Selenium in higher plants: understanding mechanisms for biofortification and phytoremediation

Voyage of selenium from environment to life: Beneficial or toxic?

Selenium (Se), a naturally occurring metalloid, is an essential micronutrient for life as it is incorporated as selenocysteine in proteins. Although beneficial at low doses, Se is hazardous at high concentrations and poses a serious threat to various ecosystems. Due to this contrasting 'dual' nature, Se has garnered the attention of researchers wishing to unravel its puzzling properties. In this review, we describe the impact of selenium's journey from environment to diverse biological systems, with an emphasis on its chemical advantage. We describe the uneven distribution of Se and how this affects the bioavailability of this element, which, in turn, profoundly affects the habitat of a region. Once taken up, the subsequent incorporation of Se into proteins as selenocysteine and its antioxidant functions are detailed here. The causes of improved protein function due to the incorporation of redox-active Se atom (instead of S) are examined. Subsequently, the reasons for the deleterious effects of Se, which depend on its chemical form (organo-selenium or the inorganic forms) in different organisms are elaborated. Although Se is vital for the function of many antioxidant enzymes, how the pro-oxidant nature of Se can be potentially exploited in different therapies is highlighted. Furthermore, we succinctly explain how the presence of Se in biological systems offsets the toxic effects of heavy metal mercury. Finally, the different avenues of research that are fundamental to expand our understanding of selenium biology are suggested.

Enrichment characteristics and dietary evaluation of selenium in navel orange fruit from the largest navel orange-producing area in China (southern Jiangxi)

Diet is the main intake source of selenium (Se) in the body. Southern Jiangxi is the largest navel orange-producing area in China, and 25.98% of its arable land is Se-rich. However, studies on the Se-rich characteristics and Se dietary evaluation of navel orange fruits in the natural environment of southern Jiangxi have not been reported. This study was large-scale and in situ samplings (n = 492) of navel oranges in southern Jiangxi with the goal of investigating the coupling relationships among Se, nutritional elements, and quality indicators in fruits and systematically evaluating Se dietary nutrition to the body. The results indicated that the average content of total Se in the flesh was 4.92 μg⋅kg^-1, and the percentage of Se-rich navel oranges (total Se ≥ 10 μg⋅kg^-1 in the flesh) was 7.93%, of which 66.74% of the total Se was distributed in the pericarp and 33.26% in the flesh. The average content of total Se in the flesh of Yudu County was the highest at 5.71 μg⋅kg^-1. There was a significant negative correlation (p < 0.05) between Se, Cu, and Zn in the Se-rich flesh. According to the Se content in the flesh, the Se dietary nutrition evaluation was carried out, and it was found that the Se-enriched navel orange provided a stronger Se nutritional potential for the human body. These findings will help to identify Se enrichment in navel orange fruit in China's largest navel orange-producing area and guide the selection of Se-rich soils for navel orange production in the future.

Novel mechanistic insights of selenium induced microscopic, histochemical and physio-biochemical changes in tomato (Solanum lycopersicum L.) plant. An account of beneficiality or toxicity

Selenium (Se) is a well-known beneficial element in plants. The window of Se between toxic and optimal concentration is narrow and uneven which fluctuates with plants species. This experiment was aimed to investigate the morpho-physiological, microscopic and histochemical responses of two different varieties of tomato (S-22 and PKM-1), exposed to different concentrations of Se (0, 10, 40 or 80 µM), applied to soil at 30 days after transplantation (DAT). At 40 DAT, it was observed that high concentrations (40 or 80 µM) of Se radically increased oxidative stress examined by elevated reactive oxygen species (ROS), malondialdehyde (MDA) content, cell death, electrolyte leakage and decreased chlorophyll content leading phenotypic symptoms of Se-induced toxicity like stunted growth and chlorosis. Furthermore, high doses of Se altered the chloroplast and stomatal organisation, and adversely affected the photosynthetic performance of plants. But low concentration of Se improved the plant dry mass, photosynthesis, Rubisco activity, protein content and maintained the steady-state equilibrium among ROS generation and antioxidant enzymes like catalase, peroxidase and superoxide dismutase. Our outcomes proposed that high concentration of Se generated toxicity (phyto-selenosis), whereas lower concentration of Se-triggered positive impact by improving growth, photosynthetic traits and maintaining steady-state equilibrium between scavenging-system and ROS generation.

