Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite

Using the method of dynamic factors for assessing the transfer of chemical elements from soil to plants from various perspectives

Environmental, biological, and ecosystem-specific properties may influence the transfer of chemical elements (CEs) from soils to plants, including the variation in the chemical elements' concentration, their types, and physiological parameters, such as biotransformation ability in the plants. The interface between the soil and a plant, or the concentration of a particular chemical element in a plant with respect to its concentration in the soil, is the basis for a widely used biological absorption coefficient, also known as the transfer factor, bioaccumulation factor, mobility ratio, or plant-soil coefficient, which is expressed in terms of the chemical element's concentration in the plant and soil. However, from the biogeochemical perspective, these coefficients/factors can provide a comparison of the chemical element (CE) concentration in different media (plants and soil), but only in a particular place (under typical environmental conditions) and at a particular time. However, factors that highlight the variation in the processes, rather than the variation in the chemical element quantity under the conditions of the environmental variation, are required. The second-level or dynamic factors can be used for this purpose. A quantitative method, using the dynamic factors of bioaccumulation, biophilicity, translocation, bioavailability, and phytoremediation, is offered to assess the variation in the process of the uptake of chemical elements by different plants, to evaluate the influence of soil modification on their participation in the plants' metabolism and to perform quantitative evaluation of phytoremediation efficiency over a particular period of time. The use of dynamic factors for describing the chemical elements' uptake by plants in various cases, representing aerogenic and edaphic chemical elements' transfer, is discussed.

Translocation of elements and sugars in wheat genotypes at vegetative and generative stages under continuous selenium exposure

BACKGROUND

Biofortification with selenium (Se) elevates its concentration in feed and fodder plants and helps to prevent health problems in animals and humans. The aim of this study was to describe Se-induced modifications in the accumulation of elements important for the proper functioning of wheat, one of the most popular cereals. The presence of Se correlated with carbohydrate synthesis and electron paramagnetic resonance (EPR). This explained the mechanisms of Se's antioxidant activity.

RESULTS

Selenium accumulation in vegetative and generative leaves, and in the grains of three wheat genotypes (cv. Parabola, cv. Raweta and cv. Manu), differing in their stress tolerance and grown hydroponically in the presence of 10 or 20 μM Na₂ SeO_4, , was proportional to its content in the medium. Stronger Se accumulation was typical of a stress-sensitive genotype. Selenium generally promoted the uptake of macronutrients and micronutrients but their distribution depended on tissue and genotype. Changes in the Se-induced EPR signals of paramagnetic metals and organic radicals corresponded with stress tolerance of the tested genotypes.

CONCLUSIONS

Se application increased the accumulation of nutrients and carbohydrates that are vital for proper plant growth and development. Accelerated uptake of molybdenum (Mo), an element improving dietary properties of grains, may be an additional advantage of Se fertilization. The mechanisms of Se-induced changes in removing Mn and iron (Fe) ions from macromolecules may be one of the factors that differentiate plant tolerance to oxidative stress. © 2019 Society of Chemical Industry.

Influence of selenium biofortification on the bioactive compounds and antioxidant activity of wheat microgreen extract

The study was conducted to confirm the effects of selenium biofortification on the bioactive compounds and antioxidant activity of wheat microgreen extract. The microgreens were cultivated in the DFT hydroponic system with different concentrations of Se (0 [control], 0.125, 0.25, 0.50, and 1.00 mg/L from sodium selenite) in a growth chamber by controlling temperature (25/20 °C, day/night), light (12 h light/dark; intensity 150 µmol‧m^-2‧s^-1 with white fluorescence lamp), and humidity (60%) for 10 days. Se biofortification increased the germination rate and decreased microgreen length and yield. Chlorophyll and carotenoid levels increased in the Se-biofortified microgreen extract. Bioactive compounds such as phenolics, flavonoids, vitamin C, and anthocyanin significantly increased in 0.25-0.50 mg/L of Se-biofortified microgreen extracts. Antioxidant (ABTS, DPPH, NSA and SOD-like) activity also increased at moderate levels (0.25-0.50 mg/L) of Se biofortification. Therefore, Se biofortification may be useful for the industrial manufacture of new products from wheat microgreen extract.

The reduction of selenium(IV) by boreal Pseudomonas sp. strain T5-6-I - Effects on selenium(IV) uptake in Brassica oleracea

Selenium (Se) is an essential micronutrient but toxic when taken in excessive amounts. Therefore, understanding the metabolic processes related to selenium uptake and bacteria-plant interactions coupled with selenium metabolism are of high importance. We cultivated Brassica oleracea with the previously isolated heterotrophic aerobic Se(IV)-reducing Pseudomonas sp. T5-6-I strain to better understand the phenomena of bacteria-mediated Se(IV) reduction on selenium availability to the plants. B. oleracea grown on Murashige and Skoog medium (MS-salt agar) with and without of Pseudomonas sp. were amended with Se(IV)/⁷⁵Se(IV), and selenium transfer into plants was studied using autoradiography and gamma spectroscopy. XANES was in addition used to study the speciation of selenium in the B. oleracea plants. In addition, the effects of Se(IV) on the protein expression in B. oleracea was studied using HPLC-SEC. TEM and confocal microscopy were used to follow the bacterial/Se-aggregate accumulation in plants and the effects of bacterial inoculation on root-hair growth. In the tests using ⁷⁵Se(IV) on average 130% more selenium was translocated to the B. oleracea plants grown with Pseudomonas sp. compared to the plants grown with selenium, but without Pseudomonas sp.. In addition, these bacteria notably increased root hair density. Changes in the protein expression of B. oleracea were observed on the ∼30-58 kDa regions in the Se(IV) treated samples, probably connected e.g. to the oxidative stress induced by Se(IV) or expression of proteins connected to the Se(IV) metabolism. Based on the XANES measurements, selenium appears to accumulate in B. oleracea mainly in organic C-Se-H and C-Se-C bonds with and without bacteria inoculation. We conclude that the Pseudomonas sp. T5-6-I strain seems to contribute positively to the selenium accumulation in plants, establishing the high potential of Se⁰-producing bacteria in the use of phytoremediation and biofortification of selenium.

The defensive benefit and flower number cost of selenium accumulation in Brassica juncea

Some plant species accumulate selenium in their tissues in quantities far above soil concentrations, and experiments demonstrate that selenium can serve as a defence against herbivores and pathogens. However, selenium may also cause oxidative stress and reduce growth in plants. We measured growth, selenium accumulation and herbivory in four varieties of the selenium accumulator Brassica juncea to investigate the cost of accumulation as well as its benefit in reducing herbivory. We measured selenium levels, plant size and flower number in four varieties of B. juncea watered with sodium selenate or treated as controls. We also conducted no-choice herbivory trials on leaves from both treatments with the specialist herbivore Pieris rapae. The selenate treatment slightly increased leaf number over the control, but tissue concentrations of selenium and flower number were negatively correlated in some varieties. In herbivory trials, leaves from the plants in the selenate treatment lost less leaf tissue, and the majority of larvae given leaves from selenate-treated plants ate very little leaf tissue at all. In the variety with the highest selenium accumulation, leaves from selenate-treated plants that showed reduced flower production also experienced less herbivory in feeding trials. The protective advantage of greater selenium accumulation may be offset by negative effects on reproduction, and the relatively low level of selenium accumulation in this species as compared to more extreme hyperaccumulators could reflect the minimum level necessary to enhance protection from herbivory.

