An Overview of Selenium Uptake, Metabolism, and Toxicity in Plants

Comparative orchestrating response of four oilseed rape (Brassica napus) cultivars against the selenium stress as revealed by physio-chemical, ultrastructural and molecular profiling

Selenium (Se) is an essential micro-element for human and animals. In higher plants, Se essentiality or phyto-toxicity is less explored. Therefore, we aimed to examine the effects of Se (0, 25, 50, and 100 µM) as sodium selenite on the physio-chemical, cell ultra-structural and genomic alterations in hydroponically grown seedlings of four cultivars of B. napus (cvs. Zheda 619, Zheda 622, ZS 758, and ZY 50). Results showed that excessive (100 µM) Se (IV) exhibited significant reduction in plant growth parameters, declined pigment contents, lower water-soluble protein levels, and overproduction of H₂O₂ and MDA contents. A significant increase in antioxidant enzyme activities and transcript levels of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), and glutathione reductase (GR), except catalase (CAT) were noticed in the leaves and roots. Non-enzymatic antioxidants including glutathione (GSH) and oxidized glutathione (GSSG), except GSSG in roots were enhanced under higher Se (IV) levels. Transmission electron microscopy analysis revealed the ultrastructural damages in leaf mesophyll and root tip cells induced by excessive Se (IV). Less-significant phytotoxic effects were observed in above-mentioned parameters at 50 µM Se (IV). Overall, Se (IV) supplementation at 25 µM displayed marginal beneficial effect by enhancing plant growth, pigment contents, protein levels and restrict H₂O₂ and MDA overproduction. A marginal increase/decrease in ROS-detoxifying enzymes (except CAT activity) and elevated GSH and GSSG levels were noticed. The accumulation of Se (IV) was much higher in roots as compared to leaves. This accumulation was maximum in Zheda 622 and minimum in ZS 758, followed by Zheda 619 and ZY 50. Overall findings showed that Zheda 622 was the most sensitive and ZS 758 as most tolerant to Se (IV) phyto-toxicity. In addition, Se (IV) was found beneficial until 25 µM Se (IV) but phytotoxic at higher Se levels especially at 100 µM Se (IV).

Nitro-Oxidative Stress Correlates with Se Tolerance of Astragalus Species

At high concentrations, selenium (Se) exerts phytotoxic effects in non-tolerant plant species partly due to the induction of nitro-oxidative stress; however, these processes are not fully understood. In order to obtain a more accurate view of the involvement of nitro-oxidative processes in plant Se sensitivity, this study aims to characterize and compare Se-triggered changes in reactive oxygen (ROS) and nitrogen species (RNS) metabolism and the consequent protein tyrosine nitration as a marker of nitrosative stress in the non-accumulator Astragalus membranaceus and the Se hyperaccumulator Astragalus bisulcatus. The observed parameters (Se accumulation, microelement homeostasis, tissue-level changes in the roots, germination, biomass production, root growth and cell viability) supported that A. membranaceus is Se sensitive while the hyperaccumulator A. bisulcatus tolerates high Se doses. We first revealed that in A. membranaceus, Se sensitivity coincides with the Se-induced disturbance of superoxide metabolism, leading to its accumulation. Furthermore, Se increased the production or disturbed the metabolism of RNS (nitric oxide, peroxynitrite and S-nitrosoglutathione), consequently resulting in intensified protein tyrosine nitration in sensitive A. membranaceus. In the (hyper)tolerant and hyperaccumulator A. bisulcatus, Se-induced ROS/RNS accumulation and tyrosine nitration proved to be negligible, suggesting that this species is able to prevent Se-induced nitro-oxidative stress.

Cytokinin is involved in TPS22-mediated selenium tolerance in Arabidopsis thaliana

Background and Aims

Excess selenium (Se) is toxic to plants, but relatively little is known about the regulatory mechanism of plant Se tolerance. This study explored the role of the TPS22 gene in Se tolerance in Arabidopsis thaliana.

Methods

Arabidopsis wild type and XVE mutant seeds were grown on half-strength MS media containing Na2SeO3 for screening of the Se-tolerant mutant tps22. The XVE T-DNA-tagged genomic sequence in tps22 was identified by TAIL-PCR. The TPS22 gene was transformed into the mutant tps22 and wild type plants using the flower infiltration method. Wild type, tps22 mutant and transgenic seedlings were cultivated on vertical plates for phenotype analysis, physiological index measurement and gene expression analysis.

