Role of phytohormones in aluminium rhizotoxicity

Differences in gas exchange, chlorophyll fluorescence, and modulated reflection of light at 820 nm between two rhododendron cultivars under aluminum stress conditions

Aluminum (Al) toxicity is an important factor restricting the normal growth of plants in acidic soil. Rhododendron (Ericaceae) can grow relatively well in acidic soil. To uncover the adaptive mechanisms of photosynthesis under Al stress, the influence of Al stress on the photosynthetic activities of Al-sensitive (Baijinpao) and Al-resistant (Kangnaixin) rhododendron cultivars was examined by measuring gas exchange, chlorophyll fluorescence, and the modulated reflection of light at 820 nm. Under Al stress conditions, the net photosynthetic rate and stomatal conductance of the rhododendron leaves decreased, whereas the intercellular CO2 concentration increased. The Al stress treatment damaged the oxygen-evolving complex of the rhododendron seedlings, while also inhibiting electron transport on the photosystem II (PSII) donor side. In addition, the exposure to Al stress restricted the oxidation of plastocyanin (PC) and the photosystem I (PSI) reaction center (P700) and led to the re-reduction of PC+ and P700+. The comparison with Kangnaixin revealed an increase in the PSII connectivity in Baijinpao. Additionally, the donor-side electron transport efficiency was more inhibited and the overall activity of PSII, PSI, and the intersystem electron transport chain decreased more extensively in Baijinpao than in Kangnaixin. On the basis of the study findings, we concluded that Al stress adversely affects photosynthesis in rhododendron seedlings by significantly decreasing the activity of PSII and PSI. Under Al stress, Kangnaixin showed stronger tolerance compared with Baijinpao.

Aluminium stress tolerance by Citrus plants: a consolidated review

Aluminium, a metallic element abundant in soils as aluminosilicates minerals, poses a toxic threat to plants, particularly in acidic soil conditions, thereby affecting their growth and development. Given their adaptability to diverse soil and climate conditions, Citrus plants have gained significant attention regarding their tolerance to Aluminium toxicity. In the North-eastern region of India, where soils are often slightly acidic with elevated aluminium levels, Citrus species are predominantly found. Understanding the tolerance mechanisms of these Citrus fruits and screening wild Citrus species for their adaptability to abiotic stresses is crucial for enhancing fruit production. Numerous investigations have demonstrated that Citrus species exhibit remarkable tolerance to aluminium contamination, surpassing the typical threshold of 30% incidence. When cultivated in acidic soils, Citrus plants encounter restricted root growth and reduced nutrient and moisture uptake, leading to various nutrient deficiency symptoms. However, promisingly, certain Citrus species such as Citrus jambhiri (Rough lemon), Poncirus trifoliata, Citrus sinensis, and Citrus grandis have shown considerable aluminium tolerance. This comprehensive review delves into the subject of aluminium toxicity and its implications, while also shedding light on the mechanisms through which Citrus plants develop tolerance to this element.

Gibberellin-Mediated Sensitivity of Rice Roots to Aluminum Stress

Aluminum toxicity poses a significant constraint on crop production in acidic soils. While phytohormones are recognized for their pivotal role in mediating plant responses to aluminum stress, the specific involvement of gibberellin (GA) in regulating aluminum tolerance remains unexplored. In this study, we demonstrate that external GA exacerbates the inhibitory impact of aluminum stress on root growth of rice seedlings, concurrently promoting reactive oxygen species (ROS) accumulation. Furthermore, rice plants overexpressing the GA synthesis gene SD1 exhibit enhanced sensitivity to aluminum stress. In contrast, the slr1 gain-of-function mutant, characterized by impeded GA signaling, displays enhanced tolerance to aluminum stress, suggesting the negative regulatory role of GA in rice resistance to aluminum-induced toxicity. We also reveal that GA application suppresses the expression of crucial aluminum tolerance genes in rice, including Al resistance transcription factor 1 (ART1), Nramp aluminum transporter 1 (OsNramp4), and Sensitive to Aluminum 1 (SAL1). Conversely, the slr1 mutant exhibits up-regulated expression of these genes compared to the wild type. In summary, our results shed light on the inhibitory effect of GA in rice resistance to aluminum stress, contributing to a theoretical foundation for unraveling the intricate mechanisms of plant hormones in regulating aluminum tolerance.

