Selenate bioreduction in a large in situ field trial.

Removing selenium (Se) from mine effluent is a common challenge. A long-term, in situ experiment was conducted to bioremediate large volumes (up to 7500 mc d-1) of Se(VI)-contaminated water (mean 87 µg L-1) by injecting the water into a saturated waste rock fill (SRF) at a coal mining operation in Elk Valley, British Columbia, Canada. To stimulate/maintain biofilm growth in the SRF, labile organic carbon (methanol) and nutrients were added to the water prior to its injection. A conservative tracer (Br-) was also added to track the migration of injected water across the SRF, identify wells with minimal dilution and used to quantify the extent of bioreduction. The evolution of the Se species through the SRF was monitored in time and space for 201 d. Selenium concentrations of <3.8 µg L-1 were attained in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Concentrations of Se species in water samples from complementary long-term (351-498 d) column experiments using influent Se(VI) concentrations of 1.0 mg L-1 were consistent with the results of the in situ experiment. Solid samples collected at the completion of the column experiments confirmed the presence of indigenous Se-reducing bacteria and that the sequestered Se was present as insoluble Se(0), likely in Se-S ring compounds. Based on the success of this ongoing bioremediation experiment, this technology is being applied at other mine sites.

Modulation of oxidative stress machinery determines the contrasting ability of cyanobacteria to adapt to Se(VI) or Se(IV).

Excess of selenium (Se) in aquatic ecosystems has necessitated thorough investigations into the effects/consequences of this metalloid on the autochthonous organisms exposed to it. The molecular details of Se-mediated adaptive response remain unknown in cyanobacteria. This study aims to uncover the molecular mechanisms driving the divergent physiological responses of cyanobacteria on exposure to selenate [Se(VI)] or selenite [Se(IV)], the two major water-soluble oxyanions of Se. The cyanobacterium, Anabaena PCC 7120, withstood 0.4 mM of Se(VI), whereas even 0.1 mM of Se(IV) was detrimental, affecting photosynthesis and enhancing endogenous ROS. Surprisingly, Anabaena pre-treated with Se(VI), but not Se(IV), showed increased tolerance to oxidative stress mediated by H2O2/methyl viologen. RNA-Seq analysis showed Se(VI) to elevate transcription of genes encoding anti-oxidant proteins and Fe-S cluster biogenesis, whereas the photosynthesis-associated genes, which were mainly downregulated by Se(IV), remained unaffected. Specifically, the content of typical 2-Cys-Prx (Alr4641), a redox-maintaining protein in Anabaena, was elevated with Se(VI). In comparison to the wild-type, the Anabaena strain over-expressing the Alr4641 protein (An4641+) showed enhanced tolerance to Se(VI) stress, whereas the corresponding knockdown-strain (KD4641) was sensitive to this stressor. Incidentally, among these strains, only An4641+ was better protected from the ROS-mediated damage caused by high dose of Se(VI). These results suggest that altering the content of the antioxidant protein 2-Cys-Prx, could be a potential strategy for modulating resistance to selenate. Thus, involvement of oxidative stress machinery appears to be the major determinant, responsible for the contrasting physiological differences observed in response to selenate/selenite in cyanobacteria.

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.

Isolation of Saccharomycopsis species from plant material.

Saccharomycopsis species are natural organic sulphur auxotrophs. Their genomes do not encode genes for the uptake and assimilation of sulphate and thus these species cannot grow on media lacking e.g. methionine. Due to the similarity between sulphate and selenate, uptake and assimilation of selenate occurs through the same pathway starting from sulphate transporters encoded by the homologs of the SUL1 and SUL2 genes in S. cerevisiae. Lack of these transporters renders Saccharomycopsis species resistant to selenate levels that are toxic to other microorganisms. We used this feature to enrich environmental samples for Saccharomycopsis species. This led to the isolation of S. schoenii, S. lassenensis and a hitherto undescribed Saccharomycopsis species with limited by-catch of other yeasts, mainly belonging to Metschnikowia and Hanseniaspora. We performed growth and predation assays to characterize the potential of these new isolates as predacious yeasts. Most Saccharomycopsis species are temperature sensitive and cannot grow at 37°C; with the exception of S. lassenensis strains. Predation assays with S. schoenii and S. cerevisiae as prey indicated that predation was enhanced at 20°C compared to 30°C. We crossed an American isolate of S. schoenii with our German isolate using marker directed breeding. Viable progeny indicated that both strains are interfertile and belong to the same biological species. S. lassenensis is heterothallic, while S. schoenii and the new Saccharomycopsis isolate, for which we suggest the name S. geisenheimensis sp. nov., are homothallic.