Key Results

We identified an Arabidopsis Se-tolerant mutant tps22 from the XVE pool lines, and cloned the gene which encodes the terpenoid synthase (TPS22). TPS22 was downregulated by Se stress, and loss-of-function of TPS22 resulted in decreased Se accumulation and enhanced Se tolerance; by contrast, overexpression of TPS22 showed similar traits to the wild type under Se stress. Further analysis revealed that TPS22 mediated Se tolerance through reduction of Se uptake and activation of metabolism detoxification, which decreased transcription of high-affinity transporters PHT1;1, PHT1;8 and PHT1;9 and significantly increased transcription of selenocysteine methyltransferase (SMT), respectively. Moreover, loss-of-function of TPS22 resulted in reduced cytokinin level and repression of cytokinin signalling components AHK3 and AHK4, and upregulation of ARR3, ARR15 and ARR16. Exogenous cytokinin increased transcription of PHT1;1, PHT2;1 and SMT and decreased Se tolerance of the tps22 mutant. In addition, enhanced Se resistance of the tps22 mutant was associated with glutathione (GSH).

Conclusions

Se stress downregulated TPS22, which reduced endogenous cytokinin level, and then affected the key factors of Se uptake and metabolism detoxification. This cascade of events resulted in reduced Se accumulation and enhanced Se tolerance.

Effect of Selenium on the Responses Induced by Heat Stress in Plant Cell Cultures

High temperatures are a significant stress factor for plants. In fact, many biochemical reactions involved in growth and development are sensitive to temperature. In particular, heat stress (HS) represents a severe issue for plant productivity and strategies to obtain high yields under this condition are important goals in agriculture. While selenium (Se) is a nutrient for humans and animals, its role as a plant micronutrient is still questioned. Se can prevent several abiotic stresses (drought, heat, UV, salinity, heavy metals), but the action mechanisms are poorly understood. Se seems to regulate reactive oxygen species (ROS) and to inhibit heavy metals transport. In addition, it has been demonstrated that Se is essential for a correct integrity of cell membranes and chloroplasts, especially the photosynthetic apparatus. Previous results showed that in tobacco (Nicotiana tabacum cv. Bright-Yellow 2) cultures HS (5 min at 50 °C) induced cell death with apoptotic features, accompanied by oxidative stress and changes in the levels of stress-related proteins. In this work we investigated the effect of Se on the responses induced by HS. The obtained results show that Se markedly reduces the effects of HS on cell vitality, cytoplasmic shrinkage, superoxide anion production, membrane lipids peroxidation, activity of caspase-3-like proteases, and the levels of some stress-related proteins (Hsp90, BiP, 14-3-3s, cytochrome c).

Effects of Selenium Supplementation on Glucosinolate Biosynthesis in Broccoli

Selenium (Se)-enriched broccoli has health-beneficial selenium-containing compounds, but it may contain reduced amounts of chemopreventive glucosinolates. To investigate the basis by which Se treatment influences glucosinolate levels, we treated two broccoli cultivars with 25 μM Na₂SeO₄. We found that Se supplementation suppressed the accumulation of total glucosinolates, particularly glucoraphanin, the direct precursor of a potent anticancer compound, in broccoli florets and leaves. We showed that the suppression was not associated with plant sulfur nutrition. The levels of the glucosinolate precursors methionine and phenylalanine as well as the expression of genes involved in glucosinolate biosynthesis were greatly decreased following Se supplementation. Comparative proteomic analysis identified proteins in multiple metabolic and cellular processes that were greatly affected and detected an enzyme affecting methionine biosynthesis that was reduced in the Se-biofortified broccoli. These results indicate that Se-conferred glucosinolate reduction is associated with negative effects on precursor amino acid biosynthesis and glucosinolate-biosynthetic-gene expression and provide information for a better understanding of glucosinolate accumulation in response to Se supplementation in broccoli.

