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

Aluminium stress tolerance by Citrus plants: a consolidated review

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

Metabolomics reveals the phytotoxicity mechanisms of foliar spinach exposed to bulk and nano sizes of PbCO3

PbCO3 is an ancient raw material for Pb minerals and continues to pose potential risks to the environment and human health through mining and industrial processes. However, the specific effects of unintentional PbCO3 discharge on edible plants remain poorly understood. This study unravels how foliar application of PbCO3 induces phytotoxicity by potentially influencing leaf morphology, photosynthetic pigments, oxidative stress, and metabolic pathways related to energy regulation, cell damage, and antioxidant defense in Spinacia oleracea L. Additionally, it quantifies the resultant human health risks. Plants were foliarly exposed to PbCO3 nanoparticles (NPs) and bulk products (BPs), as well as Pb2+ at 0, 5, 10, 25, 50, and 100 mg·L-1 concentrations once a day for three weeks. The presence and localization of PbCO3 NPs inside the plant cells were confirmed by TEM-EDS analysis. The maximum accumulation of total Pb was recorded in the root (2947.77 mg·kg-1 DW for ion exposure), followed by the shoot (942.50 mg·kg-1 DW for NPs exposure). The results revealed that PbCO3 and Pb2+ exposure had size- and dose-dependent inhibitory effects on spinach length, biomass, and photosynthesis attributes, inducing impacts on the antioxidase activity of CAT, membrane permeability, and nutrient elements absorption and translocation. Pb2+ exhibited pronounced toxicity in morphology and chlorophyll; PbCO3 BP exposure accumulated the most lipid peroxidation products of MDA and H2O2; and PbCO3 NPs triggered the largest cell membrane damage. Furthermore, PbCO3 NPs at 10 and 100 mg·L-1 induced dose-dependent metabolic reprogramming in spinach leaves, disturbing the metabolic mechanisms related to amino acids, antioxidant defense, oxidative phosphorylation, fatty acid cycle, and the respiratory chain. The spinach showed a non-carcinogenic health risk hierarchy: Pb2+ > PbCO3 NPs > PbCO3 BPs, with children more vulnerable than adults. These findings enhance our understanding of PbCO3 particle effects on food security, emphasizing the need for further research to minimize their impact on human dietary health.

Coordinated Regulation of Central Carbon Metabolism in Pyroligneous Acid-Treated Tomato Plants under Aluminum Stress

Aluminum (Al) toxicity is a major threat to global crop production in acidic soils, which can be mitigated by natural substances such as pyroligneous acid (PA). However, the effect of PA in regulating plant central carbon metabolism (CCM) under Al stress is unknown. In this study, we investigated the effects of varying PA concentrations (0, 0.25 and 1% PA/ddH₂O (v/v)) on intermediate metabolites involved in CCM in tomato (Solanum lycopersicum L., 'Scotia') seedlings under varying Al concentrations (0, 1 and 4 mM AlCl₃). A total of 48 differentially expressed metabolites of CCM were identified in the leaves of both control and PA-treated plants under Al stress. Calvin-Benson cycle (CBC) and pentose phosphate pathway (PPP) metabolites were considerably reduced under 4 mM Al stress, irrespective of the PA treatment. Conversely, the PA treatment markedly increased glycolysis and tricarboxylic acid cycle (TCA) metabolites compared to the control. Although glycolysis metabolites in the 0.25% PA-treated plants under Al stress were comparable to the control, the 1% PA-treated plants exhibited the highest accumulation of glycolysis metabolites. Furthermore, all PA treatments increased TCA metabolites under Al stress. Electron transport chain (ETC) metabolites were higher in PA-treated plants alone and under 1 mM, Al but were reduced under a higher Al treatment of 4 mM. Pearson correlation analysis revealed that CBC metabolites had a significantly strong positive (r = 0.99; p < 0.001) association with PPP metabolites. Additionally, glycolysis metabolites showed a significantly moderate positive association (r = 0.76; p < 0.05) with TCA metabolites, while ETC metabolites exhibited no association with any of the determined pathways. The coordinated association between CCM pathway metabolites suggests that PA can stimulate changes in plant metabolism to modulate energy production and biosynthesis of organic acids under Al stress conditions.

