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

GmMYB183, a R2R3-MYB Transcription Factor in Tamba Black Soybean (Glycine max. cv. Tamba), Conferred Aluminum Tolerance in Arabidopsis and Soybean

Aluminum (Al) toxicity is one of the environmental stress factors that affects crop growth, development, and productivity. MYB transcription factors play crucial roles in responding to biotic or abiotic stresses. However, the roles of MYB transcription factors in Al tolerance have not been clearly elucidated. Here, we found that GmMYB183, a gene encoding a R2R3 MYB transcription factor, is involved in Al tolerance. Subcellular localization studies revealed that GmMYB183 protein is located in the nucleus, cytoplasm and cell membrane. Overexpression of GmMYB183 in Arabidopsis and soybean hairy roots enhanced plant tolerance towards Al stress compared to the wild type, with higher citrate secretion and less Al accumulation. Furthermore, we showed that GmMYB183 binds the GmMATE75 gene promoter encoding for a plasma-membrane-localized citrate transporter. Through a dual-luciferase reporter system and yeast one hybrid, the GmMYB183 protein was shown to directly activate the transcription of GmMATE75. Furthermore, the expression of GmMATE75 may depend on phosphorylation of Ser36 residues in GmMYB183 and two MYB sites in P3 segment of the GmMATE75 promoter. In conclusion, GmMYB183 conferred Al tolerance by promoting the secretion of citrate, which provides a scientific basis for further elucidating the mechanism of plant Al resistance.

Citrus sinensis manganese tolerance: Insight from manganese-stimulated secretion of root exudates and rhizosphere alkalization

We used manganese (Mn)-tolerant 'Xuegan' (Citrus sinensis) seedlings as materials and examined the characterization of Mn uptake and Mn-activated-release of root exudates under hydroponic conditions. We observed that root and shoot Mn bioaccumulation factor (BCF) reduced with the increase of Mn supply, and that Mn transfer factor (Tf) reduced greatly as Mn supply increased from 0 to 500 μM, beyond which Tf slightly increased with increasing Mn supply, suggesting that Mn supply reduced the ability to absorb and accumulate Mn in roots and shoots, as well as root-to-shoot Mn translocation. Without Mn, roots alkalized the solution pH from 5.0 to above 6.2, while Mn supply reduced root-induced alkalization. As Mn supply increased from 0 to 2000 μM, the secretion of root total phenolics (TPs) increased, while the solution pH decreased. Mn supply did not alter the secretion of root total free amino acids, total soluble sugars, malate, and citrate. Mn-activated-release of TPs was inhibited by low temperature and anion channel inhibitors, but not by protein biosynthesis inhibitor. Using widely targeted metabolome, we detected 48 upregulated [35 upregulated phenolic compounds + 13 other secondary metabolites (SMs)] and three downregulated SMs, and 39 upregulated and eight downregulated primary metabolites (PMs). These findings suggested that reduced ability to absorb and accumulate Mn in roots and shoots and less root-to-shoot Mn translocation in Mn-toxic seedlings, rhizosphere alkalization, and Mn-activated-release of root exudates (especially phenolic compounds) contributed to the high Mn tolerance of C. sinensis seedlings.

Regulation on copper-tolerance in Citrus sinensis seedlings by boron addition: Insights from root exudates, related metabolism, and gene expression

Boron (B) can alleviate Citrus copper (Cu)-toxicity. However, the underlying mechanism by which B mitigates Cu-toxicity is unclear. 'Xuegan' (Citrus sinensis) seedlings were exposed to 0.5 (control) or 350 (Cu-toxicity) µM Cu and 2.5 or 25 µM B for 24 weeks. Thereafter, we investigated the secretion of low molecular weight compounds [LMWCs; citrate, malate, total soluble sugars (TSS), total phenolics (TP), and total free amino acids (TFAA)] by excised roots and their concentrations in roots and leaves, as well as related enzyme gene expression and activities in roots and leaves. Cu-stress stimulated root release of malate and TFAA, which might contribute to citrus Cu-tolerance. However, B-mediated-mitigation of Cu-stress could not be explained in this way, since B addition failed to further stimulate malate and TFAA secretion. Indeed, B addition decreased Cu-stimulated-secretion of malate. Further analysis suggested that Cu-induced-exudation of malate and TFAA was not regulated by their levels in roots. By contrast, B addition increased malate, citrate, and TFAA concentrations in Cu-toxic roots. Cu-toxicity increased TP concentration in 25 μM B-treated leaves, but not in 2.5 μM B-treated leaves. Our findings suggested that the internal detoxification of Cu by LMWCs played a role in B-mediated-alleviation of Cu-toxicity.

Combining transcriptomics and metabolomics to identify key response genes for aluminum toxicity in the root system of Brassica napus L. seedlings

By integrating QTL mapping, transcriptomics and metabolomics, 138 hub genes were identified in rapeseed root response to aluminum stress and mainly involved in metabolism of lipids, carbohydrates and secondary metabolites. Aluminum (Al) toxicity has become one of the important abiotic stress factors in areas with acid soil, which hinders the absorption of water and nutrients by roots, and consequently retards the growth of crops. A deeper understanding of the stress-response mechanism of Brassica napus may allow us to identify the tolerance gene(s) and use this information in breeding-resistant crop varieties. In this study, a population of 138 recombinant inbred lines (RILs) was subjected to aluminum stress, and QTL (quantitative trait locus) mapping was used to preliminarily locate quantitative trait loci related to aluminum stress. Root tissues from seedlings of an aluminum-resistant (R) line and an aluminum-sensitive (S) line from the RIL population were harvested for transcriptome sequencing and metabolome determination. By combining the data on quantitative trait genes (QTGs), differentially expressed genes (DEGs), and differentially accumulated metabolites (DAMs), key candidate genes related to aluminum tolerance in rapeseed were determined. The results showed that there were 3186 QTGs in the RIL population, 14,232 DEGs and 457 DAMs in the comparison between R and S lines. Lastly, 138 hub genes were selected to have a strong positive or negative correlation with 30 important metabolites (|R|≥ 0.95). These genes were mainly involved in the metabolism of lipids, carbohydrates and secondary metabolites in response to Al toxicity stress. In summary, this study provides an effective method for screening key genes by combining QTLs, transcriptome sequencing and metabolomic analysis, but also lists key genes for exploring the molecular mechanism of Al tolerance in rapeseed seedling roots.

Speciation and distribution of chromium (III) in rice root tip and mature zone: The significant impact of root exudation and iron plaque on chromium bioavailability

Evidence on the contribution of root regions with varied maturity levels in iron plaque (IP) formation and root exudation of metabolites and their consequences for uptake and bioavailability of chromium (Cr) remains unknown. Therefore, we applied combined nanoscale secondary ion mass spectrometry (NanoSIMS) and synchrotron-based techniques, micro-X-ray fluorescence (µ-XRF) and micro-X-ray absorption near-edge structure (µ-XANES) to examine the speciation and localisation of Cr and the distribution of (micro-) nutrients in rice root tip and mature region. µ-XRF mapping revealed that the distribution of Cr and (micro-) nutrients varied between root regions. Cr K-edge XANES analysis at Cr hotspots attributed the dominant speciation of Cr in outer (epidermal and sub-epidermal) cell layers of the root tips and mature root to Cr(III)-FA (fulvic acid-like anions) (58-64%) and Cr(III)-Fh (amorphous ferrihydrite) (83-87%) complexes, respectively. The co-occurrence of a high proportion of Cr(III)-FA species and strong co-location signals of ⁵²Cr¹⁶O and ¹³C¹⁴N in the mature root epidermis relative to the sub-epidermis indicated an association of Cr with active root surfaces, where the dissolution of IP and release of their associated Cr are likely subject to the mediation of organic anions. The results of NanoSIMS (poor ⁵²Cr¹⁶O and ¹³C¹⁴N signals), dissolution (no IP dissolution) and µ-XANES (64% in sub-epidermis >58% in the epidermis for Cr(III)-FA species) analyses of root tips may be indicative of the possible re-uptake of Cr by this region. The results of this research work highlight the significance of IP and organic anions in rice root systems on the bioavailability and dynamics of heavy metals (e.g. Cr).

Aluminum and Fluoride Stresses Altered Organic Acid and Secondary Metabolism in Tea (Camellia sinensis) Plants: Influences on Plant Tolerance, Tea Quality and Safety

Tea plants have adapted to grow in tropical acidic soils containing high concentrations of aluminum (Al) and fluoride (F) (as Al/F hyperaccumulators) and use secret organic acids (OAs) to acidify the rhizosphere for acquiring phosphorous and element nutrients. The self-enhanced rhizosphere acidification under Al/F stress and acid rain also render tea plants prone to accumulate more heavy metals and F, which raises significant food safety and health concerns. However, the mechanism behind this is not fully understood. Here, we report that tea plants responded to Al and F stresses by synthesizing and secreting OAs and altering profiles of amino acids, catechins, and caffeine in their roots. These organic compounds could form tea-plant mechanisms to tolerate lower pH and higher Al and F concentrations. Furthermore, high concentrations of Al and F stresses negatively affected the accumulation of tea secondary metabolites in young leaves, and thereby tea nutrient value. The young leaves of tea seedlings under Al and F stresses also tended to increase Al and F accumulation in young leaves but lower essential tea secondary metabolites, which challenged tea quality and safety. Comparisons of transcriptome data combined with metabolite profiling revealed that the corresponding metabolic gene expression supported and explained the metabolism changes in tea roots and young leaves via stresses from high concentrations of Al and F. The study provides new insight into Al- and F-stressed tea plants with regard to responsive metabolism changes and tolerance strategy establishment in tea plants and the impacts of Al/F stresses on metabolite compositions in young leaves used for making teas, which could influence tea nutritional value and food safety.