Foliar application of biosynthetic nano-selenium alleviates the toxicity of Cd, Pb, and Hg in Brassica chinensis by inhibiting heavy metal adsorption and improving antioxidant system in plant

Biosynthetic nano-selenium (bio-SeNP), as a plant growth regulator, has better bioavailability and lower toxicity than selenite and selenate. This study investigated the beneficial role of bio-SeNP in mitigating the adverse effects of multiple heavy metals (HMs, e.g., Cd, Pb, and Hg) on growth and yield of pak choi (Brassica chinensis) grown in slightly or heavily polluted (SP or HP) soil by regulating metabolic and antioxidant systems. The results revealed that foliar application of bio-SeNP (5, 10, 20 mg L^-1 Se) at the 6-leaf stage greatly reduced the levels of Cd, Pb, and Hg in shoots and roots of pak choi. Application of 5 mg L^-1 bio-SeNP significantly (p < 0.05) decreased the translocation factor (TF) of Cd, Pb, and Hg from root to shoot by 9.83%, 44.21%, and 46.99% for SP soil, 24.17%, 56.00%, and 39.36% for HP soil, respectively. Meanwhile, all bio-SeNP treatments led to a significant improvement in plants growth by enhancing the antioxidant defense system (e.g., AsA-GSH) and promoting chlorophyll synthesis as well as suppressed the lipid peroxidation products contents (MDA) in shoots. Moreover, the enhanced levels of mineral nutrient elements (e.g., Ca, Mg, Fe, or Zn) and organic selenium (e.g., selenocystine, Se-methylselenocysteine, and selenomethionine) in the edible shoots of bio-SeNP-treated pak choi plant under multiple HMs stress indicated the positive impacts of bio-SeNP on the improvement of shoot quality and nutritional values. Collectively, our results indicated that bio-SeNP play an important role in the management of multiple HMs-induced adverse effects on pak choi. Foliar application of bio-SeNP at appropriate concentration (≤ 5 mg L^-1 Se) can be considered as a promising agronomic measure for safety leafy vegetable production in multiple HMs polluted soils when bio-SeNP application.

Discriminative Long-Distance Transport of Selenate and Selenite Triggers Glutathione Oxidation in Specific Subcellular Compartments of Root and Shoot Cells in Arabidopsis

Selenium is an essential trace element required for seleno-protein synthesis in many eukaryotic cells excluding higher plants. However, a substantial fraction of organically bound selenide in human nutrition is directly or indirectly derived from plants, which assimilate inorganic selenium into organic seleno-compounds. In humans, selenium deficiency is associated with several health disorders Despite its importance for human health, selenium assimilation and metabolism is barely understood in plants. Here, we analyzed the impact of the two dominant forms of soil-available selenium, selenite and selenate, on plant development and selenium partitioning in plants. We found that the reference plant Arabidopsis thaliana discriminated between selenate and selenite application. In contrast to selenite, selenate was predominantly deposited in leaves. This explicit deposition of selenate caused chlorosis and impaired plant morphology, which was not observed upon selenite application. However, only selenate triggered the accumulation of the macronutrient sulfur, the sister element of selenium in the oxygen group. To understand the oxidation state-specific toxicity mechanisms for selenium in plants, we quantified the impact of selenate and selenite on the redox environment in the plastids and the cytosol in a time-resolved manner. Surprisingly, we found that selenite first caused the oxidation of the plastid-localized glutathione pool and had a marginal impact on the redox state of the cytosolic glutathione pool, specifically in roots. In contrast, selenate application caused more vigorous oxidation of the cytosolic glutathione pool but also impaired the plastidic redox environment. In agreement with the predominant deposition in leaves, the selenate-induced oxidation of both glutathione pools was more pronounced in leaves than in roots. Our results demonstrate that Se-species dependent differences in Se partitioning substantially contribute to whole plant Se toxicity and that these Se species have subcellular compartment-specific impacts on the glutathione redox buffer that correlate with toxicity symptoms.

Synergistic Effect of Melatonin and Selenium Improves Resistance to Postharvest Gray Mold Disease of Tomato Fruit