Aluminum in plant: Benefits, toxicity and tolerance mechanisms

Aluminum (Al) is the third most ubiquitous metal in the earth's crust. A decrease in soil pH below 5 increases its solubility and availability. However, its impact on plants depends largely on concentration, exposure time, plant species, developmental age, and growing conditions. Although Al can be beneficial to plants by stimulating growth and mitigating biotic and abiotic stresses, it remains unknown how Al mediates these effects since its biological significance in cellular systems is still unidentified. Al is considered a major limiting factor restricting plant growth and productivity in acidic soils. It instigates a series of phytotoxic symptoms in several Al-sensitive crops with inhibition of root growth and restriction of water and nutrient uptake as the obvious symptoms. This review explores advances in Al benefits, toxicity and tolerance mechanisms employed by plants on acidic soils. These insights will provide directions and future prospects for potential crop improvement.

Appraisal of Abelmoschus esculentus L. Response to Aluminum and Barium Stress

Trace metal element (TME) pollution is a major threat to plants, animals and humans. Agricultural products contaminated with metals may pose health risks for people; therefore, international standards have been established by the FAO/WHO to ensure food safety as well as the possibility of crop production in contaminated soils. This study aimed to assess the accumulating potential of aluminum and barium in the roots, shoots and fruits of Abelmoschus esculentus L., and their effect on growth and mineral nutrition. The content of proline and some secondary metabolites was also evaluated. After treating okra plants with aluminum/barium (0, 100, 200, 400 and 600 µM) for 45 days, the results showed that Al stimulated the dry biomass production, whereas Ba negatively affected the growth and the fructification yield. The okra plants retained both elements and exhibited a preferential accumulation in the roots following the sequence: roots > shoots > fruits, which is interesting for phytostabilization purposes. Al or Ba exposure induced a decline in mineral uptake (K, Ca, Mg, Zn and Fe), especially in roots and shoots. In order to cope with the stress conditions, the okra plants enhanced their proline and total phenol amounts, offering better adaptability to stress.

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.

Molecular mechanisms for pH-mediated amelioration of aluminum-toxicity revealed by conjoint analysis of transcriptome and metabolome in Citrus sinensis roots

Little is known about the effects of pH-aluminum (Al) interactions on gene expression and/or metabolite profiles in plants. Eleven-week-old seedlings of Citrus sinensis were fertilized with nutrient solution at an Al level of 0 or 1 mM and a pH of 3.0 or 4.0 for 18 weeks. Increased pH mitigated Al-toxicity-induced accumulation of callose, an Al-sensitive marker. In this study, we identified more differentially expressed genes and differentially abundant metabolites in pH 4.0 + 1 mM Al-treated roots (P4AR) vs pH 4.0 + 0 mM Al-treated roots (P4R) than in pH 3.0 + 1 mM Al-treated roots (P3AR) vs pH 3.0 + 0 mM Al-treated roots (P3R), suggesting that increased pH enhanced root metabolic adaptations to Al-toxicity. Further analysis indicated that increased pH-mediated mitigation of root Al-toxicity might be related to several factors, including: enhanced capacity to maintain the homeostasis of phosphate and energy and the balance between generation and scavenging of reactive oxygen species and aldehydes; and elevated accumulation of secondary metabolites such as polyphenol, proanthocyanidins and phenolamides and adaptations of cell wall and plasma membrane to Al-toxicity.

Evaluation of Physiological and Biochemical Parameters and Some Bioindicators of Barium Tolerance in Limbarda crithmoides and Helianthus annuus

