Simultaneous exposure of sulphur and calcium hinder As toxicity: Up-regulation of growth, mineral nutrients uptake and antioxidants system

Excess supply of sulfur mitigates thallium toxicity to rice (Oryza sativa L.) growth in hydroponic experiment

Sulfur (S) is an essential element for the growth of rice plants (Oryza sativa L.), crucial for enhancing crop yield and grain quality. However, its potential in mitigating thallium (Tl) toxicity in rice remains unclear. In this study, a hydroponic experiment was performed to investigate the effects of low, medium and high S application levels (LS, MS, HS) on Tl accumulation in rice at three Tl exposure levels (0, 0.5 and 1 mg·L-1). Our findings reveal that the exogenous S application could alleviate Tl toxicity, enhancing fresh weight and shoot length of rice plant. Additionally, HS (HS, SO42- content was 387.84 mg·L-1) group significantly increased chlorophyll and glutathione (GSH) content by 6.46 to 21.38 % and 2.15 to 7.31 % respectively, while reducing malondialdehyde (MDA) levels by 17.43 to 28.48 %, compared to MS (MS, SO42- content was 193.41 mg·L-1) group. Fe content in rice roots and iron plaque consistently increased with S provision under Tl-free and Tl-contaminated conditions. In Tl exposure environment, HS and LS (LS, SO42- content was 1.02 mg·L-1) groups exhibited significant differences in Fe contents and iron plaque in rice root. Moreover, in Tl exposure environment, S application reduced Tl concentration in iron plaque, root, and shoot, HS treatment showed Tl content reduction from 16.29 % to 25.89 %, compared to LS treatment. Our findings underscore the potential of S application in hydroponic environment to promote rice growth and mitigate Tl accumulation, offering insights for developing effective Tl remediation strategies by using S-contained fertilizers.

Biochar as a partner of plants and beneficial microorganisms to assist in-situ bioremediation of heavy metal contaminated soil

Synergistic remediation of heavy metal (HM) contaminated soil using beneficial microorganisms (BM) and plants is a common and effective in situ bioremediation method. However, the shortcomings of this approach are the low colonisation of BM under high levels of heavy metal stress (HMS) and the poor state of plant growth. Previous studies have overlooked the potential of biochar to mitigate the above problems and aid in-situ remediation. Therefore, this paper describes the characteristics and physicochemical properties of biochar. It is proposed that biochar enhances plant resistance to HMS and aids in situ bioremediation by increasing colonisation of BM and HM stability. On this basis, the paper focuses on the following possible mechanisms: specific biochar-derived organic matter regulates the transport of HMs in plants and promotes mycorrhizal colonisation via the abscisic acid signalling pathway and the karrikin signalling pathway; promotes the growth-promoting pathway of indole-3-acetic acid and increases expression of the nodule-initiating gene NIN; improvement of soil HM stability by ion exchange, electrostatic adsorption, redox and complex precipitation mechanisms. And this paper summarizes guidelines on how to use biochar-assisted remediation based on current research for reference. Finally, the paper identifies research gaps in biochar in the direction of promoting beneficial microbial symbiotic mechanisms, recognition and function of organic molecules, and factors affecting practical applications.

Strigolactone and nitric oxide collaborate synergistically to boost tomato seedling resilience to arsenic toxicity via modulating physiology and antioxidant system

Arsenic (As) poses a significant environmental threat as a metalloid toxin, adversely affecting the health of both plants and animals. Strigolactones (SL) and nitric oxide (NO) are known to play crucial roles in plant physiology. Therefore, the present experiment was designed to investigate the potential cumulative role of SL (GR24-0.20 μM) and NO (100 μM) in mitigating the adverse effect of AsV (53 μM) by modulating physiological mechanisms in two genotypes of tomato (Riogrand and Super Strain 8). A sample randomized design with four replicates was used to arrange the experimental pots in the growth chamber. 45-d old both tomato cultivars under AsV toxicity exhibited reduced morphological attributes (root and shoot length, root and shoot fresh weight, and root and shoot dry weight) and physiological and biochemical characteristics [chlorophyll (Chl) a and b content, activity of δ-aminolevulinic acid dehydratase activity (an enzyme responsible for Chl biosynthesis), and carbonic anhydrase activity (an enzyme responsible for photosynthesis), and enhanced Chl degradation, overproduction of reactive oxygen species (ROS) and lipid peroxidation due to enhanced malondialdehyde (MDA) content. However, the combined application of SL and NO was more effective in enhancing the tolerance of both varieties to AsV toxicity compared to individual application. The combined application of SL and NO improved growth parameters, biosynthesis of Chls, NO and proline. However, the combined application significantly suppressed cellular damage by inhibiting MDA and overproduction of ROS in leaves and roots, as confirmed by the fluorescent microscopy study and markedly upregulated the antioxidant enzymes (catalase, peroxidase, superoxide dismutase, ascorbate dismutase and glutathione reductase) activity. This study provides clear evidence that the combined application of SL and NO supplementation significantly improves the resilience of tomato seedlings against AsV toxicity. The synergistic effect of SL and NO was confirmed by the application of cPTIO (an NO scavenger) with SL and NO. However, further molecular studies could be imperative to conclusively validate the simultaneous role of SL and NO in enhancing plant tolerance to abiotic stress.