Protective Effect of Selenium-Enriched Red Radish Sprouts on Carbon Tetrachloride-Induced Liver Injury in Mice

This study aimed to investigate the effect of Se (Selenium) treatment on nutritional quality in radish sprouts. The results showed that 15 µM sodium selenite significantly increased phenolics compounds, flavonoids compounds, anthocyanins, and some essential amino acid content, while improving the total antioxidant capacity of radish sprouts. Besides, the Se-enriched radish sprouts significantly alleviated the liver damage caused by carbon tetrachloride (CCl₄ ) in mice and improved the antioxidant capacity of the liver in mice, whereas the Se-enriched radish sprouts alleviated the inflammatory reaction and apoptosis caused by CCl₄ . These results imply that Se-enriched radish sprouts have a positive impact on mice with CCl₄ -induced liver injury, and that in future Se-enriched radish sprouts could be developed into an effective food and health care product for the liver injury prevention. PRACTICAL APPLICATION: Because selenium is an essential trace element in the human body, selenium-enriched sprouts can help eliminate free radicals in the body, relieve aging, and selenium-deficient diseases. They are easy to grow and have low costs. Hence, selenium-enriched sprouts have a great potential of being widely consumed.

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.

Disentangling the complexity and diversity of crosstalk between sulfur and other mineral nutrients in cultivated plants

A complete understanding of ionome homeostasis requires a thorough investigation of the dynamics of the nutrient networks in plants. This review focuses on the complexity of interactions occurring between S and other nutrients, and these are addressed at the level of the whole plant, the individual tissues, and the cellular compartments. With regards to macronutrients, S deficiency mainly acts by reducing plant growth, which in turn restricts the root uptake of, for example, N, K, and Mg. Conversely, deficiencies in N, K, or Mg reduce uptake of S. TOR (target of rapamycin) protein kinase, whose involvement in the co-regulation of C/N and S metabolism has recently been unravelled, provides a clue to understanding the links between S and plant growth. In legumes, the original crosstalk between N and S can be found at the level of nodules, which show high requirements for S, and hence specifically express a number of sulfate transporters. With regards to micronutrients, except for Fe, their uptake can be increased under S deficiency through various mechanisms. One of these results from the broad specificity of root sulfate transporters that are up-regulated during S deficiency, which can also take up some molybdate and selenate. A second mechanism is linked to the large accumulation of sulfate in the leaf vacuoles, with its reduced osmotic contribution under S deficiency being compensated for by an increase in Cl uptake and accumulation. A third group of broader mechanisms that can explain at least some of the interactions between S and micronutrients concerns metabolic networks where several nutrients are essential, such as the synthesis of the Mo co-factor needed by some essential enzymes, which requires S, Fe, Zn and Cu for its synthesis, and the synthesis and regulation of Fe-S clusters. Finally, we briefly review recent developments in the modelling of S responses in crops (allocation amongst plant parts and distribution of mineral versus organic forms) in order to provide perspectives on prediction-based approaches that take into account the interactions with other minerals such as N.

Difference between selenite and selenate in selenium transformation and the regulation of cadmium accumulation in Brassica chinensis

Se can regulate Cd accumulation and translocation in plants; however, such effects can be controversial because of the differences in plant species and Se species. In this study, pak choi was cultured under hydroponic conditions, and the effects of selenite and selenate on Cd accumulation were investigated in the edible parts of this vegetable. The results showed gradual improvements in the effects of the two Se species on the Cd content in pak choi shoots at the four assessed growing stages. Selenite did not lead to significant changes in Cd accumulation in the shoots until day 40, when it significantly reduced the accumulation by 34%. Selenate was always found to increase the Cd content in the shoots, and the differences on days 19 and 40 were 16% and 45%, respectively, compared with those of the Cd (only) treatment. Accordingly, selenate invariably enhanced Cd translocation from the roots to the shoots, whereas selenite insignificantly reduced the translocation only on day 40. Generally, selenomethionine (SeMet) accounted for much larger proportions in selenite-treated plants, while SeO₄^2- was the dominant Se species in selenate-treated plants. However, under both Se treatments, the SeMet proportion increased substantially from day 19 to day 40 when that of SeO₄^2- exhibited a drastic decrease; therefore, the relative proportion of seleno-amino acids to SeO₄^2- may be the key factor for the regulation of Cd accumulation in pak choi via treatment with selenite and selenate at the different growing stages.

Selenate removal by Fe0 coupled with ferrous iron, hydrogen peroxide, sulfidation, and weak magnetic field: A comparative study

Dosing ferrous ions (ZVI/Fe²⁺), combining with oxidants (e.g., H₂O₂) (ZVI/H₂O₂), sulfidation treatment (S-ZVI), and introducing a weak magnetic field (ZVI/WMF) have been widely used to enhance the performance of zerovalent iron (ZVI) for reductive removal of contaminants. Taking Se(VI) as a probe contaminant, this study systematically compared the performances of different ZVI systems (i.e., ZVI/Fe²⁺, ZVI/H₂O₂, S-ZVI, and ZVI/WMF) for contaminant removal. All the four tested methods could greatly improve the performance of ZVI for Se(VI) removal. Se(VI) was removed by S-ZVI at S/Fe molar ratio of 0.05 with a much greater rate constant than other enhanced-ZVI technologies while the maximum amount of Se(VI) removal was obtained in ZVI/Fe²⁺ system with Fe²⁺ applied at 0.5 mM among the four tested enhanced-ZVI technologies at initial pH 6.0. In addition, Se(VI) removal by ZVI/Fe²⁺ was least influenced by initial pH compared to the other tested enhanced-ZVI systems, implying its good adaptability of pH. The application of these tested methods could significantly increase the electron efficiency from ∼0.5% to 4.06-8.72% and Fe²⁺ application was much more efficient in enhancing the electron efficiency than the other three methods. Finally, the perspective of these enhanced-ZVI technologies was compared in terms of their reactivity, selectivity, chemical cost, and pH adaptability and some suggestions for their possible application were provided.

Selenium Modulates the Level of Auxin to Alleviate the Toxicity of Cadmium in Tobacco

Cadmium (Cd) is an environmental pollutant that potentially threatens human health worldwide. Developing approaches for efficiently treating environmental Cd is a priority. Selenium (Se) plays important role in the protection of plants against various abiotic stresses, including heavy metals. Previous research has shown that Se can alleviate Cd toxicity, but the molecular mechanism is still not clear. In this study, we explore the function of auxin and phosphate (P) in tobacco (Nicotiana tabacum), with particular focus on their interaction with Se and Cd. Under Cd stress conditions, low Se (10 μM) significantly increased the biomass and antioxidant capacity of tobacco plants and reduced uptake of Cd. We also measured the auxin concentration and expression of auxin-relative genes in tobacco and found that plants treated with low Se (10 μM) had higher auxin concentrations at different Cd supply levels (0 μM, 20 μM, 50 μM) compared with no Se treatment, probably due to increased expression of auxin synthesis genes and auxin efflux carriers. Overexpression of a high affinity phosphate transporter NtPT2 enhanced the tolerance of tobacco to Cd stress, possibly by increasing the total P and Se content and decreasing Cd accumulation compared to that in the wild type (WT). Our results show that there is an interactive mechanism among P, Se, Cd, and auxin that affects plant growth and may provide a new approach for relieving Cd toxicity in plants.

