Fluoride Stress Research Articles

Fluoride Toxicity Imposes Differential Reprogramming of the Representative Intermediates and Enzymes Belonging to Nitrogen Metabolism in Two indica Rice Varieties, Varying in their Pattern of Fluoride Stress Response

Fluoride Toxicity Imposes Differential Reprogramming of the Representative Intermediates and Enzymes Belonging to Nitrogen Metabolism in Two indica Rice Varieties, Varying in their Pattern of Fluoride Stress Response

Extensive cross-talk among stress-regulated protective metabolites, biogenic-amines and phytohormone-signalling, co-ordinated by dopamine-mediated seed-priming, governs tolerance against fluoride stress in rice.

Seed priming with dopamine reduced fluoride bioaccumulation, induced endogenous dopamine level, thereby orchestrating phytohormone homeostasis and biogenic amine metabolism, and modulating osmolyte and antioxidant machinery to enhance fluoride tolerance in rice. The aim of this study was to decipher the efficacy of seed priming with dopamine in curtailing the adverse impacts of fluoride toxicity in rice seedlings. Fluoride-stressed seedlings exhibited severe growth retardation, high fluoride bioaccumulation, electrolyte leakage and marked cellular injuries. Dopamine priming stimulated the overall physiological growth parameters during stress, via reduced formation of H2O2, malondialdehyde and methylglyoxal, due to lesser fluoride-accumulation. Fluoride stress-induced endogenous dopamine level was further induced upon dopamine priming, marked by the up regulated DOPA decarboxylase expression. Additionally, dopamine treatment led to escalated activity of catalase, superoxide dismutase and glutathione peroxidase in the stressed seedlings, concomitant with altered CAT, SOD and GPX expression. The higher accumulation of protective osmolytes (proline and total amino acids) and non-enzymatic antioxidants (phenolics, flavonoids, anthocyanins, glutathione and carotenoids), upon dopamine priming, during fluoride stress, could be linked with the altered expression pattern of the respective genes. Dopamine promoted active utilization of the biogenic amine (polyamines and ϒ-amino butyric acid) pools for toxicity mitigation, correlated with the modulation of the concerned enzyme activity and gene expression. Dopamine stimulated the accumulation of phytohormones like gibberellin and salicylic acid, via inducing the biosynthetic genes like gibberellin-3-oxidase (GA3ox) and isochorismate synthase (ICS), respectively, while depreciating the abscisic acid and melatonin level during fluoride stress. To our knowledge, this is the first documented report for the remedial role of dopamine priming against fluoride stress in any plant species. This study will open new arenas in sustainable agriculture for the exploitation of this pulsating biomolecule against fluoride stress.

Toward a molecular understanding of fluoride stress in a model Pseudomonas strain.

Toward a molecular understanding of fluoride stress in a model Pseudomonas strain.

Endogenous calcium mediates seedling growth and fluoride stress tolerance in four bean genotypes

Fluoride (F) pollution is a global environmental problem representing a severe risk for food and vegetables grown in contaminated soils. Phaseolus vulgaris L. is widely cultivated in arid and semi-arid regions and in F contaminated areas of the world. For that reason, F tolerance during germination and seedling growth was evaluated for four bean genotypes: Borlotto nano (commercial variety) and three African genotypes (Lyamungu 85, Lyamungu 90 and Jesca). Seeds were grown in sand enriched with NaF or KF at three different levels (0, 80 and 200 mg kg-1). NaCl was used as a benchmark to determine the potential effect of different Na levels in plant. Total F content and mineral accumulation (Na, K and Ca) in roots and shoots were measured. The translocation factor, growth ratio, F tolerance index were evaluated to estimate plant-salt response. Germination rate decreased with increased F level. Borlotto was more F sensitive (0% germination with 200 mg kg-1of KF and NaF) than the African genotypes. Under the highest F concentration (200 mg kg-1), F preferentially accumulated in shoots (Jesca 75.7 mg kg-1, Lyamungu 85 100.1 mg kg-1 and Lyamungu 90 115.4 mg kg-1). Ca content in roots was negatively correlated to F absorption, suggesting its antagonistic role to F mobility. Based on these parameters, Jesca and Lyamungu 85 were the most tolerant species, recording a low F uptake and a high Ca content in the root. This study highlighted the central role of Ca, as a key secondary messenger in regulating the plant growth and development under F stress. Highlights - F stress negatively affected bean germination and seedling growth - F mainly accumulated in the shoots of bean varieties - Ca concentration in the roots played a crucial role in mitigating F accumulation