Soils and water resources of our ecosystems may contain Barium (Ba), a toxic metal naturally existent in the Earth’s crust and also can be derived from recycled wastes produced of several anthropogenic activities. As a result of this fact, the accumulation of Ba in agriculture soils would increase to reach the crops and eventually end up in the human food chain. The purpose of this work was to study tolerance and accumulation abilities in Limbarda crithmoides and Helianthus annuus treated with increasing concentrations of barium (from 0 to 500 µM) for 45 days. In order to evaluate the response of these species to Ba stress, the biomass production, the water status, and the accumulation of the secondary metabolites, macronutrients, total inorganic nitrogen (TIN), and Ba in shoots and roots, as well as chlorophyll levels, and metal tolerance index of the entire plant were assessed. Results showed an increase in plant biomass production and tolerance index in the two species with increasing Ba concentration. A significant increase in polyphenols and flavonoids levels was also shown with no negative effect on the macronutrients and TIN; however, the latter were found reduced in roots of L. crithmoides. Chlorophylls also were not affected. An average of 3000 µg·g−1 DW of Ba was accumulated in each organ of L. crithmoides while H. annuus accumulated up to 1350 µg·g−1 DW in the shoots. Our findings proved that L. crithmoides and H. annuus were susceptible to tolerate Ba-induced stress with high levels of Ba accumulation in the aboveground parts as well as in the roots during the 45 days of the experiments.

Molecular and Physiological Responses of Citrus sinensis Leaves to Long-Term Low pH Revealed by RNA-Seq Integrated with Targeted Metabolomics

Low pH-induced alterations in gene expression profiles and organic acids (OA) and free amino acid (FAA) abundances were investigated in sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] leaves. We identified 503 downregulated and 349 upregulated genes in low pH-treated leaves. Further analysis indicated that low pH impaired light reaction and carbon fixation in photosynthetic organisms, thereby lowering photosynthesis in leaves. Low pH reduced carbon and carbohydrate metabolisms, OA biosynthesis and ATP production in leaves. Low pH downregulated the biosynthesis of nitrogen compounds, proteins, and FAAs in leaves, which might be conducive to maintaining energy homeostasis during ATP deprivation. Low pH-treated leaves displayed some adaptive responses to phosphate starvation, including phosphate recycling, lipid remodeling, and phosphate transport, thus enhancing leaf acid-tolerance. Low pH upregulated the expression of some reactive oxygen species (ROS) and aldehyde detoxifying enzyme (peroxidase and superoxidase) genes and the concentrations of some antioxidants (L-tryptophan, L-proline, nicotinic acid, pantothenic acid, and pyroglutamic acid), but it impaired the pentose phosphate pathway and V_E and secondary metabolite biosynthesis and downregulated the expression of some ROS and aldehyde detoxifying enzyme (ascorbate peroxidase, aldo-keto reductase, and 2-alkenal reductase) genes and the concentrations of some antioxidants (pyridoxine and γ-aminobutyric acid), thus disturbing the balance between production and detoxification of ROS and aldehydes and causing oxidative damage to leaves.

Metabolic and DNA checkpoints for the enhancement of Al tolerance

Acidic soils are a major limiting factor for food production in many developing countries. High concentrations of soluble Al cations, particularly Al³⁺, inhibit cell division and root elongation in plants. Al³⁺ damages several biomolecules, including DNA, impairing gene expression and cell cycle progression. Notably, the loss-of-function mutants of DNA checkpoints may mediate Al tolerance. Furthermore, mitochondrial organic acids play key roles in neutralizing Al³⁺ within the cell and around the rhizosphere. Here, we provide knowledge synthesis on interactions between checkpoints related to mitochondrial organic acid homeostasis and DNA integrity mediating Al tolerance in land plants. These interactions, coupled with remarkable advances in tools related to metabolism and cell cycle, may facilitate the development of next-generation productive crops under Al toxicity.

The aluminum distribution and translocation in two citrus species differing in aluminum tolerance

BACKGROUND

Many citrus orchards of south China suffer from soil acidification, which induces aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species are not well understood.

RESULTS

The seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] and 'Shatianyou' [Citrus grandis (L.) Osbeck], that differ in Al tolerance, were hydroponically treated with a nutrient solution (Control) or supplemented by 1.0 mM Al³⁺ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species followed the order: lateral roots > primary roots > leaves > stems. The concentration of Al extracted from the cell wall (CW) of lateral roots was found to be about 8 to 10 times higher than in the lateral roots under Al toxicity, suggesting that the CW was the primary Al-binding site at the subcellular level. Furthermore, the Al distribution in CW components of the lateral roots showed that pectin had the highest affinity for binding Al. The relative expression level of genes directly relevant to Al transport indicated a dominant role of Cs6g03670.1 and Cg1g021320.1 in the Al distribution of two citrus species. Compared to C. grandis, C. sinensis had a significantly higher Al concentration on the CW of lateral roots, whereas remarkably lower Al levels in the leaves and stems. Furthermore, Al translocation revealed by the absorption kinetics of the CW demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root CW than C. grandis. According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by a modification in the structure and components of the citrus lateral root CW.