Aluminum-Immobilizing Rhizobacteria Modulate Root Exudation and Nutrient Uptake and Increase Aluminum Tolerance of Pea Mutant E107 (brz)

It is well known that plant-growth-promoting rhizobacteria (PGPRs) increase the tolerance of plants to abiotic stresses; however, the counteraction of Al toxicity has received little attention. The effects of specially selected Al-tolerant and Al-immobilizing microorganisms were investigated using pea cultivar Sparkle and its Al-sensitive mutant E107 (brz). The strain Cupriavidus sp. D39 was the most-efficient in the growth promotion of hydroponically grown peas treated with 80 µM AlCl₃, increasing the plant biomass of Sparkle by 20% and of E107 (brz) by two-times. This strain immobilized Al in the nutrient solution and decreased its concentration in E107 (brz) roots. The mutant showed upregulated exudation of organic acids, amino acids, and sugars in the absence or presence of Al as compared with Sparkle, and in most cases, the Al treatment stimulated exudation. Bacteria utilized root exudates and more actively colonized the root surface of E107 (brz). The exudation of tryptophan and the production of IAA by Cupriavidus sp. D39 in the root zone of the Al-treated mutant were observed. Aluminum disturbed the concentrations of nutrients in plants, but inoculation with Cupriavidus sp. D39 partially restored such negative effects. Thus, the E107 (brz) mutant is a useful tool for studying the mechanisms of plant-microbe interactions, and PGPR plays an important role in protecting plants against Al toxicity.

High abscisic acid and low root hydraulic conductivity may explain low leaf hydration in 'Mandarin' lime exposed to aluminum

The first symptom of aluminum (Al) toxicity is the inhibition of root growth, which has been associated with low leaf hydration, with negative consequences for leaf gas exchange including stomatal conductance (gs) observed in many plant species. Here we asked whether low leaf hydration occurs before or after the inhibition of root growth of Citrus × limonia Osbeck ('Mandarin' lime) cultivated for 60 days in nutrient solution with 0 and 1480 μM Al. The length, diameter, surface area and biomass of roots of plants exposed to Al were lower than control plants only at 30 days after treatments (DAT). Until the end of the study, estimated gs (measured by sap flow techniques) was lower than in control plants from 3 DAT, total plant transpiration (Eplant) and root hydraulic conductivity (Lpr) at 7 DAT, and midday leaf water potential (Ψmd) and relative leaf water content at 15 DAT. Abscisic acid (ABA) in leaves was twofold higher in Al-exposed plants 1 DAT, and in roots a twofold higher peak was observed at 15 DAT. As ABA in leaves approached values of control plants after 15 DAT, we propose that low gs of plants exposed to Al is primarily caused by ABA, and the maintenance of low gs could be ascribed to the low Lpr from 7 DAT until the end of the study. Therefore, the low leaf hydration in 'Mandarin' lime exposed to Al does not seem to be caused by root growth inhibition or by a simple consequence of low water uptake due to a stunted root system.

Elevated pH-mediated mitigation of aluminum-toxicity in sweet orange (Citrus sinensis) roots involved the regulation of energy-rich compounds and phytohormones