Methane Oxidation Coupled to Selenate Reduction in a Membrane Bioreactor under Oxygen-Limiting Conditions.

Microbial methane oxidation coupled to a selenate reduction process has been proposed as a promising solution to treat contaminated water, yet the underlying microbial mechanisms are still unclear. In this study, a novel methane-based membrane bioreactor system integrating hollow fiber membranes for efficient gas delivery and ultrafiltration membranes for biomass retention was established to successfully enrich abundant suspended cultures able to perform methane-dependent selenate reduction under oxygen-limiting conditions. The microbial metabolic mechanisms were then systematically investigated through a combination of short-term batch tests, DNA-based stable isotope probing (SIP) microcosm incubation, and high-throughput sequencing analyses of 16S rRNA gene and functional genes (pmoA and narG). We confirmed that the methane-supported selenate reduction process was accomplished by a microbial consortia consisting of type-II aerobic methanotrophs and several heterotrophic selenate reducers. The mass balance and validation tests on possible intermediates suggested that methane was partially oxidized into acetate under oxygen-limiting conditions, which was consumed as a carbon source for selenate-reducing bacteria. High-throughput 16S rRNA gene sequencing, DNA-SIP incubation with 13CH4, and subsequent functional gene (pmoA and narG) sequencing results collectively proved that Methylocystis actively executed partial methane oxidation and Acidovorax and Denitratisoma were dominant selenate-reducing bacteria, thus forming a syntrophic partnership to drive selenate reduction. The findings not only advance our understanding of methane oxidation coupled to selenate reduction under oxygen-limiting conditions but also offer useful information on developing methane-based biotechnology for bioremediation of selenate-contaminated water.

Transcriptome analysis reveals different mechanisms of selenite and selenate regulation of cadmium translocation in Brassica rapa.

Selenium (Se) inhibits cadmium (Cd) root-to-shoot translocation and accumulation in the shoots of pak choi; however, the mechanism by which Se regulates Cd retention in roots is still poorly understood. A time-dependent hydroponic experiment was conducted to compare the effects of selenite and selenate on Cd translocation and retention in the roots. The underlying mechanisms were investigated regarding Se biotransformation and metal transportation in roots using HPLC and transcriptome analyses. Selenite showed reducing effects on Cd translocation and accumulation in shoots earlier than selenate. Selenite is mainly biotransformed into selenomethionine (80% of total Se in roots) at 72 h, while SeO42- was the dominant species in the selenate treatments (68% in shoots). Selenite up-regulated genes involved in the biosynthesis of lignin, suberin, and phytochelatins and those involved in stress signaling, thereby helping to retain Cd in the roots, whereas essentially, selenate had opposite effects and impaired the symplastic and apoplastic retention of Cd. These results suggest that cell-wall reinforcement and Cd retention in roots may be the key processes by which Se regulates Cd accumulation, and faster biotransformation into organic seleno-compounds could lead to earlier effects.

Selenium Uptake, Translocation, and Metabolization Pattern during Barley Malting: A Comparison of Selenate, Selenite, and Selenomethionine.