CONCLUSIONS

A higher Al-retention, mainly attributable to pectin of the root CW, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis. The aluminum distribution and translocation of two citrus species differing in aluminum tolerance were associated with the transcriptional regulation of genes related to Al transport and the structural modification of root CW.

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

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

Metabolomics combined with physiology and transcriptomics reveals how Citrus grandis leaves cope with copper-toxicity

Limited data are available on metabolic responses of plants to copper (Cu)-toxicity. Firstly, we investigated Cu-toxic effects on metabolomics, the levels of free amino acids, NH₄⁺-N, NO₃^--N, total nitrogen, total soluble proteins, total phenolics, lignin, reduced glutathione (GSH) and malondialdehyde, and the activities of nitrogen-assimilatory enzymes in 'Shatian' pummelo (Citrus grandis) leaves. Then, a conjoint analysis of metabolomics, physiology and transcriptomics was performed. Herein, 59 upregulated [30 primary metabolites (PMs) and 29 secondary metabolites (SMs)] and 52 downregulated (31 PMs and 21 SMs) metabolites were identified in Cu-toxic leaves. The toxicity of Cu to leaves was related to the Cu-induced accumulation of NH₄⁺ and decrease of nitrogen assimilation. Metabolomics combined with physiology and transcriptomics revealed some adaptive responses of C. grandis leaves to Cu-toxicity, including (a) enhancing tryptophan metabolism and the levels of some amino acids and derivatives (tryptophan, phenylalanine, 5-hydroxy-l-tryptophan, 5-oxoproline and GSH); (b) increasing the accumulation of carbohydrates and alcohols and upregulating tricarboxylic acid cycle and the levels of some organic acids and derivatives (chlorogenic acid, quinic acid, d-tartaric acid and gallic acid o-hexoside); (c) reducing phospholipid (lysophosphatidylcholine and lysophosphatidylethanolamine) levels, increasing non-phosphate containing lipid [monoacylglycerol ester (acyl 18:2) isomer 1] levels, and inducing low-phosphate-responsive gene expression; and (d) triggering the biosynthesis of some chelators (total phenolics, lignin, l-trytamine, indole, eriodictyol C-hexoside, quercetin 5-O-malonylhexosyl-hexoside, N-caffeoyl agmatine, N'-p-coumaroyl agmatine, hydroxy-methoxycinnamate and protocatechuic acid o-glucoside) and vitamins and derivatives (nicotinic acid-hexoside, B₁ and methyl nicotinate). Cu-induced upregulation of many antioxidants could not protect Cu-toxic leaves from oxidative damage. To conclude, our findings corroborated the hypothesis that extensive reprogramming of metabolites was carried out in Cu-toxic C. grandis leaves in order to cope with Cu-toxicity.

Behavior of Cucumis sativus L. in presence of aluminum stress: Germination, plant growth, and antioxidant enzymes

Aluminum (Al) is an ubiquitously present element in soil; it is considered as a major problem in crop production that affects plant growth and development on acid soils. The aim of this work was to determine the germination parameters, to quantify the water status and growth, to assess the Al accumulation, and antioxidant enzyme activities in plants to evaluate the stress exerted by aluminum in Cucumis sativus L. For germination test, increasing doses of Al were used (0, 200, 500, 1,000, and 2,000 μM). Results showed that germination was stimulated with 500 of Al. Aluminum effects on development were studied by treating the plants with different concentrations of Al (100, 200, 300, and 500 µM, Al) during 45 days. As regards to the plant's growth, water content, and dry biomass production there was a slight increase. On the other hand, the activities of the antioxidant enzymes were disturbed by aluminum stress. Data indicate that the catalase (CAT) activity showed a decrease in the different parts of the plant. However, guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) activities were significantly stimulated. Studying the effects of Al-induced stress allowed us to conclude that cucumber has a high ability to accumulate this element in the roots.