Differences in the bioaccumulation of selenium by two earthworm species (Pheretima guillemi and Eisenia fetida)

Information on the bioaccumulation of selenium (Se) in soil invertebrates (e.g. earthworms) is rather scarce. In the present study, bioaccumulation of Se in two eco-physiologically different earthworms, namely anecic Pheretima guillemi and epigeic Eisenia fetida, was determined after 28 days exposure to a successive doses of Se-spiked soil, specifically 0.5, 5, 50, and 200 μg Se g^-1 soil. The results showed that Se concentration in earthworms elevated with increasing exposure levels, and maximums were up to 54.6 and 83.0 μg g^-1 dry weight in Pheretima guillemi and Eisenia fetida, respectively, after 4 weeks exposure to 200 μg Se g^-1 soil. Exposure to Se caused significant inhibition on earthworm growth, with the fresh weight loss ranging from 8.9% to 80.5%. Bioaccumulation factors (BAFs), empirically-derived and non-steady state, ranged from 0.12 to 4.17 and generally declined at higher exposure levels. Moreover, BAFs of Pheretima guillemi were higher than those of Eisenia fetida in low-dose Se-spiked soils, but the opposite was true in high-dose soils, indicating there is a species-specific response to exposure of Se between different earthworms. Further research is thus needed to reveal the accumulation pattern of Se in a wider range of earthworm species other than Eisenia fetida, which allows a better risk assessment of excessive Se to soil invertebrates and higher order organisms.

Selenium Improves Physiological Parameters and Alleviates Oxidative Stress in Strawberry Seedlings under Low-Temperature Stress

Here, we investigated the effects of selenium (Se) applications on two strawberry varieties, Akihime and Benihoppe, under chilling stress and recovery conditions. Changes in photosynthetic parameters, antioxidant enzyme activities, ascorbate (AsA)-glutathione (GSH) cycle-related enzyme activities, and low-molecular-mass antioxidant contents were determined. Foliar spraying with Se alleviated the decline in the net photosynthetic rate and chlorophyll content and increased the malondialdehyde and hydrogen peroxide contents of strawberry seedlings’ leaves under chilling stress. As the time under chilling stress increased, the stomatal conductance decreased and intercellular CO₂ concentration increased, suggesting that nonstomatal factors had major limiting effects on the net photosynthetic rate’s decrease. Se applications significantly alleviated the adverse impacts of chilling stress on changes in stomatal conductance and intercellular CO₂ concentration. Se, especially at lower concentrations, significantly increased superoxide dismutase, catalase, and peroxide enzyme activities during chilling stress. Approximately 5 mg·L⁻¹ of sodium selenite solution had the greatest stress-alleviating effects. Among the AsA-GSH cycle-related enzymes, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase (MDHAR) treatments, coupled with an appropriate dose of Se, significantly enhanced ascorbate peroxidase and MDHAR activities, which suggested that Se applications played important roles in strawberry leaves by affecting AsA-GSH cycle-related defenses against the oxidative damage caused by chilling stress. Furthermore, MDHAR was the key enzyme required to maintain the balance between AsA consumption and regeneration that may assist in protecting strawberry seedlings in a low-temperature environment.

Biofortification of Cereals With Foliar Selenium and Iodine Could Reduce Hypothyroidism

Concurrent selenium and iodine deficiencies are widespread, in both developing and developed countries. Salt iodisation is insufficient to ensure global iodine adequacy, with an estimated one-third of humanity at risk of hypothyroidism and associated iodine deficiency disorders (IDD). Agronomic biofortification of food crops, especially staples such as cereals, which are consumed widely, may be an effective component of a food system strategy to reduce selenium and iodine malnutrition. Iodine and selenium are needed in the optimum intake range for thyroid health, hence joint biofortification makes sense for areas deficient in both. Foliar application is recommended as the most effective, efficient, least wasteful method for selenium and iodine biofortification. Currently, selenium is easier to increase in grain, fruit, and storage roots by this method, being more phloem mobile than iodine. Nevertheless, strategic timing (around heading is usually best), use of surfactants and co-application with potassium nitrate can increase the effectiveness of foliar iodine biofortification. More research is needed on iodine transporters and iodine volatilisation in plants, bioavailability of iodine in biofortified plant products, and roles for nano selenium and iodine in biofortification. For adoption, farmers need an incentive such as access to a premium functional food market, a subsidy or increased grain yield resulting from possible synergies with co-applied fertilisers, enhancers, fungicides, and insecticides. Further research is needed to inform these aspects of foliar agronomic biofortification.