Subcellular compartmentalization of aluminum reduced its hazardous impact on rye photosynthesis

Aluminum (Al) toxicity limits crops growth and production in acidic soils. Compared to roots, less is known about the toxic effects of Al in leaves. Al subcellular compartmentalization is also largely unknown. Using rye (Secale cereale L.) Beira (more tolerant) and RioDeva (more sensitive to Al) genotypes, we evaluated the patterns of Al accumulation in leaf cell organelles and the photosynthetic and metabolic changes to cope with Al toxicity. The tolerant genotype accumulated less Al in all organelles, except the vacuoles. This suggests that Al compartmentalization plays a role in Al tolerance of Beira genotype. PSII efficiency, stomatal conductance, pigment biosynthesis, and photosynthesis metabolism were less affected in the tolerant genotype. In the Calvin cycle, carboxylation was compromised by Al exposure in the tolerant genotype. Other Calvin cycle-related enzymes, phoshoglycerate kinase (PGK), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), triose-phosphate isomerase (TPI), and fructose 1,6-bisphosphatase (FBPase) activities decreased in the sensitive line after 48 h of Al exposure. Consequentially, carbohydrate and organic acid metabolism were affected in a genotype-specific manner, where sugar levels increased only in the tolerant genotype. In conclusion, Al transport to the leaf and compartmentalization in the vacuoles tolerant genotype's leaf cells provide complementary mechanisms of Al tolerance, protecting the photosynthetic apparatus and thereby sustaining growth.

Characterization of copper-induced-release of exudates by Citrus sinensis roots and their possible roles in copper-tolerance

Copper (Cu) excess is often observed in old Citrus orchards. Little information is available on the characterization of Cu-induced-release of root exudates and their possible roles in plant Cu-tolerance. Using sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] seedlings as materials, we investigated the impacts of 0, 0.5, 25, 150, 350, 550, 1000, 2000 or 5000 μM CuCl₂ (pH 4.8) on Cu uptake, root exudates [malate, citrate, total phenolics (TP), total soluble sugars (TSS) and total free amino acids (TFAA)], electrolyte leakage and malondialdehyde, and solution pH under hydroponic conditions; the time-course of root exudates and solution pH in response to Cu; and the impacts of protein synthesis and anion-channel inhibitors, and temperature on Cu-induced-secretion of root exudates and solution pH. About 70% of Cu was accumulated in 0 and 0.5 μM Cu-exposed roots, while over 97% of Cu was accumulated in ≥25 μM Cu-exposed roots. Without Cu, the seedlings could alkalize the solution pH from 4.8 to above 6.0. Cu-stimulated-secretion of root exudates elevated with the increment of Cu concentration from 0 to 1000 μM, then decreased or remained unchanged with the further increment of Cu concentration, while root electrolyte leakage and malondialdehyde (root-induced alkalization) increased (lessened) with the increment of Cu concentration from 0 to 5000 μM. Further analysis indicated that Cu-stimulated-secretion of root exudates was an energy-dependent process and could repressed by inhibitors, and that there was no discernible delay between the onset of exudate release and the addition of Cu. To conclude, both root-induced alkalization and Cu-stimulated-release of root exudates played a key role in sweet orange Cu-tolerance via increasing root Cu accumulation and reducing Cu uptake and phytotoxicity.

The root apoplastic pH as an integrator of plant signaling

Plant nutrition, growth, and response to environmental stresses are pH-dependent processes that are regulated at the apoplastic and subcellular levels. The root apoplastic pH is especially sensitive to external cues and can also be modified by intracellular inputs, such as hormonal signaling. Optimal crosstalk of the mechanisms involved in the extent and span of the apoplast pH fluctuations promotes plant resilience to detrimental biotic and abiotic factors. The fact that variations in local pHs are a standard mechanism in different signaling pathways indicates that the pH itself can be the pivotal element to provide a physiological context to plant cell regions, allowing a proportional reaction to different situations. This review brings a collective vision of the causes that initiate root apoplastic pHs variations, their interaction, and how they influence root response outcomes.

Tomato Oxalyl-CoA Synthetase Degrades Oxalate and Affects Fruit Quality

Acyl activating enzyme 3 (AAE3) encodes oxalyl-CoA synthetase involved in oxalate degradation. In this study, we investigated the role of AAE3 (SlAAE3) in the fruit quality of tomato (Solanum lycopersicum). The purified recombinant SlAAE3 protein from Escherichia coli exhibited a high activity toward oxalate, with a K _m of 223.8 ± 20.03 μm and V _max of 7.908 ± 0.606 μmol mg^-1 protein min^-1. Transient expression of SlAAE3-green fluorescent protein (GFP) fusion proteins suggests that SlAAE3 is a soluble protein without specific subcellular localization. The expression of SlAAE3 is both tissue- and development-dependent, and increased during fruit ripping. The Slaae3 knockout mutants had improved fruit quality as evidenced by the increased sugar-acid ratio and mineral nutrient content. To find the mechanism by which SlAAE3 affects fruit quality, transcriptome, and metabolome were employed on SlAAE3 over-expressed line and wide type fruits. The transcriptomic and metabolic profiles indicated that SlAAE3 in fruits mainly functions at 20 days post-anthesis (20 DPA) and mature green (MG) stages, resulting in up-regulation of amino acid derivatives, nucleotides, and derivatives, but down-regulation of lipid compounds. However, differentially expressed genes (DEGs) were mainly enriched at redox pathways. Taken together, both in vivo and in vitro results suggest that SlAAE3-encoded protein acts as an oxalyl-CoA synthetase, which also participates in redox metabolism. These data provide a further understanding of the mechanism by which SlAAE3 participates in tomato fruit quality.

Genome-Wide Identification and Characterisation of Wheat MATE Genes Reveals Their Roles in Aluminium Tolerance

The Multidrug and toxin efflux (MATE) gene family plays crucial roles in plant growth and development and response to adverse stresses. This work investigated the structural and evolutionary characteristics, expression profiling and potential functions involved in aluminium (Al) tolerance from a genome-wide level. In total, 211 wheat MATE genes were identified, which were classified into four subfamilies and unevenly distributed on chromosomes. Duplication analysis showed that fragments and tandem repeats played the main roles in the amplification of TaMATEs, and Type II functional disproportionation had a leading role in the differentiation of TaMATEs. TaMATEs had abundant Al resistance and environmental stress-related elements, and generally had a high expression level in roots and leaves and in response to Al stress. The 3D structure prediction by AlphaFold and molecular docking showed that six TaMATE proteins localised in the plasmalemma could combine with citrate via amino acids in the citrate exuding motif and other sites, and then transport citrate to soil to form citrate aluminium. Meanwhile, citrate aluminium formed in root cells might be transported to leaves by TaMATEs to deposit in vacuoles, thereby alleviating Al toxicity.

A Comprehensive Review on the Heavy Metal Toxicity and Sequestration in Plants

Heavy metal (HM) toxicity has become a global concern in recent years and is imposing a severe threat to the environment and human health. In the case of plants, a higher concentration of HMs, above a threshold, adversely affects cellular metabolism because of the generation of reactive oxygen species (ROS) which target the key biological molecules. Moreover, some of the HMs such as mercury and arsenic, among others, can directly alter the protein/enzyme activities by targeting their –SH group to further impede the cellular metabolism. Particularly, inhibition of photosynthesis has been reported under HM toxicity because HMs trigger the degradation of chlorophyll molecules by enhancing the chlorophyllase activity and by replacing the central Mg ion in the porphyrin ring which affects overall plant growth and yield. Consequently, plants utilize various strategies to mitigate the negative impact of HM toxicity by limiting the uptake of these HMs and their sequestration into the vacuoles with the help of various molecules including proteins such as phytochelatins, metallothionein, compatible solutes, and secondary metabolites. In this comprehensive review, we provided insights towards a wider aspect of HM toxicity, ranging from their negative impact on plant growth to the mechanisms employed by the plants to alleviate the HM toxicity and presented the molecular mechanism of HMs toxicity and sequestration in plants.

The Endophytic Pseudomonas sp. S57 for Plant-Growth Promotion and the Biocontrol of Phytopathogenic Fungi and Nematodes

Oregano from Socoroma (Atacama Desert) is characterized by its unique organoleptic properties and distinctive flavor and it is produced using ancestral pesticide-free agricultural practices performed by the Aymara communities. The cultivation in this zone is carried out under extreme conditions where the standard production of different crops is limited by several environmental factors, including aridity, high concentration of salts, and boron among others. However, oregano plants are associated with microorganisms that mitigate biotic and abiotic stresses present in this site. In this work, the S57 strain (member of the Pseudomonas genus that is closely related to Pseudomonas lini) was isolated from roots of oregano plants, which are grown in soils with high content of non-sodium salts and aluminum. This bacterium stimulates the growth of Micro-Tom tomato plants irrigated with saline-boric water. Moreover, it controls the growth of phytopathogenic fungi Fusarium oxysporum and Botrytis cinerea and the nematode Meloidogyne incognita under saline-boric conditions. Together with the high levels of bacterial biomass (~47 g/L), these results allow the establishment of the bases for developing a potential new agricultural bioproduct useful for arid and semiarid environments where commercial biological products show erratic behavior.