Simultaneous Biofortification of Wheat with Zinc, Iodine, Selenium, and Iron through Foliar Treatment of a Micronutrient Cocktail in Six Countries

Field experiments were conducted on wheat to study the effects of foliar-applied iodine(I) alone, Zn (zinc) alone, and a micronutrient cocktail solution containing I, Zn, Se (selenium), and Fe (iron) on grain yield and grain concentrations of micronutrients. Plants were grown over 2 years in China, India, Mexico, Pakistan, South Africa, and Turkey. Grain-Zn was increased from 28.6 mg kg^-1 to 46.0 mg^-1 kg with Zn-spray and 47.1 mg^-1 kg with micronutrient cocktail spray. Foliar-applied I and micronutrient cocktail increased grain I from 24 μg kg^-1 to 361 μg kg^-1 and 249 μg kg^-1, respectively. Micronutrient cocktail also increased grain-Se from 90 μg kg^-1 to 338 μg kg^-1 in all countries. Average increase in grain-Fe by micronutrient cocktail solution was about 12%. The results obtained demonstrated that foliar application of a cocktail micronutrient solution represents an effective strategy to biofortify wheat simultaneously with Zn, I, Se and partly with Fe without yield trade-off in wheat.

Effect of phosphate and silicate on selenite uptake and phloem-mediated transport in tomato (Solanum lycopersicum L.)

The ambiguous mechanism that selenite seems to be absorbed by roots via phosphorus (P) and silicon (Si) transporters signifies P and Si may affect selenite uptake. However, the role of P and Si in phloem-mediated selenium (Se) transport within plant tissue is unknown. Therefore, in this work, tomato (Solanum lycopersicum L.) seedlings were exposed to selenite under different hydroponic conditions firstly. And then, split-root experiments were conducted. Results showed that Se uptake decreased as external pH increased. At pH 8, more selenite in the form of SeO₃^2- was assimilated under P-deficient conditions than under P-normal conditions. Silicate inhibited Se uptake only at pH 3 (27.5% H₂SeO₃ +72.5% HSeO₃^-). The results of split-root experiments showed that Se concentrations in seedlings increased under heterogeneously high P or Si. Selenium transport from shoots to roots immersed in solution without selenite was also enhanced. This study illustrated that the affinity of tomato roots to assimilate selenite species followed the order of H₂SeO₃ >HSeO₃^- >SeO₃^2-. H₂SeO₃ was absorbed into roots via Si transporters, whereas HSeO₃^- and a portion of SeO₃^2- were absorbed via low- and high-affinity P transporters, respectively. In addition, heterogeneously high P or Si concentrations in environmental media could enhance phloem-mediated Se redistribution.

The behaviour of irrigation induced Se in the groundwater-soil-plant system in Punjab, India

Selenium is of special interest in different research fields due to its narrow range between beneficial and toxic effects. On a global scale, Se deficiency is more widespread. Biofortification measures have successfully been applied to specifically increase Se concentrations in food crops. Still not much is known about the behaviour and long-term fate of externally supplied Se. Over many years, natural but external selenate is regularly introduced into the soil-plant system via irrigation at our study sites in Punjab which makes it also an ideal natural analogue to investigate the long term effect of biofortification. For our study, we combined total and species specific analysis of Se in soil and plant material. Selenium is clearly enriched in all investigated topsoils (0-15 cm) with concentrations of 1.5-13.0 mg kg-1 despite similar background Se concentrations (0.5 ± 0.1 mg kg-1) below 15 cm depth. Irrigation is indicated to be the primary source of excess Se. Processes like Se species transformation, uptake by plants and plant material decomposition further influence both the Se speciation and extent of Se enrichment in the soils. The Se concentration in different plants and plant parts is alarmingly high showing concentrations of up to 738 mg kg-1 in wheat. Irrigation induced selenate can be considered as an easily available short term pool of Se for plants and thus strongly controls their total Se concentration and speciation. The long-term pool of Se in the topsoil mainly consists of selenite and organic Se species. These species are readily retained but still sufficiently mobile to be taken up by plants. The formation of elemental Se can be considered as a non-available Se pool and is thus, the major cause of Se immobilization and long-term enrichment of Se in the soils. Our study clearly shows that biofortification with selenate, despite its effectiveness, bears the risk of easily increasing Se levels in plants to toxic levels and producing food with less favourable inorganic Se species if not done with care. Excess selenate is either lost due to biomethylation or immobilized within the soil which has to be considered as highly negative from both an economic and ecological point of few.

Selenium translocation in the soil-rice system in the Enshi seleniferous area, Central China

Rice is an important source of selenium (Se) exposure; however, the transformation and translocation of Se in the soil-rice system remain poorly understood. Here, we investigated the speciation of Se in Se-rich soils from Enshi, Central China and assessed which Se species is bioavailable for rice grown in Enshi. Extremely high Se concentrations (0.85 to 11.46 mg/kg) were observed in the soils. The soil Se fractions, which include water-soluble Se (0.2 to 3.4%), ligand-exchangeable Se (4.5 to 15.0%), organically bound Se (57.8 to 80.0%) and residual Se (6.1 to 32.9%), are largely controlled by soil organic matter (SOM) levels. Decomposition of SOM promotes the transformation of organically bound Se to water-soluble Se and ligand-exchangeable Se, thereby increasing the bioavailability of Se. The bioaccumulation factors (BAFs) of Se decrease in the following order: roots (0.84 ± 0.30) > bran (0.33 ± 0.17) > leaves (0.18 ± 0.09) > polished rice (0.14 ± 0.07) > stems (0.12 ± 0.07) > husks (0.11 ± 0.07). Selenium levels in rice plants are affected by multiple soil Se fractions in the soil. Water-soluble, ligand-exchangeable and organically bound Se fractions are the major sources of Se in rice tissues.