Melatonin (MT) is a ubiquitous hormone molecule that is commonly distributed in nature. MT not only plays an important role in animals and humans but also has extensive functions in plants. Selenium (Se) is an essential micronutrient for animals and humans, and is a beneficial element in higher plants at low concentrations. Postharvest diseases caused by fungal pathogens lead to huge economic losses worldwide. In this study, tomato fruits were treated with an optimal sodium selenite (20 mg/L) and melatonin (10 μmol/L) 2 h and were stored for 7 days at room temperature simulating shelf life, and the synergistic effects of Se and MT collectively called Se-Mel on gray mold decay in tomato fruits by Botrytis cinerea was investigated. MT did not have antifungal activity against B. cinerea in vitro, while Se significantly inhibited gray mold development caused by B. cinerea in tomatoes. However, the interaction of MT and Se showed significant inhibition of the spread and growth of the disease, showing the highest control effect of 74.05%. The combination of MT with Se treatment enhanced the disease resistance of fruits by improving the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as increasing the gene expression level of pathogenesis-related (PR) proteins. Altogether, our results indicate that the combination of MT and Se would induce the activation of antioxidant enzymes and increase the expression of PR proteins genes that might directly enhance the resistance in tomato fruit against postharvest pathogenic fungus B. cinerea.

Multiomics understanding of improved quality in cherry radish (Raphanus sativus L. var. radculus pers) after foliar application of selenium nanomaterials

A selenium (Se)-nanoenabled agriculture strategy was established in this work to improve crop yield and quality. The results demonstrated that Se engineering nanomaterials (Se ENMs, 10 mg·L^-1) were absorbed and translocated in cherry radish (Raphanus sativus L. var. radculus pers) from shoots to taproots after foliar application. RNA-Seq and metabolomic results indicated that the glucolysis, pyruvate and tricarboxylic acid (TCA) cycle metabolism pathways were accelerated by exposure to Se ENMs, resulting in increased production of flavonoids (3.2-fold), amino acids (1.4-fold), and TCA (2.5-fold) compared with the control. Moreover, Se content was enhanced by 5.4 and 2.6 times in pericarp and pulp upon Se ENMs exposure, respectively, which was more efficient (2.2 and 1.1 times) than SeO₃^2- treatment. Additionally, the yield of cherry radish was increased by 67.6% under Se ENMs, whereas SeO₃^2- exposure only led to an increase of 7.4%. Therefore, the application of Se ENMs could reduce the amount of fertilizer used to minimize the environmental impact in agriculture while improve crop production and quality. These findings highlighted the significant potential of Se ENMs-enabled agriculture practices as an eco-friendly and sustainable crop strategy.

Influence of Clay Mineral Amendments Characteristics on Heavy Metals Uptake in Vetiver Grass (Chrysopogon zizanioides L. Roberty) and Indian Mustard (Brassica juncea L. Czern)

Phytoremediation is limited when heavy metals reduce soil quality and, subsequently, inhibit plant growth. In this study, we evaluated the use of attapulgite and bentonite as amendments in soil contaminated with multiple metals, to improve the phytoremediation capacity of Vetiver grass and Indian mustard. A 21-day greenhouse study was undertaken, to investigate plant tolerance in heavy-metal-contaminated soil, as well as heavy-metal absorption in plant roots and shoots. The results showed a generally higher root-uptake rate for Cr, Cu, Co, Ni, and Zn in Vetiver grass. Overall, the highest absorption for Ni, Cr, Co, Cu, and Zn was 1.37, 2.79, 1.39, 2.48 and 3.51 mg/kg, respectively, in the roots of Vetiver grass. Clay minerals inhibited the translocation of some heavy metals. The addition of attapulgite improved the phytoremediation capacity of Vetiver for Ni, Cr, and Co, while bentonite improved Vetiver’s absorption of Cu and Zn. The translocation factor for Ni in one of the attapulgite treatments was 2, indicating that attapulgite improved the phytoextraction of Ni by Vetiver grass. Our results confirm that attapulgite at 2.5% (w/w) can successfully improve the phytostabilization of heavy metals by Vetiver grass. Indian mustard showed no significant metal uptake that could be detected by inductively coupled plasma optical emission spectrometry (ICP-OES), despite the addition of attapulgite and bentonite. This research contributes to the knowledge repository of suitable amendments that improve the phytoremediation properties of Vetiver grass.

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.

Bioaccessibility and bioavailability of selenium species in Se-enriched leeks (Allium Porrum) cultivated by hydroponically

The Allium genus vegetables are of special interest since being potentially sources for selenium. In this study, the metabolization of selenite and selenate fortification at low and high levels in hydroponically cultivated Allium porrum (Leek) was investigated. The total Se analysis of nutritional solutions which was used in cultivation medium revealed that leeks had potential to accumulate Se above over 1000 mg/kg without any growth disturbance which was proved by comparing dry masses of control group with the ones fortified by Se species. Speciation analyses performed in edible parts which are leaves and stems showed that approximately 90% of total selenium was biotransformed into organo-selenium species in which MeSeCys and SeMet were found to be the most dominant in Se(IV) fortified leeks. However, selenate was found to be the most abundant species in edible parts of selenate fortified leeks especially at high levels. Although bioavailability rate of total selenium in selenate fortified leeks was found to be higher, lower amount of inorganic selenium and higher amount of MeSeCys were found to be bioavailable in Se(IV) fortified.