Enhanced aluminum tolerance in sugarcane: evaluation of SbMATE overexpression and genome-wide identification of ALMTs in Saccharum spp

BACKGROUND

A major limiting factor for plant growth is the aluminum (Al) toxicity in acidic soils, especially in tropical regions. The exclusion of Al from the root apex through root exudation of organic acids such as malate and citrate is one of the most ubiquitous tolerance mechanisms in the plant kingdom. Two families of anion channels that confer Al tolerance are well described in the literature, ALMT and MATE family.

RESULTS

In this study, sugarcane plants constitutively overexpressing the Sorghum bicolor MATE gene (SbMATE) showed improved tolerance to Al when compared to non-transgenic (NT) plants, characterized by sustained root growth and exclusion of aluminum from the root apex based on the result obtained with hematoxylin staining. In addition, genome-wide analysis of the recently released sugarcane genome identified 11 ALMT genes and molecular studies showed potential new targets for aluminum tolerance.

CONCLUSIONS

Our results indicate that the transgenic plants overexpressing the Sorghum bicolor MATE has an improved tolerance to Al. The expression profile of ALMT genes revels potential candidate genes to be used has an alternative for agricultural expansion in Brazil and other areas with aluminum toxicity in poor and acid soils.

Insights into the Interactions among Roots, Rhizosphere, and Rhizobacteria for Improving Plant Growth and Tolerance to Abiotic Stresses: A Review

Abiotic stresses, such as drought, salinity, heavy metals, variations in temperature, and ultraviolet (UV) radiation, are antagonistic to plant growth and development, resulting in an overall decrease in plant yield. These stresses have direct effects on the rhizosphere, thus severely affect the root growth, and thereby affecting the overall plant growth, health, and productivity. However, the growth-promoting rhizobacteria that colonize the rhizosphere/endorhizosphere protect the roots from the adverse effects of abiotic stress and facilitate plant growth by various direct and indirect mechanisms. In the rhizosphere, plants are constantly interacting with thousands of these microorganisms, yet it is not very clear when and how these complex root, rhizosphere, and rhizobacteria interactions occur under abiotic stresses. Therefore, the present review attempts to focus on root–rhizosphere and rhizobacterial interactions under stresses, how roots respond to these interactions, and the role of rhizobacteria under these stresses. Further, the review focuses on the underlying mechanisms employed by rhizobacteria for improving root architecture and plant tolerance to abiotic stresses.

Aluminum or Low pH - Which Is the Bigger Enemy of Barley? Transcriptome Analysis of Barley Root Meristem Under Al and Low pH Stress

Aluminum (Al) toxicity is considered to be the most harmful abiotic stress in acidic soils that today comprise more than 50% of the world’s arable lands. Barley belongs to a group of crops that are most sensitive to Al in low pH soils. We present the RNA-seq analysis of root meristems of barley seedlings grown in hydroponics at optimal pH (6.0), low pH (4.0), and low pH with Al (10 μM of bioavailable Al³⁺ ions). Two independent experiments were conducted: with short-term (24 h) and long-term (7 days) Al treatment. In the short-term experiment, more genes were differentially expressed (DEGs) between root meristems grown at pH = 6.0 and pH = 4.0, than between those grown at pH = 4.0 with and without Al treatment. The genes upregulated by low pH were associated mainly with response to oxidative stress, cell wall organization, and iron ion binding. Among genes upregulated by Al, overrepresented were those related to response to stress condition and calcium ion binding. In the long-term experiment, the number of DEGs between hydroponics at pH = 4.0 and 6.0 were lower than in the short-term experiment, which suggests that plants partially adapted to the low pH. Interestingly, 7 days Al treatment caused massive changes in the transcriptome profile. Over 4,000 genes were upregulated and almost 2,000 genes were downregulated by long-term Al stress. These DEGs were related to stress response, cell wall development and metal ion transport. Based on our results we can assume that both, Al³⁺ ions and low pH are harmful to barley plants. Additionally, we phenotyped the root system of barley seedlings grown in the same hydroponic conditions for 7 days at pH = 6.0, pH = 4.0, and pH = 4.0 with Al. The results correspond to transcriptomic data and show that low pH itself is a stress factor that causes a significant reduction of root growth and the addition of aluminum further increases this reduction. It should be noted that in acidic arable lands, plants are exposed simultaneously to both of these stresses. The presented transcriptome analysis may help to find potential targets for breeding barley plants that are more tolerant to such conditions.

Responses of Swamp Cypress (Taxodium distichum) and Chinese Willow (Salix matsudana) Roots to Periodic Submergence in Mega-Reservoir: Changes in Organic Acid Concentration

Organic acids are critical as secondary metabolites for plant adaption in a stressful situation. Oxalic acid, tartaric acid, and malic acid can improve plant tolerance under waterlogged conditions. Two prominent woody species (Taxodium distichum-Swamp cypress and Salix matsudana-Chinese willow) have been experiencing long-term winter submergence and summer drought in the Three Gorges Reservoir. The objectives of the present study were to explore the responses of the roots of two woody species during flooding as reflected by root tissue concentrations of organic acids. Potted sample plants were randomly divided into three treatment groups: control, moderate submergence, and deep submergence. The concentrations of oxalic acid, tartaric acid, and malic acid in the main root and lateral roots of the two species were determined at four stages. The results showed that T. distichum and S. matsudana adapted well to the water regimes of the reservoir, with a survival rate of 100% during the experiment period. After experiencing a cycle of submergence and emergence, the height and base diameter of the two species showed increasing trends. Changes in base diameter showed insignificant differences between submergence treatments, and only height was significant under deep submergence. The concentrations of three organic acids in the roots of two species were influenced by winter submergence. After emergence in spring, two species could adjust their organic acid metabolisms to the normal level. Among three organic acids, tartaric acid showed the most sensitive response to water submergence, which deserved more studies in the future. The exotic species, T. distichum, had a more stable metabolism of organic acids to winter flooding. However, the native species, S. matsudana, responded more actively to long-term winter flooding. Both species can be considered in vegetation restoration, but it needs more observations for planting around 165 m above sea level, where winter submergence is more than 200 days.

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.

Raised pH conferred the ability to maintain a balance between production and detoxification of reactive oxygen species and methylglyoxal in aluminum-toxic Citrus sinensis leaves and roots

Little is known about interactive effects of pH-aluminum (Al) on reactive oxygen species (ROS) and methylglyoxal (MG) metabolisms in plants. Citrus sinensis seedlings were fertilized with nutrient solution at an Al concentration of 1 or 0 mM and a pH of 4.0, 3.5, 3.0 or 2.5 for 18 weeks. Thereafter, gas exchange and chlorophylls in leaves, H₂O₂ generation, electrolyte leakage, total soluble proteins, MG, malondialdehyde (MDA), antioxidants, sulfur-containing compounds, enzymes [viz., antioxidant enzymes, sulfur metabolism-related enzymes, ascorbate oxidase, phosphomannose isomerase, glyoxalase I and glyoxalase II] involved in ROS and MG detoxification in leaves and roots were measured. Effects of low pH and Al-toxicity on these parameters displayed obvious synergism. Without Al-toxicity, low pH increased H₂O₂ production, electrolyte leakage, MDA and MG concentrations by 45.7%-90.3% (52.4%-73.6%), 24.3%-74.5% (26.7%-86.2%), 18.6%-44.8% (35.6%-53.7%) and 16.3%-47.1% (13.8%-51.7%) in leaves (roots) relative to pH 4, respectively; low pH-induced upregulation of enzymes involved in ROS and MG detoxification and sulfur-containing compounds in leaves and/or roots could not protect them against oxidative damage. At pH 2.5-3.0, Al-toxicity increased H₂O₂ production, electrolyte leakage, MDA and MG concentrations by 34.2%-35.5% (23.9%-72.7%), 10.2%-29.5% (23.7%-56.8%), 15.6%-35.7% (27.5%-33.9%) and 21.5%-26.8% (21.0%-49.2%) in leaves (roots), respectively, and decreased total soluble protein concentration by 46.2%-47.4% (18.8%-20.8%) in leaves (roots); at pH 3.5-4.0, Al-toxicity did not affect significantly the five parameters in leaves and roots except for Al-induced increases in root MDA concentration at pH 3.5-4.0 and root electrolyte leakage at pH 3.5, and Al-induced decrease in root total soluble protein concentration at pH 4.0. Raised pH conferred the ability to maintain a balance between production and detoxification of ROS and MG in leaves and roots, thus protecting them against oxidative damage, and hence alleviating Al-induced increase in electrolyte leakage and decrease in total soluble protein level.

