inhibition in Arabidopsis. However, the relationship between WRKY and ALMT genes and their involvement in JA-mediated root growth inhibition during Al stress in tomato remain unknown. Here, we demonstrate a similar phenomenon that JA enhances Al-induced root growth inhibition in tomato (Solanum lycopersicum). By analyzing RNA-seq data and tissue-specific expression data from public databases, we selected 17 WRKY and 6 ALMT family genes to identify the genes participated in this process. The promoters of many of the selected genes contained MeJA responsive element, G-box (target site of MYC2, a core TF of JA signaling), and W-box (target site for WRKY). Quantitative real-time PCR was performed to evaluate the expression levels of selected WRKY and ALMT genes under AlCl3 and Methyl jasmonate (MeJA) treatment. SlMYC2-VIGS seedlings and jasmonic acid-insensitive1 (jai1) mutant were also employed to analyze the expression patterns of selected genes. We find that SlALMT3 is responsible for the crosstalk regulatory mechanism between Al and JA in root growth inhibition, and 6 SlWRKYs may act as the upstream regulators of SlALMT3 in this crosstalk response. This study is initial and informative in exploring the crosstalk regulatory mechanism between JA and Al in tomato.

LaALMT1 mediates malate release from phosphorus-deficient white lupin root tips and metal root to shoot translocation.

Under phosphorus (P) deficiency, Lupinus albus (white lupin) releases large amounts of organic acid anions from specialized root structures, so-called cluster or proteoid roots, to mobilize and acquire sparingly soluble phosphates from a restricted soil volume. The molecular mechanisms underlying this release and its regulation are, however, poorly understood. Here, we identified a gene belonging to the aluminium (Al)-activated malate transporter (ALMT) family that specifically contributes to malate, but not citrate release. This gene, LaALMT1, was most prominently expressed in the root apices under P deficiency, including those of cluster roots and was also detected in the root stele. Contrary to several ALMT homologs in other species, the expression was not stimulated, but moderately repressed by Al. Aluminium-independent malate currents were recorded from the plasma membrane localized LaALMT1 expressed in Xenopus oocytes. In composite lupins with transgenic roots, LaALMT1 was efficiently mutated by CRISPR-Cas9, leading to diminished malate efflux and lower xylem sap malate concentrations. When grown in an alkaline P-deficient soil, mutant shoot phosphate concentrations were similar, but iron and potassium concentrations were diminished in old leaves, suggesting a role for ALMT1 in metal root to shoot translocation, a function that was also supported by growth in hydroponics.

Melatonin ameliorates aluminum toxicity through enhancing aluminum exclusion and reestablishing redox homeostasis in roots of wheat.

Melatonin is a universal regulator modulating plant development and responses to abiotic stresses. The alteration and potential roles of melatonin in mediating aluminum (Al) tolerance were investigated in two wheat genotypes differing in Al resistance. Using the high-resolution mass spectrometry, we observed that melatonin contents in Xi Aimai-1 were 1.7-fold higher than that in Yangmai-5. Application of melatonin conferred Al resistance in both genotypes. Melatonin treatment scavenged reactive oxygen species (ROS) accumulation and alleviated Al-induced oxidative damage to lipids and proteins by stimulating antioxidant enzymes and augmenting antioxidants. Additionally, melatonin treatment decreased root tip-Al contents by 19.0% and 15.5% in Xi Aimai-1 and Yangmai-5, respectively. Malate efflux, however, was not altered by melatonin under Al stress. The amount of cell wall polysaccharide and pectin methylesterase activity was significantly increased by Al treatment; but suppressed by melatonin. Melatonin synthesis inhibitor, p-CPA, significantly increased the amount of the Al binding in cell walls of the tolerant genotype, whereas exogenous melatonin decreased cell wall Al content in the sensitive genotype. These results suggest that melatonin alleviated Al toxicity through augmenting antioxidants and inducing antioxidant enzymes to control ROS and enhancing exclusion of Al from root apex by altering cell wall polysaccharides in wheat.