Preliminary Protein Profiling of Sodium Fluoride treated Lactobacillus acidophilus and Lactobacillus salivarius

Sodium fluoride is a trace element required for human beings to prevent early dental disorders and to meet body’s minimum Fluoride levels. It is signified as a nutritional supplement for the prevention of dental caries in children of areas with inadequate Fluoride concentration in the drinking water. When Fluoride concentration exceeds required levels in the body, it commences bacteriostatic activity against beneficial flora in the gastrointestinal tract. Two such commonly affected organisms are L. acidophilus and L. salivarius. These are probiotic organisms that help to maintain immunogenic gut against several pathogenic organisms. In our previous study, Minimum inhibitory concentration (MIC) and growth dynamics were assessed on L. acidophilus and L. salivarius, in the presence of different Sodium fluoride concentrations. L. acidophilus and L. salivarius were observed to be inhibited at 20 mM and 40 mM Sodium fluoride concentrations respectively. These inhibitory concentrations were selected for further analysis. The proteins were isolated from such Sodium fluoride treated and untreated cells, the protein concentration was estimated by Bradford assay and protein profiling was done by 1D Gel Electrophoresis. The protein concentration is found to be higher in Sodium fluoride untreated organisms and below 3 kDa proteins of Sodium fluoride treated samples. Whereas low protein concentration was observed in the above 3 kDa protein samples (L.acidophilus treated protein sample above 3 kDa and L.salivarius treated protein sample above 3 kDa) of fluoride treated organisms. L. acidophilus and L. salivarius showed difference in protein expression under fluoride stress. Protein expression is high in L. salivarius than L. acidophilus. This is an indication that these strains have different capabilities for adapting to varying environmental conditions. we conclude that there is no impact on below 3 kDa protein samples in Sodium fluoride treated organisms and impact was there on the above 3 Kda proteins which are inhibited.

Calibration of polyvinylidene fluoride (PVDF) stress gauges under high-impact dynamic compression by machine learning

Calibration of stress gauges is of great importance for understanding the behaviors of materials under high dynamic impacts. However, commonly used calibration models have little transferability due to ignoring the influences of the gauge parameters. In this work, we propose a systematic approach that can generate effective and transferable calibration models including multiple independent variables by machine learning. Specifically, we conduct high-impact dynamic compression experiments using polyvinylidene fluoride (PVDF) stress gauges with two different thicknesses and varying remnant polarizations at shock levels from 0.3 to 10 GPa. To best characterize the comprehensive calibration relationship, we select a set of five features (combined by strain, remnant polarization, and film thickness) by feature engineering and use Lasso with the bagging ensemble as an algorithm to train the machine learning model. For comparison, we also propose semiempirical models that calibrate PVDF gauges effectively, but without including thickness and remnant polarization. Our results show that the machine learning model is more precise and more reasonable in physics. The predicted dependences of the calibration curves on remnant polarization and film thickness by the machine learning model are qualitatively consistent with the physics scenario. This work reveals the potential of machine learning methods to improve gauge calibration for better performance and transferability. The method used in this work is applicable to the calibration of any stress gauges with multiple variables.