Selenium biofortification enhances the growth and alters the physiological response of lamb's lettuce grown under high temperature stress

We examined the possibility to enhance the growth and the physiological tolerance of lamb's lettuce (Valerianella locusta L.) grown under heat stress (HS) by biofortification with selenium (Se). The plants were grown at optimal (22/19 °C; day/night) or high (35/22 °C; day/night) temperature and Se was applied via foliar or soil treatment. The HS reduced plant biomass and photosynthetic pigment concentration and impaired some parameters of chlorophyll a fluorescence. The lamb's lettuce grown under HS accumulated large amounts of H2O2 in the leaves, especially in younger ones. The Se fertilization (both foliar and soil) at HS was beneficial to plant growth, whilst the concentration of photosynthetic pigments and the analysed parameters of chlorophyll a fluorescence were unaffected by the Se supply. The application of Se enhanced the thermo-tolerance of plants through cooperative action of antioxidant enzymes, such as guaiacol peroxidase (GPOX; EC 1.11.1.7) and catalase (CAT; EC 1.11.1.6), and reduced glutathione (GSH) among low-molecular-weight non-enzymatic antioxidants, in removal of excess of H2O2. Although under HS the content of different phenolic compounds in the leaves was higher than under normal temperature (NT), the application of Se did not affect their concentration at stress conditions. On the other hand, at NT the Se-biofortified plants accumulated significantly more phenolic compounds with health-promoting properties than Se-untreated plants. Therefore, biofortification of lamb's lettuce with Se can be beneficial in terms of plants yield and their nutritional value under both NT and HS.

Mechanisms of selenium hyperaccumulation in plants: A survey of molecular, biochemical and ecological cues

BACKGROUND

Selenium (Se) is a micronutrient required for many life forms, but toxic at higher concentration. Plants do not have a Se requirement, but can benefit from Se via enhanced antioxidant activity. Some plant species can accumulate Se to concentrations above 0.1% of dry weight and seem to possess mechanisms that distinguish Se from its analog sulfur (S). Research on these so-called Se hyperaccumulators aims to identify key genes for this remarkable trait and to understand ecological implications.

SCOPE OF REVIEW

This review gives a broad overview of the current knowledge about Se uptake and metabolism in plants, with a special emphasis on hypothesized mechanisms of Se hyperaccumulation. The role of Se in plant defense responses and the associated ecological implications are discussed.

MAJOR CONCLUSIONS

Hyperaccumulators have enhanced expression of S transport and assimilation genes, and may possess transporters with higher specificity for selenate over sulfate. Genes involved in antioxidant reactions and biotic stress resistance are also upregulated. Key regulators in these processes appear to be the growth regulators jasmonic acid, salicylic acid and ethylene. Hyperaccumulation may have evolved owing to associated ecological benefits, particularly protection against pathogens and herbivores, and as a form of elemental allelopathy.

GENERAL SIGNIFICANCE

Understanding plant Se uptake and metabolism in hyperaccumulators has broad relevance for the environment, agriculture and human and animal nutrition and may help generate crops with selenate-specific uptake and high capacity to convert selenate to less toxic, anticarcinogenic, organic Se compounds.

Prospecting for Microelement Function and Biosafety Assessment of Transgenic Cereal Plants

Microelement contents and metabolism are vitally important for cereal plant growth and development as well as end-use properties. While minerals phytotoxicity harms plants, microelement deficiency also affects human health. Genetic engineering provides a promising way to solve these problems. As plants vary in abilities to uptake, transport, and accumulate minerals, and the key enzymes acting on that process is primarily presented in this review. Subsequently, microelement function and biosafety assessment of transgenic cereal plants have become a key issue to be addressed. Progress in genetic engineering of cereal plants has been made with the introduction of quality, high-yield, and resistant genes since the first transgenic rice, corn, and wheat were born in 1988, 1990, and 1992, respectively. As the biosafety issue of transgenic cereal plants has now risen to be a top concern, many studies on transgenic biosafety have been carried out. Transgenic cereal biosafety issues mainly include two subjects, environmental friendliness and end-use safety. Different levels of gene confirmation, genomics, proteomics, metabolomics and nutritiomics, absorption, metabolism, and function have been investigated. Also, the different levels of microelement contents have been measured in transgenic plants. Based on the motivation of the requested biosafety, systematic designs, and analysis of transgenic cereal are also presented in this review paper.