For the first time, we used targeted metabolome to investigate the effects of pH-aluminum (Al) interactions on energy-rich compounds and their metabolites (ECMs) and phytohormones in sweet orange (Citrus sinensis) roots. The concentration of total ECMs (TECMs) was reduced by Al-toxicity in 4.0-treated roots, but unaffected significantly in pH 3.0-treated roots. However, the concentrations of most ECMs and TECMs were not lower in pH 4.0 + 1.0 mM Al-treated roots (P4AR) than in pH 3.0 + 1.0 mM Al-treated roots (P3AR). Increased pH improved the adaptability of ECMs to Al-toxicity in roots. For example, increased pH improved the utilization efficiency of ECMs and the conversion of organic phosphorus (P) from P-containing ECMs into available phosphate in Al-treated roots. We identified upregulated cytokinins (CKs), downregulated jasmonic acid (JA), methyl jasmonate (MEJA) and jasmonates (JAs), and unaltered indole-3-acetic acid (IAA) and salicylic acid (SA) in P3AR vs pH 3.0 + 0 mM Al-treated roots (P3R); upregulated JA, JAs and IAA, downregulated total CKs, and unaltered MEJA and SA in P4AR vs pH 4.0 + 0 mM Al-treated roots (P4R); and upregulated CKs, downregulated JA, MEJA, JAs and SA, and unaltered IAA in P3AR vs P4AR. Generally viewed, raised pH-mediated increments of JA, MEJA, total JAs, SA and IAA concentrations and reduction of CKs concentration in Al-treated roots might help to maintain nutrient homeostasis, increase Al-toxicity-induced exudation of organic acid anions and the compartmentation of Al in vacuole, and reduce oxidative stress and Al uptake, thereby conferring root Al-tolerance. In short, elevated pH-mediated mitigation of root Al-stress involved the regulation of ECMs and phytohormones.

Identification of microRNAs Responding to Aluminium, Cadmium and Salt Stresses in Barley Roots

Plants are frequently exposed to various abiotic stresses, including aluminum, cadmium and salinity stress. Barley (Hordeum vulgare) displays wide genetic diversity in its tolerance to various abiotic stresses. In this study, small RNA and degradome libraries from the roots of a barley cultivar, Golden Promise, treated with aluminum, cadmium and salt or controls were constructed to understand the molecular mechanisms of microRNAs in regulating tolerance to these stresses. A total of 525 microRNAs including 198 known and 327 novel members were identified through high-throughput sequencing. Among these, 31 microRNAs in 17 families were responsive to these stresses, and Gene Ontology (GO) analysis revealed that their targeting genes were mostly highlighted as transcription factors. Furthermore, five (miR166a, miR166a-3p, miR167b-5p, miR172b-3p and miR390), four (MIR159a, miR160a, miR172b-5p and miR393) and three (miR156a, miR156d and miR171a-3p) microRNAs were specifically responsive to aluminum, cadmium and salt stress, respectively. Six miRNAs, i.e., miR156b, miR166a-5p, miR169a, miR171a-5p, miR394 and miR396e, were involved in the responses to the three stresses, with different expression patterns. A model of microRNAs responding to aluminum, cadmium and salt stresses was proposed, which may be helpful in comprehensively understanding the mechanisms of microRNAs in regulating stress tolerance in barley.

Alleviating aluminum toxicity in plants: Implications of reactive oxygen species signaling and crosstalk with other signaling pathways

Aluminum (Al) toxicity is a major limiting factor for plant growth and productivity in acidic soil. At pH lower than 5.0 (pH < 5.0), the soluble and toxic form of Al (Al³⁺ ions) enters root cells and inhibits root growth and uptake of water and nutrients. The organic acids malate, citrate, and oxalate are secreted by the roots and chelate Al³⁺ to form a non-toxic Al-OA complex, which decreases the entry of Al³⁺ into the root cells. When Al³⁺ enters, it leads to the production of reactive oxygen species (ROS) in cells, which are toxic and cause damage to biomolecules like lipids, carbohydrates, proteins, and nucleic acids. When ROS levels rise beyond the threshold, plants activate an antioxidant defense system that comprises of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione S-transferase (GST), ascorbic acid (ASA), phenolics and alkaloids etc., which protect plant cells from oxidative damage by scavenging and neutralizing ROS. Besides, ROS also play an important role in signal transduction and influence many molecular and cellular process like hormone signaling, gene expression, cell wall modification, cell cycle, programed cell death (PCD), and development. In the present review, the mechanisms of Al-induced ROS generation, ROS signaling, and crosstalk with other signaling pathways helping to combat Al toxicity have been summarized, which will help researchers to understand the intricacies of Al-induced plant response at cellular level and plan research for developing Al-toxicity tolerant crops for sustainable agriculture in acid soil-affected regions of the world.

