Aluminum Toxicity Research Articles

Using brefeldin A to disrupt cell wall polysaccharide components in rice and nitric oxide to modify cell wall structure to change aluminum tolerance.

The components and structure of cell wall are closely correlated with aluminum (Al) toxicity and tolerance for plants. However, the cell wall assembly and function construction in response to Al is not known. Brefeldin A (BFA), a macrolide, is used to disrupt cell wall polysaccharide components, and nitric oxide (NO), a signal molecule, is used to modify the cell wall structure. Pretreatment with BFA accelerated Al accumulation in root tips and Al-induced inhibition of root growth of two rice genotypes of Nipponbare and Zhefu 802, and significantly decreased the cell wall polysaccharide content including pectin, hemicellulose 1, and hemicellulose 2, indicating that BFA inhibits the biosynthesis of components in the cell wall and makes the root cell wall lose the ability to resist Al. The addition of NO donor (SNP) significantly alleviated the toxic effects of Al on root growth, Al accumulation, and oxidative damage, and decreased the content of pectin polysaccharide and functional groups of hydroxyl, carboxyl, and amino in the cell wall via FTIR analysis, while had no significant effect on hemicellulose 1 and hemicellulose 2 content compared with Al treatment. Furthermore, NO didn't change the inhibition effect of BFA-induced cell wall polysaccharide biosynthesis and root growth. Taken together, BFA disrupts the integrity of cell wall and NO modifies partial cell wall composition and their functional groups, which change the Al tolerance in rice.

ZmNRAMP4 Enhances the Tolerance to Aluminum Stress in Arabidopsis thaliana.

Aluminum (Al) toxicity causes severe reduction in crop yields in acidic soil. The natural resistance-associated macrophage proteins (NRAMPs) play an important role in the transport of mineral elements in plants. Recently, OsNrat1 and SbNrat1 were reported specifically to transport trivalent Al ions. In this study, we functionally characterized ZmNRAMP4, a gene previously identified from RNA-Seq data from Al-treated maize roots, in response to Al exposure in maize. ZmNRAMP4 was predominantly expressed in root tips and was specifically induced by Al stress. Yeast cells expressing ZmNRAMP4 were hypersensitive to Al, which was associated with Al accumulation in yeast. Furthermore, overexpression of ZmNRAMP4 in Arabidopsis conferred transgenic plants with a significant increase in Al tolerance. However, expression of ZmNRAMP4, either in yeast or in Arabidopsis, had no effect on the response to cadmium stress. Taken together, these results underlined an internal tolerance mechanism involving ZmNRAMP4 to enhance Al tolerance via cytoplasmic sequestration of Al in maize.

A zinc finger protein SlSZP1 protects SlSTOP1 from SlRAE1-mediated degradation to modulate aluminum resistance.

In acidic soils, aluminum (Al) toxicity is the main factor inhibiting plant root development and reducing crops yield. STOP1 (SENSITIVE TO PROTON RHIZOTOXICITY 1) was a critical factor in detoxifying Al stress. Under Al stress, STOP1 expression was not induced, although STOP1 protein accumulated, even in the presence of RAE1 (STOP1 DEGRADATION E3-LIGASE). How the Al stress triggers and stabilises the accumulation of STOP1 is still unknown. Here, we characterised SlSTOP1-interacting zinc finger protein (SlSZP1) using a yeast-two-hybrid screening, and generated slstop1, slszp1 and slstop1/slszp1 knockout mutants using clustered regularly interspaced short palindromic repeats (CRISPR) in tomato. SlSZP1 is induced by Al stress but it is not regulated by SlSTOP1. The slstop1, slszp1 and slstop1/slszp1 knockout mutants exhibited hypersensitivity to Al stress. The expression of SlSTOP1-targeted genes, such as SlRAE1 and SlASR2 (ALUMINUM SENSITIVE), was inhibited in both slstop1 and slszp1 mutants, but not directly regulated by SlSZP1. Furthermore, the degradation of SlSTOP1 by SlRAE1 was prevented by SlSZP1. Al stress increased the accumulation of SlSTOP1 in wild-type (WT) but not in slszp1 mutants. The overexpression of either SlSTOP1 or SlSZP1 did not enhance plant Al resistance. Altogether, our results show that SlSZP1 is an important factor for protecting SlSTOP1 from SlRAE1-mediated degradation.