A sterile hydroponic system for characterising root exudates from specific root types and whole-root systems of large crop plants

BackgroundPlant roots release a variety of organic compounds into the soil which alter the physical, chemical and biological properties of the rhizosphere. Root exudates are technically challenging to measure in soil because roots are difficult to access and exudates can be bound by minerals or consumed by microorganisms. Exudates are easier to measure with hydroponically-grown plants but, even here, simple compounds such as sugars and organic acids can be rapidly assimilated by microorganisms. Sterile hydroponic systems avoid this shortcoming but it is very difficult to maintain sterility for long periods especially for larger crop species. As a consequence, studies often use small model species such as Arabidopsis to measure exudates or use seedlings of crop plants which only have immature roots systems.ResultsWe developed a simple hydroponic system for cultivating large crop plants in sterile conditions for more than 30 days. Using this system wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) plants were grown in sterile conditions for 30 days by which time they had reached the six-leaf stage and developed mature root systems with seminal, nodal and lateral roots. To demonstrate the utility of this system we characterized the aluminium-activated exudation of malate from the major types of wheat roots for the first time. We found that all root types measured released malate but the amounts were two-fold greater from the seminal and nodal axile roots compared with the lateral roots. Additionally, we showed that this sterile growth system could be used to collect exudates from intact whole root systems of barley.ConclusionsWe developed a simple hydroponic system that enables cereal plants to be grown in sterile conditions for longer periods than previously recorded. Using this system we measured, for the first time, the aluminium-activated efflux of malate from the major types of wheat roots. We showed the system can also be used for collecting exudates from intact root systems of 30-day-old barley plants. This hydroponic system can be modified for various purposes. Importantly it enables the study of exudates from crop species with mature root systems.

AtALMT3 is Involved in Malate Efflux Induced by Phosphorus Deficiency in Arabidopsis thaliana Root Hairs.

Under phosphorus (P)-deficient conditions, organic acid secretion from roots plays an important role in P mobilization from insoluble P in the soil. In this study, we characterized AtALMT3, a homolog of the Arabidopsis thaliana aluminum-activated malate transporter family gene. Among the 14 AtALMT family genes, only AtALMT3 was significantly up-regulated in P-deficient roots. AtALMT3 promoter::β-glucuronidase is expressed in the epidermis in roots, especially in root hair cells. AtALMT3 protein was localized in the plasma membrane and in small vesicles. Fluorescence of AtALMT3::GFP was not observed on the vacuole membrane of protoplast after lysis, indicating that AtALMT3 localizes mainly in the plasma membrane. Compared with the wild-type (WT) line, malate exudation in the AtALMT3-knockdown line (atalmt3-1) and overexpression line (atalmt3-2) under P deficiency were, respectively, 37% and 126%. In contrast, no significant difference was found in citrate exudation among these lines. The complementation of the atalmt3-1 line with AtALMT3 recovered the malate exudation to the level of the WT. Taken together, these results suggest that AtALMT3 localized in root hair membranes is involved in malate efflux in response to P deficiency.

Elevated CO2 (free-air CO2 enrichment) increases grain yield of aluminium-resistant but not aluminium-sensitive wheat (Triticum aestivum) grown in an acid soil.

Soil acidity currently limits root growth and crop production in many regions, and climate change is leading to uncertainties regarding future food supply. However, it is unknown how elevated CO2 (eCO2) affects the performance of wheat crops in acid soils under field conditions. We investigated the effects of eCO2 on plant growth and yield of three pairs of near-isogenic hexaploid wheat lines differing in alleles of aluminium-resistant genes TaALMT1 (conferring root malate efflux) and TaMATE1B (conferring citrate efflux). Plants were grown until maturity in an acid soil under ambient CO2 (aCO2; 400 µmol mol-1) and eCO2 (550 µmol mol-1) in a soil free-air CO2 enrichment facility (SoilFACE). Growth parameters and grain yields were measured. Elevated CO2 increased grain yield of lines carrying TaMATE1B by 22 % and lines carrying only TaALMT1 by 31 %, but did not increase the grain yield of Al3+-sensitive lines. Although eCO2 promoted tiller formation, coarse root length and root biomass of lines carrying TaMATE1B, it did not affect ear number, and it therefore limited yield potential. By contrast, eCO2 decreased or did not change these parameters for lines carrying only TaALMT1, and enhanced biomass allocation to grains thereby resulting in increased grain yield. Despite TaMATE1B being less effective than TaALMT1 at conferring Al3+ resistance based on root growth, the gene promoted grain yield to a similar level to TaALMT1 when the plants were grown in acid soil. Furthermore, TaALMT1 and TaMATE1B were not additive in their effects. As atmospheric CO2 increases, it is critical that both Al3+-resistance genes (particularly TaALMT1) should be maintained in hexaploid wheat germplasm in order for yield increases from CO2 fertilization to be realized in acid soils.