Phytohormone signalling and cross-talk to alleviate aluminium toxicity in plants

Aluminium (Al) is one of the most abundant metals in earth crust, which becomes toxic to the plants growing in acidic soil. Phytohormones like ethylene, auxin, cytokinin, abscisic acid, jasmonic acid and gibberellic acid are known to play important role in regulating Al toxicity tolerance in plants. Exogenous applications of auxin, cytokinin and abscisic acid have shown significant effect on Al-induced root growth inhibition. Moreover, ethylene and cytokinin act synergistically with auxin in responding against Al toxicity. A number of studies showed that phytohormones play vital roles in controlling root responses to Al toxicity by modulating reactive oxygen species (ROS) signalling, cell wall modifications, organic acid exudation from roots and expression of Al responsive genes and transcription factors. This review provides a summary of recent studies related to involvement of phytohormone signalling and cross-talk with other pathways in regulating response against Al toxicity in plants.

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.

Recent Advances in Understanding Mechanisms of Plant Tolerance and Response to Aluminum Toxicity

Aluminum (Al) toxicity is a major environmental stress that inhibits plant growth and development. There has been impressive progress in recent years that has greatly increased our understanding of the nature of Al toxicity and its mechanisms of tolerance. This review describes the transcription factors (TFs) and plant hormones involved in the adaptation to Al stress. In particular, it discusses strategies to confer plant resistance to Al stress, such as transgenic breeding, as well as small molecules and plant growth-promoting rhizobacteria (PGPRs) to alleviate Al toxicity. This paper provides a theoretical basis for the enhancement of plant production in acidic soils.

Computational Systems Biology of Alfalfa - Bacterial Blight Host-Pathogen Interactions: Uncovering the Complex Molecular Networks for Developing Durable Disease Resistant Crop

Medicago sativa (also known as alfalfa), a forage legume, is widely cultivated due to its high yield and high-value hay crop production. Infectious diseases are a major threat to the crops, owing to huge economic losses to the agriculture industry, worldwide. The protein-protein interactions (PPIs) between the pathogens and their hosts play a critical role in understanding the molecular basis of pathogenesis. Pseudomonas syringae pv. syringae ALF3 suppresses the plant's innate immune response by secreting type III effector proteins into the host cell, causing bacterial stem blight in alfalfa. The alfalfa-P. syringae system has little information available for PPIs. Thus, to understand the infection mechanism, we elucidated the genome-scale host-pathogen interactions (HPIs) between alfalfa and P. syringae using two computational approaches: interolog-based and domain-based method. A total of ∼14 M putative PPIs were predicted between 50,629 alfalfa proteins and 2,932 P. syringae proteins by combining these approaches. Additionally, ∼0.7 M consensus PPIs were also predicted. The functional analysis revealed that P. syringae proteins are highly involved in nucleotide binding activity (GO:0000166), intracellular organelle (GO:0043229), and translation (GO:0006412) while alfalfa proteins are involved in cellular response to chemical stimulus (GO:0070887), oxidoreductase activity (GO:0016614), and Golgi apparatus (GO:0005794). According to subcellular localization predictions, most of the pathogen proteins targeted host proteins within the cytoplasm and nucleus. In addition, we discovered a slew of new virulence effectors in the predicted HPIs. The current research describes an integrated approach for deciphering genome-scale host-pathogen PPIs between alfalfa and P. syringae, allowing the researchers to better understand the pathogen's infection mechanism and develop pathogen-resistant lines.

