Selenium ameliorates arsenic induced oxidative stress through modulation of antioxidant enzymes and thiols in rice (Oryza sativa L.)

Migration, transformation of arsenic, and pollution controlling strategies in paddy soil-rice system: A comprehensive review

Arsenic pollution in paddy fields has become a public concern by seriously threatening rice growth, food security and human health. In this review, we delve into the biogeochemical behaviors of arsenic in paddy soil-rice system, systemically revealing the complexity of its migration and transformation processes, including the release of arsenic from soil to porewater, uptake and translocation of arsenic by rice plants, as well as transformation of arsenic species mediated by microorganism. Especially, microbial processes like reduction, oxidation and methylation of arsenic, and the coupling of arsenic with carbon, iron, sulfur, nitrogen cycling through microbes and related mechanisms were highlighted. Environmental factors like pH, redox potential, organic matter, minerals, nutrient elements, microorganisms and periphyton significantly influence these processes through different pathways, which are discussed in this review. Furthermore, the current progress in remediation strategies, including agricultural interventions, passivation, phytoremediation and microbial remediation is explored, and their potential and limitations are analyzed to address the gaps. This review offers comprehensive perspectives on the complicated behaviors of arsenic and influence factors in paddy soil-rice system, and provides a scientific basis for developing effective arsenic pollution control strategies.

Cooperation of selenium, iron and phosphorus for simultaneously minimizing cadmium and arsenic concentrations in rice grains

Cadmium (Cd) and arsenic (As), two toxic elements to humans, are ubiquitously coexisting contaminant found in paddy fields. The accumulation of Cd and As in rice, a major food source for many people around the world, can pose a serious threat to food safety and human health. Therefore, it is crucial to be aware of these contaminants and take adequate measures to reduce the accumulation of these two elements in rice. Developing an effective method to simultaneously reduce the accumulation of Cd) and As in rice is challenging. In this study, a pot experiment was conducted to investigate the synergistic effects of selenium (Se), iron (Fe) and phosphorus (P) on the uptake, transport and accumulation of cadmium and arsenic in rice by analyzing the physical and chemical properties of the soil, the elemental concentrations and their interrelationships in the rice tissues, and the composition and morphology of the iron plaque (IP). The results showed that the combined treatments of Se, Fe and P had positive effects on reducing Cd and As accumulation in rice, reducing Cd concentrations in brown rice by 3.86-51.88 % and As concentrations by 25.37-40.81 %. The possible mechanisms for the reduction of As and Cd concentrations in rice grains were: (i) Combined application of Fe, P and Se can effectively reduce the soil available Cd and As concentration. (ii) Combined application significantly improved the formation of IP at the tillering stage and increased the crystalline iron oxides in IP, promoting the deposition of SiO2 in rice roots, thereby effectively inhibiting the uptake of Cd and As by rice roots. (iii) Interplay and interaction between elements facilitated by transporter proteins could contribute to the synergistic mitigation of Cd and As by Se, Fe and P. This study provides a valuable new approach for effective control of Cd and As concentration of rice grown in co-contaminated soil.

Melatonin differentially refines the metabolome to improve seed formation during grain developmental stages and enhances yield in two contrasting rice cultivars, grown in arsenic-contaminated soil

The manuscript revealed the ameliorative effects of exogenous melatonin in two distinct reproductive stages, i.e., developing grains (20 days after pollination) and matured grains (40 days after pollination) in two contrasting indica rice genotypes, viz., Khitish (arsenic-susceptible) and Muktashri (arsenic-tolerant), irrigated with arsenic-contaminated water throughout their life-cycle. Melatonin administration improved yield-related parameters like rachis length, primary and secondary branch length, number of grains per panicle, number of filled and empty grains per panicle, grain length and breadth and 1000-grain per weight. Expression of GW2, which negatively regulates grain development, was suppressed, along with concomitant induction of positive regulators like GIF1, DEP1 and SPL14 in both Khitish and Muktashri. Melatonin lowered arsenic bioaccumulation in grains and tissue biomass, more effectively in Khitish. Unregulated production of reactive oxygen species, leading to cellular necrosis caused by arsenic, was reversed in presence of melatonin. Endogenous melatonin level was stimulated due to up-regulation of the key biosynthetic genes, SNAT and ASMT. Melatonin enhanced the production of diverse antioxidants like anthocyanins, flavonoids, total phenolics and ascorbic acid and also heightened the production of thiol-metabolites (cysteine, reduced glutathione, non-protein thiols and phytochelatin), ensuring effective chelation and arsenic detoxification. Altogether, our observation, supported by principal component analysis, proved that melatonin re-programs the antioxidative metabolome to enhance plant resilience against arsenic stress to mitigate oxidative damages and reduce arsenic translocation from the soil to tissue biomass and edible grains.

Phytofabricated gold nanoparticles as modulators of salt stress responses in spinach: implications for redox homeostasis, biochemical and physiological adaptation

Introduction

The utilization of plant material for synthesizing nanoparticles effectively triggers physiological and biochemical responses in plants to combat abiotic stresses. Salt stress, particularly caused by NaCl, significantly affects plant morphology and physiology, leading to reduced crop yields. Understanding the mechanisms of salt tolerance is crucial for maintaining crop productivity.

Methods

In this study, we examined the effects of 150 μM spinach-assisted gold nanoparticles (S-AuNPs) on various parameters related to seed germination, growth attributes, photosynthetic pigments, stomatal traits, ion concentrations, stress markers, antioxidants, metabolites, and nutritional contents of spinach plants irrigated with 50 mM NaCl.

Results

Results showed that S-AuNPs enhanced chlorophyll levels, leading to improved light absorption, increased photosynthates production, higher sugar content, and stimulated plant growth under NaCl stress. Stomatal traits were improved, and partially closed stomata were reopened with S-AuNPs treatment, possibly due to K+/Na+ modulation, resulting in enhanced relative water content and stomatal conductance. ABA content decreased under S-AuNPs application, possibly due to K+ ion accumulation. S-AuNPs supplementation increased proline and flavonoid contents while reducing ROS accumulation and lipid peroxidation via activation of both non-enzymatic and enzymatic antioxidants. S-AuNPs also regulated the ionic ratio of K+/Na+, leading to decreased Na+ accumulation and increased levels of essential ions in spinach plants under NaCl irrigation.

Discussion

Overall, these findings suggest that S-AuNPs significantly contribute to salt stress endurance in spinach plants by modulating various physiological attributes.

Synergistic influence of selenium and silicon supplementation prevents the oxidative effects of arsenic stress in wheat

Influence of supplementation of selenium (Se, 1 and 5 µM) and silicon (Si, 0.1 and 0.5 mM) was investigated in wheat under arsenic (30 µM As) stress. Plants grown under As stress exhibited a significant decline in growth parameters however, Se and Si supplementation mitigated the decline significantly. Treatment of Se and Si alleviated the reduction in the intermediate components of chlorophyll biosynthesis pathway and the content of photosynthetic pigments. Arsenic stressed plants exhibited increased reactive oxygen species accumulation and the NADPH oxidase activity which were lowered significantly due to Se and Si treatments. Moreover, Se and Si supplementation reduced lipid peroxidation and activity of lipoxygenase and protease under As stress. Supplementation of Se and Si significantly improved the antioxidant activities and the content of cysteine, tocopherol, reduced glutathione and ascorbic acid. Treatment of Se and Si alleviated the reduction in nitrate reductase activity. Exogenously applied Se and Si mitigated the reduction in mineral elements and reduced As accumulation. Hence, supplementation of Se and Si is beneficial in preventing the alterations in growth and metabolism of wheat under As stress.