NRT1.1B improves selenium concentrations in rice grains by facilitating selenomethinone translocation

Selenium (Se) is an essential trace element for humans and other animals, yet approximately one billion people worldwide suffer from Se deficiency. Rice is a staple food for over half of the world's population that is a major dietary source of Se. In paddy soils, rice roots mainly take up selenite. Se speciation analysis indicated that most of the selenite absorbed by rice is predominantly transformed into selenomethinone (SeMet) and retained in roots. However, the mechanism by which SeMet is transported in plants remains largely unknown. In this study, SeMet uptake was found to be an energy-dependent symport process involving H⁺ transport, with neutral amino acids strongly inhibiting SeMet uptake. We further revealed that NRT1.1B, a member of rice peptide transporter (PTR) family which plays an important role in nitrate uptake and transport in rice, displays SeMet transport activity in yeast and Xenopus oocyte. The uptake rate of SeMet in the roots and its accumulation rate in the shoots of nrt1.1b mutant were significantly repressed. Conversely, the overexpression of NRT1.1B in rice significantly promoted SeMet translocation from roots to shoots, resulting in increased Se concentrations in shoots and rice grains. With vascular-specific expression of NRT1.1B, the grain Se concentration was 1.83-fold higher than that of wild type. These results strongly demonstrate that NRT1.1B holds great potential for the improvement of Se concentrations in grains by facilitating SeMet translocation, and the findings provide novel insight into breeding of Se-enriched rice varieties.

Dual behavior of selenium: Insights into physio-biochemical, anatomical and molecular analyses of four Brassica napus cultivars

Selenium (Se) is a prerequisite metalloid for humans and animals. But, its essentialness or phytotoxicity is still obscure. Here, we investigated the dual effects of sodium selenite (0, 25, 50 or 100 μM) on the physio-biochemical, anatomical and molecular alterations in different Brassicca napus L. cultivars (viz. Zheda 619, Zheda 622, ZY 50, and ZS 758). Findings revealed that Se-supplementation markedly boosted the plant growth and biomasses by improving mineral uptake, water-soluble protein, sugar, photosynthetic efficiency regarding the pigments and gas exchange parameters. Higher Se-levels impaired the photosynthetic efficiency, deplete nutrients-uptake, osmotic stress by proline accumulation and higher Se-accumulation in roots led to growth and biomass reduction. Se-supplementation minimized the accumulation of ROS (hydrogen peroxide, superoxide radical), malondialdehyde and methylglyoxal (MG) levels by activating the enzymes engaged in AsA-GSH cycle and ROS-MG detoxification. But, elevated-Se impaired the oxidative metabolism by desynchronizing the antioxidants as revealed by decreasing levels of ascorbic acid, activities and expression levels of catalase, glutathione reductase, and dehydro-ascorbate reductase. Up-regulation of secondary metabolites genes (PAL, PPO) revealed the role of Se in regulating transcriptional networks involved in oxidative stress. The damages in leaf and root ultra-structures disclosed the Se-phytotoxicity. Together, outcomes uncovered the protective mechanism of Se (till 25 μM) by reinforcing the plant morphology, photosynthesis, osmo-protection, redox balance, enzyme activities for ROS-MG detoxification by reducing ROS and MG components. Excessive-Se prompt phytotoxicity by impairing above mentioned parameters, especially at 100 μM Se. Among all B. napus cultivars, Zheda 622 was discovered as highly-susceptible and ZS 758 showed greatest-tolerance against Se stress.

Selenium alleviated chromium stress in Chinese cabbage (Brassica campestris L. ssp. Pekinensis) by regulating root morphology and metal element uptake

Excessive chromium (Cr) causes toxicity to plants, while the beneficial effects of selenium (Se) have been verified in plants under various adverse conditions. Under Cr stress, the impacts of exogenous Se on root morphology and metal element uptake were investigated in root of Chinese cabbage by cellular and biochemical approaches. Exogenous Se alleviated Cr-induced irreversible damage to root morphology, plasma membrane integrity and ultrastructure of root tip cells. Compared with Cr treatment alone, exogenous Se reduced root Cr content by 17%. Se supply changed the subcellular distribution of Cr in root, and the concentration of Cr was reduced in the fractions of plastids and mitochondria, while increased in soluble fraction. Besides, exogenous Se counteracted the nutrient elements (Na, Ca, Fe, Mn, Cu and Zn) loss induced by Cr. For plant with Se pretreatment, the increase rate of Cr influx was lower than that of plant without Se pretreatment, particularly in solution containing high concentration (100-400 μmol L^-1) of Cr. In addition, higher K_m value was observed in plant with Se pretreatment, which indicated a lower Cr affinity than that of plant without Se pretreatment. The results suggest that Se modified root morphology and regulated nutrient elements uptake by root, which might play a combined role in reducing Cr uptake by root, consequently alleviating Cr stress and maintaining plant growth.

Effects of phosphorus on absorption and transport of selenium in rice seedlings

The effects of different concentrations of phosphorus (P) on absorption and transfer of selenium (Se) in rice seedlings were studied by hydroponics experiment. The interaction between iron plaque and phosphorus on absorption and transport of selenium were studied by adding a large amount of iron-induced iron plaque, to provide a theoretical basis for rational application of phosphate fertilizer in the selenium bio-strengthening process of rice. The results showed that phosphorus deficiency may result in the formation of reddish brown iron oxide coating on the root surface of rice. The formation of root iron plaque of rice is related to concentration of phosphorus, and low concentration of phosphorus (0-1.5 mmol L^-1) can increase the amount of root iron plaque. Compared P deficiency culture and 2 mmol L^-1 P culture, Se content in the shoots and roots decreased by 76 and 47%, respectively. Addition of Fe²⁺ significantly reduced biomass of shoot and had no significant effect on the roots; when the P concentration increased from 0.1 to 0.3 mmol L^-1, transfer coefficient of Se decreased. Therefore, both root iron plaques induced by phosphorus deficiency and iron addition have a strong adsorption effect on selenium, which reduces the transport of selenium from the rice roots to the shoots. In the lower range of phosphorus concentration, low phosphorus can promote selenium content of rice shoot, while higher on the contrary. In the practice of rice production, proper management of phosphorus nutrient is of great significance to control selenium content in rice grain.

Coupling Langmuir with Michaelis-Menten-A practical alternative to estimate Se content in rice?

Selenium plays an important, but vastly neglected role in human nutrition with a narrow gap between dietary deficiency and toxicity. For a potential biofortification of food with Se, as well as for toxicity-risk assessment in sites contaminated by Se, modelling of local and global Se cycling is essential. As bioavailability of Se for rice plants depends on the speciation of Se and the resulting interactions with mineral surfaces as well as the interaction with Se uptake mechanisms in plants, resulting plant Se content is complex to model. Unfortunately, simple experimental models to estimate Se uptake into plants from substrates have been lacking. Therefore, a mass balance of Se transfer between lithosphere (represented by kaolinite), hydrosphere (represented by a controlled nutrient solution), and biosphere (represented by rice plants) has been established. In a controlled, closed, lab-scale system, rice plants were grown hydroponically in nutrient solution supplemented with 0–10 000 μg L^-1 Se of either selenate or selenite. Furthermore, in a series of batch experiments, adsorption and desorption were studied for selenate and selenite in competition with each of the major nutrient oxy-anions, nitrate, sulfate and phosphate. In a third step, the hydroponical plants experiments were coupled with sorption experiments to study synergy effects. These data were used to develop a mass balance fitting model of Se uptake and partitioning. Adsorption was well-described by Langmuir isotherms, despite competing anions, however, a certain percentage of Se always remained bio-unavailable to the plant. Uptake of selenate or selenite by transporters into the rice plant was fitted with the non-time differentiated Michaelis-Menten equation. Subsequent sequestration of Se to the shoot was better described using a substrate-inhibited variation of the Michaelis-Menten equation. These fitted parameters were then integrated into a mass balance model of Se transfer.