Root-Associated Microbiota Response to Ecological Factors: Role of Soil Acidity in Enhancing Citrus Tolerance to Huanglongbing.

The citrus orchards in southern China are widely threatened by low soil pH and Huanglongbing (HLB) prevalence. Notably, the lime application has been used to optimize soil pH, which is propitious to maintain root health and enhance HLB tolerance of citrus; however, little is known about the interactive effects of soil acidity on the soil properties and root-associated (rhizoplane and endosphere) microbial community of HLB-infected citrus orchard. In this study, the differences in microbial community structures and functions between the acidified and amended soils in the Gannan citrus orchard were investigated, which may represent the response of the host-associated microbiome in diseased roots and rhizoplane to dynamic soil acidity. Our findings demonstrated that the severity of soil acidification and aluminum toxicity was mitigated after soil improvement, accompanied by the increase in root activity and the decrease of HLB pathogen concentration in citrus roots. Additionally, the Illumina sequencing-based community analysis showed that the application of soil amendment enriched functional categories involved in host-microbe interactions and nitrogen and sulfur metabolisms in the HLB-infected citrus rhizoplane; and it also strongly altered root endophytic microbial community diversity and structure, which represented by the enrichment of beneficial microorganisms in diseased roots. These changes in rhizoplane-enriched functional properties and microbial composition may subsequently benefit the plant's health and tolerance to HLB disease. Overall, this study advances our understanding of the important role of root-associated microbiota changes and ecological factors, such as soil acidity, in delaying and alleviating HLB disease.

Evaluating Three Histochemical Stains (Solochrome Azurine Stain (Asa), Walton Stain, and Modified Hematoxylin) used in Tissue Aluminium Detection.

Aluminium in recent times has been considered a possible risk factor in some diseases in humans, animals and plants, and exposure to aluminium may pose a health hazard. Studies have pointed to the fact that increasing acidification of the environment has made aluminium more bio-available and therefore, able to cause disturbances in the function of human and animal organisms. More importantly also, is the use of aluminium as based adjuvants in human vaccinations, and its fate being unclear. Our study aimed to evaluate histochemical stains currently used to detect Al in tissue samples for their sensitivity using agar blocks as a preliminary study to validate the Walton histological stain use in detecting aluminium toxicity in fish. Visual estimation (colour change and staining intensity) of aluminium-stained sections using the Solochrome Azurine stain (ASA), Walton stain, and the modified haematoxylin were carried out. All three stains indicated the presence or absence of aluminium through colour changes, but the ASA gave more distinct dose- response intensity in staining.

Development of ammonium nitrate free nutrient media for aluminium toxicity tolerance screening of rice genotypes from North-Eastern India

North-Eastern India is blessed with a vast diversity of rice genotypes with varying yielding abilities. However, the predominant strong soil acidity induced nutrient stresses from the toxicity of aluminum (Al3+) ions often causes sub-optimal rice productivity. The lack of suitable nutrient media for the screening of aluminum (Al) toxicity tolerance of rice genotypes is one of the limiting factors in Al tolerant varieties' development. Modified Magnavaca's solution has been the most recommended nutrient solution for this purpose where ammonium nitrate is the primary nitrogen source. However, strict regulations related to the handling and storage of ammonium nitrate in India limit the preparation of Modified Magnavaca’s solution for laboratory use. Here, a modified ammonium nitrate-free formulation based upon the Magnavaca's solution has been proposed. The modified formulation was found to have 160.9 µM of active Al3+ concentration when 550 µM of aluminum chloride hexahydrate (AlCl3.6H2O) was added at pH 4.1 to the solution. Through a plant growth screening experiment using ten diverse rice genotypes a significant difference in the response of various genotypes to differential Al toxicity levels on root growth performance indicators was observed. Based on the response, we could able to categorize the genotypes into tolerant and sensitive in relative terms. Thus, the present experiment provided an important nutrient formulation suitable for screening rice genotypes under Al toxicity conditions. Moreover, the selected tolerant and sensitive genotypes can further pave the way for studying the molecular mechanism of Al toxicity response in rice and their use in the breeding program.