Selenium (Se) is an essential trace element for human and animal health. Understanding the uptake and translocation of Se in crops is critical from the perspective of Se biofortification. In this study, barley was malted to investigate the uptake, translocation, and metabolism of exogenous Se including Na2SeO4, Na2SeO3, and selenomethionine (Se-Met). The results showed that the uptake rates of different forms of Se in barley decreased in the following order: Se-Met > Na2SeO3 > Na2SeO4, with the peak uptake occurring at the end of the steeping stages. In the early stages of germination, Se was mainly distributed in the husk and endosperm. Exogenous Se upregulated the transcription levels of Se transport and metabolic enzyme genes in the barley to varying degrees, which promoted Se transformation in various tissues, and improved Se bioeffectiveness. Compared to the Na2SeO3 and Se-Met groups, more Se was transferred from husk and endosperm to acrospire and rootlets in the Na2SeO4 group during the germination stage. Na2SeO4 and Se-Met stimulated the development of rootlets, and accelerated Se metabolism, resulting in a higher Se loss rate. Thus, these comparative findings provide new insights into Se uptake, transformation, and metabolization in barley.

Isolation of novel chemical components and their plant target proteins under selenium stress.

Selenium is recognized as a beneficial nutrient in living organisms. Excessive amounts of selenium, however, can have a significant negative impact on organisms. Screening of novel chemical compounds that regulate and/or moderate selenium in plants was conducted. The present chapter discusses (1) the design of a chemical screening strategy, (2) methods used to identify and select candidate chemicals, and (3) the identification of chemical-binding target proteins. We identified a novel chemical compound, C9H8N2OS2, in our screening program that enhances selenate accumulation and stress tolerance. The target protein, beta-glucosidase 23, in Arabidopsis was found to regulate selenium accumulation, as well as plant response to selenate stress.

Direct and indirect selenium speciation in biofortified wheat: A tale of two techniques.

Wheat can be biofortified with different inorganic selenium (Se) forms, selenite or selenate. The choice of Se source influences the physiological response of the plant and the Se metabolites produced. We looked at selenium uptake, distribution and metabolization in wheat exposed to selenite, selenate and a 1:1 molar mixture of both to determine the impact of each treatment on the Se speciation in roots, shoots, and grains. To achieve a comprehensive quantification of the Se species, the complementarity of high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy was exploited. This approach allowed the identification of the six main selenium species: selenomethionine, selenocysteine, selenocystine, selenite, selenate, and elemental selenium. The three treatments resulted in similar total selenium concentration in grains, 90-150 mg Se kg-1 , but produced different effects in the plant. Selenite enhanced root accumulation (66% of selenium) and induced the maximum toxicity, whereas selenate favored shoot translocation (46%). With the 1:1 mixture, selenium was distributed along the plant generating lower toxicity. Although all conditions resulted in >92% of organic selenium in the grain, selenate produced mainly C-Se-C forms, such as selenomethionine, while selenite (alone or in the mixture) enhanced the production of C-Se-Se-C forms, such as selenocystine, modifying the selenoamino acid composition. These results provide a better understanding of the metabolization of selenium species which is key to minimize plant toxicity and any concomitant effect that may arise due to Se-biofortification.

Dual RNA and 16S ribosomal DNA sequencing reveal arbuscular mycorrhizal fungi-mediated mitigation of selenate stress in Zea mays L. and reshaping of soil microbiota.