The role of cytokinin in selenium stress response in Arabidopsis

Cytokinins (CKs) regulate many developmental processes and environmental stress responses in plants. In this study, our data provide evidence that CK negatively regulates Arabidopsis selenium (Se) stress response. CK-deficient plant ipt1 3 5 7 exhibited enhanced Se tolerance which was abolished by exogenous benzylaminopurine (BA) application, while CK- receptor -deficient mutants ahk2 and ahk3 were sensitive to Se stress. Further investigation suggested that CK regulated Se tolerance of ipt1 3 5 7 through reduction of Se uptake and activation of metabolism detoxification, which had significantly lower transcriptions of high-affinity transporters PHT1;1, PHT1;8, PHT1;9 and the higher transcription of selenocysteine methyltransferase (SMT) respectively. Moreover, Se tolerance of ipt1 3 5 7 was associated with the enhanced antioxidant levels which had the higher catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX) activities as well as the higher glutathione (GSH) content. On the other hand, loss-of-function mutations in single CK receptor genes could increase Se uptake and reactive oxygen species (ROS) accumulation, which caused Se sensitivity in ahk2 and ahk3 mutants. Taken together, these findings provide new insights to the role of CK in Se stress response in Arabidopsis.

Accumulation, mobilization, and transformation of selenium in rice grain provided with foliar sodium selenite

BACKGROUND

Selenium is an indispensable trace element for humans and its deficiency can lead to serious health complications. Nearly 70% of the area of China faces selenium deficiency. To deal with this problem, selenium-enriched rice has been increasingly incorporated into everyday diets. However, there is a lack of in-depth studies of the absorption, translocation, and transformation of selenium in the different parts of the rice plant when sprayed with sodium selenite.

RESULTS

Foliar sodium selenite applied at critical growth stages can significantly improve the total and organic selenium content of plants. Application of 10 mg L^-1 sodium selenite led to the most organic selenium (0.03 mg kg^-1 ) in polished rice. Correlation studies of sodium selenite applied to leaves and other plant parts showed that total selenium accumulated most in glume, followed by rice bran, then polished rice, and finally embryo. The behavior of organic selenium was different. Organic selenium accumulated most in polished rice, then embryo, then rice bran, and finally glume. Moreover, 75-85% of the Se found in polished rice and embryo was organic in nature.

CONCLUSIONS

We propose that 10 mg L^-1 sodium selenite can be recommended as appropriate for foliar fertilization in the organic selenium biofortification of Se-free rice. © 2018 Society of Chemical Industry.

A comprehensive review on environmental transformation of selenium: recent advances and research perspectives

Selenium (Se) is an important micronutrient and essential trace element for both humans and animals, which exist in the environment ubiquitously. Selenium deficiency is an important issue worldwide, with various reported cases of its deficiency. Low selenium contents in some specific terrestrial environments have resulted in its deficiency in humans. However, high levels of selenium in the geochemical environment may have harmful influences and can cause a severe toxicity to living things. Due to its extremely narrow deficiency and toxicity limits, selenium is becoming a serious matter of discussion for the scientists who deals with selenium-related environmental and health issues. Based on available relevant literature, this review provides a comprehensive data about Se sources, levels, production and factors affecting selenium bioavailability/speciation in soil, characteristics of Se, biogeochemical cycling, deficiency and toxicity, and its environmental transformation to know the Se distribution in the environment. Further research should focus on thoroughly understanding the concentration, speciation, Se cycling in the environment and food chain to effectively utilize Se resources, remediate Se deficiency/toxicity, and evaluate the Se states and eco-effects on human health.

Uptake kinetics and interaction of selenium species in tomato (Solanum lycopersicum L.) seedlings

Selenite and selenate are two main selenium (Se) forms absorbed by plants. The comparative effects of selenite and/or selenate on Se uptake and translocation in plants in spite of their coexistence in the environment are still unclear. Therefore, tomato (Solanum lycopersicum L.) seedlings were grown in a hydroponic solution with exogenous selenite, selenate, or selenite and selenate mixed, and Se concentrations in shoots, roots, and xylem sap were measured after harvest. Results showed that selenite (> 0.1 mg Se L^-1) could cause phytotoxicity more easily than selenate (> 1 mg Se L^-1) under hydroponic conditions. And the absorbability rate of tomato to selenate was higher than that to selenite when Se application level was 0.0175-0.2998 mg L^-1, while the opposite result was observed in other Se concentrations. More Se accumulated in roots and Se(VI) in the xylem sap decreased when both Se forms supplied. This study demonstrated that the difference between selenite and selenate on Se uptake and translocation in tomatoes depended on exogenous Se concentration. And selenite could inhibit the absorption and translocation of selenate when supplied with both Se forms.

Effects of selenate and red Se-nanoparticles on the photosynthetic apparatus of Nicotiana tabacum

Selenium (Se) is a natural trace element, which shifts its action in a relatively narrow concentration range from nutritional role to toxicity. Although it has been well established that in plants chloroplasts are among the primary targets, the mechanism of toxicity on photosynthesis is not well understood. Here, we compared selenate and red-allotrope elemental selenium nanoparticles (red nanoSe) in in vitro tobacco cultures to investigate their effects on the structure and functions of the photosynthetic machinery. Selenate at 10 mg/L concentration retarded plant growth; it also led to a decreased chlorophyll content, accompanied with an increase in the carotenoid-to-chlorophyll ratio. Structural examinations of the photosynthetic machinery, using electron microscopy, small-angle neutron scattering and circular dichroism spectroscopy, revealed significant perturbation in the macro-organization of the pigment-protein complexes and sizeable shrinkage in the repeat distance of granum thylakoid membranes. As shown by chlorophyll a fluorescence transient measurements, these changes in the ultrastructure were associated with a significantly diminished photosystem II activity and a reduced performance of the photosynthetic electron transport, and an enhanced capability of non-photochemical quenching. These changes in the structure and function of the photosynthetic apparatus explain, at least in part, the retarded growth of plantlets in the presence of 10 mg/L selenate. In contrast, red nanoSe, even at 100 mg/L and selenate at 1 mg/L, exerted no negative effect on the growth of plantlets and affected only marginally the thylakoid membrane ultrastructure and the photosynthetic functions.