Biochemical and molecular responses of the ascorbate-glutathione cycle in wheat seedlings exposed to different forms of selenium

Biofortification aims to increase selenium (Se) concentration and bioavailability in edible parts of crops such as wheat (Triticum aestivum L.), resulting in increased concentration of Se in plants and/or soil. Higher Se concentrations can disturb protein structure and consequently influence glutathione (GSH) metabolism in plants which can affect antioxidative and other detoxification pathways. The aim of this study was to elucidate the impact of five different concentrations of selenate and selenite (0.4, 4, 20, 40 and 400 mg kg-1) on the ascorbate-glutathione cycle in wheat shoots and roots and to determine biochemical and molecular tissue-specific responses. Content of investigated metabolites, activities of detoxification enzymes and expression of their genes depended both on the chemical form and concentration of the applied Se, as well as on the type of plant tissue. The most pronounced changes in the expression level of genes involved in GSH metabolism were visible in wheat shoots at the highest concentrations of both forms of Se. Obtained results can serve as a basis for further research on Se toxicity and detoxification mechanisms in wheat. New insights into the Se impact on GSH metabolism could contribute to the further development of biofortification strategies.

Effects of selenium on biogeochemical cycles of cadmium in rice from flooded paddy soil systems in the alluvial Indus Valley of Pakistan

This study delves into the pollution status, assesses the effects of Se on Cd biogeochemical pathways, and explores their interactions in nutrient-rich paddy soil-rice ecosystems through 500 soil-rice samples in Pakistan. The results showed that 99.6 % and 12.8 % of soil samples exceeded the World Health Organization (WHO) allowable Se and Cd levels (7 and 0.35 mg/kg). In comparison, 23 % and 6 % of the grain samples exceeded WHO's allowable Se and Cd levels (0.3 and 0.2 mg/kg), respectively. Geographically Weighted Regression (GWR) model results further revealed spatial nonstationarity, confirming diverse associations between dependent variables (Se and Cd in rice grain) and independent variables from paddy soil and plant tissues (root and shoot), such as Soil Organic Matter (SOM), pH, Se, and Cd concentrations. High Se:Cd molar ratios (>1) and a negative correlation (r = -0.16, p < 0.01) between the Cd translocation factor (Cd in rice grain/Cd in root) and Se in roots suggest that increased root Se levels inhibit the transfer of Cd from roots to grains. The inverse correlation between Se and Cd in paddy grains was further characterized as Se deficiency, no risk, high Cd risk, Se risk, Cd risk, and Se-Cd co-exposure risk. There was no apparent risk for human co-consumption in 42.6 % of grain samples with moderate Se and low Cd. The remaining categories indicate differing degrees of risk. In the study area, 31 % and 20 % of grain samples with low Se and Cd indicate Se deficiency and risk, respectively. High Se and low Cd levels in rice samples suggest a potential hazard for severe Se exposure due to frequent rice consumption. This study not only systematically evaluates the pollution status of paddy-soil systems in Pakistan but also provides a reference to thoroughly contemplate the development of a scientific approach for evaluating human risks and the potential dangers associated with paddy soils and rice, specifically in regions characterized by low Se and low Cd concentrations, as well as those with moderate Se and high Cd concentrations. SYNOPSIS: This study is significant for understanding the effects of Se on Cd geochemical cycles and their interactions in paddy soil systems in Pakistan.

Exogenous γ-aminobutyric acid and Bacillus pumilus reduce arsenic uptake and toxicity in rice

In this study, the addition of γ-aminobutyric acid (GABA), Bacillus pumilus, or both, was found to enhance rice growth and yield while significantly decreasing arsenic (As) accumulation in Oryza sativa rice tissues. GABA emerged as a regulator of iron (Fe) homeostasis, acting as a signaling modulator that influenced phytosiderophore secretions in the plant. Meanwhile, B. pumilus directly increased Fe levels through siderophore production, promoting the development of Fe-rich rice plants. Subsequently, Fe competed with As uptake at the root surface, leading to decreased As levels and translocation to the grains. Furthermore, the addition of GABA and B. pumilus optimized rice indole-3 acetic acid (IAA) contents, thereby adjusting cell metabolite balance under As stress. This adjustment results in low malondialdehyde (MDA) contents in the leaves and roots during the early and late vegetative phases, effectively reducing oxidative stress. When added to As-contaminated soil, GABA and B. pumilus effectively maintained endogenous GABA levels and exhibited low ROS generation, similar to normal soil. Concurrently, GABA and B. pumilus significantly downregulated the activity of OsLsi1, OsLsi2, and OsABCC1 in roots, reducing As uptake through roots, shoots, and grains, respectively. These findings suggest that GABA and B. pumilus additions impede As translocation through grains, ultimately enhancing rice productivity under As stress.

Impact of ethylene diamine tetraacetic acid on physiochemical parameters and yield attribute in two varieties of Brassica juncea under lead stress

Lead (Pb) is one of the most toxic elements on earth. The harmful effects of Pb at higher concentrations were seen on plant vegetation because plants are directly exposed towards it. Indian mustard, a well-known hyperaccumulator plant is the most promising crop for the environment, engaged in a variety of scenarios for ecological cleanup. In the present study, we used ethylene diamine tetraacetic acid (EDTA), a chelating agent that is of remarkable efficiency. The pot experiments were conducted in soil pretreated with 1000 mgkg-1 Pb with different concentrations of EDTA (2-10 mmol). All the growth parameters were reduced significantly in the plants treated with Pb and EDTA, however, a non-significant effect was observed in 5 mmol EDTA compared to Pb alone treatment. Photosynthetic pigments yield, nitrate reductase activity and NPK content were affected negatively; in contrast, superoxide dismutase and catalase activity was increased in Pb and Pb+EDTA treated in both the varieties. The Pb accumulation was elevated significantly by the augmentation of 5 mmol EDTA in both varieties. Accumulation of Pb in the shoot was higher in PM 25 than in P. Vijay, whereas root Pb accumulation showed the opposite, i.e., more Pb in roots of P. Vijay than PM 25. Moreover, The Pb accumulation per plant was observed more in P. Vijay as compared to PM 25. Hence, the present study implies that the augmentation of Pb-polluted soil with EDTA works well while dealing with B. juncea assisted phytoremediation and P. Vijay to be a stronger variety than PM 25. Further, 5 mmol of EDTA was optimum for phytoremediation of the soil polluted with up to 1000 mg Pb kg-1 soil.

Coupling of different antioxidative systems in rice under the simultaneous influence of selenium and cadmium

Selenium (Se) elevates the antioxidant ability of rice against cadmium (Cd) stress, but previous studies only focused on the variation in antioxidant enzymes or nonenzymatic substances induced by Se under Cd stress and ignored the relationships between different antioxidant parameters during the interaction. Here, hydroponic experiments with rice were performed by adding both Cd and Se at doses in the range of 0-50 μM to explore the physiological responses of rice and their relationships in the presence of different levels of Se and Cd. Exogenous Cd markedly promoted the activity of antioxidant enzymes with the exception of catalase (CAT) and the concentration of nonenzymatic substances in aerial parts. Se enhanced the antioxidant capacity by improving the activities of all the enzymes tested in this study and increasing the concentrations of nonenzymatic compounds. The couplings among different antioxidant substances within paddy rice were then determined based on cluster and linear fitting results and their metabolic process and physiological functions. The findings specifically highlight that couplings among the ascorbic acid (AsA)-glutathione (GSH) cycle, glutathione synthase (GS)-phytochelatin synthetase (PCS) coupling system and glutathione peroxidase (GPX)-superoxide dismutase (SOD) coupling system in aerial parts helps protect plants from Cd stress. These coupling systems form likely due to the fact that one enzyme generated a product that could be the substrate for another enzyme. Noticeably, such coupling systems do not emerge in roots because the stronger damage to roots than other organs activates the ascorbate peroxidase (APX)-GPX-CAT and PCS-GS-SOD systems with distinct functions and structures. This study provides new insights into the detoxification mechanisms of rice caused by the combined effect of Se and Cd.