Genetic diversity of VIR Raphanus sativus L. collections on aluminum tolerance

Radish and small radish (Raphanus sativus L.) are popular and widely cultivated root vegetables in the world, which occupy an important place in human nutrition. Edaphic stressors have a significant impact on their productivity and quality. The main factor determining the phytotoxicity of acidic soils is the increased concentration of mobile aluminum ions in the soil solution. The accumulation of aluminum in root tissues disrupts the processes of cell division, initiation and growth of the lateral roots, the supply of plants with minerals and water. The study of intraspecific variation in aluminum resistance of R. sativus is an important stage for the breeding of these crops. The purpose of this work was to study the genetic diversity of R. sativus crops including 109 accessions of small radish and radish of various ecological and geographical origin, belonging to 23 types, 14 varieties of European, Chinese and Japanese subspecies on aluminum tolerance. In the absence of a rapid assessment methodology specialized for the species studied, a method is used to assess the aluminum resistance of cereals using an eriochrome cyanine R dye, which is based on the recovery or absence of restoration of mitotic activity of the seedlings roots subjected to shock exposure to aluminum. The effect of various concentrations on the vital activity of plants was revealed: a 66-mM concentration of AlCl₃ · 6Н₂О had a weak toxic effect on R. sativus accessions slowing down root growth; 83 mM contributed to a large differentiation of the small radish accessions and to a lesser extent for radish; 99 mM inhibited further root growth in 13.0 % of small radish accessions and in 7.3 % of radish and had a highly damaging effect. AlCl₃ · 6Н₂О at a concentration of 99 mM allowed us to identify the most tolerant small radish and radish accessions that originate from countries with a wide distribution of acidic soils. In a result, it was possible to determine the intraspecific variability of small radish and radish plants in the early stages of vegetation and to identify genotypes that are contrasting in their resistance to aluminum. We recommend the AlCl₃ · 6Н₂О concentration of 83 mM for screening the aluminum resistance of small radish and 99 mM for radish. The modified method that we developed is proposed as a rapid diagnosis of aluminum tolerance for the screening of a wide range of R. sativus genotypes and a subsequent study of contrasting forms during a longer cultivation of plants in hydroponic culture (including elemental analysis of roots and shoots, contrasting in resistance of accessions) as well as reactions of plants in soil conditions.

Citrus CitERF6 Contributes to Citric Acid Degradation via Upregulation of CitAclα1, Encoding ATP-Citrate Lyase Subunit α

Citric acid is the most abundant organic acid in citrus fruit, and the acetyl-CoA pathway potentially plays an important role in citric acid degradation, which occurs during fruit ripening. Analysis of transcripts during fruit development of key genes in the acetyl-CoA pathway and transient overexpression assay in citrus leaves indicated that CitAclα1 could be a potential target gene involved in citrate degradation. In order to understand more about CitAclα1, 23 transcription factors coexpressed with CitAclα1 in citrus fruit were identified by RNA-seq. Using dual-luciferase assays, CitERF6 was shown to trans-activate the promoter of CitAclα1 and electrophoretic mobility shift assays (EMSAs) showed that CitERF6 directly bound to a 5'-CAACA-3' motif in the CitAclα1 promoter. Furthermore, citric acid content was significantly reduced when CitERF6 was overexpressed in transgenic tobacco leaves. Taken together, these results indicate an important role for CitERF6 in transcriptional regulation of CitAclα1 and control of citrate degradation.

Proanthocyanidin-Aluminum Complexes Improve Aluminum Resistance and Detoxification of Camellia sinensis

Aluminum (Al) influences crop yield in acidic soil. The tea plant (Camellia sinensis) has high Al tolerance with abundant monomeric catechins in its leaves, especially epigallocatechin gallate (EGCG), and polymeric proanthocyanidins in its roots (rPA). The role of these polyphenols in the Al resistance of tea plants is unclear. In this study, we observed that these polyphenols could form complexes with Al in vitro, and complexation capacity was positively influenced by high solution pH (pH 5.8), polyphenol type (rPA and EGCG), and high Al concentration. In the ²⁷Al nuclear magnetic resonance (NMR) experiment, rPA-Al and EGCG-Al complex signals could be detected both in vitro and in vivo. The rPA-Al and EGCG-Al complexes were detected in roots and old leaves, respectively, of both greenhouse seedlings and tea garden plants. Furthermore, in seedlings, Al accumulated in roots and old leaves and mostly existed in the apoplast in binding form. These results indicate that the formation of complexes with tea polyphenols in vivo plays a vital role in Al resistance in the tea plant.

Interactive effects of pH and aluminum on the secretion of organic acid anions by roots and related metabolic factors in Citrus sinensis roots and leaves

Low pH and aluminum (Al)-toxicity often coexist in acidic soils. Citrus sinensis seedlings were treated with nutrient solution at a pH of 2.5, 3.0, 3.5 or 4.0 and an Al concentration of 0 or 1 mM for 18 weeks. Thereafter, malate, citrate, isocitrate, acid-metabolizing enzymes, and nonstructural carbohydrates in roots and leaves, and release of malate and citrate from roots were measured. Al concentration in roots and leaves increased under Al-toxicity, but it declined with elevating nutrient solution pH. Al-toxicity increased the levels of glucose, fructose, sucrose and total soluble sugars in leaves and roots at each given pH except for a similar sucrose level at pH 2.5-3.0, but it reduced or did not alter the levels of starch and total nonstructural carbohydrates (TNC) in leaves and roots with the exception that Al improved TNC level in roots at pH 4.0. Levels of nonstructural carbohydrates in roots and leaves rose with reducing pH with a few exceptions with or without Al-toxicity. A potential model for the possible role of root organic acid (OA) metabolism (anions) in C. sinensis Al-tolerance was proposed. With Al-toxicity, the elevated pH upregulated the OA metabolism, and increased the flow of carbon to OA metabolism, and the accumulation of malate and citrate in roots and subsequent release of them, thus reducing root and leaf Al and hence eliminating Al-toxicity. Without Al-toxicity, low pH stimulated the exudation of malate and citrate, an adaptive response of Citrus to low pH. The interactive effects of pH and pH on OA metabolism were different between roots and leaves.

Biological mechanisms of cadmium accumulation in edible Amaranth (Amaranthus mangostanus L.) cultivars promoted by salinity: A transcriptome analysis

Strategies to prevent cadmium (Cd) mobilization by crops under salinity conditions differs among distinct genotypes, but the biological mechanisms of Cd accumulation in different genotype crops promoted by salinity have remained scarce. In this study, we investigated the biological mechanisms of Cd accumulation in two quite different amaranth cultivars of low-Cd accumulator Quanhong (QH) and high-Cd accumulator Liuye (LY) in response to salt stress. Transcriptomes analysis was carried out on leaves and roots tissues of LY and QH grown with exchangeable Cd 0.27 mg kg^-1 and salinity 3.0 g kg^-1 treatment or control conditions, respectively. A total of 3224 differentially expressed genes (DEGs) in LY (1119 in roots, 2105 in leaves) and 848 in QH (207 in roots, 641 in leaves) were identified. Almost in each fold change category (2-2⁵, 2⁵-2¹⁰, >2¹⁰), the numbers of DEGs induced by salinity in LY treatments were much more than those in QH treatments, indicating that LY is more salt sensitive. Gene ontology (GO) analysis revealed that salinity stress promoted soil acidification and Cd mobilization in LY treatments through the enhancive expression of genes related to adenine metabolism (84-fold enrichment) and proton pumping ATPase (50-fold enrichment) in roots, and carbohydrate hydrolysis (2.5-fold enrichment) in leaves compared with that of whole genome, respectively. The genes expression of organic acid transporter (ALMT) was promoted by 2.71- to 3.94-fold in roots, facilitating the secretion of organic acids. Salt stress also inhibited the expression of key enzymes related to cell wall biosynthesis in roots, reducing the physical barriers for Cd uptake. All these processes altered in LY were more substantially compared with that of QH, suggesting that salt sensitive cultivars might accumulate more Cd and pose a higher health risk.

Improving cadmium mobilization by phosphate-solubilizing bacteria via regulating organic acids metabolism with potassium

Organic acids secreted by phosphorus-solubilizing bacteria (PSB) is one of the main biological metabolites with cadmium (Cd) mobilization capacity in the conversion of insoluble precipitate forms to bioavailable forms in contaminated soil. However, the fluctuating concentrations of nutrient elements caused by agricultural activities may result in the substantial variances of carbohydrate metabolism of microorganisms involved in Cd remediation, it is therefore essential to study how metabolic strategies, especially for organic acids, affected by the environmentally friendly fertilizers, such as potassium (K). In this study, adding K⁺ (KCl) concentrations from 0.0 to 100.0 mg/L in medium clearly accelerated Cd mobilization from 15.9 to 35.9 mg/L via inducing the secretion of tartaric acid, 3-hydroxybutyrate, fumaric and succinic acids, increased by 10.0-, 7.5-, 4.3- and 4.1-fold changes, respectively. Current data revealed that the significant differences of metabolic pathways and genes expressions with the varied K⁺ concentrations included: ⅰ) K⁺ induces a substantial up-regulation in metabolic pathway of pyruvic acid to oxaloacetate and tartaric acids; ⅱ) the varied expression of genes involved in encoding enzymes of tricarboxylic acid cycle result in the up-regulated fumaric acid, succinic acid and 3-hydroxybutyrate; ⅲ) the expression of genes related enzyme cysteine and glutamate metabolism processes promoted with the increasing bioavailable Cd concentrations. Besides, P-type ATPase activity increased with K⁺ levels, indicating that H⁺ efflux and medium acidification were strengthened. In general, an appropriate enhancement of K based fertilizer is an effective manner for soil Cd remediation via the regulation of organic acids metabolism and H⁺ secretion of PSB.