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 Distribution and Translocation in Rice (Oryza sativa L.) under Different Naturally Seleniferous Soils

Selenium (Se) accumulation in plant foods may be providing dietary Se to minimize the health problems related to Se deficiency. In this study, rice plants were cultivated in different naturally seleniferous soils (0.5–1.5 mg Se kg−1). Se concentration in rice plant tissues was analysed, and the distribution and translocation of Se in rice were also studied. The effect of exogenous Se on yield and Se concentration in rice grain was also investigated by spraying Na2SeO3 (15 mg L−1, 15 g ha−1). Results show that Se concentration in root, straw and grain of rice was increased with increased concentrations of Se in seleniferous soils. The root accumulated higher Se than straw and grain under the same naturally seleniferous soil. Spraying Se significantly increased Se concentration in grain, hull, brown rice and polished rice compared with spraying water. Se concentration in the grain fractions was in the following order: Bran > brown rice > whole grain > polished rice > hull. About 13.7% Se in wholegrain was discarded by milling process if about 6.9% of it was polished as bran. Se-enriched rice could be produced in naturally seleniferous soils with Se concentration from 0.5 to 1.0 mg kg−1, and this polished rice would provide enough Se (60–80 μg day−1) to satisfy the human requirement. Therefore, naturally seleniferous soils may be an effective way to produce Se-enriched rice without spraying Se fertilizer, which will be more economically feasible and environmentally friendly for without exogenous Se added to the soils or plants. However, the polished rice and brown rice, produced by spraying Na2SeO3 (15 g ha−1) or grown in soil with total Se upto 1.5 mg kg−1 was not suitable for daily human consumption, unless diluted with Se-deficient rice to meet the standard (≤0.3 mg Se kg−1). This study imparted a better understanding of the utilization of seleniferous soils and Se-enriched rice for human health and food safety.

Combating Micronutrient Deficiency and Enhancing Food Functional Quality Through Selenium Fortification of Select Lettuce Genotypes Grown in a Closed Soilless System

Selenium (Se) is an essential trace element for human nutrition and a key component of selenoproteins having fundamental biological and nutraceutical functions. We currently examined lettuce biofortification with Se in an open-gas-exchange growth chamber using closed soilless cultivation for delivering Se-rich food. Morphometric traits, minerals, phenolic acids, and carotenoids of two differently pigmented Salanova cultivars were evaluated in response to six Se concentrations (0-40 μM) delivered as sodium selenate in the nutrient solution. All treatments reduced green lettuce fresh yield slightly (9%), while a decrease in red lettuce was observed only at 32 and 40 μM Se (11 and 21% respectively). Leaf Se content increased in both cultivars, with the red accumulating 57% more Se than the green. At 16 μM Se all detected phenolic acids increased, moreover a substantial increase in anthocyanins (184%) was recorded in red Salanova. Selenium applications slightly reduced the carotenoids content of green Salanova, whereas in red Salanova treated with 32 μM Se violaxanthin + neoxanthin, lutein and β-cryptoxanthin spiked by 38.6, 27.4, and 23.1%, respectively. Lettuce constitutes an ideal target crop for selenium biofortification and closed soilless cultivation comprises an effective tool for producing Se-enriched foods of high nutraceutical value.

Plant selenium toxicity: Proteome in the crosshairs

The metalloid element, selenium (Se) is in many ways special and perhaps because of this its research in human and plant systems is of great interest. Despite its non-essentiality, higher plants take it up and metabolize it via sulfur pathways, but higher amounts of Se cause toxic symptoms in plants. However, the molecular mechanisms of selenium phytotoxicity have been only partly revealed; the data obtained so far point out that Se toxicity targets the plant proteome. Besides seleno- and oxyproteins, nitroproteins are also formed due to Se stress. In order to minimize proteomic damages induced by Se, certain plants are able to redirect selenocysteine away from protein synthesis thus preventing Se-protein formation. Additionally, the damaged or malformed selenoproteins, oxyproteins and nitroproteins may be removed by proteasomes. Based on the literature this review sets Se toxicity mechanisms into a new concept and it draws attention to the importance of Se-induced protein-level changes.

In Situ Selective Measurement of SeIV in Waters and Soils: Diffusive Gradients in Thin-Films with Bi-Functionalized Silica Nanoparticles

The speciation of selenium (Se) controls its fate and behavior, determining both its biological and environmental activities. However, in situ monitoring of Se^IV presents a significant challenge due to its sensitivity to redox change. A novel diffusive gradients in thin films (DGT) technique containing mercapto-, amino-bifunctionalized SBA15 mesoporous silica nanoparticles was developed and evaluated in a series of laboratory and field deployment tests. The SBA-DGT exhibited a linear accumulation of Se^IV ( r² > 0.997) over a 72 h deployment, with negligible accumulation of Se^VI(<5%). Consistent prediction of Se^IV occurred within ionic strength and pH ranges of 0.1-200 mmol L^-1 and 3.6-8, respectively. Limits of detection of the SBA-DGT were 0.03 μg Se^IV L^-1, which is suitable for natural waters. Moreover, the properties of the bifunctionalized SBA15 enable it to be fabricated within ultrathin (0.05 mm) gel layers for use in conjunction with O₂ planar optode imaging. This new sandwich sensor technology with SBA-DGT was validated by mapping the two-dimensional distribution of Se^IV and oxygen simultaneously in rice rhizospheres. This study shows that SBA-DGT provides a selective measurement of Se^IV in situ, demonstrating its potential for both environmental monitoring and as a research tool for improving our understanding of Se biogeochemical processes.

Selenium (Se) uptake and dynamic changes of Se content in soil-plant systems

In this study, we collected crop plants and associated soil samples and determined these for selenium (Se) content to analyze the uptake, enrichment, and translocation of Se in the different soil-plant systems of an agricultural production area, elucidate the dynamic mechanisms relating to Se content in plants and soil during different growth periods, and screen plants for high Se enrichment ability. Bioconcentration factor determinations indicated that the grains of rice have the strongest Se enrichment ability, followed by soybean and corn. Translocation factor analysis indicated that the grains of rice and corn have similar low translocation abilities for Se compared with soybean. Within the study area, the Se content in plants was closely related to the soil available Se content and varied considerably among different growth periods and plant organs. This study provides a theoretical basis for the development and utilization of local agricultural products.

Influence of selenite and selenate on growth, leaf physiology and antioxidant defense system in wheat (Triticum aestivum L.)

BACKGROUND

Selenium (Se) induced oxidative stress as well as synthesis of non-specific selenoproteins has been attributed to its toxicity in plants. Selenium toxicity can affect growth, chlorophyll and protein synthesis and crop yield. This study reveals the effects of different sources (sodium selenite and sodium selenate) and levels (2 and 4 mg Se kg^-1 soil) of Se on its uptake, leaf physiology, antioxidant defense system, isoenzymic patterns and mitochondrial activity in wheat cultivar PBW621 at tillering and ear-initiation stages.

RESULTS

Higher Se accumulation in leaves of wheat plants was observed in selenate than control and selenite treatments. Selenium tolerance index, chlorophyll, photosynthetic efficiency, mitochondrial reduction test, electron transport system activity, lipid peroxidation, proline and glutathione in Se-treated wheat plants decreased significantly as compared to control. Significant increase in hydrogen peroxide and activities of antioxidant enzymes, namely catalase, peroxidase, superoxide dismutase, glutathione reductase in leaves was due to the presence of Se-induced oxidative stress in wheat plants.

CONCLUSION

Wheat cultivar PBW621 could adapt to applied selenite concentrations by developing antioxidant defense system but selenate treated plants could exhibit toxicity tolerance up to 2 mg kg^-1 and died at high concentrations due to damage to tissue development and function. © 2018 Society of Chemical Industry.