Size-dependent transformation, uptake, and transportation of SeNPs in a wheat-soil system

Foliar application of selenium nanoparticles (SeNPs) has been used to enhance Se concentration in winter wheat, but soil application of SeNPs on Se uptake in the crop and their transformation in soil are still limited. This study investigated the effects of varying sizes (50, 100, 200 nm) and concentrations (0, 2, 5, 25, 100 mg kg^-1) of chemical synthesized SeNPs in soil on uptake and accumulation of Se in the crop at maturity and related mechanisms. SeNPs not only posed very low toxic to plant growth, except for leaf, but also significantly enhanced grain Se concentration. Regardless of concentration of SeNPs added to soil, the transformation rate of the larger sized SeNPs (200 nm) in soil was significantly (p < 0.05) higher than that of the smaller one, which is mainly due to the latter was more easily adsorbed onto soil organic matter and reluctant to be oxidized. Significantly higher grain Se concentration under the larger sized SeNPs contributed to significantly higher transformation rate of SeNPs and concentration of available Se in soil. The present study showed that the larger sized SeNPs in soil had significant advantages including higher grain Se concentration and Se utilization efficiency for wheat Se biofortification.

Selenium fertigation with nanobubbles influences soil selenium residual and plant performance by modulation of bacterial community

Although selenium (Se) is an essential microelement for humans and animals, it is a potentially toxic element due to its bioaccumulation potential. In this study, Se fertilizer was supplied in a greenhouse vegetable (cucumber) plantation using an innovative system consisting of nanobubbles (NB_Se) and compared to that under conventional conditions of fertigation (C_Se) with six doses. The results revealed that NB_Se significantly reduced soil Se accumulation (38%-144%) and increased cucumber Se content compared with the C_Se treatments at the same Se dose. NB_Se significantly lowered the soil bacterial diversity, with an initial increase and then decrease with the Se doses. Bacterial associations and potential keystone taxa also differed between the NB_Se and C_Se. The greater abundance of oxidizing bacteria (indicated by the function composition of bacterial community) and the improved soil redox environment created by NBs sustained more available Se for plants, leading to a reduction in soil Se residual and an increase in the plant Se content. Our results highlight the feasibility and efficiency of NB_Se and demonstrate the important implications of Se for the maintenance of soil health and sustainability.

Selenium in Soil-Plant-Microbe: A Review

Selenium (Se) plays an important role in geochemistry and is an essential trace element for humans and animals. This review summarizes the transformation and accumulation of Se in the plant-soil-microbe system. As one of the important reservoirs of Se, soil is an important material basis of its entry into the food chain through plants. Soil with an appropriate amount of Se is beneficial for plant growth and plays a valuable role in a stress-resistant environment. Among the many migration and transformation pathways, the transformation of Se by microorganisms is particularly important and is the main form of Se transformation in the soil environment. In this review, the role and form transformation of Se in plants, soil, and microorganisms; the role of Se in plants; the form, input, and output of Se in soil; the absorption and transformation of Se by plants; and the role of microorganisms in Se transformation are presented. In addition to describing the migration and transformation laws of Se in the environment, this review expounds on the main directions and trends of Se research in the agricultural field as well as current gaps and difficulties in Se-related research. Overall, this reviews aims to provide necessary information and theoretical references for the development of Se-rich agriculture.

The beneficial and hazardous effects of selenium on the health of the soil-plant-human system: An overview

Selenium (Se), which can be both hazardous and beneficial to plants, animals and humans, plays a pivotal role in regulating soil-plant-human ecosystem functions. The biogeochemical behavior of Se and its environmental impact on the soil-plant-human system has received broad attention in the last decades. This review provides a comprehensive understanding of Se biogeochemistry in the soil-plant-human system. The speciation, transformation, bioavailability as well as the beneficial and hazardous effects of Se in the soil-plant-human system are summarized. Several important aspects in Se in the soil-plant-human system are detailed mentioned, including (1) strategies for biofortification in Se-deficient areas and phytoremediation of soil Se in seleniferous areas; (2) factors affecting Se uptake and transport by plants; (3) metabolic pathways of Se in the human body; (4) the interactions between Se and other trace elements in plant and animals, in particular, the detoxification of heavy metals by Se. Important research hotspots of Se biogeochemistry are outlined, including (1) the coupling of soil microbial activity and the Se biogeochemical cycle; (2) the molecular mechanism of Se metabolic in plants and animals; and (3) the application of Se isotopes as a biogeochemical tracer in research. This review provides up-to-date knowledge and guidelines on Se biogeochemistry research.