Physiological effects induced by aluminium and fluoride stress in tall fescue (Festuca arundinacea Schreb)

Aluminium (Al) and fluoride (F) are phytotoxic elements that can inhibit plant growth and development. Al3+ and F- can react with each other to form complexes in the soil which will induce alteration of toxicity of single element. However, the mechanisms of plant response to aluminium fluoride induced toxicity are not very clear. In the present study, tall fescue (Festuca arundinacea Schreb) cultivar ‘Houndog 5′ was treated by 0, 0.4, 4, 20 mg·L−1 Al2(SO4)3 and 0, 0.5, 5 mg·L−1 NaF, respectively. After 25 days of treatment, leaf samples were collected for physiological evaluation. The results showed that several forms of Al-OH and Al-F complexes such as Al(OH)2+, AlOH2+, Al(OH)3, Al(OH)4-, Al2(OH)24+, Al3(OH)45+, AlF2+, AlF2+, AlF3 and AlF4- were formed in Al3+ and F- combined solution. The nutrient uptake including Al, P and K were improved by Al3+ and F-. Under Al3+ stress, the MDA (malondialdehyde) content and EL (electrolyte leakage) dramatically increased after high concentration of F- treatment, while relative low concentration of F induced decrease of MDA content and EL. On the contrary, chlorophyll content decreased significantly after high concentration of F treatment. The photosynthesis efficiency parameters, including φP0 (Fv/Fm), δR0 and PIABS, decreased remarkably after high concentration of Al and F treatment. However, L-band incresed after high concentration of Al3+ and F- treatment. The results of correlation analysis showed that MDA content and EL negatively correlated with other indexes, and Al-F complex significantly correlated with MDA, Pro and EL but negatively correlated with Chl and φP0. These results suggested that low concentration of F could alleviate the damage induced by Al stress in tall fescue, but high concentration of Al and F combined solution had negative effects on the growth and development of tall fescue.

Functional and molecular characterization of fluoride exporter (FEX) from rice and its constitutive overexpression in Nicotiana benthamiana to promote fluoride tolerance.

Early induction of OsFEX was insufficient for fluoride adaptation in IR-64. Overexpression of OsFEX in yeast and Nicotiana benthamiana enhanced fluoride tolerance. The present study delineates the regulation of fluoride exporter (FEX) in the fluoride-sensitive rice cultivar, IR-64 and its efficacy in generating high fluoride tolerance in transgenic Nicotiana benthamiana. Gene and protein expression profiling revealed that OsFEX exhibited early induction during fluoride stress in the vegetative and reproductive tissues of IR-64, although the expression was suppressed upon prolonged stress treatment. Analysis of OsFEX promoter in transgenic N. benthamiana, using β-glucuronidase reporter assay confirmed its early inducible nature, since the reporter expression and activity peaked at 12h of NaF stress, after which it was lowered. OsFEX expression was up regulated in the presence of gibberellic acid (GA) and melatonin, while it was suppressed by abscisic acid (ABA). Complementation of ΔFEX1ΔFEX2 yeast mutants with OsFEX enabled high fluoride tolerance, thus validating the functional efficiency of the transgene. Bioassay of transgenic N. benthamiana lines, expressing OsFEX either under its own promoter or under CaMV35S promoter, established that constitutive overexpression, rather than early induction of OsFEX was essential and crucial for generating fluoride tolerance in the transgenics. Overall, the suppression of OsFEX in the later growth phases of stressed IR-64 due to enhanced ABA conservation and lowered synthesis of GA, as supported by the application of the respective phytohormone biosynthetic inhibitors, such as sodium tungstate and paclobutrazol, accounted for the fluoride-hyperaccumulative nature of the rice cultivar.

Salicylic acid-mediated alleviation of fluoride toxicity in rice by restricting fluoride bioaccumulation and strengthening the osmolyte, antioxidant and glyoxalase systems.