Selenium alleviates cadmium and copper toxicity in Gracilaria lemaneiformis (rhodophyta) with contrasting detoxification strategies

Selenium (Se) is a beneficial element for plants, and can be used to mitigate the toxicity of heavy metals. However, the detoxification of Se in macroalgae, a crucial part of aquatic ecosystem productivity, has rarely been reported. In the present study, a red macroalga Gracilaria lemaneiformis was exposed to non-essential metal cadmium (Cd) or essential metal copper (Cu) with addition of different levels of Se. We then examined the changes in growth rate, metal accumulation, metal uptake rate, subcellular distribution, as well as thiol compound induction in this alga. Se addition alleviated Cd/Cu-induced stress in G. lemaneiformis by regulating cellular metal accumulation and intracellular detoxification. Specifically, supplementation of low-level Se displayed a significant decrease in Cd accumulation, and thus alleviated the growth inhibition induced by Cd. This may be caused by the inhibitory effect of endogenous Se instead of exogenous Se on Cd uptake. Although Se addition increased bioaccumulation of Cu in G. lemaneiformis, the important intracellular metal chelators, phytochelatins (PCs), were massively induced to alleviate Cu-induced growth inhibition. High-dose Se addition did not deteriorate but failed to normalize the growth of algae under metal stress conditions. Reduction in Cd accumulation or induction of PCs by Cu could not suppress the toxicity of Se above safe levels. Se addition also altered metal subcellular distribution in G. lemaneiformis, which might affect the subsequent metal trophic transfer. Our results demonstrated that the detoxification strategies of Se between Cd and Cu were different in macroalgae. Elucidating the protective mechanisms of Se against metal stress may help us better apply Se to regulate metal accumulation, toxicity, and transfer in aquatic environment.

Selenium Biofortification Enhanced Grain Yield and Alleviated the Risk of Arsenic and Cadmium Toxicity in Rice for Human Consumption

Arsenic (As) and Cadmium (Cd) are toxic to rice plants. However, selenium (Se) has the potential to regulate As and Cd toxicity. The present study aimed to evaluate the co-exposure to As⁵⁺ and Se⁶⁺ species in two rice cultivars, BRS Pampa and EPAGRI 108. The plants were divided into six groups and cultivated until complete maturation of the grains, under greenhouse conditions. Regarding total As and inorganic As (i-As) accumulation in grains, the highest concentrations were found for BRS Pampa. For Se, EPAGRI 108 presented the highest concentration of inorganic and organic Se (i-Se and o-Se). The exposure assessments showed that Se biofortification can mitigate the As accumulation in rice and, consequently, the risk of As and Cd toxicity in grains for human consumption. The combined effect of As and Se in rice plants could represent an alternative to biofortify this food in a safe way and with a higher percentage of bioavailable Se. Although Se is able to mitigate As toxicity in rice plants, in the present study we showed that co-exposure in different cultivars under the same growing conditions may present different responses to As and Se exposure.

The role of selenium and nano selenium on physiological responses in plant: a review

Selenium (Se), being an essential micronutrient, enhances plant growth and development in trace amounts. It also protects plants against different abiotic stresses by acting as an antioxidant or stimulator in a dose-dependent manner. Knowledge of Se uptake, translocation, and accumulation is crucial to achieving the inclusive benefits of Se in plants. Therefore, this review discusses the absorption, translocation, and signaling of Se in plants as well as proteomic and genomic investigations of Se shortage and toxicity. Furthermore, the physiological responses to Se in plants and its ability to mitigate abiotic stress have been included. In this golden age of nanotechnology, scientists are interested in nanostructured materials due to their advantages over bulk ones. Thus, the synthesis of nano-Se or Se nanoparticles (SeNP) and its impact on plants have been studied, highlighting the essential functions of Se NP in plant physiology. In this review, we survey the research literature from the perspective of the role of Se in plant metabolism. We also highlight the outstanding aspects of Se NP that enlighten the knowledge and importance of Se in the plant system.

Graphical abstract

Oligochitosan fortifies antioxidative and photosynthetic metabolism and enhances secondary metabolite accumulation in arsenic-stressed peppermint

The current study was designed to investigate whether application of irradiated chitosan (ICn), a recently established plant growth promoter, can prove effective in alleviating arsenic (As) stress in peppermint, a medicinally important plant. This study investigated how foliar application of ICn alleviated As toxicity in peppermint (Mentha piperita L.). Peppermint plants were treated with ICn (80 mg L^-1) alone or in combination with As (10, 20, or 40 mg kg^-1 of soil, as Na₂HAsO₄·7H₂O) 40 days after transplantation (DAT), and effects on the growth, photosynthesis, and antioxidants were assessed at 150 DAT as stress severely decreases plant growth, affects photosynthesis, and alters enzymatic (ascorbate peroxidase, superoxide dismutase) and non-enzymatic (glutathione) antioxidants. When applied at 40 mg kg^-1, ICn significantly decreased the content of essential oil (EO) and total phenols in peppermint by 13.8 and 16.0%, respectively, and decreased phenylalanine ammonia lyase (PAL) and deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) activities by 12.8 and 14.6%, respectively. Application of ICn mitigated the disadvantageous effects caused by As toxicity in peppermint by enhancing activities of antioxidative enzymes and photosynthesis and increased accretion of secondary metabolism products (EOs and phenols). An enhancement of total phenols (increased by 17.3%) and EOs (36.4%) is endorsed to ICn-stimulated enhancement in the activities of PAL and DXR (65.9 and 28.9%, respectively) in comparison to the control. To conclude, this study demonstrated that foliar application of ICn (80 mgL^-1) effectively promoted the growth and physiology of peppermint and eliminated As-induced toxicity to achieve high production of EO-containing crops grown in metal-contaminated soils.

Molecular Hydrogen Improves Rice Storage Quality via Alleviating Lipid Deterioration and Maintaining Nutritional Values

Improvement of the storage quality of rice is a critical challenge for the scientific community. This study assesses the effects of the irrigation with hydrogen nanobubble water (HNW) on the storage quality of rice (Oryza sativa 'Huruan1212'). Compared with ditch water control, after one year of storage at 25 °C and 70% RH, the HNW-irrigated rice had higher contents of essential amino acids, especially lysine. Importantly, the generation of off-flavors in the stored rice was significantly decreased, which was confirmed by the lower levels of volatile substances, including pentanal, hexanal, heptanal, octanal, 1-octen-3-ol, and 2-heptanone. The subsequent results showed that the HNW-irrigated rice not only retained lower levels of free fatty acid values, but also had increased antioxidant capacity and decreased lipoxygenase activity and transcripts, thus resulting in decreased lipid peroxidation. This study opens a new window for the practical application of HNW irrigation in the production and subsequent storage of crops.

Application of Pteris vittata L. for phytoremediation of arsenic and biomonitoring of the process through cyto-genetic biomarkers of Trigonella foenum-graecum L

The arsenic (As) contamination demands its remediation from the environment which is naturally possible by the application of Pteris vittata L. However, biomonitoring of phytoremediation potential of P. vittata at chromosomal and DNA level is still meager. The present study was designed to biomonitor the phytoremediation efficiency of P. vittata through phytotoxic and cyto-genotoxic biomarkers assessment using Trigonella foenum-graecum L. (Fenugreek; Methi) as test system. Study revealed hyperaccumulation potential of P. vittata which extracted arsenic in its tissues. Biomonitoring evaluation depicted that phytotoxic damage was reduced in Trigonella exposed to remediated soil, which was revealed through reduced electrolyte leakage, hydrogen peroxide and MDA content. Moreover, cyto-genetic endpoints like mitotic depression (44.03%), relative abnormality rate (16.6%) and chromosomal abnormality frequency (1.06%) were also lesser in test plants grown in remediated soil compared to those grown in non-remediated soil. Along with this various chromosomal aberrations like stickiness, breaks, laggards, bridges, fragmentations and micronuclei were also augmented in test plants exposed to non-remediated arsenic enriched soil. It was evident that arsenic enriched soil caused toxicity to plants in dose-dependent manner that was assessable through the analysis of biochemical parameters and cyto-genetic biomarkers. The cyto-genetic biomarkers are very efficient, simple and non-expensive tools to biomonitor arsenic toxicity at chromosomal as well as DNA level to assess the remediation potential of P. vittata in field conditions.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

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.