Excessively high concentrations of selenium (Se) in soil are toxic to crop plants, and inoculation with arbuscular mycorrhizal fungi (AMF) can reverse Se stress in maize (Zea mays L.). To investigate the underlying mechanisms, maize seedlings were treated with sodium selenate (5 mg Se[VI] kg-1) and/or AMF (Funneliformis mosseae and Claroideoglomus etunicatum). Dual RNA sequencing in mycorrhiza and 16 S ribosomal DNA sequencing in soil were performed. The results showed that Se(VI) application alone decreased plant dry weight, but increased plant Se concentration, total Se content (mainly selenocysteine), and root superoxide content. Inoculation with either F. mosseae or C. etunicatum increased plant dry weight, decreased Se accumulation and selenocysteine proportion, enhanced root peroxidase activity, and alleviated oxidative stress in Se(VI)-treated plants. Inoculation also downregulated the expression of genes encoding Se transporters, assimilation enzymes, and cysteine-rich receptor-like kinases in Se(VI)-stressed plants, similar to plant-pathogen interaction and glutathione metabolism related genes. Conversely, genes encoding selenium-binding proteins and those related to phenylpropanoid biosynthesis were upregulated in inoculated plants under Se(VI) stress. Compared with Se(VI)-free plants, Se tolerance index, symbiotic feedback percentage on plant dry weight, and root colonization rate were all increased in inoculated plants under Se(VI) stress, corresponding to upregulated expression of 'key genes' in symbiosis. AMF inoculation increased bacterial diversity, decreased the relative abundances of selenobacteria related to plant Se absorption (e.g., Proteobacteria and Firmicutes), and improved bacterial network complexity in Se(VI)-stressed soils. We suggest that stress-mediated enhancement of mycorrhizal symbiosis contributed to plant Se(VI) tolerance, whereas AMF-mediated reshaping of soil bacterial community structure prevented excessive Se accumulation in maize.

Combined metabolome and transcriptome analysis reveal the mechanism of selenate influence on the growth and quality of cabbage (Brassica oleracea var. capitata L.).

Selenium is an essential trace element for human and animal health, and an appropriate amount of Se can promote the growth and development of plants. Cabbage is a popular cruciferous vegetable with a good ability to accumulate Se, and Se-enriched cabbage can be used as an important Se source for humans. However, the effects of Se-enriched cultivation and the Se accumulation mechanism in cabbage are still unclear. In this study, the effects of different concentrations (0, 0.1, 0.2, 0.4, 0.8, and 1.6 mmol/L) of selenate on cabbage growth and quality were explored. A low concentration of selenate (0.1 mmol/L) promoted growth and nutritional quality. The contents of total Se, S, selenocystine, and selenomethionine significantly increased following selenate application. Important secondary metabolites, namely glucosinolates, phenolic acids, and flavonoids, participate in the response to selenate in cabbage. Comparative transcriptome and metabolomics analysis revealed that SULTR2.2, SULTR3.1, APS, APK2, HMT, MMT, and NTR2 played important roles in Se absorption and conversion. Additionally, the SUR1, UGT74B1, and ST5b genes and cytochrome P450 family genes CYP83A1, CYP79A2, and CYP79F1 may be the crucial genes in the glucosinolates biosynthesis and regulation pathway. The PAL, 4CL, CAD, CHS3, FLS, and CYP73A5 genes were involved in flavonoid and phenolic acid accumulation under selenate treatment. These results reveal the internal relationships in the regulatory network of Se metabolism and secondary metabolite biosynthesis in cabbage and help further the understanding of the physiological and molecular mechanism of how Se biofortification affects cabbage quality, thereby providing genetic resources and a technical basis for the breeding and cultivation of Se-enriched cabbage with excellent nutritional quality.

Selenium alleviates physiological traits, nutrient uptake and nitrogen metabolism in rice under arsenate stress.

A green house experiment was conducted to evaluate the efficacy of soil application of selenium (Se) in modulating metabolic changes in rice under arsenic (As) stress. Rice plants were grown over soil amended with sodium arsenate (25, 50 and 100 µM kg-1 soil) with or without sodium selenate @ 0.5 and 1 mg kg-1 soil in a complete randomized experimental design, and photosynthetic efficiency, nutrient uptake and nitrogen metabolism in rice leaves were estimated at tillering and grain filling stages. Se treatments significantly improved the toxic effects of As on plant height, leaf dry weight and grain yield. Arsenate treatment reduced uptake of Na, Mg, P, K, Ca, Mn, Fe and Zn and lowered chlorophyll, carotenoids and activities of enzymes of nitrogen metabolism (nitrate reductase, nitrite reductase, glutamine synthase and glutamate synthase) in rice leaves at both the stages in a dose-dependent fashion. Se application along with As improved photosynthesis, nutrient uptake and arsenate-induced effects on activities of enzymes of nitrogen metabolism with maximum impact shown by As50 + Se1 combination. Application of Se can modulate photosynthetic efficiency, nutrient uptake and alterations in nitrogen metabolism in rice Cv PR126 due to As stress that helped plants to adapt to excess As and resulted in improved plant growth.