The aim of the manuscript was to demonstrate the efficacy of salicylic acid (SA) in abrogating the fluoride-induced oxidative damages in the susceptible rice cultivar, MTU1010. Prolonged exposure of seedlings to sodium fluoride (25 mg L-1) severely impaired growth and overall physiological parameters like germination percentage, biomass and root and shoot length and incited the formation of hydrogen peroxide that enhanced electrolyte leakage, formation of cytotoxic products like malondialdehyde and methylglyoxal and lipoxygenase activity. Exogenous application of SA (0.5 mM) enhanced the endogenous level of SA that restored the chlorophyll content and catalase activity and further escalated the activity of other enzymatic antioxidants (superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase, glutathione peroxidase and glutathione S-transferase), formation of non-enzymatic antioxidants (anthocyanins, carotenoids, flavonoids, phenolics, ascorbate and reduced glutathione) and osmolytes (proline, amino acids and glycine betaine) that cumulatively maintained the integrity of membrane structure and homeostatic balance of the cells by scavenging the accumulated hydrogen peroxide. SA-mediated formation of proline and flavonoids was linked with the enhanced activity of Δ1-pyrroline-5-carboxylate synthetase and phenylalanine ammonia lyase. Fluoride stress enhanced the activity of enzymes like glyoxalase I and glyoxalase II which were further aggravated in the seedlings upon treatment with SA, effectively detoxifying the methylglyoxal formed during stress. Overall, the manuscript depicts the pivotal role played by exogenous SA in ameliorating the effects of fluoride-induced damages in the seedlings and proves its potentiality as a protective chemical against fluoride stress when applied exogenously in rice.

Differential Responses of Vigna radiata and Vigna mungo to Fluoride-Induced Oxidative Stress and Amelioration via Exogenous Application of Sodium Nitroprusside

The manuscript aimed to explore the differential responses of Vigna radiata and Vigna mungo to fluoride (25 mg L−1 NaF) stress and validate the protective roles of sodium nitroprusside (SNP) (50 µM). V. radiata showed 53.5% higher bioaccumulation of fluoride compared to V. mungo. The overall growth in V. radiata was more affected (32.0%) compared with V. mungo (6.3%), which on the contrary displayed regulated fluoride entry and fluoride-adaptive phenotype. Fluoride bioaccumulation in V. radiata significantly triggered H2O2 and methylglyoxal accumulation, electrolyte leakage, chlorophyll degradation and lipid peroxidation, which accounted for its fluoride sensitivity. Exogenous SNP alleviated fluoride toxicity in both the species by synthesizing osmolytes (proline, glycine-betaine and total amino acids), non-enzymatic antioxidants (anthocyanins, flavonoids, total phenolics, carotenoids, ascorbate and glutathione) and activating antioxidative enzymes (APX, SOD, GPX, GST, GPOX and GR). Fluoride toxicity inhibited the CAT activity to a higher degree in V. radiata (51.0%) than V. mungo (31.3%). However, SNP supplementation rescued the CAT activity, lowered the fluoride uptake and membrane damage, and also alleviated oxidative stress in both the species, along with accelerated growth recovery and chlorophyll conservation. Cytotoxic methylglyoxal was efficiently detoxified in presence of SNP due to enhanced activity of the glyoxalases. Exogenous SNP also triggered endogenous NO formation along with up regulating the activity of P5CS and PAL, which stimulated proline and flavonoid biosynthesis during stress. Statistical analyses revealed that the ameliorating potential of SNP was more prominent in the susceptible species. Overall, our investigation established V. radiata and V. mungo as fluoride-sensitive and fluoride-tolerant species, respectively, and also illustrated the efficacy of SNP in abating stress-induced damages in Vigna by augmenting the defence machinery.