Mitigation effects of exogenous melatonin-selenium nanoparticles on arsenic-induced stress in Brassica napus

Melatonin (MT) and selenium (Se) application known to decrease heavy metal uptake and toxicity in plants. By mixing the Se in MT medium a new complex MT-Se nanoparticles (MT-Se NPs) was synthesized and we investigated the role of MT-Se NPs on B. napus growth and tolerance against As stress. The MT-Se particles significantly enhanced the plant growth and other associated physiological attributes under As stress. The As treatment at 80 μM was more phytotoxic, however MT-Se NPs application resulted in a substantial increase in leaf chlorophyll fluorescence, biomass accumulation, and decreased ROS relative to As stressed plants. The use of MT-Se NPs to As stressed plants reduced photosynthetic inhibition and oxidative stress and attenuated the increase in MDA and H₂O₂ contents. The application of MT-Se NPs also boosted the antioxidant enzymes activities such as SOD, POD and CAT as well as the APX, GR and GSH activates under As stress. The results also showed MT-Se NPs treatments alleviated the growth inhibition induced by As and reduced the accumulation of As in leaves and roots of B. napus seedlings. Moreover, treatment with MT-Se NPs improved the plant growth more successfully than treatment of MT and Se alone. This study explored the mechanism of melatonin and selenium efficiency in the composition can be jointly encouraged to exert synergistic effects and boost plant enzymatic activities.

Selenium-agarose hybrid hydrogel as a recyclable natural substrate for selenium-enriched cultivation of mung bean sprouts

Selenium (Se) is an essential trace element for human beings and animals. Traditional plant Se enrichment technology suffers from selenium pollution. Herein, environmentally friendly Se-agarose (Se-Agar) hybrid hydrogels are prepared by simply mixing agar with different Se species including selenocarrageenan (SeCA), selenite and Se yeast under heating and stirring for 0.5 h without any other reagent. Such Se-Agar hybrid hydrogels with excellent biocompatibility were used as natural substrates for the cultivation of Se-enriched mung bean sprouts. Compared with Se yeast, SeCA and selenite show a better Se enrichment effect on mung bean sprouts. Furthermore, the growth indices including plant weight and plant height of mung bean sprouts were investigated with different concentrations and sources of Se. Notably, the Se-Agar hybrid hydrogels could be easily regenerated and reused for multiple cycles. The results indicated that Se-Agar hybrid hydrogels as recyclable natural substrates offer a simple, sustainable and affordable strategy for plant Se enrichment.

Uptake and translocation mechanisms of different forms of organic selenium in rice (Oryza sativa L.)

Selenium (Se) is an essential trace element for human and animal health, and toward an understanding of the uptake and translocation of Se in plants is important from the perspective of Se biofortification. In this study, we conducted hydroponic experiments to investigate the mechanisms of organic Se [selenomethionine (SeMet) and selenomethionine-oxide (SeOMet)] uptake, translocation, and the interactions between SeMet and SeOMet in rice. We also investigated differences in the dynamics of organic and inorganic Se uptake by rice roots. Concentration-dependent kinetic results revealed that SeMet uptake during a 1 h exposure was 3.19-16.0 times higher than that of three other Se chemical forms, with uptake capacity (V_max ) values ordered as follows: SeMet>SeOMet>selenite>selenate. Furthermore, time-dependent kinetic analysis revealed that SeMet uptake by roots and content in shoots were initially clearly higher than those of SeOMet, although the differences gradually diminished with prolonged exposure time; while no significant difference was found in the transfer factor of Se from rice roots to shoots between SeMet and SeOMet. Root uptake of SeOMet was significantly inhibited by carbonyl cyanide 3-chlorophenylhydrazone (CCCP) (30.4%), AgNO₃ (41.8%), and tetraethylammonium chloride (TEACl) (45.6%), indicating that SeOMet uptake is a metabolically active process, and that it could be mediated via aquaporins and K⁺ channels. Contrarily, SeMet uptake was insensitive to CCCP, although markedly inhibited by AgNO₃ (93.1%), indicating that rice absorbs SeMet primarily via aquaporins. Furthermore, Se uptake and translocation in rice treated simultaneously with both SeMet and SeOMet were considerably lower than those in rice treated with SeMet treatment alone and notably lower than the theoretical quantity, indicating interactions between SeMet and SeOMet. Our findings provide important insights into the mechanisms underlying the uptake and translocation of organic Se within plants.