How candidate genes respond to aluminum toxicity in Citrus x limonia Osbeck?

How candidate genes respond to aluminum toxicity in Citrus x limonia Osbeck?

Soil Acidity in Ethiopia: Its Causes, Effects on Crop Production and Management Experiences So Far- Review

Soil acidity is a serious chemical problem that limits agricultural productivity in most of the highlands of Ethiopia as well as in the western part of the country. These acidic soils, which cover an estimated 41% of the arable land in Ethiopia, are inherently infertile and exhibit aluminium (Al) or manganese (Mn) toxicity, which are generally considered to be the major limiting factor for plant growth in acidic soils. The major factors leading to acid soils in western Ethiopia include erosion of topsoil by heavy rains and high temperatures, which increase the greatest loss of organic matter and leaching of exchangeable basic cations (Ca 2+ , Mg 2+ , Na + and K + ). Organic matter can be easily degraded and lost through conventional land clearing practices such as burning and direct sun and rain exposure, which is exacerbated by improper agricultural practices. If these soils are not properly managed after clearing, they can quickly lose much of their original fertility and beneficial physical as well as biological properties. It has been suggested that maintaining production on these soils requires an effort far beyond the means of poor farmers. Because low pH affects the availability of nutrients, especially phosphorus and other macronutrients, correcting low pH through liming and/or the application of organic matter (such as compost and vermicompost) is critical to the sustainable management of these acidic soils. The recently introduced technology of organic soil amendment with vermicompost has proven to be a potential resource to significantly improve soil fertility and soil health in various soil types around the globe. Therefore, farmers should be encouraged to increase the productivity of acidic soils through organic amendments such as compost/vermicompost. Vermicomposting, which is still very young and rarely practiced by farmers, should be supported and strengthened by more research and extension than we currently have to produce high quality vermicompost to amend acidic soils of Ethiopia and restore soil life. Keywords: Acidic soils, lime, vermicompost, organic materials DOI: 10.7176/JNSR/13-14-02 Publication date: July 31 st 2022

Response of Rice Variety Nagina 22 (N22) and its Putative Mutants to Aluminium Toxicity Conditions in North-East India

The study was performed to evaluate the wildtype genotype Nagina 22 (N22) and its mutants generated by ethyl methyl sulphonate (EMS)for aluminium (Al) toxicity tolerance. Based on performance under upland acidic field conditions on the basis of twelve different traits,among several available M4N22 mutants,four putative mutants N 714, N 721, N 4249 and N 4487 were selected for hydroponics evaluation. The seeds of the four selected lines were subjected to germination and grown in modified Magnacava’s broth supplemented with varying doses of AlCl3 under aseptic conditionsand phenotypic variations for traits like root length, shoot length, root biomass and shoot biomass were recorded. Haematoxylin staining of roots were also carried out on the seedlings grown under control as well as Al toxicity conditions which further revealed that putative mutants N 721 and N 4249 were better performers when compared to N22. Additionally, twelve candidate gene based markers targeting four known Al toxicity tolerance genes resulted in detection of gel based polymorphism for markers AR051-2, AR051-3 and OsFRLD4-1. These putative variations need further validation by running the markers on a set of at least 10 individual putative mutants and sequencing. Al toxicity tolerance in rice is a quantitative trait, these identified mutants can be used to obtain better understanding of tolerance mechanisms and then if possibleuse them synergistically to obtain better tolerance.