Metabolics and ionomics responses of tea leaves (Camellia sinensis (L.) O. Kuntze) to fluoride stress

Tea plant (Camellia sinensis (L.) O. Kuntze) is known to accumulate high concentrations of fluoride (F) in its leaves; however, the underlying mechanism of F accumulation remains unclear. The main objective of this study was to investigate the homeostatic self-defense mechanisms of tea leaves to F supplementation (0, 5, 20, and 50 mgL−1) by metabolomics and ionomics. We identified a total of 96 up-regulated and 40 down-regulated metabolites in tea leaves treated with F. Of these different compounds, minor polypeptides, carbohydrates and amino acids played valuable roles in the F-tolerating mechanism of tea plant. After F treatments, the concentrations of sodium (Na), ferrum (Fe), manganese (Mn), and molybdenum (Mo) were significantly increased in tea leaves, whereas the aluminum (Al) was decreased. These findings suggest that the ionic balance and metabolites are attributable to the development of F tolerance, providing new insight into tea plant adaptation to F stress.

Fluoride toxicity variably affects overall physiology and grain development in three contrasting rice genotypes, representing a potential biohazard.

Ingestion of fluoride through consumption of contaminated food grains has been regarded to be hazardous for consumer health. The current study indicated the possible occurrence of such biohazard due to fluoride bioaccumulation in rice grains and straw (cattle feed). The effects of fluoride toxicity at three stages of grain development in three rice genotypes, viz., IR-64, Gobindobhog (aromatic), and Khitish, were also studied. Irrigation with fluoride-infested water inhibited grain formation in IR-64 and reduced grain yield in Gobindobhog. Fluoride toxicity promoted seed sterility in IR-64 by triggering reactive oxygen species (ROS) production and cellular necrosis, suppressing genes like GIF1, DEP1, and SPL14 (positively controlling seed formation) and inducing GW2 (negatively mediating grain development). Gobindobhog showed intermediate fluoride sensitivity and accumulated high levels of proline, anthocyanins, flavonoids, and phenolics due to the induction of genes like P5CS, ANS, and PAL in developing grains. The agronomic attributes in Khitish were unaffected by fluoride stress due to regulated fluoride uptake and high expression of GIF1, DEP1, and SPL14 along with an increased synthesis of anthocyanins, flavonoids, and phenolics. Khitish also accumulated low ROS as a result of which lowest lipoxygenase expression (among selected cultivars) was observed in developing grains. Fluoride entry was accelerated in the straw of Khitish, possibly due to the absence of regulated uptake mechanism in dead seedlings. Furthermore, the ecological concerns regarding fluoride bioaccumulation and reduced grain yield at the varietal level were also established, based on statistical modelling.

Fluoride Stress-Mediated Regulation of Tricarboxylic Acid Cycle and Sugar Metabolism in Rice Seedlings in Absence and Presence of Exogenous Calcium

The aim of the current study was to investigate the regulation of tricarboxylic acid (TCA) cycle and sugar metabolism in the seedlings of the rice variety, Khitish, during fluoride stress, either in absence or presence of exogenous calcium compounds, viz., calcium hydroxide and calcium nitrate. The inhibitory effect of fluoride stress on energy production was reflected by lowered levels of pyruvic acid, citric acid and malic acid, substantial interference with the respiratory enzymes such as PDH, CS, IDH and SDH, and inhibiting the expression of the concerned genes. Seed priming with both calcium compounds elevated the organic acid levels, restored the activity of the above enzymes and expression of the corresponding genes. On the contrary, the enhanced fumarase and MDH activity and MDH gene expression, during fluoride stress, was lowered by calcium pre-treatment. The sucrose level, together with SPS and SS activity and expression of the corresponding genes were restored to higher levels, concomitant with lowered INV activity and gene expression, in presence of calcium compounds, suggesting lesser utilization of sucrose to combat fluoride stress. The efficacy of calcium compounds in conserving starch as the major food reserve, without provoking its catabolic breakdown, was established on the basis of enhanced starch content with lowered α-amylase and starch phosphorylase activity and corresponding gene expression. The present communication for the first time documented the detrimental impact of fluoride stress on TCA cycle metabolites and majority of the enzymes, and endogenous sucrose and starch accumulation, as well as highlighted the reversal of the negative effects with the exogenous application of calcium compounds.