Anatomical and morphological changes in Pinus sylvestris and Larix sibirica needles under impact of emissions from a large aluminum enterprise

Species-specific anatomical and morphological characteristics of Pinus sylvestris and Larix sibirica needles were studied at different levels of tree stand pollution by aluminum smelter emissions. The anatomical characteristics of the needle were studied using light microscopy. The level of tree stand pollution was determined using the cluster analysis outcomes of the pollutant elements content (fluorine, sulfur, and heavy metals) in the needles. Four levels of tree stand pollution were separated: low, moderate, high, and critical, as well as background tree stand in unpolluted areas. It was found that the state of tree phytomass deteriorated with increasing levels of pollution (from low to critical): pine crown defoliation increased to 85%, and larch defoliation increased to 65%. The life span of pine needles was reduced to 2-3 years, with a background value of 6-7 years. The change of morphological parameters was more pronounced in P. sylvestris: the weight and length of the 2-year-old shoot decreased by 2.7-3.1 times compared to the background values; the weight of needles on the shoot and the number of needle pairs on the shoot-by 1.9-2.1 times. The length of the needle and shoot and the number of L. sibirica brachyblasts decreased by 1.8-1.9 times. The anatomical parameters of the needle also changed to a greater extent in P. sylvestris. Up to the high level of tree pollution, we observed a decrease in the cross-sectional area of the needle, central cylinder, vascular bundle, area and thickness of mesophyll, number and diameter of resin ducts by 18-66% compared to background values. At the critical pollution level, when the content of pollutant elements in pine needles reached maximum values, the anatomical parameters of the remaining few green needles were close to background values. In our opinion, this may be due to the activation of mechanisms aimed at maintaining the viability of trees. A reduction in thickness and area of assimilation tissue in the L. sibirica needle was detected only at the critical pollution level. An upward trend in these parameters was found at low, medium, and high pollution levels of tree stand, which may indicate an adaptive nature. The results suggested that at a similar pollution level of trees, the greatest amount of negative anatomical and morphological changes were recorded in pine needles, which indicates a greater sensitivity of this species to technogenic emissions.

Calcium and L-glutamate present the opposite role in managing arsenic in barley

Arsenic contamination in agricultural soils has posed tremendous threat to sustainable crop production and human health via food chain. Calcium and Glutamate have been well-documented in metal(loid)s detoxification, but it is poorly understood how they regulate arsenic-induced toxicity to plants. In this study, the effect of glutamate and calcium at high concentration on arsenic toxicity and accumulation in barley seedling was accessed in terms of plant growth, photosynthetic efficacy, arsenic uptake, translocation and accumulation, antioxidant defense, nutrient uptake and the expression of As transporters. Our results have demonstrated that calcium could effectively ameliorate arsenic toxicity to barley seedlings, which is mainly attributed to its beneficial effect on increasing nutrient uptake, reducing the aboveground arsenic accumulation and enhancing antioxidative defense capacity. However, it is unexpected that glutamate considerably exacerbated the arsenic toxicity to barley seedlings. More importantly, for the first time, glutamate was observed to tremendously facilitate the root-to-shoot translocation of arsenic by 41.8- to 60.8-fold, leading to 90% of the total amount of As accumulating in barley shoots. The reason of this phenomenon can be well explained by the glutamate-triggered enormous upregulation of genes involved in arsenic uptake (HvPHT1;1, HvPHR2 and HvNIP3;2), reduction (HvHAC1;1), translocation (HvABCC7, HvNIP1;1 and HvNIP3;3) and intracellular sequestration (HvABCC1). These findings suggest that calcium and glutamate function as the opposite player in managing arsenic, with calcium being an effective alleviator of arsenic stress to ensure the safe production of crops; while glutamate being a highly efficient phytoextraction agent for phytoremediation of arsenate-contaminated soils.