Transcriptomic responses to aluminum stress in tea plant leaves

Tea plant (Camellia sinensis) is a well-known Al-accumulating plant, showing a high level of aluminum (Al) tolerance. However, the molecular mechanisms of Al tolerance and accumulation are poorly understood. We carried out transcriptome analysis of tea plant leaves in response to three different Al levels (0, 1, 4 mM, for 7 days). In total, 794, 829 and 585 differentially expressed genes (DEGs) were obtained in 4 mM Al vs. 1 mM Al, 0 Al vs. 1 mM Al, and 4 mM Al vs. 0 Al comparisons, respectively. Analysis of genes related to polysaccharide and cell wall metabolism, detoxification of reactive oxygen species (ROS), cellular transport, and signal transduction were involved in the Al stress response. Furthermore, the transcription factors such as zinc finger, myeloblastosis (MYB), and WRKY played a critical role in transcriptional regulation of genes associated with Al resistance in tea plant. In addition, the genes involved in phenolics biosynthesis and decomposition were overwhelmingly upregulated in the leaves treated with either 0 Al and 4 mM Al stress, indicating they may play an important role in Al tolerance. These results will further help us to understand mechanisms of Al stress and tolerance in tea plants regulated at the transcriptional level.

Molecular and physiological mechanisms underlying magnesium-deficiency-induced enlargement, cracking and lignification of Citrus sinensis leaf veins

Little is known about the physiological and molecular mechanisms underlying magnesium (Mg)-deficiency-induced enlargement, cracking and lignification of midribs and main lateral veins of Citrus leaves. Citrus sinensis (L.) Osbeck seedlings were irrigated with nutrient solution at a concentration of 0 (Mg-deficiency) or 2 (Mg-sufficiency) mM Mg(NO3)2 for 16 weeks. Enlargement, cracking and lignification of veins occurred only in lower leaves, but not in upper leaves. Total soluble sugars (glucose + fructose + sucrose), starch and cellulose concentrations were less in Mg-deficiency veins of lower leaves (MDVLL) than those in Mg-sufficiency veins of lower leaves (MSVLL), but lignin concentration was higher in MDVLL than that in MSVLL. However, all four parameters were similar between Mg-deficiency veins of upper leaves (MDVUL) and Mg-sufficiency veins of upper leaves (MSVUL). Using label-free, liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we identified 1229 and 492 differentially abundant proteins (DAPs) in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively. Magnesium-deficiency-induced alterations of Mg, nonstructural carbohydrates, cell wall components, and protein profiles were greater in veins of lower leaves than those in veins of upper leaves. The increased concentration of lignin in MDVLL vs MSVLL might be caused by the following factors: (i) repression of cellulose and starch accumulation promoted lignin biosynthesis; (ii) abundances of proteins involved in phenylpropanoid biosynthesis pathway, hormone biosynthesis and glutathione metabolism were increased; and (iii) the abundances of the other DAPs [viz., copper/zinc-superoxide dismutase, ascorbate oxidase (AO) and ABC transporters] involved in lignin biosynthesis were elevated. Also, the abundances of several proteins involved in cell wall metabolism (viz., expansins, Rho GTPase-activating protein gacA, AO, monocopper oxidase-like protein and xyloglucan endotransglucosylase/hydrolase) were increased in MDVLL vs MSVLL, which might be responsible for the enlargement and cracking of leaf veins.

Phosphorus-mediated alleviation of aluminum toxicity revealed by the iTRAQ technique in Citrus grandis roots

Citrus grandis seedlings were irrigated with nutrient solutions with four Al-P combinations [two Al levels (0 mM and 1.2 mM AlCl3·6H2O) × two P levels (0 μM and 200 μM KH2PO4)] for 18 weeks. Al dramatically inhibited the growth of C. grandis seedlings, as revealed by a decreased dry weight of roots and shoots. Elevating P level could ameliorate the Al-induced growth inhibition and organic acid (malate and citrate) secretion in C. grandis. Using a comparative proteomic approach revealed by the isobaric tags for relative and absolute quantification (iTRAQ) technique, 318 differentially abundant proteins (DAPs) were successfully identified and quantified in this study. The possible mechanisms underlying P-induced alleviation of Al toxicity in C. grandis were proposed. Furthermore, some DAPs, such as GLN phosphoribosyl pyrophosphate amidotransferase 2, ATP-dependent caseinolytic (Clp) protease/crotonase family protein, methionine-S-oxide reductase B2, ABC transporter I family member 17 and pyridoxal phosphate phosphatase, were reported for the first time to respond to Al stress in Citrus plants. Our study provides some proteomic details about the alleviative effects of P on Al toxicity in C. grandis, however, the exact function of the DAPs identified herein in response to Al tolerance in plants must be further investigated.