Characteristics of Time-Dependent Selenium Biofortification of Rice ( Oryza sativa L.)

The application of selenite to soil has increasingly been used to produce Se-enriched food. This study investigated the biofortification characteristics of Se in rice after application of selenite to soil at different growth stages. The results showed that the application of Se during booting stage resulted in the highest concentration of Se in brown rice due to the highest upward translocation of Se. More than 90% of Se in the brown rice was organic species, with selenomethionine predominated. The proportion of selenomethionine in the brown rice decreased with the delay in application time. The rice grown in the acidic soil had higher Se concentrations than in the neutral soil. With increasing soil Cd level, Se accumulation and the proportion of Se-methylselenocysteine in the brown rice were reduced. This study provides a theoretical basis for the production of Se-enriched rice in clean soil or slightly to moderately Cd-contaminated soil.

Speciation of Selenium in Brown Rice Fertilized with Selenite and Effects of Selenium Fertilization on Rice Proteins

Foliar Selenium (Se) fertilizer has been widely used to accumulate Se in rice to a level that meets the adequate intake level. The Se content in brown rice (Oryza sativa L.) was increased in a dose-dependent manner by the foliar application of sodium selenite as a fertilizer at concentrations of 25, 50, 75, and 100 g Se/ha. Selenite was mainly transformed to organic Se, that is, selenomethionine in rice. Beyond the metabolic capacity of Se in rice, inorganic Se also appeared. In addition, four extractable protein fractions in brown rice were analyzed for Se concentration. The Se concentrations in the glutelin and albumin fractions saturated with increasing Se concentration in the fertilizer compared with those in the globulin and prolamin fractions. The structural analyses by fluorescence spectroscopy, Fourier transform infrared spectrometry, and differential scanning calorimetry suggest that the secondary structure and thermostability of glutelin were altered by the Se treatments. These alterations could be due to the replacements of cysteine and methionine to selenocysteine and selenomethionine, respectively. These findings indicate that foliar fertilization of Se was effective in not only transforming inorganic Se to low-molecular-weight selenometabolites such as selenoamino acids, but also incorporating Se into general rice proteins, such as albumin, globulin glutelin, and prolamin, as selenocysteine and selenomethionine in place of cysteine and methionine, respectively.

Selenium application alters soil cadmium bioavailability and reduces its accumulation in rice grown in Cd-contaminated soil

Selenium (Se) alleviates cadmium (Cd) accumulation in several plants. Nevertheless, it is still unclear why it has such effect. Thus, this study aimed to investigate the effects of Se on soil Cd bioavailability, and Cd accumulation in flooded rice plants, and to determine the mechanisms underlying these effects. Concentration of Cd and Se in different rice tissues was determined along Cd and Se concentrations in the soil solution and soil Cd fractions. Results showed that exogenous selenite and selenate treatments significantly increased rice grain Se by 4.25- and 2.39-fold and decreased Cd by 36.5% and 25.3% relative to control treatment, respectively. The addition of Se to Cd-contaminated soil significantly decreased total Cd concentration in the soil solution by 11.2-13.0%, increased soil pH by 0.06-0.32 units, and enhanced soil Cd immobilization in relation to control. Exogenous Se also reduced diethylenetriaminepentaacetic acid-Cd, exchangeable, and residual Cd but increased the levels of Cd bound to carbonate and iron and manganese oxides. Thus, amending Cd-contaminated soil with Se may help decrease Cd content as well as increase Se levels in rice grain, as Se may mitigate Cd accumulation in rice plants by increasing soil pH, reducing Cd bioavailability, and inhibiting Cd translocation from roots to shoots.

Selenate tolerance and selenium hyperaccumulation in the monocot giant reed (Arundo donax), a biomass crop plant with phytoremediation potential

The response of giant reed (Arundo donax L.) to selenium (Se), added as selenate, was studied. The development, stress response, uptake, translocation, and accumulation of Se were documented in three giant reed ecotypes STM (Hungary), BL (USA), and ESP (Spain), representing different climatic zones. Plantlets regenerated from sterile tissue cultures were grown under greenhouse conditions in sand supplemented with 0, 2.5, 5, and 10 mg Se kg^-1 added as sodium selenate. Total Se content was measured in different plant parts using hydride generation atomic fluorescence spectroscopy. All plants developed normally in the 0-5.0 mg Se kg^-1 concentration range regardless of ecotype, but no growth occurred at 10.0 mg Se kg^-1. There were no signs of chlorosis or necrosis, and the photosynthetic machinery was not affected as evidenced by no marked differences in the structure of thylakoid membranes. There was no change in the maximum quantum yield of photosystem II (F_v/F_m ratio) in the three ecotypes under Se stress, except for a significant negative effect in the ESP ecotype in the 5.0 mg Se kg^-1 treatment. Glutathione peroxidase (GPx) activity increased as the Se concentration increased in the growth medium. GPx activity was higher in the shoot system than the root system in all Se treatments. All ecotypes showed great capacity of take up, translocate and accumulate selenium in their stem and leaf. Relative Se accumulation is best described as leaf ˃˃ stem ˃ root. The ESP ecotype accumulated 1783 μg g^-1 in leaf, followed by BL with 1769 μg g^-1, and STM with 1606 μg g^-1 in the 5.0 mg Se kg^-1 treatment. All ecotypes showed high values of translocation and bioaccumulation factors, particularly the ESP ecotype (10.1 and 689, respectively, at the highest tolerated Se supplementation level). Based on these findings, Arundo donax has been identified as the first monocot hyperaccumulator of selenium, because Se concentration in the leaves of all three ecotypes, and also in the stem of the ESP ecotype, is higher than 0.1% (dry weight basis) under the conditions tested. Tolerance up to 5.0 mg Se kg^-1 and the Se hyperaccumulation capacity make giant reed a promising tool for Se phytoremediation.

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).

Mineral Composition and Antioxidant Status of Tomato with Application of Selenium

This experiment was carried out in a greenhouse to evaluate the effects of selenium application (as Na2SeO3) on mineral concentration (as N, P, K, Ca and Se), biomass, yield and total antioxidant status (TAS) of tomato fruit. The study consisted of two experiments: an irrigation experiment with the application of selenium at 0, 2.5 and 5 mg L−1 on the fertilizer solution in soil and perlite; and the foliar application experiment with selenium application at 0, 10, and 20 mg L−1 in foliar spray every 20 days. Results showed that mineral content (as K, Ca, Mg and P) was not modified by selenium application. However, N decreased due to the Se applied in fertilizer solution 5 mg L−1, and a negative correlation was found between the selenium applied in foliar form and the nitrogen concentration. The Se concentration, TAS, and biomass increased in plants in all Se treatments. However, the best response in TAS and Se in fruits was observed with foliar spray every 20 days at concentrations of 10 mg L−1, without negative responses in biomass or mineral content.