Selenium alleviates arsenic induced stress by modulating growth, oxidative stress, antioxidant defense and thiol metabolism in rice seedlings

This study aims to investigate the potentiality of selenium in modulating arsenic stress in rice seedlings. Arsenate accumulation along with its transformation to arsenite was enhanced in arsenate exposed seedlings. Arsenite induced oxidative stress and severely affected the growth of the seedlings. Arsenate exposure caused an elevation in ascorbate and glutathione levels along with the activities of their metabolizing enzymes viz., ascorbate peroxidase, glutathione reductase, glutathione-S-transferase, and glutathione peroxidase. Phytochelatins content was increased under arsenic stress to subdue the toxic effects in the test seedlings. Co-application of arsenate and selenate in rice seedlings manifested pronounced alteration of oxidative stress, antioxidant defense, and thiol metabolism as compared to arsenate treatment only. ANOVA analysis (Tukey's HSD test) demonstrated the relevance of using selenate along with arsenate to maintain the normal growth and development of rice seedlings. Thus, exogenous supplementation of selenium will be a beneficial approach to cultivate rice seedlings in arsenic polluted soil.

Two new selenite reducing bacterial isolates from paddy soil and the potential Se biofortification of paddy rice

Selenium (Se) is an essential element for human health. Se-enriched agricultural products can promote people's intake of Se. Microorganisms play an important role in Se cycling. In this study, two new bacterial strains were isolated from paddy soil and were identified as Chitinophaga sp. and Comamonas testosteroni, respectively. More than 44% and 39% of 1.0 mM selenite were reduced in 84 h by them using yeast extract as carbon source, respectively. Scanning electron microscope (SEM) and Energy dispersive X-ray spectrometry (EDS) results indicated that the reduction product of selenite was nanometer Se. These strains could promote the available Se in soil and the content of Se in rice plants in pot experiments. Organic combined Se in soils was increased up to 35%, accompanied by the 92% and 130% increase of Se in rice plants. To our best knowledge, this is the first report of Se reduction by Chitinophaga. This work might provide a prospective strategy for microbial fortification of Se in corps.

Heterologous expression of cyanobacterial PCS confers augmented arsenic and cadmium stress tolerance and higher artemisinin in Artemisia annua hairy roots

The present study provides the first report of heterologous expression of phytochelatin synthase from Anabaena PCC 7120 (anaPCS) into the hairy roots of Artemisia annua. Transformed hairy roots of A. annua expressing anaPCS gene showed better tolerance to heavy metals, viz., arsenic (As) and cadmium (Cd) owing to 143 and 191% more As- and Cd-accumulation, respectively, as compared to normal roots with a bioconcentration factor (BCF) of 9.7 and 21.1 for As and Cd, respectively. Under As and Cd stresses, transformed hairy roots possessed significantly higher amounts of phytochelatins and thiols probably due to the presence of both AaPCS (Artemisia annua PCS) and anaPCS. In addition, artemisinin synthesis was also induced in transformed hairy roots under heavy metals stresses. In-silico analysis revealed the presence of conserved motifs in both AaPCS and anaPCS sequences as well as structural modelling of PCS functional domain was conducted. Interaction of AaPCS and anaPCS proteins with CdCl₂ and sodium arsenate gene ontology analysis gave insights to anaPCS functioning in transformed hairy roots of A. annua. The study provides transformed hairy roots of A. annua as an efficient tool for effective phytoremediation with added advantages of artemisinin extraction from hairy roots used for phytoremediation.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11816-021-00682-5.

Application of inorganic selenium to reduce accumulation and toxicity of heavy metals (metalloids) in plants: The main mechanisms, concerns, and risks

Anthropogenic activities such as mining, industrialization and subsequent emission of industrial waste, and agricultural practices have led to an increase in the accumulation of metal(loid)s in agricultural soils and crops, which threatens the health of people; the risk is more pronounced for individuals whose survival depends on food sources from several contaminated regions. Selenium (Se) is an element essential for the normal functioning of the human body and is a beneficial element for plants. Se deficiency in the diet is a common issue in many countries around the world, such as China and Egypt. >40 diseases are associated with Se deficiency. In practice, Se compounds have been applied through foliar sprays or via base application of fertilizers to increase Se concentration in the edible parts of crops and to satisfy the daily Se intake. Moreover, Se at low concentrations has been used to mitigate the toxicity of many metal(loid)s. In this review, we present an overview of the latest knowledge and practices with regards to the utilization of Se to reduce the uptake/toxicity of metal(loid)s in plants. We have focused on the following issues: 1) the current status of understanding the mechanisms of detoxification and uptake restriction of metal(loid)s regulated by Se; 2) the optimal dose and speciation of Se, and stage of plant growth that is optimal for application; 3) the differences in the efficiency of different application methods of Se including seed priming, base application, and foliar spray of Se fertilizers; 4) the possibility of using Se along with other methods to reduce multiple metal(loid) accumulation in crops; and 5) potential risks when Se is used to reduce metal(loid) accumulation in crops.

Selenium Supplementation and Crop Plant Tolerance to Metal/Metalloid Toxicity

Selenium (Se) supplementation can restrict metal uptake by roots and translocation to shoots, which is one of the vital stress tolerance mechanisms. Selenium can also enhance cellular functions like membrane stability, mineral nutrition homeostasis, antioxidant response, photosynthesis, and thus improve plant growth and development under metal/metalloid stress. Metal/metalloid toxicity decreases crop productivity and uptake of metal/metalloid through food chain causes health hazards. Selenium has been recognized as an element essential for the functioning of the human physiology and is a beneficial element for plants. Low concentrations of Se can mitigate metal/metalloid toxicity in plants and improve tolerance in various ways. Selenium stimulates the biosynthesis of hormones for remodeling the root architecture that decreases metal uptake. Growth enhancing function of Se has been reported in a number of studies, which is the outcome of improvement of various physiological features. Photosynthesis has been improved by Se supplementation under metal/metalloid stress due to the prevention of pigment destruction, sustained enzymatic activity, improved stomatal function, and photosystem activity. By modulating the antioxidant defense system Se mitigates oxidative stress. Selenium improves the yield and quality of plants. However, excessive concentration of Se exerts toxic effects on plants. This review presents the role of Se for improving plant tolerance to metal/metalloid stress.

Silicon nutrition modulates arsenic-inflicted oxidative overload and thiol metabolism in wheat (Triticum aestivum L.) seedlings

A hydroponic experiment was conducted to establish the response of exogenous silicon [Si] in alleviating arsenate [As (V)] prompted alterations on antioxidant enzyme activities and thiol metabolism in wheat (Triticum aestivum L. cv PBW 343) seedlings. Objective of the work was to validate the hypothesis whether silicate may alleviate arsenate-provoked oxidative stress in wheat through diverse metabolic pathways with an endeavor to improve food safety and health. Arsenate treatment significantly enhanced oxidative stress and was associated with modifications in non-enzymatic and enzymatic antioxidants. The activities of arsenate reductase [AR] and the enzymes related to thiol metabolism revealed dose-dependent enhancements with increase in arsenate along with enhanced production of phytochelatins [PCs] in the cultivar. Simultaneous supplementations of silicate with arsenate in the nutrient formulation reduced arsenate uptake along with arsenate reductase activity and consequently lowered arsenite [As (III)] accumulation. The antioxidative defense was upregulated and phytochelatin production was lowered causing an appreciable revival from the arsenate-imposed consequences that eventually augmented growth.

Antioxidant defense system in lettuces tissues upon various As species exposure

Characterization of antioxidant response is essential to elucidate the mechanism for plants tolerating arsenic (As) stress. Ten-day old lettuces were exposed to 50, 100, and 200 μg L-1 of arsenite (As(III)), arsenate (As(V)) or dimethylarsinic acid (DMA) for 50 days in hydroponic culture. The activities of superoxide dismutase, catalase, peroxidase, glutathione peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase, as well as the glutathione concentration in tissues, were monitored. And the speciation and occurrence of As were concurrently analyzed in roots and leaves. The results showed that As(III) was the predominant As species in lettuces upon inorganic As exposure, while DMA was the primary As species upon DMA exposure. DMA presented higher mobility than inorganic As. The reduction of As(V) in roots upon As(V) exposure and in leaves upon As(III) exposure were suggested. The alterations of enzymatic antioxidant activities and non-enzymatic antioxidant contents showed that the antioxidant responses were As species-dependent, dose-dependent and tissue-dependent. And upon As(V) and DMA exposures, antioxidant responses were more intense than that upon As(III) exposure. Further the results indicated that the antioxidant responses in lettuce were associated with the conversion and transport of As species.