Silicon nanoparticle-pulsing mitigates fluoride stress in rice by fine-tuning the ionomic and metabolomic balance and refining agronomic traits

The present manuscript investigates the roles of silicon nanoparticles (SiNPs) in ameliorating fluoride toxicity in the susceptible rice cultivar, IR-64. Fluoride toxicity reduced overall growth and yield by suppressing grain development. Fluoride stress alarmingly increased the accumulation of cobalt, which together with fluoride triggered electrolyte leakage, malondialdehyde, methylglyoxal and hydrogen peroxide accumulation and NADPH oxidase activity. The overall photosynthesis was compromised due to chlorosis and inhibited Hill activity. Nano-Si-priming efficiently ameliorated molecular injuries and restored yield by reducing fluoride bioaccumulation particularly in the grains. The level of non-enzymatic antioxidants like anthocyanins, flavonoids, phenolics and glutathione was stimulated upon SiNP-priming. Nano-Si-pulsing removed fluoride-mediated inhibition of glutathione synthesis by activating glutathione reductase. Glutathione was utilized to activate glyoxalases and associated enzymes like glutathione-S-transferase and glutathione peroxidase. Uptake of nutrients like silicon, potassium, zinc, copper, iron, nickel, manganese, selenium and vanadium improved seedling health even during prolonged fluoride stress. Nano-Si-pulsing produced a nanozymatic effect, since high level of crucial co-factors like copper, zinc and iron stimulated the activity of superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase, which synergistically with other enzymatic and non-enzymatic antioxidants scavenged reactive oxygen species and promoted fluoride tolerance. Overall, the study supported by statistical modelling using principal component analysis, t-distributed stochastic neighbour embedding and multidimensional scaling, established the potential of SiNP to promote safe rice cultivation and precision farming even in fluoride-infested environments.

A deep insight into the transcriptome of midgut and fat body reveals the toxic mechanism of fluoride exposure in silkworm

Fluoride generally exists in the natural environment, and has been reported to induce serious environmental hazard to animals, plants, and even humans via ecological cycle. Silkworm, Bombyx mori, which showed significant growth and reproductivity reduction when exposed to fluoride, has become a model to evaluate the toxicity of fluoride. However, the detailed mechanism underlying fluoride toxicity and corresponding transport proteins remain unclear. In this study, we performed RNA-seq of the larval midgut and fat body with fluoride exposure and normal treatment. Differential analysis showed that there were 4405 differentially expressed genes in fat body and 4430 DEGs in midgut with fluoride stress. By Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses, we identified several key pathways involved in the fluoride exposure and poisoning. We focused on the oxidative phosphorylation and MAPK signal pathway. QRT-PCR confirmed that oxidative phosphorylation process was remarkably inhibited by fluoride exposure and resulted in the blocking of ATP synthesis. The MAPK signal pathway was stimulated via phosphorylation signal transduction. Moreover, by protein structure analysis combined with the DEGs, we screen 36 potential membrane proteins which might take part in transporting fluoride. Taken together, the results of our study expanded the underlying mechanisms of fluoride poisoning on silkworm larval growth and development, and implied potential fluoride transport proteins in silkworm.