Cadmium dynamics in soil pore water and uptake by rice: Influences of soil-applied selenite with different water managements

Cadmium (Cd) in rice grains is a potential threat to human health. This study investigated the effects of selenite fertilisation (0 mg kg^-1, 0.5 mg kg^-1, and 1.0 mg kg^-1) on soil solution Cd dynamics and rice uptake. Rice was grown in two Cd-contaminated soils in Jiangxi and Hunan Provinces under two different sets of conditions: aerobic and flooded. The experiments were conducted in pots. The plants were harvested at the seedling stage and at maturity to determine their Cd levels. Soil solutions were also extracted during the growing season to monitor Cd dynamics. The results showed that in the Jiangxi soil (pH 5.25), Cd concentrations in the soil solutions, seedlings, and mature rice plants were higher under aerobic than under flooded water management conditions. In the Hunan soil (pH 7.26), however, flooding decreased Cd levels in the rice seedlings but not in mature plants. Selenite additions to the Hunan soil decreased Cd concentrations in the soil solutions and in the mature rice plants. These effects were not observed for the solutions or the plants from Jiangxi soil amended with selenite. Relative to the control treatment, 0.5 mg kg^-1 selenite decreased the rice grain Cd content by 45.2% and 67.7% under aerobic and flooding conditions, respectively. The results demonstrated that water management regimes affected rice Cd uptake more effectively in Jiangxi than in Hunan soil, whereas selenite addition was more effective in Hunan than in Jiangxi soil. Selenite addition was also more effective at reducing rice grain Cd levels when it was applied under flooding than under aerobic conditions.

Action of selenium against Sclerotinia sclerotiorum: Damaging membrane system and interfering with metabolism

Selenium (Se) in soil is beneficial for environmental stress tolerance of plants, and it has widespread toxic effects on pathogens. Based on the fact that Se significantly inhibited the growth of Sclerotinia sclerotiorum, we set experiments with different concentrations of Se to investigate the action of Se against S. sclerotiorum in this study. The results showed that Se (>0.5 mg L^-1) changed the morphology of S. sclerotiorum mycelia, and higher Se concentrations severely damaged mycelial structures. Fourier transform infrared spectroscopy (FTIR) analysis indicated that Se treatment induced the chemical composition of mycelia with much abundance of functional groups such as alcohols, ketones, ammonium and esters, and 0.5 mg L^-1 Se maximized their concentrations. Under Se treatments, the electrical conductivity of mycelia increased in a time-dependent manner, and osmolyte concentrations of mycelia increased as well. Se supplementation significantly reduced polymethylgalacturonase (PMG) and carboxymethylcellulase (Cx) activities, which protecting plants from infection, and increased the energy expenditure in S. sclerotiorum. Combined action of Se damage on membrane system, osmoregulation, reduction of cell wall degrading enzymes activities and improvement of energy expenditure resulted in the inhibition of S. sclerotiorum growth. Findings in this study provided evidences for using Se as a potential fungicide to control S. sclerotiorum.

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.

Foliar application of zinc sulphate and zinc EDTA to wheat leaves: differences in mobility, distribution, and speciation

Foliar application of zinc (Zn) to crops is an effective way to increase the grain concentration of Zn. However, the development of more efficient foliar Zn fertilizers is limited by a lack of knowledge regarding the distribution, mobility, and speciation of Zn in leaves once it is taken up by the plant. We performed an experiment using radiolabelled Zn (65Zn), and in situ time-resolved elemental imaging using synchrotron X-ray fluorescence microscopy (XFM), to investigate the behaviour of two commonly used Zn foliar fertilizers (Zn sulphate and ZnEDTA) in wheat (Triticum aestivum) leaves. Both experiments showed that Zn had limited mobility in leaves, moving <25 mm from the application point after 24 h. Although limited, the translocation of Zn occurred quickly for both treatments; moving more between 3 h and 12 h after application than between 12 h and 24 h. Speciation analysis using synchrotron-based X-ray absorption near-edge structure (XANES) showed that ZnEDTA was in fact taken up in chelated form and not as ionic Zn (Zn2+). The XANES data also showed that Zn, from both treatments, was then complexed by ligands in the leaf (e.g. phytate and citrate), potentially in response to localized Zn toxicity. The results of the present study provide important insights into the behaviour of commonly used foliar-applied Zn fertilizers, and can be used to optimize current fertilization strategies and contribute to the development of more efficient foliar Zn fertilizers.

A rapid and sensitive IC-ICP-MS method for determining selenium speciation in natural waters

Selenium (Se) is an element monitored by water quality agencies worldwide. The challenge of assessing its presence in aquatic systems is its low concentrations (parts per trillion) and the need for determining its chemical speciation. A method was developed using an ion chromatograph (IC) paired with a quadrupole inductively coupled plasma mass spectrometer (ICP-MS) equipped with a hydrogen reaction cell to provide analysts with a rapid and sensitive method to measure Se speciation with suitable accuracy and precision. The Se species selenite (SeIV) and selenate (SeVI) were separated within a 5 min span using dilute nitric acid as a mobile phase in a step-wise gradient (50–400 mmol L−1) and quantified using 80Se isotope that yielded low limits of detection (<10 ng L−1). Spectral interference from plasma generated diatomic argon ions (40Ar2+; m/z = 80) on 80Se was eliminated by hydrogen gas (H2) in the reaction cell. Polyatomic 79Br1H+ (m/z = 80) did not interfere with 80Se for quantification of common aquatic Se species (SeVI and SeIV) due to different column retention times. Two organic species (methylselenocysteine and selenomethionine) commonly found in aquatic and terrestrial plant tissues were also tested to rule out possible chromatographic interference and explore the potential application to biological samples. Urban rainwater and Canadian river water samples were analyzed for Se species to demonstrate the applicability of the method. Owing to its ability to rapidly determine Se species in water samples at environmentally relevant concentrations, the method may be useful for monitoring agencies to routinely measure Se species in freshwater aquatic systems.

Promoter analysis and functional implications of the selenium binding protein (SBP) gene family in Arabidopsis thaliana

Selenium Βinding Protein (SBP, originally termed SBP56) was identified in mouse liver as a cytosolic protein that could bind radioactive selenium. SBPs are highly conserved proteins present in a wide array of species across all kingdoms and are likely to be involved in selenium metabolism. In Arabidopsis, the selenium binding protein (SBP) gene family comprises three genes (AtSBP1, AtSBP2 and AtSBP3). AtSBP1 and AtSBP2 are clustered in a head-to-tail arrangement on chromosome IV, while AtSBP3 is located on chromosome III. In this work, we studied the promoter activity of the Arabidopsis SBP genes, determined their tissue specificity and showed that they are differentially regulated by sodium selenite and sodium selenate. All three SBP genes are upregulated in response to externally applied selenium compounds and the antioxidant NAC selectively downregulates SBP2. Although the effect on SBP2 levels was the most prominent, in all cases, the concurrent exposure of plants to selenite and the antioxidant supressed the expression of the SBP genes. We provide evidence that (at least) SBP1 expression is tightly linked to detoxification processes related to oxidative stress, since it is downregulated in the presence of NAC in selenium-treated plants. Furthermore, our results suggest that SBP genes may participate in the mechanisms that sense redox imbalance.