A multidrug and toxic compound extrusion (MATE) transporter modulates auxin levels in root to regulate root development and promotes aluminium tolerance

MATE (multidrug and toxic compound extrusion) transporters play multiple roles in plants including detoxification, secondary metabolite transport, aluminium (Al) tolerance, and disease resistance. Here we identify and characterize the role of the Arabidopsis MATE transporter DETOXIFICATION30. AtDTX30 regulates auxin homeostasis in Arabidopsis roots to modulate root development and Al-tolerance. DTX30 is primarily expressed in roots and localizes to the plasma membrane of root epidermal cells including root hairs. dtx30 mutants exhibit reduced elongation of the primary root, root hairs, and lateral roots. The mutant seedlings accumulate more auxin in their root tips indicating role of DTX30 in maintaining auxin homeostasis in the root. Al induces DTX30 expression and promotes its localization to the distal transition zone. dtx30 seedlings accumulate more Al in their roots but are hyposensitive to Al-mediated rhizotoxicity perhaps due to saturation in root growth inhibition. Increase in expression of ethylene and auxin biosynthesis genes in presence of Al is absent in dtx30. The mutants exude less citrate under Al conditions, which might be due to misregulation of AtSTOP1 and the citrate transporter AtMATE. In conclusion, DTX30 modulates auxin levels in root to regulate root development and in the presence of Al indirectly modulates citrate exudation to promote Al tolerance.

Polyamines modulate aluminum-induced oxidative stress differently by inducing or reducing H2O2 production in wheat

Polyamines are important bioactive molecules involved in regulating H₂O₂ homeostasis, which is recognized as a major stimulus of oxidative stress under aluminum (Al) exposure. In this study, we investigated the involvement of spermidine oxidation in Al-induced oxidative stress, and its modulation by exogenous putrescine (Put) in two wheat genotypes differing in Al tolerance. Aluminum caused more severe oxidative damage at the root apexes in the Al-sensitive genotype Yangmai-5 than in the tolerant Xi Aimai-1, but these effects were significantly reversed by exogenous Put and polyamine oxidase (PAO) inhibitors. Aluminum caused a more significant increase in cell wall-bound PAO (CW-PAO) activity in Yangmai-5 than in Xi Aimai-1. Inhibiting of CW-PAO reduced H₂O₂ accumulation, restored Spd decline in both genotypes, indicating its potential role in Al-induced H₂O₂ production through catalyzing Spd oxidation. Additionally, Al significantly increased the activity of plasma membrane-NADPH oxidase, another H₂O₂ generator, in wheat roots. Put application significantly inhibited the activity of CW-PAO and plasma membrane-NADPH oxidase, and reduced H₂O₂ accumulation in Al-stressed wheat roots. Antioxidant enzymes were significantly stimulated by Al, but not Put. Overall, Put may protect wheat roots against Al-induced oxidative stress through regulating H₂O₂ production by inhibiting CW-PAO and plasma membrane-NADPH oxidase.

Endophytic Microbial Consortia of Phytohormones-Producing Fungus Paecilomyces formosus LHL10 and Bacteria Sphingomonas sp. LK11 to Glycine max L. Regulates Physio-hormonal Changes to Attenuate Aluminum and Zinc Stresses

The compatible microbial consortia containing fungal and bacterial symbionts acting synergistically are applied to improve plant growth and eco-physiological responses in extreme crop growth conditions. However, the interactive effects of phytohormones-producing endophytic fungal and bacterial symbionts plant growth and stress tolerance under heavy metal stress have been least known. In the current study, the phytohormones-producing endophytic Paecilomyces formosus LHL10 and Sphingomonas sp. LK11 revealed potent growth and tolerance during their initial screening against combined Al and Zn (2.5 mM each) stress. This was followed with their co-inoculation in the Al- and Zn-stressed Glycine max L. plants, showing significantly higher plant growth attributes (shoot/root length, fresh/dry weight, and chlorophyll content) than the plants solely inoculated with LHL10 or LK11 and the non-inoculated (control) plants under metal stresses. Interestingly, under metal stress, the consortia exhibited lower metal uptake and inhibited metal transport in roots. Metal-induced oxidative stresses were modulated in co-inoculated plants through reduced hydrogen peroxide, lipid peroxidation, and antioxidant enzymes (catalase and superoxide dismutase) in comparison to the non-inoculated plants. In addition, endophytic co-inoculation enhanced plant macronutrient uptake (P, K, S, and N) and modulated soil enzymatic activities under stress conditions. It significantly downregulated the expression of heavy metal ATPase genes GmHMA13, GmHMA18, GmHMA19, and GmPHA1 and upregulated the expression of an ariadne-like ubiquitin ligase gene GmARI1 under heavy metals stress. Furthermore, the endogenous phytohormonal contents of co-inoculated plants revealed significantly enhanced gibberellins and reduced abscisic acid and jasmonic acid contents, suggesting that this endophytic interaction mitigated the adverse effect of metal stresses in host plants. In conclusion, the co-inoculation of the endophytic fungus LHL10 and bacteria LK11 actively contributed to the tripartite mutualistic symbiosis in G. max under heavy metal stresses; this could be used an excellent strategy for sustainable agriculture in the heavy metal-contaminated fields.