Quantification of methionine and selenomethionine in biological samples using multiple reaction monitoring high performance liquid chromatography tandem mass spectrometry (MRM-HPLC-MS/MS)

Quantification of selenated amino-acids currently relies on methods employing inductively coupled plasma mass spectrometry (ICP-MS). Although very accurate, these methods do not allow the simultaneous determination of standard amino-acids, hampering the comparison of the content of selenated versus non-selenated species such as methionine (Met) and selenomethionine (SeMet). This paper reports two approaches for the simultaneous quantification of Met and SeMet. In the first approach, standard enzymatic hydrolysis employing Protease XIV was applied for the preparation of samples. The second approach utilized methanesulfonic acid (MA) for the hydrolysis of samples, either in a reflux system or in a microwave oven, followed by derivatization with diethyl ethoxymethylenemalonate. The prepared samples were then analyzed by multiple reaction monitoring high performance liquid chromatography tandem mass spectrometry (MRM-HPLC-MS/MS). Both approaches provided platforms for the accurate determination of selenium/sulfur substitution rate in Met. Moreover the second approach also provided accurate simultaneous quantification of Met and SeMet with a low limit of detection, low limit of quantification and wide linearity range, comparable to the commonly used gas chromatography mass spectrometry (GC-MS) method or ICP-MS. The novel method was validated using certified reference material in conjunction with the GC-MS reference method.

Effects of Foliar Selenite on the Nutrient Components of Turnip (Brassica rapa var. rapa Linn.)

We administered foliar applications of 50, 100, and 200 mg L^-1 selenium (Se, selenite) on turnip (Brassica rapa var. rapa Linn.) and detected the changes in the main nutrient components in fleshy roots. Results showed that the foliar application of Se (IV) significantly increased the Se content in turnip, and Se (IV) positively affected the uptake of several mineral elements, including magnesium, phosphorus, iron, zinc, manganese, and copper. Se (IV) treatments also improved the synthesis of protein and multiple amino acids instead of crude fat and total carbohydrate in turnip, indicating that the foliar application of Se (IV) could enhance Se biofortification in turnip and promote its nutritional value. We recommended 50-100 mg L^-1 Se treatment for foliar application on turnip based on the daily intake of Se for adults (96-139 μg person^-1 day^-1) and its favorable effects on the nutrient components of turnip.

A critical review of selenium biogeochemical behavior in soil-plant system with an inference to human health

Selenium (Se) is an essential trace element for humans and animals, although controversial for different plant species. There exists a narrow line between essential, beneficial and toxic levels of Se to living organisms which greatly varies with Se speciation, as well as the type of living organisms. Therefore, it is crucial to monitor its solid- and solution-phase speciation, exposure levels and pathways to living organisms. Consumption of Se-laced food (cereals, vegetables, legumes and pulses) is the prime source of Se exposure to humans. Thus, it is imperative to assess the biogeochemical behavior of Se in soil-plant system with respect to applied levels and speciation, which ultimately affect Se status in humans. Based on available relevant literature, this review traces a plausible link among (i) Se levels, sources, speciation, bioavailability, and effect of soil chemical properties on selenium bioavailability/speciation in soil; (ii) role of different protein transporters in soil-root-shoot transfer of Se; and (iii) speciation, metabolism, phytotoxicity and detoxification of Se inside plants. The toxic and beneficial effects of Se to plants have been discussed with respect to speciation and toxic/deficient concentration of Se. We highlight the significance of various enzymatic (catalase, peroxidase, superoxide dismutase, ascorbate peroxidase, glutathione peroxidase) and non-enzymatic (phytochelatins and glutathione) antioxidants which help combat Se-induced overproduction of reactive oxygen species (ROS). The review also delineates Se accumulation in edible plant parts from soils containing low or high Se levels; elucidates associated health disorders or risks due to the consumption of Se-deficient or Se-rich foods; discusses the potential role of Se in different human disorders/diseases.

Selenium distribution in the Chinese environment and its relationship with human health: A review

This paper reviewed the Se in the environment (including total Se in soil, water, plants, and food), the daily Se intake and Se content in human hair were also examined to elucidate Se distribution in the environment and its effects on human health in China. Approximately 51% of China is Se deficiency in soil, compared with 72% in the survey conducted in 1989. Low Se concentrations in soil, water, plants, human diet and thus human hair were found in most areas of China. The only significant difference was observed between Se-rich and Se-excessive areas for Se contents in water, staple cereal, vegetables, fruits, and animal-based food, no remarkable contrast was found among other areas (p>0.05). This study also demonstrated that 39-61% of Chinese residents have lower daily Se intakes according to WHO/FAO recommended value (26-34μg/day). Further studies should focus on thoroughly understanding the concentration, speciation, and distribution of Se in the environment and food chain to successfully utilize Se resources, remediate Se deficiency, and assess the Se states and eco-effects on human health.