Seed priming with selenium and zinc nanoparticles modifies germination, growth, and yield of direct-seeded rice (Oryza sativa L.).

Direct-seeded rice (DSR) seeds are often exposed to multiple environmental stresses in the field, leading to poor emergence, growth and productivity. Appropriate seed priming agents may help to overcome these challenges by ensuring uniform seed germination, and better seedling stand establishment. To examine the effectiveness of sodium selenite (Na-selenite), sodium selenate (Na-selenate), zinc oxide nanoparticles (ZnO-NPs), and their combinations as priming agents for DSR seeds, a controlled pot experiment followed by a field experiment over two consecutive years was conducted on a sandy clay loam soil (Inceptisol) in West Bengal, India. Priming with combinations of all priming agents had advantages over the hydro-priming treatment (control). All the combinations of the three priming agents resulted in the early emergence of seedlings with improved vigour. In the field experiment, all the combinations increased the plant chlorophyll, phenol and protein contents, leaf area index and duration, crop growth rate, uptake of nutrients (N, P, K, B, Zn and Si), and yield of DSR over the control. Our findings suggest that seed priming with the combination of ZnO-NPs, Na-selenite, and Na-selenate could be a viable option for the risk mitigation in DSR.

Enhanced phytoremediation of selenium using genetically engineered rice plants.

Selenium (Se) is a micronutrient essential for human and animal health. However, Se is toxic at high levels because the nonspecific substitution of cysteine by selenocysteine could lead to protein malfunction. In an attempt to prevent nonspecific selenocysteine incorporation into proteins, we simultaneously overexpressed the gene encoding selenocysteine lyase from Homo sapiens (HsSL), which specifically catalyzes the decomposition of selenocysteine into elemental Se0 and alanine, and the gene encoding selenocysteine methyltransferase from Astragalus bisulcatus (AbSMT), which methylates selenocysteine into methylselenocysteine in rice. The transgenic plants showed normal growth under standard conditions. Se treatment resulted in higher levels of alanine and methylselenocysteine in transgenic plants than in wild-type plants, which indicated that this approach might have successfully redirected Se flow in the plant. Overexpression of HsSL and AbSMT in rice also endows transgenic plants with hyposensitivity to Se stress at the seed germination stage. The transgenic plants showed enhanced selenate and selenite tolerance, which was simultaneously supported by fresh weight values. Moreover, our phytoremediation assay revealed that the transgenic plants exhibited greatly improved Se elimination capabilities and accumulated about 38.5% and 128.6% more Se than wild-type plants when treated with selenate and selenite, respectively. This study offers hope that genetically modified plants could play a role in the restoration of Se-contaminated environment.

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.

Arsenic toxicity in the environment is a global concern, causes chronic signs of poisoning to plants and humans, leads to ecological imbalance. Selenium is known for its antagonistic characteristics and has been found to be effective in combating the adversities of arsenic at low concentrations (5 µM). The present study was performed to explore the comparative responses of rice seedlings during the joint application of selenium and arsenic in terms of growth, generation of oxidative stress, antioxidant defense, and thiol metabolism. Although the molecular basis of arsenic­selenium interaction is widely known a small number of reports were listed about the physio-chemical role of selenium against arsenic stress. Thus, we investigated the influence of selenium to alleviate arsenic-induced toxic effects by modulating the activities of antioxidant enzymes and reducing the levels of oxidative stress markers. It has been noted that selenium regulates thiol metabolism which is known to play a key role in growth preservation by restriction of arsenic translocation. The outcome from the study would be useful in field trials for sustainable agriculture in arsenic-contaminated soil.

Microbial ecology in selenate-reducing biofilm communities: Rare biosphere and their interactions with abundant phylotypes.