Effects of selenium on antioxidant enzyme activity and bioaccessibility of arsenic in arsenic-stressed radish

Consuming arsenic (As)-contaminated vegetables is the main route of As exposure in humans. The present study focused on the alterations in antioxidant enzymatic activities and As bioaccessibility in As-contaminated radish subjected to Se. Compared to the CK group, the total As content in raw radish was reduced by 27.5 ± 1.3%, and the bioaccessibility of As was reduced by 21.9 ± 2.3% in the 6 mg Se kg^-1 treatment group. The total As content in the treatment groups decreased first but then increased with increasing Se application in raw radish, gastric (G) fraction and gastrointestinal (GI) fraction, while the antioxidant activity exhibited the opposite trend. The results revealed that a low amount of Se effectively blocks the accumulation of As in radish, improves the antioxidant activity in radish and reduces the bioaccessibility of As. These findings provide new ideas for effectively alleviating the spread of As to the human body through the food chain.

Efficiency and risks of selenite combined with different water conditions in reducing uptake of arsenic and cadmium in paddy rice

The co-contamination of arsenic (As) and cadmium (Cd) in soils is a common problem. Selenium (Se) can reduce the uptake of As and Cd in plants, and in practice, the alternate wetting and drying is a common culture mode in rice production. However, it is unknown whether Se can efficiently reduce As and Cd concentrations in crops suffering from a high-level contamination of As and Cd under different soil water conditions. In this study, we assessed the efficiency and risks of selenite [Se(IV)], in a pot experiment, to reduce the uptake of As and Cd in a rice plant (YangDao No 6) growing in a heavily contaminated soil by As and Cd (pH 7.28) under different soil water conditions. The results showed that Se(IV) failed to control the grain total As and Cd concentrations within their individual limited standard (0.2 mg kg^-1) despite that Se(IV) significantly reduced the grain total As and Cd concentrations. The soil drying treatment alone could reduce the accumulation of arsenite [As(III)] in the grains, but additional Se(IV) stimulated the accumulation of As(III) in the grains under soil drying conditions. In addition, the addition of Se(IV) enhanced the As and Cd concentrations in the shoots and/or roots of rice plants under certain conditions. The above results all suggested that the utilization of Se(IV) in a high contaminated soil by As and Cd cannot well control the total concentrations of As and Cd in plants. In this study, the available concentrations of As and Cd in the rhizosphere soil, the rhizosphere soil pH, the formation of root iron/manganese plaques and the concentrations of essential elements in the grains were monitored, and the related mechanisms on the changes of these parameters were also discussed. This study will give a guideline for the safe production of rice plants in a heavily co-contaminated soil by As and Cd.

Transcriptome and proteome analyses reveal selenium mediated amelioration of arsenic toxicity in rice (Oryza sativa L.)

Arsenic (As), a chronic poison and non-threshold carcinogen, is a food chain contaminant in rice, posing yield losses as well as serious health risks. Selenium (Se), a trace element, is a known antagonist of As toxicity. In present study, RNA seq. and proteome profiling, along with morphological analyses were performed to explore molecular cross-talk involved in Se mediated As stress amelioration. The repair of As induced structural deformities involving disintegration of cell wall and membranes were observed upon Se supplementation. The expression of As transporter genes viz., NIP1;1, NIP2;1, ABCG5, NRAMP1, NRAMP5, TIP2;2 as well as sulfate transporters, SULTR3;1 and SULTR3;6, were higher in As + Se compared to As alone exposure, which resulted in reduced As accumulation and toxicity. The higher expression of regulatory elements like AUX/IAA, WRKY and MYB TFs during As + Se exposure was also observed. The up-regulation of GST, PRX and GRX during As + Se exposure confirmed the amelioration of As induced oxidative stress. The abundance of proteins involved in photosynthesis, energy metabolism, transport, signaling and ROS homeostasis were found higher in As + Se than in As alone exposure. Overall, present study identified Se responsive pathways, genes and proteins involved to cope-up with As toxicity in rice.

Phyto-genotoxicity of arsenic contaminated soil from Lakhimpur Kheri, India on Vicia faba L

The present experiment was designed to evaluate physico-chemical characteristics and phyto-genotoxicity of arsenic (As) contaminated soil collected from different sites of Lakhimpur, Uttar Pradesh (UP), India by employing Vicia faba L. The analyses included various biochemical as well as cyto-genotoxicity assays. The results showed that soil pH was slightly acidic to neutral in nature. The bulk density (1.18-1.23 gcm^-3), particle density (2.51-2.59 gcm^-3) and porosity (44-53%) varied in different places. The level of available nutrients, nitrogen, phosphorus and potassium was found to vary as 124-165 mgkg-1, 173-186 mgkg-1 and 48-98 mgkg-1, respectively. The maximum As levels were found in soil of Fulvareya (27.13 mgkg-1) and Atareya (24.12 mgkg-1), the level of As in water samples of these sites were 0.19 mgl^-1and 0.21 mgl^-1, respectively. Phytotoxicity of the As present in soils was evident through significant increases in stress metabolites, hydrogen peroxide (H₂O₂), malondialdehyde (MDA) and carbonyl groups in root and shoot of V. faba. Cyto-genotoxic effects were also seen through reduced mitotic index (MI) and increased mitotic depression (MD), relative abnormality rate (RAR) as well as other chromosomal abnormalities along with micronuclei in root meristematic cells of V. faba. The phytotoxicity and cyto-genotoxicity assessment suggests the harmful soil properties that might affect biota.

Selenomethionine induces oxidative stress and modifies growth in rice (Oryza sativa L.) seedlings through effects on hormone biosynthesis and primary metabolism

Although the chemical characteristics of selenomethionine (SeMet) are similar to those of methionine (Met), the physiological activity of SeMet apparently differs in its ability to stimulate ethylene production in plant tissues. Since selenium alters root architecture of rice seedlings by modifying ethylene production, the investigation of the effect of SeMet and Met on rice growth would be a step forward towards unraveling factors that underlie selenium toxicity. Here, we report that SeMet increased concentrations of reactive oxygen species (ROS), inhibiting auxin and increasing ethylene production in rice seedlings. The effect of SeMet on seedlings was mediated by the inhibition of the abundance of transcripts encoding auxin transport and cell expansion proteins. Moreover, SeMet led to increased seedling respiration, which was positively correlated with organic acids consumption, but negatively with sugars consumption, thereby decreasing seedling growth. In contrast with SeMet treatment, Met did not affect ROS production, hormone biosynthesis and seedling growth, indicating an exclusive selenium effect. The singlet oxygen scavenger, 1,4-diazabicyclooctane, overrode the repressive effect of SeMet in seedling growth. Our results demonstrate a phytotoxic effect of SeMet for rice seedlings and reveal a relationship between reactive oxygen species, hormone homeostasis and carbon availability, which regulates growth responses.