Correlation between the presence of metals with intoxication potential, omega 3 deficiency, increased omega 6:omega 3 ratio and their associated symptoms

Objectives. The study aims to determine the correlation between the presence of heavy metals with intoxication potential (mercury, aluminum, arsenic), increasing the values for the omega 6: omega 3 ratio, decreasing the concentration of omega 3 and it’s impact on health. Another objective of this study is the use of an integrated protocol, developed by an interdisciplinary team, based on a customized alignment program and a treatment plan. Materials and methods. We evaluated in a retrospective observational study, 77 patients who came to the Nutribalance Clinic between September 2017 and December 2019, with specific symptoms of intoxication with toxic metals, especially mercury, aluminum, arsenic and omega 3 deficiencies. The manifested symptoms required a complete set of blood tests for each subject in the study group, tests that showed the presence of one, two and three toxic metals in the case of all 77 subjects. The interdisciplinary team developed a personalized treatment and diet plan, monitoring the evolution on the symptoms initially manifested by each subject. Outcomes. The results of this study shows the beneficial impact of the personalized diet plan and supplements used on the decrease in omega 6:omega 3 ratio, the significant increases in omega 3 which support the detoxification process of heavy metals and a significant improvement in the symptoms of each patient. Conclusions. This study reveals that the process of chelating or detoxifying heavy metals is a complex one and that implies the presence of a complementarity between food and nutritional supplement therapy, avoiding one single direction, as results can be obtained for a short or insignificant period of time.

Metabolome Analysis under Aluminum Toxicity between Aluminum-Tolerant and -Sensitive Rice (Oryza sativa L.).

Aluminum (Al) solubilizes into trivalent ions (Al3+) on acidic soils, inhibiting root growth. Since about 13% of global rice cultivation is grown on acidic soils, improving Al tolerance in rice may significantly increase yields. In the present study, metabolome analysis under Al toxicity between the Al-tolerant variety Nipponbare and the Al-sensitive variety H570 were performed. There were 45 and 83 differential metabolites which were specifically detected in Nipponbare and H570 under Al toxicity, respectively. Furthermore, the results showed that 16 lipids out of 45 total metabolites were down-regulated, and 7 phenolic acids as well as 4 alkaloids of 45 metabolites were up-regulated in Nipponbare, while 12 amino acids and their derivatives were specifically detected in H570, of which 11 amino acids increased, including L-homoserine and L-methionine, which are involved in cysteine synthesis, L-ornithine and L-proline, which are associated with putrescine synthesis, and 1-aminocyclopropane-1-carboxylate, which is associated with ethylene synthesis. The contents of cysteine and s-(methyl) glutathione, which were reported to be related to Al detoxification in rice, decreased significantly. Meanwhile, putrescine was accumulated in H570, while there was no significant change in Nipponbare, so we speculated that it might be an intermediate product of Al detoxification in rice. The differential metabolites detected between Al-tolerant and -sensitive rice variants in the present study might play important roles in Al tolerance. These results provide new insights in the mechanisms of Al tolerance in rice.

Critical values of soil solution Al3+ activity and pH for canola and maize cultivation in two acidic soils.

Aluminum (Al) toxicity caused by soil acidification is the main constraint for crop growth in tropical and subtropical areas of southern China. The critical values of soil solution Al3+ activity and pH for crops in acidic soils can provide a useful reference for soil acidity amelioration. A pot experiment in a greenhouse was conducted to investigate the critical values of soil solution Al3+ activity and pH for canola and maize in an Ultisol and an Alfisol. The critical values of soil solution Al3+ activity in Ultisol and Alfisol for canola were 1.5 and 10.0 μmol L-1 , and 13.9 and 30.4 μmol L-1 for maize, respectively. The Al tolerance varied with soil type for the same variety of crop. There was more biomass of roots and shoots and higher plant height under the same Al3+ activity, and thus greater critical values of soil solution Al3+ activity for both crops in Alfisol than those in Ultisol, owing to higher Ca2+ /Al3+ , Mg2+ /Al3+ and K+ /Al3+ ratios in soil solution caused by higher cation exchange capacity and exchangeable base cations in Alfisol, when compared with those in Ultisol. The critical values of soil solution pH for canola and maize in Ultisol were 5.09 and 4.72, respectively; while those in Alfisol were 4.87 and 4.54, respectively. The critical values of Al3+ activity were higher for maize than for canola and the critical values for both crops were higher in Alfisol than in Ultisol. The critical soil pH for both crops showed opposite trends to soil Al3+ activity. © 2022 Society of Chemical Industry.