Selenate (SeO42- ) reduction in hydrogen (H2 )-fed membrane biofilm reactors (H2 -MBfRs) was studied in combinations with other common electron acceptors. We employed H2 -MBfRs with two distinctly different conditions: R1, with ample electron-donor availability and acceptors SeO42- and sulfate (SO42- ), and R2, with electron-donor limitation and the presence of electron acceptors SeO42- , nitrate (NO3- ), and SO42- . Even though H2 was available to reduce all input SeO42- and SO42- in R1, SeO42- reduction was preferred over SO42- reduction. In R2, co-reduction of NO3- and SeO42- occurred, and SO42- reduction was mostly suppressed. Biofilms in all MBfRs had high microbial diversity that was influenced by the "rare biosphere" (RB), phylotypes with relative abundance less than 1%. While all MBfR biofilms had abundant members, such as Dechloromonas and Methyloversatilis, the bacterial communities were significantly different between R1 and R2. For R1, abundant genera were Methyloversatilis, Melioribacter, and Propionivibrio; for R2, abundant genera were Dechloromonas, Hydrogenophaga, Cystobacter, Methyloversatilis, and Thauera. Although changes in electron-acceptor or -donor loading altered the phylogenetic structure of the microbial communities, the biofilm communities were resilient in terms of SeO42- and NO3- reductions, because interacting members of the RB had the capacity of respiring these electron acceptors.

Selenium Biofortification: Roles, Mechanisms, Responses and Prospects.

The trace element selenium (Se) is a crucial element for many living organisms, including soil microorganisms, plants and animals, including humans. Generally, in Nature Se is taken up in the living cells of microorganisms, plants, animals and humans in several inorganic forms such as selenate, selenite, elemental Se and selenide. These forms are converted to organic forms by biological process, mostly as the two selenoamino acids selenocysteine (SeCys) and selenomethionine (SeMet). The biological systems of plants, animals and humans can fix these amino acids into Se-containing proteins by a modest replacement of methionine with SeMet. While the form SeCys is usually present in the active site of enzymes, which is essential for catalytic activity. Within human cells, organic forms of Se are significant for the accurate functioning of the immune and reproductive systems, the thyroid and the brain, and to enzyme activity within cells. Humans ingest Se through plant and animal foods rich in the element. The concentration of Se in foodstuffs depends on the presence of available forms of Se in soils and its uptake and accumulation by plants and herbivorous animals. Therefore, improving the availability of Se to plants is, therefore, a potential pathway to overcoming human Se deficiencies. Among these prospective pathways, the Se-biofortification of plants has already been established as a pioneering approach for producing Se-enriched agricultural products. To achieve this desirable aim of Se-biofortification, molecular breeding and genetic engineering in combination with novel agronomic and edaphic management approaches should be combined. This current review summarizes the roles, responses, prospects and mechanisms of Se in human nutrition. It also elaborates how biofortification is a plausible approach to resolving Se-deficiency in humans and other animals.

Selenium biofortification enhances ROS scavenge system increasing yield of coffee plants.

There are conclusive evidences of selenium (Se) deficiency in Brazilian soils and foods. Brazil is the largest producer and consumer of coffee worldwide, which favors agronomic biofortification of its coffee. This study aimed to evaluate effects of foliar application of three formulations and six rates of Se on antioxidant metabolism, agronomic biofortification and yield of coffee beans. Seven Se concentrations (0, 10, 20, 40, 80, 100 and 160 mg L-1) were applied from three formulations of Se (sodium selenate, nano-Se 1500, and nano-Se 5000). Selenium application up to 40 mg L-1 increased the concentration of photosynthetic pigments such as chlorophylls, pheophytins and carotenoids in coffee leaves. Foliar application of Se ranging from 20 to 80 mg L-1 decreased lipid peroxidation and concentration of hydrogen peroxide, but increased superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase activities in coffee leaves. These results indicated that foliar Se application stimulates antioxidative metabolism to mitigate reactive oxygen species. Foliar application of 20 mg Se L-1 of sodium selenate increased coffee yield by 38%, and 160 mg Se L-1 of nano-Se 5000 increased dramatically coffee yield by 42%. Selenium concentration in grains ranged from 0.116 to 4.47 mg kg-1 (sodium selenate), 4.84 mg kg-1 (nano-Se 1500) and 5.82 mg kg-1 (nano-Se 5000). The results suggest the beneficial effect of Se on the increment of photosynthetic pigments, antioxidative metabolism, increased coffee yield and nutritional quality of grains. The recommended foliar Se application in this study can mitigate abiotic stressors such as high temperatures resulting in higher yield of coffee plants.