The impact of elevated CO2 on acid-soil tolerance of hexaploid wheat (Triticum aestivum L.) genotypes varying in organic anion efflux

It is unclear how elevated CO2 (eCO2) affects the response of crops to soil acidity. This study examined the effect of eCO2 on acid-soil tolerance of hexaploid wheat genotypes that vary in Al3+ resistance due to differences in root efflux of citrate and malate. Three pairs of near-isogenic lines were grown for 24–25 days under ambient CO2 (400 µmol mol-1) and eCO2 (800 µmol mol-1) in acid soils and hydroponics with various Al3+ concentrations. The lines consisted of pairs that differed in alleles of the TaALMT1 and TaMATE1B genes. Plant growth parameters and rhizosphere soil properties were measured. Elevated CO2 increased the slope of negative correlations between root and shoot biomass and Al3+ concentration in the rhizosphere of a line that has an Al3+-sensitive allele of the TaALMT1 gene conditioning malate efflux (ES8), but did not change that of a near-isogenic sister line with an Al3+-resistant TaALMT1 allele (ET8). Elevated CO2 decreased the relative root length and biomass (% of limed soil) of a line that lacked both malate and citrate efflux (Egret), but did not affect lines that possessed either malate or citrate efflux. Elevated CO2 had no effect on either malate or citrate efflux from root tips of Al3+-resistant lines. Elevated CO2 also increased Al3+ concentration and decreased NH4+ concentration in rhizosphere soil, but decreased concentrations of Al and Zn in shoots. Elevated CO2 decreased acid-soil tolerance of Al3+sensitive genotypes but not of Al3+ resistant genotypes. Malate efflux played a dominant role in conferring acid-soil tolerance to hexaploid wheat.

Altered Expression of the Malate-Permeable Anion Channel OsALMT4 Reduces the Growth of Rice Under Low Radiance.

We examined the function of OsALMT4 in rice (Oryza sativa L.) which is a member of the aluminum-activated malate transporter family. Previous studies showed that OsALMT4 localizes to the plasma membrane and that expression in transgenic rice lines results in a constitutive release of malate from the roots. Here, we show that OsALMT4 is expressed widely in roots, shoots, flowers, and grain but not guard cells. Expression was also affected by ionic and osmotic stress, light and to the hormones ABA, IAA, and salicylic acid. Malate efflux from the transgenic plants over-expressing OsALMT4 was inhibited by niflumate and salicylic acid. Growth of transgenic lines with either increased OsALMT4 expression or reduced expression was measured in different environments. Light intensity caused significant differences in growth between the transgenic lines and controls. When day-time light was reduced from 700 to 300 μmol m-2s-1 independent transgenic lines with either increased or decreased OsALMT4 expression accumulated less biomass compared to their null controls. This response was not associated with differences in photosynthetic capacity, stomatal conductance or sugar concentrations in tissues. We propose that by disrupting malate fluxes across the plasma membrane carbon partitioning and perhaps signaling are affected which compromises growth under low light. We conclude that OsALMT4 is expressed widely in rice and facilitates malate efflux from different cell types. Altering OsALMT4 expression compromises growth in low-light environments.

Aluminum-Activated Malate Transporters Can Facilitate GABA Transport.

Plant aluminum-activated malate transporters (ALMTs) are currently classified as anion channels; they are also known to be regulated by diverse signals, leading to a range of physiological responses. Gamma-aminobutyric acid (GABA) regulation of anion flux through ALMT proteins requires a specific amino acid motif in ALMTs that shares similarity with a GABA binding site in mammalian GABAA receptors. Here, we explore why TaALMT1 activation leads to a negative correlation between malate efflux and endogenous GABA concentrations ([GABA]i) in both wheat (Triticum aestivum) root tips and in heterologous expression systems. We show that TaALMT1 activation reduces [GABA]i because TaALMT1 facilitates GABA efflux but GABA does not complex Al3+ TaALMT1 also leads to GABA transport into cells, demonstrated by a yeast complementation assay and via 14C-GABA uptake into TaALMT1-expressing Xenopus laevis oocytes; this was found to be a general feature of all ALMTs we examined. Mutation of the GABA motif (TaALMT1F213C) prevented both GABA influx and efflux, and resulted in no correlation between malate efflux and [GABA]i We conclude that ALMTs are likely to act as both GABA and anion transporters in planta. GABA and malate appear to interact with ALMTs in a complex manner to regulate each other's transport, suggestive of a role for ALMTs in communicating metabolic status.