Determination of aluminum concentrations in biological specimens: application in the clinical laboratory.

Aluminum enters the body primarily through diet or occupational exposure, and is cleared through urine. However, this trace element may accumulate and cause toxicity in subjects with renal insufficiency, and even in dialysis patients. The mechanism of aluminum toxicity is related to increased oxidative and inflammatory stress, iron and calcium dyshomeostasis, or cholinergic dysregulation, among other. A review was conducted on the specimens and analytical methods used to determine aluminum in biological specimens and dialysis water. This paper describes the most relevant aspects related to quality assurance. This is a practical guideline for the development and implementation of a reliable method for determination of aluminum in the clinical laboratory. Serum aluminum is the main biomarker of toxicity. For cases of chronic exposure, urine testing is recommended. At present, inductively coupled plasma mass spectrometry (ICP-MS) is the gold-standard determination method, since it has been proven to have the best quantification limits, selectivity and robustness. Clear recommendations are provided in relation to the specimens used for aluminum determination. Relevant pre-analytical, analytical, and post-analytical considerations are also presented.

Dietary aluminium intake disrupts the overall structure of gut microbiota in Wistar rats.

Approximately, 40% of ingested dietary aluminium accumulates in the intestine, which has been considered a target organ for dietary aluminium exposure. The gut microbiota may be the first protective barrier against the toxic metal aluminium and a crucial mediator of the bioavailability of metal aluminium. We previously evaluated dietary aluminium intake and its health risks in a population from Jilin Province, China, and found that the average daily intake of aluminium in the diet of residents in Jilin Province was 0.163 mg/kg after the total diet survey. In the present study, the equivalent concentration of aluminium in rats was extrapolated by the average dietary aluminium intake in the population of Jilin Province based on body surface area. Furthermore, healthy adult Wistar rats were randomly divided into four groups (n=15 for each group): a control group and three groups treated with aluminium solution (1, 10, and 100 mg/kg/day, intragastrically) for 28 days. Following treatment, necrosis of renal tubular epithelial cells, hyperplasia of bile ducts and hyperplasia of heart tissue, as well as fiber in the liver, kidney, and heart tissues of aluminium-treated rats were observed, although there were no significant changes in the spleen and brain. Subsequently, fecal samples were withdrawn for 16S rRNA gene sequence analysis. It was found that aluminium decreased the microbiota diversity and changed the overall community structure of the gut microbiota, including three phyla and four genera, together with the regulation of 12 signaling pathways. Collectively, treatment with aluminium markedly altered the structure of the gut microbiota, suggesting that the disorders of intestinal flora induced by aluminium may be an important mechanism for aluminium toxicity.

GmWRKY81 Encoding a WRKY Transcription Factor Enhances Aluminum Tolerance in Soybean.

Aluminum (Al) toxicity is an essential factor that adversely limits soybean (Glycine max (L.) Merr.) growth in acid soils. WRKY transcription factors play important roles in soybean responses to abiotic stresses. Here, GmWRKY81 was screened from genes that were differentially expressed under Al treatment in Al-tolerant soybean Baxi10 and Al-sensitive soybean Bendi2. We found that GmWRKY81 was significantly induced by 20 μM AlCl3 and upregulated by AlCl3 treatment for 2 h. In different tissues, the expression of GmWRKY81 was differentially induced. In 0–1 cm root tips, the expression of GmWRKY81 was induced to the highest level. The overexpression of GmWRKY81 in soybean resulted in higher relative root elongation, root weight, depth, root length, volume, number of root tips and peroxidase activity but lower root average diameter, malonaldehyde and H2O2 contents, indicating enhanced Al tolerance. Moreover, RNA-seq identified 205 upregulated and 108 downregulated genes in GmWRKY81 transgenic lines. Fifteen of these genes that were differentially expressed in both AlCl3-treated and GmWRKY81-overexpressing soybean had the W-box element, which can bind to the upstream-conserved WRKY domain. Overall, the combined functional analysis indicates that GmWRKY81 may improve soybean Al tolerance by regulating downstream genes participating in Al3+ transport, organic acid secretion and antioxidant reactions.