Silicon-regulated antioxidant and osmolyte defense and methylglyoxal detoxification functions co-ordinately in attenuating fluoride toxicity and conferring protection to rice seedlings

The objective of this manuscript was to demonstrate the efficacy of silicon supplementation in relieving the fluoride-induced damages in rice cultivar, Khitish. The exposure of seedlings to two different concentrations of fluoride, viz., 25 and 50 mg L−1 NaF caused increase in fluoride accumulation, as a result of which the seedlings suffered severe oxidative stress, as evident from growth inhibition, reduction in seed germination, tissue biomass, root and shoot length, decline in chlorophyll content, increases in electrolyte leakage, H2O2 content, lipid peroxidation (malondialdehyde content and lipoxygenase activity), protein carbonylation and protease activity. The extent of damage was more at higher fluoride concentration. Silicon amendment, irrespective of fluoride concentrations, led to large build-up of endogenous silicon level and brought considerable improvement in all the parameters examined with respect to fluoride stress. The fluoride-mediated enhancement in methylglyoxal level was lowered by silicon, because of the prominent activation of glyoxalase I and glyoxalase II. While the stress-mediated induction in antioxidative enzymes like GPOX, APX, SOD, GPX and GR was lowered by silicon, the inhibition in CAT activity was relieved. The antioxidative defense mechanism was also boosted up via enhanced content of total phenolics and carotenoids. However, the fluoride-mediated increase in anthocyanins, flavonoids, xanthophyll, ascorbate and reduced glutathione, and osmolytes like total amino acids, proline and glycine-betaine, were all lowered in presence of silicon, together with reduced PAL and P5CS activity. Overall, silicon reduced oxidative damages to develop fluoride-tolerant rice plants through augmentation of different antioxidant and osmolyte defense and methylglyoxal detoxification system.

Genome-Wide Identification of Genes Involved in General Acid Stress and Fluoride Toxicity in Saccharomyces cerevisiae

Hydrofluoric acid elicits cell cycle arrest through a mechanism that has long been presumed to be linked with the high affinity of fluoride to metals. However, we have recently found that the acid stress from fluoride exposure is sufficient to elicit many of the hallmark phenotypes of fluoride toxicity. Here we report the systematic screening of genes involved in fluoride resistance and general acid resistance using a genome deletion library in Saccharomyces cerevisiae. We compare these to a variety of acids – 2,4-dinitrophenol, FCCP, hydrochloric acid, and sulfuric acid – none of which has a high metal affinity. Pathways involved in endocytosis, vesicle trafficking, pH maintenance, and vacuolar function are of particular importance to fluoride tolerance. The majority of genes conferring resistance to fluoride stress also enhanced resistance to general acid toxicity. Genes whose expression regulate Golgi-mediated vesicle transport were specific to fluoride resistance, and may be linked with fluoride-metal interactions. These results support the notion that acidity is an important and underappreciated principle underlying the mechanisms of fluoride toxicity.

Comparative study of vegetative and reproductive growth of different tea varieties response to different fluoride concentrations stress

BackgroundThe amount of fluoride accumulation in tea leaves was gradually increase as the matures of tea plants, and the excessive fluoride intake can threaten people's health. Based on years of field investigations, a low level of fluoride variety Xiangbo Lǜ (XBL) and a high level of fluoride variety Zhenong 139 (ZN139) were selected. ResultsIn this study, the root, 1st and the 5th leaf of the two-year-old tea trees were used for morphological, physiological and comparative transcriptomics analysis to understand the different features of “XBL” and “ZN139” under fluoride stress conditions. The color of the 1st and 5th leaves of XBL were yellower, the activity of peroxidase, catalase and antioxidant enzyme were lower than ZN139 under the high-fluoride stress. Transcriptomics analysis indicated that core genes involved in photosynthesis rates regulation showed no significantly exchanged expression, the co-downregulation of magnesium ions transportation, while the ROS scavenging, vegetative growth and self-compatibility between the two varieties were different. Crucial genes' expression were also identified by the real-time RT-PCR. ConclusionThe tea tree is one of the few plants that has a high-fluoride content, but the different varieties respond differently to fluoride stress. High-fluoride tea tree varieties, such as ZN139, have stronger ROS scavenging abilities through the use of both their non-enzymatic and enzymatic antioxidant systems which act by increasing the expression levels of inositol-1-monophosphatases and peroxidases, among others. ZN139 can also compensate for the decrease in photosynthetic rate that is associated with the ionic imbalance caused by the reduced consumption of light energy during long-periods of high fluoride stress. Reproductive development was protected in ZN139 by the up-regulated expression of S-locus glycoprotein, Mildew resistance locus o and Phospholipase D under fluoride stress, while the vegetative development of low-fluoride varieties such as XBL was retarded. More starch and cellulose were redistributed to glucose by increasing the expression levels of glycosyl transferases and hydrolases to provide more energy for processes involved in the response and tolerance towards fluoride stress.