Phytotoxicity of silver nanoparticles on Vicia faba: Evaluation of particle size effects on photosynthetic performance and leaf gas exchange

Nanotechnology is an emerging field in science and engineering, which presents significant impacts on the economy, society and the environment. The nanomaterials' (NMs) production, use, and disposal is inevitably leading to their release into the environment where there are uncertainties about its fate, behaviour, and toxicity. Recent works have demonstrated that NMs can penetrate, translocate, and accumulate in plants. However, studies about the effects of the NMs on plants are still limited because most investigations are carried out in the initial stage of plant development. The present study aimed to evaluate and characterize the photochemical efficiency of photosystem II (PSII) of broad bean (Vicia faba) leaves when subjected to silver nanoparticles (AgNPs) with diameters of 20, 51, and 73 nm as well as to micrometer-size Ag particles (AgBulk). The AgNPs were characterized by transmission electron microscopy and dynamic light scattering. The analyses were performed by injecting the leaves with 100 mg L^-1 aqueous solution of Ag and measuring the chlorophyll fluorescence imaging, gas exchange, thermal imaging, and reactive oxygen species (ROS) production. In addition, silver ion (Ag⁺) release from Ag particles was determined by dialysis. The results revealed that AgNPs induce a decrease in the photochemical efficiency of photosystem II (PSII) and an increase in the non-photochemical quenching. The data also revealed that AgNPs affected the stomatal conductance (g_s) and CO₂ assimilation. Further, AgNPs induced an overproduction of ROS in Vicia faba leaves. Finally, all observed effects were particle diameter-dependent, increasing with the reduction of AgNPs diameter and revealing that AgBulk caused only a small or no changes on plants. In summary, the results point out that AgNPs may negatively affect the photosynthesis process when accumulated in the leaves, and that the NPs themselves were mainly responsible since negligible Ag⁺ release was detected.

Selenium-Ethylene Interplay in Postharvest Life of Cut Flowers

Selenium (Se) is considered a beneficial element in higher plants when provided at low concentrations. Recently, studies have unveiled the interactions between Se and ethylene metabolism throughout plant growth and development. However, despite the evidence that Se may provide longer shelf life in ethylene-sensitive flowers, its primary action on ethylene biosynthesis and cause-effect responses are still understated. In the present review, we discuss the likely action of Se on ethylene biosynthesis and its consequence on postharvest physiology of cut flowers. By combining Se chemical properties with a dissection of ethylene metabolism, we further highlighted both the potential use of Se solutions and their downstream responses. We believe that this report will provide the foundation for the hypothesis that Se plays a key role in the postharvest longevity of ethylene-sensitive flowers.

New insights into cadmium stressful-conditions: Role of ethylene on selenium-mediated antioxidant enzymes

Cadmium (Cd) contamination has generated an environmental problem worldwide, leading to harmful effects on human health and damages to plant metabolism. Selenium (Se) is non essential for plants, however it can improve plant growth and reduce the adverse effects of abiotic stress. In addition, ethylene may interplay the positive effects of Se in plants. In order to investigate the role of ethylene in Se-modulation of antioxidant defence system in response to Cd-stress, we tested the hormonal mutant Epinastic (epi) with a subset of constitutive activation of the ethylene response and Micro-Tom (MT) plants. For this purpose, Se mineral uptake, Cd and Se concentrations, pigments, malondialdeyde (MDA) and hydrogen peroxide (H₂O₂) contents, ethylene production, glutathione (GSH) compound, and superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR) and glutathione peroxidase (GSH-Px) activities were analysed in MT and epi plants submitted to 0.5 mM CdCl₂ and 1 μM of selenate or selenite. MT plants treated with both Se forms increased growth in the presence or not of 0.5 mM CdCl₂, but not change epi growth. Both Se forms reduced Cd uptake in MT plants and cause reverse effect in epi plants. P, Mg, S, K and Zn uptake increased in epi plants with Se application, irrespective to Cd exposure. Chlorophylls and carotenoids contents decreased in both genotypes under Cd exposure, in contrast to what was observed in epi leaves in the presence of Se. When antioxidant enzymes activities were concerned, Se application increased Mn-SOD, Fe-SOD and APX activities. In the presence of Cd, MT and epi plants exhibited decreased SOD activity and increased CAT, APX and GR activities. MT and epi plants with Se supply exhibited increased APX and GR activities in the presence of Cd. Overall, these results suggest that ethylene may be involved in Se induced-defence responses, that triggers a positive response of the antioxidant system and improve growth under Cd stress. These results showed integrative roles of ethylene and Se in regulating the cell responses to stressful-conditions and, the cross-tolerance to stress could be used to manipulate ethylene regulated gene expression to induce heavy metal tolerance.

Protective mechanisms of melatonin against selenium toxicity in Brassica napus: insights into physiological traits, thiol biosynthesis and antioxidant machinery

BACKGROUND

The ubiquitous signaling molecule melatonin (N-acetyl-5-methoxytryptamine) (MT) plays vital roles in plant development and stress tolerance. Selenium (Se) may be phytotoxic at high concentrations. Interactions between MT and Se (IV) stress in higher plants are poorly understood. The aim of this study was to evaluate the defensive roles of exogenous MT (0 μM, 50 μM, and 100 μM) against Se (IV) (0 μM, 50 μM, 100 μM, and 200 μM) stress based on the physiological and biochemical properties, thiol biosynthesis, and antioxidant system of Brassica napus plants subjected to these treatments.

RESULTS

Se (IV) stress inhibited B. napus growth and biomass accumulation, reduced pigment content, and lowered net photosynthetic rate (P_n) and PSII photochemical efficiency (Fv/Fm) in a dose-dependent manner. All of the aforementioned responses were effectively alleviated by exogenous MT treatment. Exogenous MT mitigated oxidative damage and lipid peroxidation and protected the plasma membranes from Se toxicity by reducing Se-induced reactive oxygen species (ROS) accumulation. MT also alleviated osmotic stress by restoring foliar water and sugar levels. Relative to standalone Se treatment, the combination of MT and Se upregulated the ROS-detoxifying enzymes SOD, APX, GR, and CAT, increased proline, free amino acids, and the thiol components GSH, GSSG, GSH/GSSG, NPTs, PCs, and cys and upregulated the metabolic enzymes γ-ECS, GST, and PCS. Therefore, MT application attenuates Se-induce oxidative damage in plants. MT promotes the accumulation of chelating agents in the roots, detoxifies Se there, and impedes its further translocation to the leaves.

CONCLUSIONS

Exogenous MT improves the physiological traits, antioxidant system, and thiol ligand biosynthesis in B. napus subjected to Se stress primarily by enhancing Se detoxification and sequestration especially at the root level. Our results reveal better understanding of Se-phytotoxicity and Se-stress alleviation by the adequate supply of MT. The mechanisms of MT-induced plant tolerance to Se stress have potential implications in developing novel strategies for safe crop production in Se-rich soils.

Effect of Endogenous Selenium on Arsenic Uptake and Antioxidative Enzymes in As-Exposed Rice Seedlings

Arsenic (As) and selenium (Se) are two metalloids found in the environment. As it poses a significant threat to human health and plant growth due to its prevalence and toxicity, however Se is a required micronutrient for human health. In this study hydroponic experiments were performed to investigate whether endogenous Se can mitigate As toxicity in rice (Oryza sativa L.). We found that As uptake by rice roots increased by pretreatment with selenateSe(VI) or selenite Se(IV). However, co-application of arsenate As(V) or arsenite As(III) with selenate markedly reduced the uptake of As by roots. Co- or pretreatment with Se with five µM of As(V) or one µM of As(III) significantly decreased shoot As content. Conversely, Se pretreatment before the addition of five µM of As(III) or one µM of As(V) resulted in As accumulation in the shoot compared to As and Se co-application. As translocation to the shoot was lower whereas the transfer factor was higher upon the simultaneous application of Se and As compared to Se pretreatment. Se supplementation with As(III) or pretreatment increased antioxidant enzymes: Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) increased in the root and shoot, but decreased glutathione (GSH) and malondialdehyde (MDA) contents in the shoot. Plants under As(V) treatment showed the same trend except that CAT content decreased in the root and shoot, while MDA content increased in the shoot. These results suggest that cultivating rice in the presence of Se can reduce the accumulation of toxic As in seedlings, thus ensuring the safety of this important crop for human consumption.