Effect of Selenium from Selenium-Enriched Kale Sprout Versus Other Selenium Sources on Productivity and Selenium Concentrations in Egg and Tissue of Laying Hens

A 6-week trial was conducted to compare the effect of selenium (Se) from hydroponically produced Se-enriched kale sprout (HPSeKS), sodium selenite (SS), and Se-enriched yeast (SeY) in laying hens. A total of 144 40-week-old hens were randomly divided into four groups, according to a completely randomized design. Each group consisted of four replicates with nine hens per replicate. The dietary treatments were T1 (basal diet) and T2, T3, and T4 (basal diets supplemented with 0.30 mg Se/kg from SS, SeY, and HPSeKS, respectively). Results showed that Se supplement did not affect (p > 0.05) productivity and egg quality. Hens fed Se from HPSeKS and SeY exhibited higher (p < 0.05) Se bioavailability than hens fed Se from SS. Whole egg Se concentration of hens fed Se from HPSeKS was similar (p > 0.05) to that of hens fed Se from SeY, but higher (p < 0.05) than that of hens fed Se from SS. However, the breast muscle and heart tissue Se concentrations of hens fed Se from SS, SeY, and HPSeKS were not different (p > 0.05). The results of this trial demonstrated that Se from HPSeKS and SeY was more efficient than Se from SS on Se bioavailability and whole egg Se concentration in laying hens.

Selenite-induced nitro-oxidative stress processes in Arabidopsis thaliana and Brassica juncea

Extremes of selenium (Se) exert toxic effects on plants' physiological processes; although plant species tolerate Se differently. This study focuses on the effect of Se (0, 20, 50 or 100μM sodium selenite) on secondary nitro-oxidative stress processes mainly using in situ microscopic methods in non-accumulator Arabidopsis thaliana and secondary Se accumulator Brassica juncea. Relative Se tolerance or sensitivity of the species was evaluated based on growth parameters (fresh and dry weight, root growth) and cell viability. Besides, selenite-triggered cell wall modifications (pectin, callose) and stomatal regulations were determined for the first time. In case of Arabidopsis, relative selenite sensitivity was accompanied by decreased stomatal density and induced stomatal opening, callose accumulation, pronounced oxidative stress and moderate nitrosative modifications. In contrast, the selenite-treated, relatively tolerant Brassica juncea showed larger number of more opened stomata, pectin accumulation, moderate oxidative and intense nitrosative stress. These suggest that selenite tolerance or sensitivity is rather associated with oxidative processes than secondary nitrosative modifications in higher plants.

Selenium-Induced Toxicity Is Counteracted by Sulfur in Broccoli (Brassica oleracea L. var. italica)

Selenium (Se) is an essential micronutrient for humans. Increasing Se content in food crops offers an effective approach to enhance the consumption of Se in human diets. A thoroughly understanding of the effects of Se on plant growth is important for Se biofortification in food crops. Given that Se is an analog of sulfur (S) and can be toxic to plants, its effect on plant growth is expected to be greatly affected by S nutrition. However, this remains to be further understood. Here, we evaluated the influence of Se treatments on broccoli (Brassica oleracea L. var. italica) growth when S was withheld from the growth nutrient solution. We found that Se was highly toxic to plants when S nutrition was poor. In contrast to Se treatments with adequate S nutrition that slightly reduced broccoli growth, the same concentration of Se treatments without S supplementation dramatically reduced plant sizes. Higher Se toxicity was observed with selenate than selenite under low S nutrition. We examined the bases underlying the toxicity. We discovered that the high Se toxicity in low S nutrition was specifically associated with an increased ratio of Se in proteins verse total Se level, enhanced generation of reactive oxygen species, elevated lipid peroxidation causing increased cell membrane damage, and reduced antioxidant enzyme activities. Se toxicity could be counteracted with increased supplementation of S, which is likely through decreasing non-specific integration of Se into proteins and altering the redox system. The present study provides information for better understanding of Se toxicity and shows that adequate S nutrition is important to prevent Se toxicity during biofortification of crops by Se fertilization.