Studying the Formation and Stabilization of Cur-Al+3 Complexes by Using Tartaric Acid as Catalyst

The aluminum cooking wares that were using in many countries, especially the developing countries be a source of free aluminum. Aluminum is recognized as a public health concern because of its potential toxic effects on human health. The present study was undertaken to determine the effectiveness of curcumin (CUR) in reducing the toxicity of leached aluminum through formation stable complexes. The available rhizomes in local markets contain 8.5% of curcumin as crude pigment. The maximum absorbance of curcumin at different pH values was determined. The curcumin-Al+3 complex was prepared by using tartaric acid as catalyst, at different pH values which in turn reflect the elimination percentages of Al+3 from the solution. The highly complexation was observed at pH 2.5, 3.0, 3.5 and 4.0. The Al+3 elimination at these pH values were 61.30,61.95,58.71 and 56.40% respectively. The stability of cur-Al+3 complexes at 25, 50, 75 and 100oC for 60 min at pH 2.5 represent 96.94, 96.29, 95.55 and 95.52 % of the initial concentration respectively. The identification of curcumin and its complex with Al+3 by using Fourier-transform infrared spectroscopy (FT-IR) was achieved. The leached aluminum from three regions of aluminum cooking wares (local, Syrian and Iranian) were determined at different pH values, 1.5, 2.5, 3.5, 4.5, 5.5 and 6.5 at boiling point. In general, Iranian cooking ware leached more aluminum comparing with Syrian and local cooking wares. The elimination percentages of leached aluminum by using the most leaching cooking ware (Iranian) were 61.30 and 58.71% at 2.5 and 3.5 pH respectively, which is high comparing with other pH values.

Boron contributes to excessive aluminum tolerance in trifoliate orange (Poncirus trifoliata (L.) Raf.) by inhibiting cell wall deposition and promoting vacuole compartmentation

Boron (B) is an indispensable micronutrient for plant growth that can also alleviate aluminum (Al) toxicity. However, limited data are available on the underlying mechanisms behind this phenomenon. Here, we found that a certain range of B application could alleviate the inhibitory effects of Al toxicity on citrus. Transcriptome analysis revealed that several Al stress-responsive genes and pathways were differentially affected and enriched, such as coding for the secretion of organic acid and the distribution of Al in subcellular components after B addition. Specifically, B application enhanced rhizosphere pH and induced malate exudation by expressing PtALMT4 and PtALMT9 genes occurred in Al-treated root, which ultimately reduced the absorption of Al and coincided with down-regulated the expression of PtNrat1. Moreover, B supply suppressed the pectin methyl-esterase (PME) activity and displayed a lower level of PtPME2 expression, while enhanced the PtSTAR1 expression, which is responsible for reducing cell wall (CW) Al deposition. Boron addition enhanced the PtALS1 and PtALS3 expression, accompanied by a higher proportion of vacuolar Al compartmentation during Al exposure. Collectively, the protective effects of B on root injury induced by Al is mainly by subsiding the Al uptake in the root apoplast and compartmentalizing Al into vacuole.

Silicon- and Boron-Induced Physio-Biochemical Alteration and Organic Acid Regulation Mitigates Aluminum Phytotoxicity in Date Palm Seedlings.