The paradox in accumulation behavior of cadmium and selenium at different planting times in rice

The synergistic effects of trace elements selenium (Se) and cadmium (Cd) are well known. But the reasons for the trending accumulation behavior in both trace elements are under debate in the scientific community. The present investigation was conducted to evaluate the impact of heat units on the accumulation behavior in two environments. Se and Cd were applied in three groups (T₀; 0:0, T₁; 0.4:1, T₂; 1:2) mg kg^-1. As the time of planting and heat units consumed by the crop to attain its physiological maturity appears to be different. The sunlight may contribute as an important manipulating factor for the accumulation of heavy metals in the plant. The results of the present study indicated that the behavior in the accumulation pattern of both elements appears to be distinct in the same material. The increased fertilizer treatment in soil bulk linearly increased the metal contents in rice grain. The studies between different plant parts disclosed panicle as the primary reservoir for Se and Cd accumulation. The plant sown earlier accumulates more Se than Cd while the plants uptake more Cd when sown 1 month later. The plant completes the critical developmental phases (tillering, heading, and flowering) within 20-30 days interval depending on the variety. But the late-sown plants complete these transitional phases within 9-10 days interval ultimately result in less utilization of heat units. These quick transitional changes may lead to the uptake of an ample amount of Cd contents in rice grain even at a low level of Cd stress (1 and 2 mg kg^-1), making rice unsafe for edible purposes. The proper time of planting could be effective in timely acclimatization of Se and Cd sequestration and translocation in rice various components. Graphical Abstract.

Selenium (Se) Does Not Reduce Cadmium (Cd) Uptake and Translocation in Rice (Oryza sativa L.) in Naturally Occurred Se-Rich Paddy Fields with a High Geological Background of Cd

This study examined the selenium (Se) and cadmium (Cd) uptake by rice from soil and analyzed the relationship between Se and Cd in naturally occurred Se-rich paddy fields with a high geological background of Cd. Significant correlations were observed between soil Se and plant biomass Se, but not between soil Cd and plant biomass Cd. High concentrations of Cd were detected in rice plants and particularly in rice grains, suggesting potential health risks to human. Contrary to results from other previous studies, our results showed that high soil Se did not reduce Cd uptake by rice, although it decreased the availability of Cd in soil. Rather, soil Se and internal Se pool in rice were positively correlated to the transfer of Cd from root to straw. The effect of Se on the uptake and translocation of Cd in rice in field is therefore different from those in pot experiments.

The role of selenium on mitigating arsenic accumulation, enhancing growth and antioxidant responses in metallicolous and non-metallicolous population of Isatis cappadocica Desv. and Brassica oleracea L

A hydroponic experiment was conducted to explore the interactive effects of selenium (Se) supplementation (0, 5, and 10 μM) and arsenic (As) toxicity (0, 200, and 400 μM) on the growth, accumulation, and oxidative damage along with defense mechanisms of metallicolous (MP) and non-metallicolous population (NMP) of Isatis cappadocica, an As-hyperaccumulator, and Brassica oleracea as reference brassica. The results revealed that As stress significantly hampered plant growth particularly in B. oleracea. It reduced plant growth due to enhanced oxidative load of As-stressed plants. Between the two Isatis populations, metallicolous plants accumulated significantly higher As, however with considerably low growth defects. Furthermore, Se supplementation counteracted the adverse effects of stress on growth and physiological performance of all studied plants. Addition of Se, particularly at higher dose (10 μM), significantly suppressed root As uptake and slightly its accumulation in shoots of B. oleracea plants treated with 400 μM As, and thus improved growth characteristics of stressed plants. Under As stress, Se supplementation increased the activities of enzymatic (peroxidase (POD) and glutathione reductase (GR)) and non-enzymatic (anthocyanins and total flavonoids) antioxidants, thereby suggesting relieved As stress by reduced oxidative damage. Taken together, these results support the beneficial role of Se in the regulation of As stress by improving growth, physiology, and antioxidant capacity, and highlight its significance for plants grown on such metal-contaminated soils.

Effect of Arbuscular Mycorrhizal Fungi, Selenium and Biochar on Photosynthetic Pigments and Antioxidant Enzyme Activity Under Arsenic Stress in Mung Bean (Vigna radiata)

Environmental perturbations alter biochemical compounds in food crops. Arsenic (As), a toxic metalloid, is known to affect the cultivation of food crops in many regions of the world; however, the changes in chlorophyll, catalase (CAT), and proline in response to As stress and the role of stress relief substances remain largely unknown in mung bean (Vigna radiate L.). In this study, biochar (BC), arbuscular mycorrhizal fungi (AMF), and selenium (Se) were applied to soils as stress relief substances (under 30 mg kg-1 As stress), and the effects of BC, AMF, and Se on chlorophyll a, chlorophyll b, total chlorophyll, CAT activity, and proline content were studied in different mung bean genotypes. Under As stress, the chlorophyll a, chlorophyll b, and total chlorophyll contents in BARI mung 3, BARI mung 5, and BARI mung 8 were found statistically similar. Meanwhile, CAT activity increased in comparison to the control due to the application of BC, AMF, and Se in mung bean crops. However, proline was found significantly lower in AMF, BC, and Se-treated mung bean. This indicates that oxidative stress was potentially minimized in As-stressed mung bean crops due to the application of these stress relief substances. Notably, AMF was relatively effective against As stress in comparison to BC and Se. It is concluded that BC, AMF, and Se are all highly effective in enhancing antioxidant defenses as well as the nutritional quality of mung bean crops under As stress.

Two selenium tolerant Lysinibacillus sp. strains are capable of reducing selenite to elemental Se efficiently under aerobic conditions

Microbes play important roles in the transport and transformation of selenium (Se) in the environment, thereby influencing plant resistance to Se and Se accumulation in plant. The objectives are to characterize the bacteria with high Se tolerance and reduction capacity and explore the significance of microbial origins on their Se tolerance, reduction rate and efficiency. Two bacterial strains were isolated from a naturally occurred Se-rich soil at tea orchard in southern Anhui Province, China. The reduction kinetics of selenite was investigated and the reducing product was characterized using scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy. The bacteria were identified as Lysinibacillus xylanilyticus and Lysinibacillus macrolides, respectively, using morphological, physiological and molecular methods. The results showed that the minimal inhibitory concentrations (MICs) of selenite for L. xylanilyticus and L. macrolides were 120 and 220 mmol/L, respectively, while MICs of selenate for L. xylanilyticus and L. macrolides were 800 and 700 mmol/L, respectively. Both strains aerobically reduced selenite with an initial concentration of 1.0 mmol/L to elemental Se nanoparticles (SeNPs) completely within 36 hr. Biogenic SeNPs were observed both inside and outside the cells suggesting either an intra- or extracellular reduction process. Our study implied that the microbes from Se-rich environments were more tolerant to Se and generally quicker and more efficient than those from Se-free habitats in the reduction of Se oxyanions. The bacterial strains with high Se reduction capacity and the biological synthesized SeNPs would have potential applications in agriculture, food, environment and medicine.

Accumulation, subcellular distribution, and oxidative stress of cadmium in Brassica chinensis supplied with selenite and selenate at different growth stages

Despite not being an essential element for plants, Se has been proved to reduce Cd accumulation and Cd-induced oxidative stress, although the underlying mechanisms are not fully understood. A pak choi hydroponic experiment was conducted to investigate the effects of Se on Cd accumulation, subcellular distribution, and Cd-induced oxidative stress at different growth stages. The results showed that on day 19 after germination, Cd content was significantly reduced by 32% by selenite, but was increased by 15% by selenate. Accordingly, selenite improved cell-wall Cd sequestration by 20%, whereas selenate caused enhanced translocation of Cd from the root to the shoot. However, the effects of selenite on the reduction in Cd accumulation and distribution in pak choi seedlings were completely dismissed on day 40. Nevertheless, both forms of Se enhanced antioxidative defense, as they both inhibited the accumulation of H₂O₂ and malondialdehyde. On day 19, ascorbate peroxidase and glutathione reductase activities were increased by more than 50% by selenite; additionally, superoxide dismutase, catalase, and peroxidase activities increased by up to 86%, 63%, and 24%, respectively, in the presence of selenite, when compared to Cd treatment alone. Activities of most of the antioxidants remained significantly unaffected by both forms of Se on day 40. Consequently, selenite and selenate affected Cd accumulation in pak choi seedlings by altering Cd subcellular distribution and by enhancing antioxidative defense, but such effects depended on the Se forms applied and the growth stage as well.