Aluminum stress differentially affects physiological performance and metabolic compounds in cultivars of highbush blueberry

Aluminum (Al) toxicity is one of the major factors that limit the growth and production of crops in acid soils. Highbush blueberry (Vaccinium corymbosum L.) cultivars differing in resistance to Al toxicity regarding root growth and photosynthetic performance were used. In this study, we compared the physiological and metabolic strategies to cope with Al toxicity among the highbush blueberry cultivars [two new ones (Camellia and Cargo) and three established ones (Brigitta (Al-resistant), Star and Duke)]. Aluminum concentration in roots and leaves increased in all cultivars after 24 and 48 h of exposure to Al, but less so in roots of cultivar Camellia and leaves of cultivar Cargo. These two cultivars displayed minor effects of Al exposure in terms of photosynthetic activity in comparison with the established cultivars. Furthermore, Cargo did not vary fluorescence parameters, whereas Camellia exhibited a decrease in effective quantum yield (ΦPSII) and electron transport rate (ETR) and a change in non-photochemical quenching (NPQ) and maximum quantum yield (Fv/Fm) under Al after 48 h. The Al treatment increased total phenols in leaves of Brigitta, Cargo, and Camellia, whereas antioxidant activity increased in Star and Cargo after 48 h. Aluminum exposure decreased malate concentration in roots of all cultivars, but no change was noted in fumarate concentration. The antioxidant activity correlated with photosynthetic performance and the total phenol concentration in the leaves of new cultivars exposed to Al, suggesting enhanced resistance in the short-term experiment. The principal component analysis separated the new from the established cultivars. In conclusion, the new cultivars appear to be more Al-resistant than the established ones, with Star being most Al-sensitive. Regarding the Al-resistance mechanisms of the new cultivars, it is suggested that Camellia could have a root Al-exclusion mechanism under Al toxicity. This mechanism could be explained by low Al concentration in roots, suggesting that this cultivar could exude organic acid, allowing to chelate Al in the rhizosphere. Nonetheless, further researches are needed to confirm this assumption.

Proteome Dataset of Qualea grandiflora Mart. (Vochysiaceae) by LC-MS/MS Label-Free Identification in Response to Aluminum

This dataset brief is about the descriptive proteome of Qualea grandiflora plants by label free mass spectrometry (LC-MS/MS). Q. grandiflora is a plant that accumulates aluminum (Al) in high quantities and requires it for growth and development. Although quite relevant for the understanding of Al effects on plants, the proteome of Q. grandiflora has not been studied yet. Therefore, the current proteome analysis identifies a total of 2010 proteins. Furthermore, the identified Q. grandiflora root proteins are associated with several crucial molecular functions, biological processes, and cellular sites. Hence, the proteome analysis of Q. grandiflora will contribute to unravel how plants evolved to cope with high levels of Al in soils. All data can be accessed at the Centre for Computational Mass Spectrometry - MassIVE MSV000082284 - https://massive.ucsd.edu/ProteoSAFe/dataset.jsp?task=adb9647282a5421a9cffe3124c060f46.

Hyperinsulinemia precedes insulin resistance in offspring rats exposed to angiotensin II type 1 autoantibody in utero

OBJECTIVE

Insulin resistance is highly associated with an adverse intrauterine environment. We previously reported that fetal rats exposed to angiotensin II type 1 receptor (AT₁R) autoantibody (AT1-AA) displayed increased susceptibility to metabolic diseases during middle age. However, the timing of the onset of insulin resistance remains unknown. In this study, we examined the offspring of AT1-AA-positive rats, tracking the development of insulin resistance.