Aluminum relieves fluoride stress through stimulation of organic acid production in Camellia sinensis.

Tea plants (Camellia sinensis O. Kuntze) can hyperaccumulate fluoride (F) in leaves. Although, aluminum (Al) can alleviate F toxicity in C. sinensis, the mechanisms driving this process remain unclear. Here, we measured root length, root activity, soluble proteins content, and levels of peroxidase, superoxide dismutase, catalase, malondialdehyde (MDA), and chlorophyll in tea leaves after treatment with different F concentrations. In addition, we focused on the content of organic acids, the gene transcription of malate dehydrogenase (MDH), glycolate oxidase (GO) and citrate synthase (CS) and the relative enzyme activity involved in the tolerance to F in C. sinensis. We also examined the role of Al in this process by analyzing the content of these physiological indicators in tea leaves treated with F and Al. Our results demonstrate that increased MDA content, together with decreased chlorophyll content and soluble proteins are responsible for oxidative damage and metabolism inhibition at high F concentration. Moreover, increased antioxidant enzymes activity regulates ROS damage to protect tea leaves during F stress. Furthermore, exogenous Al alleviated F stress in tea leaves through the regulation of MDA content and antioxidant enzymes activity. In addition, organic acids in exudate stimulated root growth in tea plants exposed to low F concentrations are regulated by MDH, GO, and CS. In addition, Al can stimulate the exudation of organic acids, and may participate in regulating rhizosphere pH of the roots through the interaction with F, eventually leading to the response to F stress in C. sinensis.

Gibberellic Acid-Priming Promotes Fluoride Tolerance in a Susceptible Indica Rice Cultivar by Regulating the Antioxidant and Phytohormone Homeostasis

Excessive utilization of groundwater for anthropogenic purposes has led to severe depletion of the water table, resulting in contamination of fluorides from the mineral bed. Irrigation of rice seedlings with such fluoride-infested water leads to high fluoride bioaccumulation and compromised growth physiology. In the present study, we showed that the priming of seeds with gibberellic acid 3 (GA) alleviated prolonged fluoride-induced toxicity in the fluoride-susceptible indica rice cultivar, IR-64 (grown in soil) by reducing the accumulation of the xenobiotic within the seedling biomass. The primed seeds showed improved percentage of germination during fluoride stress compared to the non-primed seeds. The stressed seedlings grown from the GA-primed seeds exhibited increased endogenous accumulation of GA and the auxin, indole-3-acetic acid which stimulated shoot and root growth and relative water content, compared to the stressed seedlings germinated from the non-primed seeds. GA-priming reduced the chlorophyll degradation, affected the homeostasis of the accessory pigments and lowered the electrolyte leakage during stress. Upon GA-priming, the fluoride-induced oxidative stress was ameliorated by an increase in proline, anthocyanin, flavonoid and total phenolic contents, reducing power, total antioxidant capacity and DPPH-radical scavenging activity. The altered activity of the antioxidative enzymes like catalase, ascorbate peroxidase and guaiacol peroxidase also enabled efficient H2O2 scavenging in the stressed plants germinated from the primed seeds. Thus, seed pre-treatment with GA promoted fluoride tolerance by activating the antioxidant machinery and elevating the endogenous level of the two most important classes of plant growth regulators, gibberellic acid and auxin.