Selenium amelioration of arsenic toxicity in rice shows genotypic variation: A transcriptomic and biochemical analysis

The toxic metalloid arsenic (As) is consumed mostly through contaminated rice. Therefore, reducing its accumulation and maintaining nutrient homeostasis in crop plants are imperative to ensure food safety. However, there is a dearth of information on the interrelationship between nutrient homeostasis and the regulatory mechanisms of arsenic-selenium (As-Se) interactive pathways responsible for stress tolerance. In the present study, experiments were conducted in hydroponically grown 12-day-old seedlings of rice (Oryza sativa L.) varieties (Pusa Basmati1 and IR64) treated with arsenite (AsIII) (150 μM), selenium (SeVI) (20 μM), and As + Se. It was observed that selenium supplementation ameliorated As toxicity by reducing its accumulation and retrieving As-induced nutrient deficiency. Significant decrease in As accumulation, H₂O₂ content, and fluorescent intensity of nitric oxide (NO), reactive oxygen species (ROS), and superoxide radical (O₂^.-) along with cell death with Se supplementation in both rice varieties demonstrated the protective role of Se as a probable ROS quencher. Addition of Se increased the enzyme activities of thiol metabolism and induced differential transcript accumulation patterns of sulfur-related genes. Nutrient level positively correlated with the differential expression pattern of NPK-related genes that play roles in metabolism and nutrient availability in both varieties. Though Pusa Basmati1 (PB1) showed higher tolerance to As, IR64 overcomes As toxicity more efficiently than PB1 in the presence of Se, which highlights that IR64 is a better performer in the presence of Se. Overall, this study provides novel insight into the role of Se in As-stressed rice genotypes through alteration of nutrient transporters and thiol-related genes.

Selenium at the redox interface of the genome, metabolome and exposome

Selenium (Se) is a redox-active environmental mineral that is converted to only a small number of metabolites and required for a relatively small number of mammalian enzymes. Despite this, dietary and environmental Se has extensive impact on every layer of omics space. This highlights a need for global network response structures to provide reference for targeted, hypothesis-driven Se research. In this review, we survey the Se research literature from the perspective of the responsive physical and chemical barrier between an organism (functional genome) and its environment (exposome), which we have previously termed the redox interface. Recent advances in metabolomics allow molecular phenotyping of the integrated genome-metabolome-exposome structure. Use of metabolomics with transcriptomics to map functional network responses to supplemental Se in mice revealed complex network responses linked to dyslipidemia and weight gain. Central metabolic hubs in the network structure in liver were not directly linked to transcripts for selenoproteins but were, instead, linked to transcripts for glucose transport and fatty acid β-oxidation. The experimental results confirm the survey of research literature in showing that Se interacts with the functional genome through a complex network response structure. The results imply that systematic application of data-driven integrated omics methods to models with controlled Se exposure could disentangle health benefits and risks from Se exposures and also serve more broadly as an experimental paradigm for exposome research.

Genotypic Variation and Biofortification with Selenium in Brazilian Wheat Cultivars

Selenium is essential to human and animal health, as it regulates glutathione peroxidase activity. Although not considered essential to plants, it may be beneficial to plant growth and development at low concentrations. This study evaluated the effect of selenate application on Se biofortification, macro- and micronutrient content, and the expression of genes involved in Se uptake and assimilation in 12 Brazilian wheat ( L.) cultivars. This nutrient-solution experiment was performed in a greenhouse and consisted of a complete 12 × 2 factorial randomized design, with 12 wheat cultivars in the absence or presence of Se in solution (13 μmol), with three replicates. The presence of Se in solution did not affect growth and yield of wheat cultivars. Selenium content and accumulation in the grain varied significantly among the different cultivars. The presence of Se affected macronutrient content more than micronutrient content, and selenate application increased S content in the shoots of eight cultivars and in the grains of five cultivars. Examination of gene expression did not allow identification of responses within the two groups of cultivars-with high or low Se contents-after selenate application. Our findings are relevant to the design of Se biofortification strategies for wheat in tropical and subtropical agroecosystems.

Enhanced arsenic tolerance and secondary metabolism by modulation of gene expression and proteome profile in Artemisia annua L. after application of exogenous salicylic acid

This study was designed to investigate the effect of exogenous application of salicylic acid (SA) on proteins pattern and secondary metabolites in arsenic (As) stressed Artemisia annua. A. annua was treated by As 100 μM, SA 100 μM and combined treatment of SA 100 μM + As 100 μM upto 3 days. Significant accumulation of As was observed in roots than shoots at As 100 μM treatment. Under As treatment, oxidative stress was induced as indicated by increased TBARS content. Biomass, carotenoid, flavonoids were enhanced whereas total chlorophyll pigment was reduced under As treatment. Combined treatment of SA 100 μM + As 100 μM was more effective for increment of biomass, total chlorophyll content, and flavonoids as compared to As 100 μM treatment. Protein profiling revealed 20 differentially abundant proteins by 2-DE PAGE and MALDI-TOF-MS analysis. Identified proteins were related to photosynthesis, energy metabolism, transcriptional regulators, secondary metabolism, lipid metabolism, transport proteins and unknown/hypothetical proteins. All identified proteins were significantly increased in abundance under combined treatments of SA 100 μM + As 100 μM. The expression analysis of key genes involved in biosynthesis of lipid metabolism, signal molecule, transcriptional regulators, artemisinin biosynthetic genes, isoprenoids pathway, terpenes and flavonoids pathway were significantly upregulated under combined treatments of SA 100 μM + As 100 μM, suggesting a fine linkage in regulation of primary and secondary metabolism to modulate tolerance capacity and to improve phytoremediation property of A. annua against arsenic toxicity.

The Level of Toxic Elements in Edible Crops from Seleniferous Area (Punjab, India)

The primary objective of the present study was to assess the level of selenium and toxic trace elements in wheat, rice, maize, and mustard from seleniferous areas of Punjab, India. The content of selenium (Se) and toxic trace elements, including aluminum (Al), arsenic (As), cadmium (Cd), mercury (Hg), nickel (Ni), lead (Pb), and tin (Sn), in crop samples was assessed using inductively coupled plasma mass-spectrometry after microwave digestion of the samples. The obtained data demonstrate that cultivation of crops on seleniferous soils significantly increased Se level in wheat, mustard, rice, and maize by a factor of more than 590, 111, 85, and 64, respectively. The study also showed that Se exposure affected toxic metal content in crops. In particular, Se-rich wheat was characterized by a significant decrease in Al, As, Ni, Pb, and Sn levels. The level of As, Cd, Ni, Pb, and Sn was significantly decreased in Se-rich rice, whereas As content was increased. In turn, the decrease in Al, As, Cd, Ni, Pb, and Sn levels in Se-rich maize was associated with a significant elevation of Hg content. Finally, Se-rich mustard was characterized by a significant increase in Al, As, and Hg levels, while the content of Ni, Pb, and Sn was significantly lower than the control levels. These findings should be taken into account while developing the nutritional strategies for correction of Se status. At the same time, the exact mechanisms underlying the observed differences are to be estimated.

Cytotoxic Assessment of Chromium and Arsenic Using Chromosomal Behavior of Root Meristem in Allium cepa L

A study was performed for phyto-genotoxic assay of chromium (Cr) and arsenic (As) through Allium cepa. Various concentrations (0, 1, 3, 6 and 12 mg L^-1) of Cr and As for 48 and 168 h time points exposed to A. cepa. The phytotoxic effects of metal(loid) were evident through inhibited root length and root protein. Metal(loid) toxicity also lead to genotoxic effects, which included depression of mitotic index and increased frequency of chromosomes aberrations like break, fragments, c-metaphase, multipolar arrangements etc. Genotoxic endpoint as progressive frequency of micronuclei in interphase of root meristem cells in treated plants was also observed. This genotoxic endpoint revealed carcinogenic nature of both aforementioned metal(loid). Along with inhibition in root length and protein content, depression in mitotic index as well as stimulation of various abnormality in mitotic cell division indicated that both metal(loid) are hazardous in nature and causing harmful effect on the environment.