METHODS

Pregnant rats were intravenously injected with AT1-AA. Afterwards, we collected serum samples and liver tissues of the offspring at various stages, including gestation day 18, 3 weeks (weaning period), 18 weeks (young adulthood), and 48 weeks (middle age) after birth.

RESULTS

Compared with saline control group, hepatic vacuolar degeneration was visible in AT1-AA offspring rats as early as 3 weeks; hyperinsulinemia and impaired glucose tolerance occurred at 18 weeks of age, however, insulin resistance was not observed until 48 weeks. At 18 weeks we detected suppressed protein levels of insulin receptor (IR) but increased levels of IR substrate 1 (IRS1) in the liver of AT1-AA group rats. Interestingly, both IR and IRS1/2 were significantly decreased at 48 weeks. Liver proteomic analysis indicated that the differences in protein expression between the AT1-AA and control rats became more pronounced with age, particularly in terms of mitochondrial energy metabolism.

CONCLUSION

Rats exposed to AT1-AA in utero developed hyperinsulinemia from young adulthood which subsequently progressed to insulin resistance, and was linked with abnormal hepatic structure and impaired IR signaling. Additionally, dysregulation of energy metabolism may play a fundamental role in predisposing offspring to insulin resistance.

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.

Photosystem II Is More Sensitive than Photosystem I to Al3+ Induced Phytotoxicity

Aluminium (Al) the most abundant metal in the earth's crust is toxic in acid soils (pH < 5.5) mainly in the ionic form of Al3+ species. The ability of crops to overcome Al toxicity varies among crop species and cultivars. Here, we report for a first time the simultaneous responses of photosystem II (PSII) and photosystem I (PSI) to Al3+ phytotoxicity. The responses of PSII and PSI in the durum wheat (Triticum turgidum L. cv. 'Appulo E') and the triticale (X Triticosecale Witmark cv. 'Dada') were evaluated by chlorophyll fluorescence quenching analysis and reflection spectroscopy respectively, under control (-Al, pH 6.5) and 148 μM Al (+Al, pH 4.5) conditions. During control growth conditions the high activity of PSII in 'Appulo E' led to a rather higher electron flow to PSI, which induced a higher PSI excitation pressure in 'Appulo E' than in 'Dada' that presented a lower PSII activity. However, under 148 μM Al the triticale 'Dada' presented a lower PSII and PSI excitation pressure than 'Appulo E'. In conclusion, both photosystems of 'Dada' displayed a superior performance than 'Appulo E' under Al exposure, while in both cultivars PSII was more affected than PSI from Al3+ phytotoxicity.

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.

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.

Proteomics in commercial crops: An overview

Proteomics is a rapidly growing area of biological research that is positively affecting plant science. Recent advances in proteomic technology, such as mass spectrometry, can now identify a broad range of proteins and monitor their modulation during plant growth and development, as well as during responses to abiotic and biotic stresses. In this review, we highlight recent proteomic studies of commercial crops and discuss the advances in understanding of the proteomes of these crops. We anticipate that proteomic-based research will continue to expand and contribute to crop improvement.

SIGNIFICANCE

Plant proteomics study is a rapidly growing area of biological research that is positively impacting plant science. With the recent advances in new technologies, proteomics not only allows us to comprehensively analyses crop proteins, but also help us to understand the functions of the genes. In this review, we highlighted recent proteomic studies in commercial crops and updated the advances in our understanding of the proteomes of these crops. We believe that proteomic-based research will continue to grow and contribute to the improvement of crops.

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.

SEM-EDS-Based Elemental Identification on the Enamel Surface after the Completion of Orthodontic Treatment: In Vitro Studies

Braces as foreign bodies in the mouth carry a risk of side effects and toxicity to the human body. This article presents the results indicating the possible toxic effects of tools used for cleaning the enamel after the completion of orthodontic treatment. The studies were carried out in vitro. The procedure of enamel etching, bonding orthodontic metal brackets, and enamel cleaning after their removal was performed under laboratory conditions. The enamel microstructure and elements present on its surface were evaluated using the scanning electron microscope (SEM). Silicon and aluminium were found in addition to the tooth building elements.