The current study aimed to understand the synergistic impacts of silicon (Si; 1.0 mM) and boron (B; 10 µM) application on modulating physio-molecular responses of date palm to mitigate aluminum (Al3+; 2.0 mM) toxicity. Results revealed that compared to sole Si and B treatments, a combined application significantly improved plant growth, biomass, and photosynthetic pigments during Al toxicity. Interestingly, Si and B resulted in significantly higher exudation of organic acid (malic acids, citric acids, and acetic acid) in the plant’s rhizosphere. This is also correlated with the reduced accumulation and translocation of Al in roots (60%) and shoots (56%) in Si and B treatments during Al toxicity compared to in sole Al3+ treatment. The activation of organic acids by combined Si + B application has significantly regulated the ALMT1, ALMT2 and plasma membrane ATPase; PMMA1 and PMMA3 in roots and shoots. Further, the Si-related transporter Lsi2 gene was upregulated by Si + B application under Al toxicity. This was also validated by the higher uptake and translocation of Si in plants. Al-induced oxidative stress was significantly counteracted by exhibiting lower malondialdehyde and superoxide production in Si + B treatments. Experiencing less oxidative stress was evident from upregulation of CAT and Cyt-Cu/Zn SOD expression; hence, enzymatic activities such as polyphenol oxidase, catalase, peroxidase, and ascorbate peroxidase were significantly activated. In the case of endogenous phytohormones, Si + B application demonstrated the downregulation of the abscisic acid (ABA; NCED1 and NCED6) and salicylic acid (SA; PYL4, PYR1) biosynthesis-related genes. Consequently, we also noticed a lower accumulation of ABA and rising SA levels under Al-stress. The current findings illustrate that the synergistic Si + B application could be an effective strategy for date palm growth and productivity against Al stress and could be further extended in field trails in Al-contaminated fields.

Growth Response and Dry Matter Partitioning of Quality Protein Maize (Zea mays L.) Genotypes under Aluminum Toxicity

In this work, 75 quality protein maize (QPM) inbred lines were evaluated for aluminum tolerance using a nutrient solution assay in a laboratory and a soil-based technique in a greenhouse tunnel. The experiment was set up in a completely randomized design with three replications in the laboratory, and a randomized complete block design was used in the greenhouse. Aluminum toxicity was generated by amending a nutrient solution with 600 µM of aluminum sulfate (Al2 [SO4]3) in the laboratory, and Al2 [SO4]3 was applied at a rate of 24 mg kg−1 of soil in the greenhouse experiment. Relative root length (RRL) and hematoxylin staining (HS) scores were used to identify tolerant genotypes in the laboratory. According to RRL, 94.7% of genotypes were tolerant and 5.3% were sensitive, while Hematoxylin (HS) classified 77.9% of the genotypes as tolerant, and 22.1% as sensitive. RRL and HS presented a very strong negative association (−0.788). In the soil-based method, the experiments were conducted twice in successive summer seasons of 2019 and 2020. Several growth traits were measured and most genotypes that exhibited tolerance in the nutrient solution also had similar tolerance in the soil-based screening technique. Genetic variability for tolerance was identified, revealing potentially useful donors of tolerance genes in breeding programs.

Boron-mediated lignin metabolism in response to aluminum toxicity in citrus (Poncirus trifoliata (L.) Raf.) root

Aluminum (Al) toxicity has conspicuous detrimental effects on citrus production whereas boron (B) has been shown to alleviate its toxicity. Lignin plays a critical role in the cell wall extensibility and root elongation under stressed conditions. Hence, the interaction between B and Al on cell wall structure and lignin-related metabolic pathway was investigated in root of trifoliate orange (Poncirus trifoliata (L.) Raf.) seedlings. The results showed B supply considerably decreased the Al content in root, particularly in cell wall, and reduced Al-induced damage on growth-related parameters and thickness of cell wall. Boron application decreased the hydrogen peroxide (H2O2), malondialdehyde (MDA), and lignin contents in the Al-treated root, which prevents the inhibitory effects of Al on the root length. Moreover, metabonomics results showed that B addition resulted in the reduction of metabolites involved in the lignin biosynthesis pathways (phenylpropanoid metabolic) i.e., shikimic acid, tyrosine, caffeic acid, chlorogenic acid, coniferyl alcohol, sinapinic acid, sinapaldehyde, and sinapyl alcohol, as well as distinctively restrain the activities of lignin biosynthesis-related enzymes (4-coumarate-CoA ligase (4CL), cinnamyl-alcohol dehydrogenase (CAD)) under Al toxicity. Collectively, our findings suggest that the positive effects of B on the resistance of Al toxicity may be it reduces Al accumulation in the cell wall, lignin biosynthesis, and cell wall thickness, thereby increasing the extensibility and elasticity of cell wall and thus promoting root elongation.