Silicon dioxide nanoparticles ameliorate the phytotoxic hazards of aluminum in maize grown on acidic soil

Aluminum (Al) toxicity is a major constraint for crop production in acid soils. Therefore, looking for sustainable solutions to increase plant tolerance to Al toxicity is needed. Although several studies addressed the potential utilization of silica or silicon dioxide nanoparticles (SNPs) to ameliorate heavy metal phytotoxicity, the exact mechanisms underlying SNPs-induced stress tolerance are still unknown. The current study investigated how SNPs could mitigate Al toxicity in maize plants grown on acidic soil. The impact of Al alone or in combination with SNPs on Al accumulation and detoxification, plant growth, photosynthetic C assimilation and redox homeostasis has been investigated. Al accumulation in stressed-maize organs reduced their growth, decreased photosynthesis related parameters and increased production of reactive oxygen species, through induced NADPH oxidase and photorespiration activities, and cell damage. These effects were more pronounced in roots than in leaves. SNPs ameliorated Al toxicity at growth, physiological and oxidative damage levels. Co-application of SNPs significantly reduced the activities of the photorespiratory enzymes and NADPH oxidase. It stimulated the antioxidant defense systems at enzymatic (superoxide dismutase, catalase, ascorbate and glutathione peroxidases) and non-enzymatic (ascorbate, glutathione, polyphenols, flavonoids, tocopherols, and FRAP) levels. Moreover, SNPs increased organic acids accumulation and metal detoxification (i.e. glutathione-S-transferase activity) in roots, as a protective mechanism against Al toxicity. The SNPs induced-protective mechanisms was dependent on the applied Al concentration and acted in organ-specific manner. Overall, the current study suggests the promising application of SNPs as an innovative approach to mitigate Al phytotoxicity in acidic soils and provides a comprehensive view of the cellular and biochemical mechanisms underlying this mitigation capacity.

Low pH effects on reactive oxygen species and methylglyoxal metabolisms in Citrus roots and leaves

BACKGROUND

Limited data are available on the responses of reactive oxygen species (ROS) and methylglyoxal (MG) metabolisms to low pH in roots and leaves. In China, quite a few of Citrus are cultivated in acidic soils (pH < 5.0). 'Xuegan' (Citrus sinensis) and 'Sour pummelo' (Citrus grandis) (C. sinensis were more tolerant to low pH than C. grandis) seedlings were irrigated daily with nutrient solution at a pH of 2.5, 3 or 5 for nine months. Thereafter, we examined low pH effects on growth, and superoxide anion production rate (SAP), malondialdehyde (MDA), MG, antioxidants, and enzymes related to ROS and MG detoxification in roots and leaves in order to (a) test the hypothesis that low pH affected ROS and MG metabolisms more in roots than those of leaves, and (b) understand the roles of ROS and MG metabolisms in Citrus low pH-tolerance and -toxicity.

RESULTS

Compared with control, most of the physiological parameters related to ROS and MG metabolisms were greatly altered at pH 2.5, but almost unaffected at pH 3. In addition to decreased root growth, many fibrous roots became rotten and died at pH 2.5. pH 2.5-induced changes in SAP, the levels of MDA, MG and antioxidants, and the activities of most enzymes related to ROS and MG metabolisms were greater in roots than those of leaves. Impairment of root ascorbate metabolism was the most serious, especially in C. grandis roots. pH 2.5-induced increases in MDA and MG levels in roots and leaves, decreases in the ratios of ascorbate/(ascorbate+dehydroascorbate) in roots and leaves and of reduced glutathione/(reduced+oxidized glutathione) in roots were greater in C. grandis than those in C. sinensis.

CONCLUSIONS

Low pH affected MG and ROS metabolisms more in roots than those in leaves. The most seriously impaired ascorbate metabolism in roots was suggested to play a role in low pH-induced root death and growth inhibition. Low pH-treated C. sinensis roots and leaves had higher capacity to maintain a balance between ROS and MG production and their removal via detoxification systems than low pH-treated C. grandis ones, thus contribute to the higher acid-tolerance of C. sinensis.

Transcriptomic Revelation of Phenolic Compounds Involved in Aluminum Toxicity Responses in Roots of Cunninghamia lanceolata (Lamb.) Hook

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important coniferous evergreen tree species in South China due to its desirable attributes of fast growth and production of strong and hardy wood. However, the yield of Chinese fir is often inhibited by aluminum (Al) toxicity in acidic soils of South China. Understanding the molecular mechanisms of Chinese fir root responses to Al toxicity might help to further increase its productivity. Here we used the Illumina Hiseq4000 platform to carry out transcriptome analysis of Chinese fir roots subjected to Al toxicity conditions. A total of 88.88 Gb of clean data was generated from 12 samples and assembled into 105,732 distinct unigenes. The average length and N50 length of these unigenes were 839 bp and 1411 bp, respectively. Among them, 58362 unigenes were annotated through searches of five public databases (Nr: NCBI non-redundant protein sequences, Swiss-Prot: A manually annotated and reviewed protein sequence database, GO: Gene Ontology, KOG/COG: Clusters of Orthologous Groups of proteins, and KEGG: the Kyoto Encyclopedia of Genes and Genomes database), which led to association of unigenes with 44 GO terms. Plus, 1615 transcription factors (TFs) were functionally classified. Then, differentially expressed genes (DEGs, |log₂(fold change)| ≥ 1 and FDR ≤ 0.05) were identified in comparisons labelled TC1 (CK-72 h/CK-1 h) and TC2 (Al-72 h/Al-1 h). A large number of TC2 DEGs group were identified, with most being down-regulated under Al stress, while TC1 DEGs were primarily up-regulated. Combining GO, KEGG, and MapMan pathway analysis indicated that many DEGs are involved in primary metabolism, including cell wall metabolism and lipid metabolism, while other DEGs are associated with signaling pathways and secondary metabolism, including flavonoids and phenylpropanoids metabolism. Furthermore, TFs identified in TC1 and TC2 DEGs represented 21 and 40 transcription factor families, respectively. Among them, expression of bHLH, C2H2, ERF, bZIP, GRAS, and MYB TFs changed considerably under Al stress, which suggests that these TFs might play crucial roles in Chinese fir root responses to Al toxicity. These differentially expressed TFs might act in concert with flavonoid and phenylpropanoid pathway genes in fulfilling of key roles in Chinese fir roots responding to Al toxicity.

Exogenous silicon alters organic acid production and enzymatic activity of TCA cycle in two NaCl stressed indica rice cultivars

The activities of TCA cycle enzymes viz., pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase as well as levels of different organic acids viz., pyruvic acid, citric acid, succinic acid and malic acid were studied in two rice cultivars viz. cv. Nonabokra and cv. MTU 1010 differing in salt tolerance grown under 25, 50 and 100 mM NaCl salinity levels. A contrasting response to salt stress on enzyme activities of TCA cycle and accumulation of organic acid was observed between two cultivars during twenty-one days period of study. Salinity caused enhanced organic acid production and increase in all five enzyme activities in cv. Nonabokra whereas in cv. MTU 1010 decrease in both organic acid production and enzymes activities were noted. Joint application of exogenous silicon along with NaCl, altered the organic acids levels and activities of enzymes in both cultivars of rice seedlings conferring tolerance against salt induced stress. Rice cv. MTU 1010 showed better response to exogenous silicon on parameters tested compared to cv. Nonabokra.

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.

Aluminum effects on photosynthesis, reactive oxygen species and methylglyoxal detoxification in two Citrus species differing in aluminum tolerance

Citrus are mainly grown in low pH soils with high active aluminum (Al). 'Xuegan' (Citrus sinensis (L.) Osbeck) and 'Shatian pummelo' (Citrus grandis (L.) Osbeck) seedlings were fertilized for 18 weeks with nutrient solution containing either 0 mM (control) or 1 mM (Al toxicity) AlCl3·6H2O. Aluminum induced decreases of biomass, leaf photosynthesis, relative water content and total soluble protein levels, and increases of methylglyoxal levels only occurred in C. grandis roots and leaves. Besides, the Al-induced decreases of pigments and alterations of chlorophyll a fluorescence transients and fluorescence parameters were greater in C. grandis leaves than those in C. sinensis leaves. Aluminum-treated C. grandis had higher stem and leaf Al levels and similar root Al levels relative to Al-treated C. sinensis, but lower Al distribution in roots and Al uptake per plant. Aluminum toxicity decreased nitrogen, phosphorus, potassium, calcium, magnesium and sulfur uptake per plant in C. grandis and C. sinensis seedlings, with the exception of Al-treated C. sinensis seedlings exhibiting increased sulfur uptake per plant and unaltered magnesium uptake per plant. Under Al-stress, macroelement uptake per plant was higher in C. sinensis than that in C. grandis. Aluminum toxicity decreased the ratios of reduced glutathione/(reduced + oxidized glutathione) and of ascorbate/(ascorbate + dehydroascorbate) only in C. grandis roots and leaves. The activities of most antioxidant enzymes, sulfur metabolism-related enzymes and glyoxalases and the levels of S-containing compounds were higher in Al-treated C. sinensis roots and leaves than those in Al-treated C. grandis ones. Thus, C. sinensis displayed higher Al tolerance than C. grandis did. The higher Al tolerance of C. sinensis might involve: (i) more Al accumulation in roots and less transport of Al from roots to shoots; (ii) efficient maintenance of nutrient homeostasis; and (iii) efficient maintenance of redox homeostasis via detoxification systems of reactive oxygen species and methylglyoxal.