Comparison of selenite and selenate in alleviation of drought stress in Nicotiana tabacum L

Exogenous selenium (Se) improves the tolerance of plants to abiotic stress. However, the effects and mechanisms of different Se species on drought stress alleviation are poorly understood. This study aims to evaluate and compare the different effects and mechanisms of sodium selenate (Na2SeO4) and sodium selenite (Na2SeO3) on the growth, photosynthesis, antioxidant system, osmotic substances and stress-responsive gene expression of Nicotiana tabacum L. under drought stress. The results revealed that drought stress could significantly inhibit growth, whereas both Na2SeO4 and Na2SeO3 could significantly facilitate the growth of N. tabacum under drought stress. However, compared to Na2SeO3, Se application as Na2SeO4 induced a significant increase in the root tip number and number of bifurcations under drought stress. Furthermore, both Na2SeO4 and Na2SeO3 displayed higher levels of photosynthetic pigments, better photosynthesis, and higher concentrations of osmotic substances, antioxidant enzymes, and stress-responsive gene (NtCDPK2, NtP5CS, NtAREB and NtLEA5) expression than drought stress alone. However, the application of Na2SeO4 showed higher expression levels of the NtP5CS and NtAREB genes than Na2SeO3. Both Na2SeO4 and Na2SeO3 alleviated many of the deleterious effects of drought in leaves, which was achieved by reducing stress-induced lipid peroxidation (MDA) and H2O2 content by enhancing the activity of antioxidant enzymes, while Na2SeO4 application showed lower H2O2 and MDA content than Na2SeO3 application. Overall, the results confirm the positive effects of Se application, especially Na2SeO4 application, which is markedly superior to Na2SeO3 in the role of resistance towards abiotic stress in N. tabacum.

Transcriptome and proteome profiling revealed molecular mechanism of selenium responses in bread wheat (Triticum aestivum L.)

BACKGROUND

Although selenium (Se) plays important roles in scavenging free radicals, alleviating oxidative stresses, and strengthening immune system, the knowledge about Se responses in bread wheat is still limited. In order to clarify the molecular mechanism of Se responses in bread wheat, 2-week-old wheat seedlings of cultivar 'Jimai22' treated with 10 μM disodium selenate (Na₂SeO₄) for 0, 3, and 24 h were collected and analyzed by transcriptional sequencing and tandem mass tag-based (TMT) quantitative proteomics.

RESULTS

At least 11,656 proteins and 133,911 genes were identified, and proteins including ATP sulfurylase (APS), cysteine synthase (CS), SeCys lyase, sulfate transporters, glutathione S-transferase (GSTs), glutathione peroxidase (GSH-Px), glutaredoxins (GRXs), superoxide dismutases (SODs), catalases (CATs), heat shock proteins (HSPs), UDP-glycose flavonoid glycosyltransferases (UFGTs), sucrose-6-phosphate hydrolases (Suc-6-PHs), archaeal phosphoglucose isomerases (APGIs), malate synthases (MSs), and endo-1,4-beta-xylanase (Xyn) in Se accumulation, ROS scavenging, secondary metabolism, and carbohydrate metabolism were significantly differently expressed.

CONCLUSIONS

This is the first complementary analyses of the transcriptome and proteome related with selenium responses in bread wheat. Our work enhances the understanding about the molecular mechanism of selenium responses in bread wheat.

Selenium-oxidizing Agrobacterium sp. T3F4 steadily colonizes in soil promoting selenium uptake by pak choi (Brassica campestris)