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.

The potential of microbes and sulfate in reducing arsenic phytoaccumulation by maize (Zea mays L.) plants

Arsenic (As) contamination in soil-plant system is an important environmental, agricultural and health issue globally. The microbe- and sulfate-mediated As cycling in soil-plant system may depend on soil sulfate levels, and it can be used as a potential strategy to reduce plant As uptake and improve plant growth. Here, we investigated the role of soil microbes (SMs) to examine As phytoaccumulation using maize as a test plant, under varying sulfate levels (S-0, S-5, S-25 mmol kg^-1) and As stress. The addition of sulfate and SMs promoted maize plant growth and reduced As concentration in shoots compared to sulfate-treated plants without SMs. Results revealed that the SMs-S-5 treatment proved to be the most promising in reducing As uptake by 27% and 48% in root and shoot of the maize plants, respectively. The SMs-S treatments, primarily with S-5, enhanced plant growth, shoot dry biomass, Chl a, b and total Chl (a + b) contents, and gas exchange attributes of maize plants. Similarly, the antioxidant defense in maize plants was increased significantly in SMs-S-treated plants, notably with SMs-S-5 treatment. Overall, the SMs-S-5-treated plants possessed improved plant growth, dry biomass, physiology and antioxidant defense system and decrease in plant shoot As concentration. The outcomes of this study suggest that sulfate supplementation in soil along with SMs could assist in reducing As accumulation by maize plants, thus providing a sustainable and eco-friendly bioremediation strategy in limiting As exposure.

Changes in the fatty acid composition of pine needle lipids under the aluminum smelter emissions

Changes in the fatty acid (FA) composition of total lipids of Pinus sylvestris needles at different pollution levels caused by emissions from a large aluminum smelter (BrAS) have been studied. In the needles of trees from unpolluted (background) territories, the FA spectrum is represented by 24 acids with prevalence of unsaturated FAs (71.6%). The main unsaturated FA are represented by oleic (C18: 1ω9), linoleic (C18: 2ω6), and α-linolenic (C18: 3ω3) acids. Under the influence of BrAS emissions, the total amount of identified FAs in the needles and the proportion of unsaturated FAs decrease, while the fraction of saturated FAs, on the contrary, increases from 25.4% in unpolluted needles to 33.2% in polluted ones. The content of palmitic FA (C16:0) in the needles exceeds background values by 1.5 times, behenic acid (C22:0) - by 1.6-2.5 times, arachidic acid (C20:0) - by 1.5 times, palmitic margaric acid (C17:0) - by 1.5-2.3 times. These FAs play the important role in the protection of plant membranes from the effects of abiotic stress factors, making them less permeable. The sum of short-chain saturated FAs (C12:0, C14:0, C15:0) increase by 4.8 times in needles of trees that are highly polluted. Pentadecanoic (C15:0) acid is found in the needles only in the background areas and at the low pollution level. With a more severe pollution, C15:0 is not identified, but lauric acid with the cis-configuration of double bonds in the structure (izo-C12:0) appears. The presence of "relict" ∆5-polymethylene FAs in the composition of pine needle membrane lipids is determined. In the background areas, they account for 12.9% of the total FAs. With the industrial pollution intensification, their total content increases and reaches 14.1%. ∆5-polymethylene FAs are also able to protect membranes against negative influences. Thus, changes in the quantitative and qualitative FA composition of pine needle total lipids indicate the activation of the stabilization mechanisms of membrane lipids due to their tight packing in a bilayer. It is one of the adaptive reactions of Pinus sylvestris in response to the impact of the aluminum industry emissions.