Low pH-responsive proteins revealed by a 2-DE based MS approach and related physiological responses in Citrus leaves

BACKGROUND

Rare data are available on the molecular responses of higher plants to low pH. Seedlings of 'Sour pummelo' (Citrus grandis) and 'Xuegan' (Citrus sinensis) were treated daily with nutrient solution at a pH of 2.5, 3, or 6 (control) for nine months. Thereafter, we first used 2-dimensional electrophoresis (2-DE) to investigate low pH-responsive proteins in Citrus leaves. Meanwhile, we examined low pH-effects on leaf gas exchange, carbohydrates, ascorbate, dehydroascorbate and malondialdehyde. The objectives were to understand the adaptive mechanisms of Citrus to low pH and to identify the possible candidate proteins for low pH-tolerance.

RESULTS

Our results demonstrated that Citrus were tolerant to low pH, with a slightly higher low pH-tolerance in the C. sinensis than in the C. grandis. Using 2-DE, we identified more pH 2.5-responsive proteins than pH 3-responsive proteins in leaves. This paper discussed mainly on the pH 2.5-responsive proteins. pH 2.5 decreased the abundances of proteins involved in ribulose bisphosphate carboxylase/oxygenase activation, Calvin cycle, carbon fixation, chlorophyll biosynthesis and electron transport, hence lowering chlorophyll level, electron transport rate and photosynthesis. The higher oxidative damage in the pH 2.5-treated C. grandis leaves might be due to a combination of factors including higher production of reactive oxygen species, more proteins decreased in abundance involved in antioxidation and detoxification, and lower ascorbate level. Protein and amino acid metabolisms were less affected in the C. sinensis leaves than those in the C. grandis leaves when exposed to pH 2.5. The abundances of proteins related to jasmonic acid biosynthesis and signal transduction were increased and decreased in the pH 2.5-treated C. sinensis and C. grandis leaves, respectively.

CONCLUSIONS

This is the first report on low pH-responsive proteins in higher plants. Thus, our results provide some novel information on low pH-toxicity and -tolerance in higher plants.

Molecular cloning of TaMATE2 homoeologues potentially related to aluminium tolerance in bread wheat (Triticum aestivum L.)

Recently, members of the MATE family have been implicated in aluminium (Al) tolerance by facilitating citrate efflux in plants. The aim of the present work was to perform a molecular characterisation of the MATE2 gene in bread wheat. Here, we cloned a member of the MATE gene family in bread wheat and named it TaMATE2, which showed the typical secondary structure of MATE-type transporters maintaining all the 12 transmembrane domains. Amplification in Chinese Spring nulli-tetrasomic and ditelosomic lines revealed that TaMATE2 is located on the long arm of homoeologous group 1 chromosomes. The transcript expression of TaMATE2 homoeologues in two diverse bread wheat genotypes, Barbela 7/72/92 (Al-tolerant) and Anahuac (Al-sensitive), suggested that TaMATE2 is expressed in both root and shoot tissues of bread wheat, but mainly confined to root rather than shoot tissues. A time-course analysis of TaMATE2 homoeologue transcript expression revealed the Al responsive expression of TaMATE2 in root apices of the Al-tolerant genotype, Barbela 7/72/92. Considering the high similarity of TaMATE2 together with similar Al responsive expression pattern as of ScFRDL2 from rye and OsFRDL2 from rice, it is likely that TaMATE2 also encodes a citrate transporter. Furthermore, the TaMATE2-D homoeologue appears to be near the previously reported locus (wPt0077) on chromosome 1D for Al tolerance. In conclusion, molecular cloning of TaMATE2 homoeologues, particularly TaMATE2-D, provides a plausible candidate for Al tolerance in bread wheat that can be used for the development of more Al-tolerant cultivars in this staple crop.

Aluminum-responsive genes revealed by RNA-Seq and related physiological responses in leaves of two Citrus species with contrasting aluminum-tolerance

Little is known about the physiological and molecular responses of leaves to aluminum (Al)-toxicity. Seedlings of Al-intolerant Citrus grandis and Al-tolerant Citrus sinensis were supplied daily with nutrient solution containing 0 mM (control) and 1.0 mM (Al-toxicity) AlCl₃·6H₂O for 18 weeks. We found that Al-treatment only decreased CO₂ assimilation in C. grandis leaves, and that the Al-induced alterations of gene expression profiles were less in C. sinensis leaves than those in C. grandis leaves, indicating that C. sinensis seedlings were more tolerant to Al-toxicity than C. grandis ones. Al concentration was similar between Al-treated C. sinensis and C. grandis roots, but it was higher in Al-treated C. grandis stems and leaves than that in Al-treated C. sinensis stems and leaves. Al-treated C. sinensis seedlings accumulated relatively more Al in roots and transported relatively little Al to shoots. This might be responsible for the higher Al-tolerance of C. sinensis. Further analysis showed that the following several aspects might account for the higher Al-tolerance of C. sinensis, including: (a) Al-treated C. sinensis leaves had higher capacity to maintain the homeostasis of energy and phosphate, the stability of lipid composition and the integrity of cell wall than did Al-treated C. grandis leaves; (b) Al-triggered production of reactive oxygen species (ROS) and the other cytotoxic compounds was less in Al-treated C. sinensis leaves than that in Al-treated C. grandis leaves, because Al-toxicity decreased CO₂ assimilation only in C. grandis leaves; accordingly, more upregulated genes involved in the detoxifications of ROS, aldehydes and methylglyoxal were identified in Al-treated C. grandis leaves; in addition, flavonoid concentration was increased only in Al-treated C. grandis leaves; (c) Al-treated C. sinensis leaves could keep a better balance between protein phosphorylation and dephosphorylation than did Al-treated C. grandis leaves; and (d) both the equilibrium of hormones and hormone-mediated signal transduction were greatly disrupted in Al-treated C. grandis leaves, but less altered in Al-treated C. sinensis leaves. Finally, we discussed the differences in Al-responsive genes between Citrus roots and leaves.

Enhancement of Low pH Stress Tolerance in Anthocyanin-Enriched Transgenic Petunia Overexpressing RsMYB1 Gene

We investigated whether the presence of anthocyanins in plants could contribute to low pH stress tolerance using anthocyanin-enriched transgenic petunia lines (PM2, PM6, and PM8) expressing RsMYB1 and wild-type (WT) plants. We examined several physiological and biochemical factors and the transcript levels of genes involved in abiotic stress tolerance. A reduction in plant growth, including plant height and fresh weight, was observed when plants (PM2, PM6, PM8, and WT) were exposed to low pH (pH 3.0) conditions compared to growth under normal (pH 5.8) conditions. A small reduction in the growth of PM6 was observed, followed by that in PM2, PM8, and WT, reflecting the anthocyanin levels in the plants (PM6 > PM2 and PM8 > WT). An analysis of physiological and biochemical factors also supports the degree of low pH tolerance in the plants (PM6 > PM2 and PM8 > WT). In addition, an enhanced expression of the genes [superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), osmotin, and vacuolar H+-ATPase (V-ATPase)] was observed in the transgenic lines (PM2, PM6, and PM8). The resultant of the enhanced transcript levels of the genes could promote antioxidant activities, proline content, and pH homeostasis involved in the mechanisms underlying abiotic stress tolerance in plants. These results suggest that anthocyanin-enriched plants overexpressing RsMYB1 enhances low pH stress tolerance by elevating the transcript levels of the relevant genes.

Sulfur-Mediated-Alleviation of Aluminum-Toxicity in Citrus grandis Seedlings

Limited data are available on the sulfur (S)-mediated-alleviation of aluminum (Al)-toxicity in higher plants. Citrus grandis seedlings were irrigated for 18 weeks with 0.5 mM MgSO₄ or 0.5 mM MgSO₄ + 0.5 mM Na₂SO₄, and 0 (-Al) or 1 mM AlCl₃·6H₂O (+Al, Al-toxicity). Under Al-toxicity, S decreased the level of Al in leaves; increased the relative water content (RWC) of roots and leaves, the contents of phosphorus (P), calcium (Ca) and magnesium (Mg) per plant, the dry weights (DW) of roots and shoots, the ratios of root DW/shoot DW, and the Al-induced secretion of citrate from root; and alleviated the Al-induced inhibition of photosynthesis via mitigating the Al-induced decrease of electron transport capacity resulting from the impaired photosynthetic electron transport chain. In addition to decreasing the Al-stimulated H₂O₂ production, the S-induced upregulation of both S metabolism-related enzymes and antioxidant enzymes also contributed to the S-mediated-alleviation of oxidative damage in Al-treated roots and leaves. Decreased transport of Al from roots to shoots and relatively little accumulation of Al in leaves, and increased leaf and root RWC and P, Ca, and Mg contents per plant might also play a role in the S-mediated-alleviation of Al-toxicity.