Insights into uptake, accumulation, and subcellular distribution of selenium among eight wheat (Triticum aestivum L.) cultivars supplied with selenite and selenate.

Selenium (Se)-enriched wheat can be improved by altering Se sources and selecting wheat cultivars. Such improvement can affect subcellular distribution and speciation of Se in wheat. Thus, a pot experiment was conducted to investigate Se uptake and distribution when Se was applied as selenite or selenate at low and high rates (1 and 10 mg kg-1, respectively). Moreover, Se's impact on the grain and biomass yield of eight wheat cultivars was also investigated. The subcellular distribution and speciation of Se were also explored to elucidate Se metabolism and micro-distribution pattern in wheat. Results showed that biomass and grain yield were decreased with the application of both selenite and selenate in almost all the cultivars, regardless of the Se rate. Application high Se rate resulted in a significant (p < 0.05) decrease in grain yield and biomass compared with low rate of Se. Compared with the low rate of selenite application, the grain and the biomass yield of ZM-9023 significantly (p < 0.05) increased by about 15% for low rate of selenate application. In addition, both selenite and selenate treatment increased the uptake of Se in each part of wheat, compared with the control. Selenium was mostly accumulated in the grain and root of wheat under selenite treatment, while more Se accumulation was found in leaves and straw for selenate application. Further investigation on the subcellular distribution of Se showed that the proportion of Se in soluble fraction was significantly (p < 0.05) higher in wheat leaves than that in organelle fraction and cell walls (46%-66%). Meanwhile, Se6+ was the main species found in soluble fraction, whereas SeMet and MeSeCys were the species predominantly stored in organelle fraction. In conclusion, wheat cultivar ZM-9023 is the most Se-rich potential cultivar, and the isolation of Se in the soluble fraction plays an important role in Se tolerance and accumulation.

Selenium toxicity stress-induced phenotypical, biochemical and physiological responses in rice plants: Characterization of symptoms and plant metabolic adjustment.

Selenium (Se) at low concentration is considered benefit element to plants. The range between optimal and toxic concentration of Se is narrow and varies among plant species. This study aimed to evaluate the phenotypic, physiological and biochemical responses of four rice genotypes (BRS Esmeralda, BRSMG Relâmpago, BRS Bonança and Bico Ganga) grown hydroponically treated with sodium selenate (1.5 mM L-1). Selenium treated plants showed a dramatically decrease of soluble proteins, chlorophylls, and carotenoids concentration, resulting in the visual symptoms of toxicity characterized as leaf chlorosis and necrosis. Selenium toxicity caused a decrease on shoot and root dry weight of rice plants. Excess Se increased the oxidative stress monitored by the levels of hydrogen peroxide and lipid peroxidation. The enzymatic antioxidant system (catalase, superoxide dismutase, and ascorbate peroxidase) increased in response to Se supply. Interestingly, primary metabolism compounds such as sucrose, total sugars, nitrate, ammonia and amino acids increased in Se-treated plants. The increase in these metabolites may indicate a defense mechanism for the osmotic readjustment of rice plants to mitigate the toxicity caused by Se. However, these metabolites were not effective to minimize the damages on phenotypic traits such as leaf chlorosis and reduced shoot and root dry weight in response to excess Se. Increased sugars profile combined with antioxidant enzymes activities can be an effective biomarkers to indicate stress induced by Se in rice plants. This study shows the physiological attributes that must be taken into account for success in the sustainable cultivation of rice in environments containing excess Se.