Role of Phytohormones in Piriformospora indica-Induced Growth Promotion and Stress Tolerance in Plants: More Questions Than Answers

Phytohormones play vital roles in the growth and development of plants as well as in interactions of plants with microbes such as endophytic fungi. The endophytic root-colonizing fungus Piriformospora indica promotes plant growth and performance, increases resistance of colonized plants to pathogens, insects and abiotic stress. Here, we discuss the roles of the phytohormones (auxins, cytokinin, gibberellins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids) in the interaction of P. indica with higher plant species, and compare available data with those from other (beneficial) microorganisms interacting with roots. Crosstalks between different hormones in balancing the plant responses to microbial signals is an emerging topic in current research. Furthermore, phytohormones play crucial roles in systemic signal propagation as well as interplant communication. P. indica interferes with plant hormone synthesis and signaling to stimulate growth, flowering time, differentiation and local and systemic immune responses. Plants adjust their hormone levels in the roots in response to the microbes to control colonization and fungal propagation. The available information on the roles of phytohormones in beneficial root-microbe interactions opens new questions of how P. indica manipulates the plant hormone metabolism to promote the benefits for both partners in the symbiosis.

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.

Transcriptome Analysis of Al-Induced Genes in Buckwheat (Fagopyrum esculentum Moench) Root Apex: New Insight into Al Toxicity and Resistance Mechanisms in an Al Accumulating Species

Relying on Al-activated root oxalate secretion, and internal detoxification and accumulation of Al, buckwheat is highly Al resistant. However, the molecular mechanisms responsible for these processes are still poorly understood. It is well-known that root apex is the critical region of Al toxicity that rapidly impairs a series of events, thus, resulting in inhibition of root elongation. Here, we carried out transcriptome analysis of the buckwheat root apex (0-1 cm) with regards to early response (first 6 h) to Al stress (20 μM), which is crucial for identification of both genes and processes involved in Al toxicity and tolerance mechanisms. We obtained 34,469 unigenes with 26,664 unigenes annotated in the NCBI database, and identified 589 up-regulated and 255 down-regulated differentially expressed genes (DEGs) under Al stress. Functional category analysis revealed that biological processes differ between up- and down-regulated genes, although 'metabolic processes' were the most affected category in both up- and down-regulated DEGs. Based on the data, it is proposed that Al stress affects a variety of biological processes that collectively contributes to the inhibition of root elongation. We identified 30 transporter genes and 27 transcription factor (TF) genes induced by Al. Gene homology analysis highlighted candidate genes encoding transporters associated with Al uptake, transport, detoxification, and accumulation. We also found that TFs play critical role in transcriptional regulation of Al resistance genes in buckwheat. In addition, gene duplication events are very common in the buckwheat genome, suggesting a possible role for gene duplication in the species' high Al resistance. Taken together, the transcriptomic analysis of buckwheat root apex shed light on the processes that contribute to the inhibition of root elongation. Furthermore, the comprehensive analysis of both transporter genes and TF genes not only deep our understanding on the responses of buckwheat roots to Al toxicity but provide a good start for functional characterization of genes critical for Al tolerance.