Selenium (Se) deficiency in soil is linked to its low content in edible crops, resulting in adverse impacts on the health of 15% of the global population. The crop mainly absorbs oxidized selenate and selenite from soil, then converts them into organic Se. However, the role of Se-oxidizing bacteria in soil Se oxidation, Se bioavailability and Se absorption into plants remains unclear. The strain Agrobacterium sp. T3F4, isolated from seleniferous soil, was able to oxidize elemental Se into selenite under pure culture conditions. The green fluorescent protein (gfp)-gene-marked strain (T3F4-GFP) and elemental Se or selenite (5 mg·kg^-1) were added to pak choi (Brassica campestris ssp. chinensis) pot cultures. Observation of the fluorescence and viable counting indicated that GFP-expressing bacterial cells steadily colonized the soil in the pots and the leaves of the pak choi, reaching up to 6.6 × 10⁶ and 2.0 × 10⁵ CFU g^-1 at 21 days post cultivation, respectively. Moreover, the total Se content (mostly organic Se) was significantly increased in the pak choi under T3F4 inoculated pot culture, with elemental Se(0) being oxidized into Se(IV), and soil Se(IV) being dissolved before being absorbed by the crop. After strain T3F4 was inoculated, no significant differences in microbial diversity were observed in the soils and roots, whereas the abundance of Rhizobium spp. was significantly increased. To our knowledge, this is the first time that Se-oxidizing Agrobacterium sp. T3F4 has been found to steadily colonize soil and plant tissues, and that its addition to soil increases the absorption of Se in plants. This study provides a potential strategy for Se biofortification.

Microbes: a potential tool for selenium biofortification

Selenium (Se) is a component of many enzymes and indispensable for human health due to its characteristics of reducing oxidative stress and enhancing immunity. Human beings take Se mainly from Se-containing crops. Taking measures to biofortify crops with Se may lead to improved public health. Se accumulation in plants mainly depends on the content and bioavailability of Se in soil. Beneficial microbes may change the chemical form and bioavailability of Se. This review highlights the potential role of microbes in promoting Se uptake and accumulation in crops and the related mechanisms. The potential approaches of microbial enhancement of Se biofortification can be summarized in the following four aspects: (1) microbes alter soil properties and impact the redox chemistry of Se to improve the bioavailability of Se in soil; (2) beneficial microbes regulate root morphology and stimulate the development of plants through the release of certain secretions, facilitating Se uptake in plants; (3) microbes upregulate the expression of certain genes and proteins that are related to Se metabolism in plants; and (4) the inoculation of microbes give rise to the generation of certain metabolites in plants contributing to Se absorption. Considering the ecological safety and economic feasibility, microbial enhancement is a potential tool for Se biofortification. For further study, the recombination and establishment of synthesis microbes is of potential benefit in Se-enrichment agriculture.

Comparative responses of cadmium accumulation and subcellular distribution in wheat and rice supplied with selenite or selenate

Cadmium (Cd) contamination of crop plants has aroused a worldwide concern because of the threats posed to human health through accumulation in the food chains. Selenium (Se) can alleviate the Cd-induced phytotoxicity, but the relevant underlying mechanisms are not fully understood. Therefore, with wheat (Triticum aestivum L.) and rice (Oryza sativa L.) chosen as the target plants in this study, the effects of selenite or selenate on Cd accumulation and subcellular distribution were investigated through greenhouse hydroponic experiments; and simultaneously, the effects of pre-Se treatment with selenite or selenate on Cd accumulation and root-to-shoot translocation in the studied plants were also included. Results showed the addition of Se slightly changed the Cd content in plant roots in a time-dependent manner; however, with the obvious decreasing trend on the Cd transfer factor (TF), its content in plant shoots was significantly reduced by selenite or selenate in a plant species-dependent manner. At 48 h of exposure, the supplementation of selenite and selenate significantly decreased the Cd content by 40.4% and 38.0% in wheat shoots, and by 72.2% and 40.9% in rice shoots, respectively. Additionally, the order of Cd proportion distributed to the different subcellular fractions of plant tissues was as follows: cell wall > soluble cytosol > organelle, irrespective of the Se treatments or the plant species. However, selenate increased the Cd percentage in soluble cytosol of wheat shoots, while selenite increased that percentage in the cell wall of rice shoots; and the Cd proportion in soluble cytosol of the studied plant roots was significantly enhanced owing to selenite or selenate addition. Moreover, similar to the co-application, the pre-Se treatment with inorganic Se also reduced the Cd accumulation and translocation both in wheat and rice. Our results proved that the inorganic Se could decline the Cd accumulation and translocation in the crop plants, although selenite was found more effective than selenate regarding such effects.