A comparative study of the effective response of di-potassium phosphate (K2HPO4) on physiological, biochemical and anatomical aspects of crops dwelling with zinc oxide nanoparticles toxicity

The dipotassium phosphate (K₂HPO₄) is a source of phosphorus (P), which is an essential micronutrient for plant growth and reproduction and also acts as a stress alleviator against abiotic stresses. Therefore, it could also become a potential mineral to cope up with zinc oxide nanoparticles' (ZnONPs) toxicity in crops. This study primarily includes synthesis, characterization and differential toxic impacts of ZnONPs on two crop plantsThis study includes synthesis, characterization and differential toxic impacts of ZnONPs on two crop plants, i.e. Triticum aestivum and Solanum lycopersicum, as well as assuage the toxic impacts of ZnONPs through nutrient management approach implied via supplementation of P. The growth and physiological changes under toxic doses of ZnONPs and ameliorative potential of P in crop plants were examined by analysing growth, intracellular Zn accumulation, photosynthetic pigment contents, the kinetics of photosystem II (PS II) photochemistry, root cell anatomy and cell viability via histochemical staining 4',6-diamidino-2-phenylindole and propidium iodide. ZnONPs at 500 and 1000 μM concentrations significantly affected the growth, photosynthetic pigment and PS II photochemistry and cell death in both the plants. It also caused deformation in root anatomy of T. aestivum and S. lycopersicum. Whereas supplementation of P caused significant improvement against ZnONPs stress by causing remarkable enhancement in growth, photosynthetic pigments and activity of PS II photochemistry and decreased cell death. Moreover, the study also discloses the tolerant nature of S. lycopersicum comparing with T. aestivum seedlings. Thus, P is comparatively more effective in managing the ZnONPs toxicity in S. lycopersicum than in T. aestivum.

Auxin metabolic network regulates the plant response to metalloids stress

Metalloids are among the major pollutants posing a risk to the environment and global food security. Plant roots uptake these toxic metalloids from the soil along with other essential minerals. Plants respond to metalloid stress by regulating the distribution and levels of various endogenous phytohormones. Recent research showed that auxin is instrumental in mediating resilience to metalloid-induced stress in plants. Exogenous supplementation of the auxin or plant growth-promoting micro-organisms (PGPMs) alleviates metalloid uptake, localization, and accumulation in the plant tissues, thereby improving plant growth under metalloid stress. Moreover, auxin triggers various biological responses such as the production of enzymatic and non-enzymatic antioxidants to combat nitro-oxidative stress induced by the metalloids. However, an in-depth understanding of the auxin stimulated molecular and physiological responses to the metalloid toxicity needs to be investigated in future studies. The current review attempts to provide an update on the recent advances and the current state-of-the-art associated with auxin and metalloid interaction, which could be used as a start point to develop biotechnological tools and create an eco-friendly environment.

Protective role of nitric oxide on nitrogen-thiol metabolism and amino acids profiling during arsenic exposure in Oryza sativa L

Nitric oxide (NO) being a signaling molecule inside the plant cells, play significant role in signaling cascades and protection against environmental stresses. However, the protective role of NO in alleviating As toxicity in rice plants is currently not available. In the present study, the level of NO, nitrogen (N), inorganic N (nitrate, ammonium), thiols {TT (Total thiols), NPT (Nonprotein thiol)} and AAs contents along with N assimilating enzymes (NR, GDH, GOGAT) were analyzed after exposure of As^III/NO treatment alone, and in combination. NO supplementation enhanced the content of N, inorganic N & thiol contents, NR, GOGAT activities, when compared with As^III exposure alone. In As^III exposed rice seedlings, content of AAs (except His, Arg, Met) reduced over the control, while supplementation of SNP improved AAs contents, compared to As^III treatment alone. In conclusion, rice seedlings supplemented with NO tolerate the As^III toxicity by reducing the N related parameters, thiol contents, altering the AA profile and enhanced the nutritional quality by increasing EAAs (essential amino acids) and NEAAs (non-essential amino acids).

Interplay of Calcium and Nitric Oxide in improvement of Growth and Arsenic-induced Toxicity in Mustard Seedlings