Root Adaptive Responses to Aluminum-Treatment Revealed by RNA-Seq in Two Citrus Species With Different Aluminum-Tolerance

Seedlings of aluminum (Al)-tolerant Citrus sinensis and Al-intolerant Citrus grandis were fertigated daily with nutrient solution containing 0 and 1.0 mM AlCl3●6H2O for 18 weeks. The Al-induced decreases of biomass and root total soluble proteins only occurred in C. grandis, demonstrating that C. sinensis had higher Al-tolerance than C. grandis. Under Al-treatment, C. sinensis roots secreted more citrate and malate than C. grandis ones; less Al was accumulated in C. sinenis than in C. grandis leaves. The Al-induced reduction of phosphorus was lesser in C. sinensis roots and leaves than in C. grandis ones, whereas the Al-induced increase of sulfur was greater in C. sinensis roots and leaves. Using RNA-seq, we isolated 1905 and 2670 differentially expressed genes (DEGs) from Al-treated C. sinensis than C. grandis roots, respectively. Among these DEGs, only 649 DEGs were shared by the two species. Further analysis suggested that the following several aspects conferred C. sinensis higher Al-tolerance: (a) Al-treated C. sinensis seedlings had a higher external Al detoxification capacity via enhanced Al-induced secretion of organic acid anions, a higher antioxidant capacity and a more efficient chelation system in roots; (b) Al-treated C. sinensis seedlings displayed a higher level of sulfur in roots and leaves possibly due to increased uptake and decreased export of sulfur and a higher capacity to maintain the cellular phosphorus homeostasis by enhancing phosphorus acquisition and utilization; (c) Cell wall and cytoskeleton metabolism, energy and carbohydrate metabolism and signal transduction displayed higher adaptative responses to Al in C. sinensis than in C. grandis roots; (d) More upregulated than downregulated genes related to fatty acid and amino acid metabolisms were isolated from Al-treated C. sinensis roots, but the reverse was the case for Al-treated C. grandis roots. These results provide a platform for further investigating the roles of genes possibly responsible for citrus Al-tolerance.

Effects of Low pH on Photosynthesis, Related Physiological Parameters, and Nutrient Profiles of Citrus

Seedlings of "Xuegan" (Citrus sinensis) and "Sour pummelo" (Citrus grandis) were irrigated daily with a nutrient solution at a pH of 2.5, 3, 4, 5, or 6 for 9 months. Thereafter, the following responses were investigated: seedling growth; root, stem, and leaf concentrations of nutrient elements; leaf gas exchange, pigment concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase activity and chlorophyll a fluorescence; relative water content, total soluble protein level, H2O2 production and electrolyte leakage in roots and leaves. This was done (a) to determine how low pH affects photosynthesis, related physiological parameters, and mineral nutrient profiles; and (b) to understand the mechanisms by which low pH may cause a decrease in leaf CO2 assimilation. The pH 2.5 greatly inhibited seedling growth, and many physiological parameters were altered only at pH 2.5; pH 3 slightly inhibited seedling growth; pH 4 had almost no influence on seedling growth; and seedling growth and many physiological parameters reached their maximum at pH 5. No seedlings died at any given pH. These results demonstrate that citrus survival is insensitive to low pH. H+-toxicity may directly damage citrus roots, thus affecting the uptake of mineral nutrients and water. H+-toxicity and a decreased uptake of nutrients (i.e., nitrogen, phosphorus, potassium, calcium, and magnesium) and water were likely responsible for the low pH-induced inhibition of growth. Leaf CO2 assimilation was inhibited only at pH 2.5. The combinations of an impaired photosynthetic electron transport chain, increased production of reactive oxygen species, and decreased uptake of nutrients and water might account for the pH 2.5-induced decrease in CO2 assimilation. Mottled bleached leaves only occurred in the pH 2.5-treated C. grandis seedlings. Furthermore, the pH 2.5-induced alterations of leaf CO2 assimilation, water-use efficiency, chlorophylls, polyphasic chlorophyll a fluorescence (OJIP) transients and many fluorescence parameters, root and leaf total soluble proteins, H2O2 production, and electrolyte leakage were all slightly greater in C. grandis than in C. sinensis seedlings. Hence, C. sinensis was slightly more tolerant to low pH than C. grandis. In conclusion, our findings provide novel insight into the causes of low pH-induced inhibition of seedling growth and leaf CO2 assimilation.

Statistical Approaches for Gene Selection, Hub Gene Identification and Module Interaction in Gene Co-Expression Network Analysis: An Application to Aluminum Stress in Soybean (Glycine max L.)

Selection of informative genes is an important problem in gene expression studies. The small sample size and the large number of genes in gene expression data make the selection process complex. Further, the selected informative genes may act as a vital input for gene co-expression network analysis. Moreover, the identification of hub genes and module interactions in gene co-expression networks is yet to be fully explored. This paper presents a statistically sound gene selection technique based on support vector machine algorithm for selecting informative genes from high dimensional gene expression data. Also, an attempt has been made to develop a statistical approach for identification of hub genes in the gene co-expression network. Besides, a differential hub gene analysis approach has also been developed to group the identified hub genes into various groups based on their gene connectivity in a case vs. control study. Based on this proposed approach, an R package, i.e., dhga (https://cran.r-project.org/web/packages/dhga) has been developed. The comparative performance of the proposed gene selection technique as well as hub gene identification approach was evaluated on three different crop microarray datasets. The proposed gene selection technique outperformed most of the existing techniques for selecting robust set of informative genes. Based on the proposed hub gene identification approach, a few number of hub genes were identified as compared to the existing approach, which is in accordance with the principle of scale free property of real networks. In this study, some key genes along with their Arabidopsis orthologs has been reported, which can be used for Aluminum toxic stress response engineering in soybean. The functional analysis of various selected key genes revealed the underlying molecular mechanisms of Aluminum toxic stress response in soybean.

Glutathione S-transferases and UDP-glycosyltransferases Are Involved in Response to Aluminum Stress in Flax

About 30% of the world's ice-free land area is occupied by acid soils. In soils with pH below 5, aluminum (Al) releases to the soil solution, and becomes highly toxic for plants. Therefore, breeding of varieties that are resistant to Al is needed. Flax (Linum usitatissimum L.) is grown worldwide for fiber and seed production. Al toxicity in acid soils is a serious problem for flax cultivation. However, very little is known about mechanisms of flax resistance to Al and the genetics of this resistance. In the present work, we sequenced 16 transcriptomes of flax cultivars resistant (Hermes and TMP1919) and sensitive (Lira and Orshanskiy) to Al, which were exposed to control conditions and aluminum treatment for 4, 12, and 24 h. In total, 44.9-63.3 million paired-end 100-nucleotide reads were generated for each sequencing library. Based on the obtained high-throughput sequencing data, genes with differential expression under aluminum exposure were revealed in flax. The majority of the top 50 up-regulated genes were involved in transmembrane transport and transporter activity in both the Al-resistant and Al-sensitive cultivars. However, genes encoding proteins with glutathione transferase and UDP-glycosyltransferase activity were in the top 50 up-regulated genes only in the flax cultivars resistant to aluminum. For qPCR analysis in extended sampling, two UDP-glycosyltransferases (UGTs), and three glutathione S-transferases (GSTs) were selected. The general trend of alterations in the expression of the examined genes was the up-regulation under Al stress, especially after 4 h of Al exposure. Moreover, in the flax cultivars resistant to aluminum, the increase in expression was more pronounced than that in the sensitive cultivars. We speculate that the defense against the Al toxicity via GST antioxidant activity is the probable mechanism of the response of flax plants to aluminum stress. We also suggest that UGTs could be involved in cell wall modification and protection from reactive oxygen species (ROS) in response to Al stress in L. usitatissimum. Thus, GSTs and UGTs, probably, play an important role in the response of flax to Al via detoxification of ROS and cell wall modification.

Aluminum Toxicity-Induced Alterations of Leaf Proteome in Two Citrus Species Differing in Aluminum Tolerance

Seedlings of aluminum-tolerant ‘Xuegan’ (Citrus sinensis) and Al-intolerant ‘sour pummelo’ (Citrus grandis) were fertigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O. Al toxicity-induced inhibition of photosynthesis and the decrease of total soluble protein only occurred in C. grandis leaves, demonstrating that C. sinensis had higher Al tolerance than C. grandis. Using isobaric tags for relative and absolute quantification (iTRAQ), we obtained more Al toxicity-responsive proteins from C. sinensis than from C. grandis leaves, which might be responsible for the higher Al tolerance of C. sinensis. The following aspects might contribute to the Al tolerance of C. sinensis: (a) better maintenance of photosynthesis and energy balance via inducing photosynthesis and energy-related proteins; (b) less increased requirement for the detoxification of reactive oxygen species and other toxic compounds, such as aldehydes, and great improvement of the total ability of detoxification; and (c) upregulation of low-phosphorus-responsive proteins. Al toxicity-responsive proteins related to RNA regulation, protein metabolism, cellular transport and signal transduction might also play key roles in the higher Al tolerance of C. sinensis. We present the global picture of Al toxicity-induced alterations of protein profiles in citrus leaves, and identify some new Al toxicity-responsive proteins related to various biological processes. Our results provide some novel clues about plant Al tolerance.

Aluminum uptake and migration from the soil compartment into Betula pendula for two different environments: a polluted and environmentally protected area of Poland

This paper presents the impact of soil contamination on aluminum (Al) concentrations in plant parts of Betula pendula and a possible way of migration and transformation of Al in the soil-root-stem-twig-leaf system. A new procedure of Al fractionation based on extraction in water phase was applied to obtain and measure the most available forms of Al in soils and B. pendula samples. In addition, total Al content was determined in biological samples and pseudo total Al content in soil samples collected under plant saplings, using atomic absorption spectrometry with flame atomization. A number of relations concerning the occurrence of Al and Ca in soils and plant parts of B. pendula (tap roots, lateral roots, stem, twigs, and leaves) were observed. Based on the research findings, the mechanism of Al migration from soil to the leaves of B. pendula can be presented. It was found that aluminum uptake may be limited in roots by high calcium concentration. The application of a new procedure based on the simple sequential extraction of water-soluble fractions (the most available and exchangeable fractions of Al) can be used as an effective tool for the estimation of aluminum toxicity in soils and plants.