In this study, Ca²⁺ mediated NO signalling was studied in response to metalloid (As) stress in Brassica seedlings. Arsenic toxicity strongly suppressed the growth (fresh weight, root and shoot length), photosynthetic pigments, Chl a fluorescence indices (Kinetic traits: F_v, F_m, F_v/F_o, F_m/F_o, ФP_o or F_v/F_m, Ψ_o, ФE_o, PI_ABS, Area and N and redox status (AsA/DHA and GSH/GSSG ratios) of the cell; whereas energy flux traits: ABS/RC, TR_o/RC, ET_o/RC and DI_o/RC along with F_o, F_o/F_v, F_o/F_m, ФD_o and S_m) were enhanced. Further, addition of EGTA (Ca²⁺ scavenger) and LaCl₃ (plasma membrane Ca²⁺ channel blocker) to As + Ca; while c‒PTIO (NO scavenger) and l‒NAME (NO synthase inhibitor) to As + SNP treated seedlings, siezed recovery on above parameters caused due to Ca²⁺ and NO supplementation, respectively to As stressed seedlings thereby indicating their signalling behaviour. Further, to investigate the link between Ca²⁺ and NO, when c‒PTIO and l‒NAME individually as well as in combination were supplemented to As + Ca treated seedlings; a sharp inhibition in above mentioned traits was observed even in presence of Ca²⁺, thereby signifying that NO plays crucial role in Ca²⁺ mediated signalling. In addition, As accumulation, ROS and their indices, antioxidant system, NO accumulation and thiol compounds were also studied that showed varied results.

Mitigation of chromium toxicity in Arabidopsis thaliana by sulfur supplementation

Chromium (Cr) contamination of soil and water has become a severe threat to human health. In this study, a series of experiments were conducted to examine the ameliorative effects of Cr toxicity, by exogenous 100 μM sodium sulfate. Our team has examined the plant growth, Cr content, chlorophyll, antioxidant index and soluble protein content, before and after the addition of sodium sulfate. The results showed that the addition of sulfur (S) can reduce the enrichment of Cr and the content of malonyldialdehyde (MDA) under Cr stress. After addition of S in the culture solution, the biomass and roots length of Arabidopsis thaliana increased under Cr stress. Furthermore, the content of chlorophyll, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione (GSH), and soluble protein increased with the addition of sulfur. Transmission electron microscope observation point to that the chloroplasts can be damaged in leaf. All data demonstrate that S supplementation should help to alleviate the negative effects caused by both Cr(III) and Cr(VI) on Arabidopsis thaliana.

Sulfur and Calcium Simultaneously Regulate Photosynthetic Performance and Nitrogen Metabolism Status in As-Challenged Brassica juncea L. Seedlings

In the present study, the role of sulfur (K2SO4: S; 60 mg S kg-1 sand) and/or calcium (CaCl2: Ca; 250 mg Ca kg-1 sand) applied alone as well as in combination on growth, photosynthetic performance, indices of chlorophyll a fluorescence, nitrogen metabolism, and protein and carbohydrate contents of Indian mustard (Brassica juncea L.) seedlings in the absence and presence of arsenic (Na2HAsO4.7H2O: As1; 15 mg As kg-1 sand and As2; 30 mg As kg-1 sand) stress was analyzed. Arsenic with its rising concentration negatively affected the fresh weight, root/shoot ratio, leaf area, photosynthetic pigments content, photosynthetic oxygen yield, and chlorophyll a fluorescence parameters: the O-J, J-I and I-P rise, QA- kinetic parameters, i.e., ΦP0, Ψ0, ΦE0, and PIABS, along with Fv/F0 and Area while increased the energy flux parameters, i.e., ABS/RC, TR0/RC, ET0/RC, and DI0/RC along with F0/Fv and Sm due to higher As/S and As/Ca ratio in test seedlings; however, exogenous application of S and Ca and their combined effect notably counteracted on As induced toxicity on growth and other important growth regulating processes. Moreover, inorganic nitrogen contents, i.e., nitrate (NO3-) and nitrite (NO2-) and the activities of nitrate assimilating enzymes, viz., nitrate reductase (NR) and nitrite reductase (NiR) and ammonia assimilating enzymes, viz., glutamine synthetase (GS) and glutamate synthase (GOGAT) along with protein and carbohydrate contents were severely affected with As toxicity; while under similar condition, ammonium (NH4+) content and glutamate dehydrogenase (GDH) activity in both root and leaves showed reverse trend. Furthermore, S and Ca supplementation alone and also in combination to As stressed seedlings ameliorated these parameters except NH4+ content and GDH activity, which showed an obvious reduction under similar conditions. These findings point out that exogenous application of S and/or Ca particularly S+Ca more favorably regulated the photosynthesis, contents of protein, carbohydrate and inorganic nitrogen, and the activities of nitrate and ammonia assimilating enzymes, which might be linked with the mitigation of As stress. Our results suggest that exogenous application of S+Ca more efficiently defends Brassica seedlings by declining As accumulation in root and shoot tissues and by maintaining the photosynthesis and nitrogen metabolism as well.