Root iTRAQ protein profile analysis of two Citrus species differing in aluminum-tolerance in response to long-term aluminum-toxicity

BACKGROUND

Limited information is available on aluminum (Al)-toxicity-responsive proteins in woody plant roots. Seedlings of 'Xuegan' (Citrus sinensis) and 'Sour pummelo' (Citrus grandis) were treated for 18 weeks with nutrient solution containing 0 (control) or 1.2 mM AlCl3 · 6H2O (+Al). Thereafter, we investigated Citrus root protein profiles using isobaric tags for relative and absolute quantification (iTRAQ). The aims of this work were to determine the molecular mechanisms of plants to deal with Al-toxicity and to identify differentially expressed proteins involved in Al-tolerance.

RESULTS

C. sinensis was more tolerant to Al-toxicity than C. grandis. We isolated 347 differentially expressed proteins from + Al Citrus roots. Among these proteins, 202 (96) proteins only presented in C. sinensis (C. grandis), and 49 proteins were shared by the two species. Of the 49 overlapping proteins, 45 proteins were regulated in the same direction upon Al exposure in the both species. These proteins were classified into following categories: sulfur metabolism, stress and defense response, carbohydrate and energy metabolism, nucleic acid metabolism, protein metabolism, cell transport, biological regulation and signal transduction, cell wall and cytoskeleton metabolism, and jasmonic acid (JA) biosynthesis. The higher Al-tolerance of C. sinensis may be related to several factors, including: (a) activation of sulfur metabolism; (b) greatly improving the total ability of antioxidation and detoxification; (c) up-regulation of carbohydrate and energy metabolism; (d) enhancing cell transport; (e) decreased (increased) abundances of proteins involved in protein synthesis (proteiolysis); (f) keeping a better balance between protein phosphorylation and dephosphorylation; and (g) increasing JA biosynthesis.

CONCLUSIONS

Our results demonstrated that metabolic flexibility was more remarkable in C. sinenis than in C. grandis roots, thus improving the Al-tolerance of C. sinensis. This provided the most integrated view of the adaptive responses occurring in Al-toxicity roots.

Leaf cDNA-AFLP analysis reveals novel mechanisms for boron-induced alleviation of aluminum-toxicity in Citrus grandis seedlings

Little information is available on the molecular mechanisms of boron (B)-induced alleviation of aluminum (Al)-toxicity. 'Sour pummelo' (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing different concentrations of B (2.5 or 20μM H3BO3) and Al (0 or 1.2mM AlCl3·6H2O). B alleviated Al-induced inhibition in plant growth accompanied by lower leaf Al. We used cDNA-AFLP to isolate 127 differentially expressed genes from leaves subjected to B and Al interactions. These genes were related to signal transduction, transport, cell wall modification, carbohydrate and energy metabolism, nucleic acid metabolism, amino acid and protein metabolism, lipid metabolism and stress responses. The ameliorative mechanisms of B on Al-toxicity might be related to: (a) triggering multiple signal transduction pathways; (b) improving the expression levels of genes related to transport; (c) activating genes involved in energy production; and (d) increasing amino acid accumulation and protein degradation. Also, genes involved in nucleic acid metabolism, cell wall modification and stress responses might play a role in B-induced alleviation of Al-toxicity. To conclude, our findings reveal some novel mechanisms on B-induced alleviation of Al-toxicity at the transcriptional level in C. grandis leaves.

A new allele of acid soil tolerance gene from a malting barley variety

Background

Acid soil is a serious limitation to crop production all over the world. Toxic aluminium (Al) cations in acid soil inhibit root growth and reduce yield. Although a gene tolerant to acid soil has been identified, it has not been used in malting barley breeding, which is partly due to the acid soil tolerance gene being linked to unfavorable malting quality traits.

Results

A Brazilian malting barley variety Br2 was identified as tolerant to acid soil. A doubled haploid (DH) population was developed from a cross between Br2 and the Australian acid-sensitive cultivar Hamelin. The DH population was tested for acid soil tolerance in native acid soil and a hydroponic system with pH 4.2, pH 4.2 + Al or pH 6.5, and genotyped using SSR, DArT and gene-specific markers. A single QTL was detected for all parameters related to acid soil tolerance. The QTL was mapped to the known HvMATE location on chromosome 4H. Sequence alignment of the HvMATE gene identified 13 INDELs and 87 SNPs, where one SNP coded for a single amino acid difference between the two varieties. A gene-specific marker was developed to detect the single nucleotide polymorphism between Hamelin and Br2. This marker co-segregated with aluminium tolerance and accounted for 79 % of phenotypic variation for acid soil tolerance.

Conclusion

The present study identified a novel source of acid soil/Al tolerance from a Brazilian malting barley cultivar Br2. This variety tolerated Al toxicity but was sensitive to low pH which is similar to most other Al-tolerant varieties. A gene-specific marker Cit7 was developed based on the HvMATE gene sequence. Cit7 will improve the efficiency of molecular-assisted selection of new barley varieties with tolerance to acid soil. Multiple alleles exist for the acid soil tolerance gene on chromosome 4H, so a malting barley variety that tolerates acid soil could be developed by selecting suitable tolerant alleles. Tolerance to low pH may play an important role for barley to adapt to acid soils.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0254-4) contains supplementary material, which is available to authorized users.

Jacks of metal/metalloid chelation trade in plants-an overview

Varied environmental compartments including soils are being contaminated by a myriad toxic metal(loid)s (hereafter termed as "metal/s") mainly through anthropogenic activities. These metals may contaminate food chain and bring irreparable consequences in human. Plant-based approach (phytoremediation) stands second to none among bioremediation technologies meant for sustainable cleanup of soils/sites with metal-contamination. In turn, the capacity of plants to tolerate potential consequences caused by the extracted/accumulated metals decides the effectiveness and success of phytoremediation system. Chelation is among the potential mechanisms that largely govern metal-tolerance in plant cells by maintaining low concentrations of free metals in cytoplasm. Metal-chelation can be performed by compounds of both thiol origin (such as GSH, glutathione; PCs, phytochelatins; MTs, metallothioneins) and non-thiol origin (such as histidine, nicotianamine, organic acids). This paper presents an appraisal of recent reports on both thiol and non-thiol compounds in an effort to shed light on the significance of these compounds in plant-metal tolerance, as well as to provide scientific clues for the advancement of metal-phytoextraction strategies.

Eliminating aluminum toxicity in an acid sulfate soil for rice cultivation using plant growth promoting bacteria

Aluminum toxicity is widely considered as the most important limiting factor for plants growing in acid sulfate soils. A study was conducted in laboratory and in field to ameliorate Al toxicity using plant growth promoting bacteria (PGPB), ground magnesium limestone (GML) and ground basalt. Five-day-old rice seedlings were inoculated by Bacillus sp., Stenotrophomonas maltophila, Burkholderia thailandensis and Burkholderia seminalis and grown for 21 days in Hoagland solution (pH 4.0) at various Al concentrations (0, 50 and 100 μM). Toxicity symptoms in root and leaf were studied using scanning electron microscope. In the field, biofertilizer (PGPB), GML and basalt were applied (4 t·ha-1 each). Results showed that Al severely affected the growth of rice. At high concentrations, the root surface was ruptured, leading to cell collapse; however, no damages were observed in the PGPB inoculated seedlings. After 21 days of inoculation, solution pH increased to >6.0, while the control treatment remained same. Field study showed that the highest rice growth and yield were obtained in the bio-fertilizer and GML treatments. This study showed that Al toxicity was reduced by PGPB via production of organic acids that were able to chelate the Al and the production of polysaccharides that increased solution pH. The release of phytohormones further enhanced rice growth that resulted in yield increase.

Molecular characterization of the citrate transporter gene TaMATE1 and expression analysis of upstream genes involved in organic acid transport under Al stress in bread wheat (Triticum aestivum)

In bread wheat, besides malate, the importance of citrate efflux for Al tolerance has also been reported. For better understanding the Al tolerance mechanism in bread wheat, here, we performed both a molecular characterization of the citrate transporter gene TaMATE1 and an investigation on the upstream variations in citrate and malate transporter genes. TaMATE1 belong to multidrug transporter protein family, which are located on the long arm of homoeologous group 4 chromosomes (TaMATE1-4A, TaMATE1-4B TaMATE1-4D). TaMATE1 homoeologues transcript expression study exhibited the preponderance of homoeologue TaMATE1-4B followed by TaMATE1-4D whereas homoeologue TaMATE1-4A seemed to be silenced. TaMATE1, particularly homoeologue TaMATE1-4B and TaALMT1 transcripts were much more expressed in the root apices than in shoots of Al tolerant genotype Barbela 7/72/92 under both control and Al stress conditions. In addition, in both tissues of Barbela 7/72/92, higher basal levels of these gene transcripts were observed than in Anahuac (Al sensitive). Noticeably, the presence of a transposon in the upstream of TaMATE1-4B in Barbela 7/72/92 seems to be responsible for its higher transcript expression where it may confer citrate efflux. Thus, promoter variations (transposon in TaMATE1-4B upstream and type VI promoter in TaALMT1) associated with higher basal transcript expression of TaMATE1-4B and TaALMT1 clearly show how different mechanisms for Al tolerance operate simultaneously in a single genotype. In conclusion, our results demonstrate that Barbela 7/72/92 has favorable alleles for these organic acids transporter genes which could be utilized through genomic assisted selection to develop improved cultivars for acidic soils.