Combined use of lime, bentonite, and biochar for immobilization of Cd and mobilization of Se in paddy soil

Remediation of soil contaminated with cadmium (Cd) that can produce rice enriched with selenium (Se) is highly significant for improving the public health in China. A key issue needing resolution in this regard is the simultaneous immobilization of Cd and mobilization of Se. To explore a potential promising method to remediate median-high Se soil that is contaminated by Cd, a potted experiment was conducted, and seven combined amendments (0.03-0.12% lime, 0.03-0.18% bentonite, and 0.3-1.2% biochar of the dry soil weight) were used to immobilize Cd in three paddy soils, in which the concentrations of Cd and Se are 0.46 and 0.45 mg/kg, 1.12 and 0.33 mg/kg, and 2.96 and 0.31 mg/kg, respectively. The soil pH increased by 1.5-2 units after the application of the amendments, and the soil organic carbon (SOC) concentration increased notably with the addition of large quantities of biochar. As the pH and SOC concentration increased, the concentrations of the available Cd in the soil decreased by 35-50%, and the FTIR spectrum showed that O-containing groups and Si-O facilitated the Cd immobilization. The concentration of Cd in brown rice decreased with a decrease in the available Cd. There was no apparent correlation between the Se concentration in the brown rice and the concentration of the available Se in the soil, although the available Se increased by 40-80% after the application of amendments. The accumulation of Se in rice grains was regulated by interactions among the lime, bentonite, biochar, and the soil. An SEM-EDS analysis showed that the biochar particles were covered with bentonite and other soil minerals that could postpone biochar aging and contribute to the longevity of the combined amendments in the soil.

Mechanisms underlying mercury detoxification in soil-plant systems after selenium application: a review

Feasible countermeasures to mitigate mercury (Hg) accumulation and its deleterious effects on crops are urgently needed worldwide. Selenium (Se) fertilizer application is a cost-effective strategy to reduce Hg concentrations, promote agro-environmental sustainability and food safety, and decrease the public health risk posed by Hg-contaminated soils and its accumulation in food crops. This holistic review focuses on the processes and detoxification mechanisms of Hg in whole soil-plant systems after Se application. The reduction of Hg bioavailability in soil, the formation of inert HgSe or/and HgSe-containing proteinaceous complexes in the rhizosphere and/or roots, and the reduction of plant root uptake and translocation of Hg in plant after Se application are systemically discussed. In addition, the positive responses in plant physiological and biochemical processes to Se application under Hg stress are presented to show the possible mechanisms for protecting the plant. However, application of high levels Se showed synergistic toxic effect with Hg and inhibited plant growth. The effectiveness of Se application methods, rates, and species on Hg detoxification is compared. This review provides a good approach for plant production in Hg-contaminated areas to meet food security demands and reduce the public health risk.

Selenium speciation and bioavailability from mine discharge to the environment: a field study in Northern Quebec, Canada

The speciation, behaviour, and bioavailability of released selenium (Se) from mine effluent discharge to sediments and plants were assessed. Discharged mine effluent containing 65±0.9 μg/L of total Se subsequently contaminated the exposed sediment with an average total Se concentration of 321 mg/kg as well as exposed Typha latifolia plants where 534 and 92 mg/kg were found in roots and leaves, respectively. The strategy of T. latifolia in Se phytoremediation consisted of a phytostabilization and accumulation of Se predominantly in roots. Se plant root uptake was promoted by synergistic effects of Cu, Pb, Zn, and Cd while Co, Fe, Mn, Ni, Na, K, and Mg had antagonistic effects. Se plant uptake was also governed by sediment characteristics mainly pH, total Se, and iron concentration. Se speciation results demonstrated that the most accumulated Se species by T. latifolia roots were selenite and selenomethionine with average concentrations of 2.68 and 2.04 mg/kg respectively while other Se species were the most translocated (average translocation factor of 1.89). Se speciation in roots was positively correlated with sediment pH, organic matter, electrical conductivity, and iron concentration. This study confirms deploying corrective measures for mine effluent treatment before discharge in a sediment-plant environment to protect living organisms from toxic effects. T. latifolia is recommended as a Se-hyperaccumulator to be used for mine soil phytoremediation in cold regions in Canada.

In Vitro Evaluation of the Inhibitory Activity of Different Selenium Chemical Forms on the Growth of a Fusarium proliferatum Strain Isolated from Rice Seedlings

In this study, the in vitro effects of different Se concentrations (5, 10, 15, 20, and 100 mg kg−1) from different Se forms (sodium selenite, sodium selenate, selenomethionine, and selenocystine) on the development of a Fusarium proliferatum strain isolated from rice were investigated. A concentration-dependent effect was detected. Se reduced fungal growth starting from 10 mg kg−1 and increasing the concentration (15, 20, and 100 mg kg−1) enhanced the inhibitory effect. Se bioactivity was also chemical form dependent. Selenocystine was found to be the most effective at the lowest concentration (5 mg kg−1). Complete growth inhibition was observed at 20 mg kg−1 of Se from selenite, selenomethionine, and selenocystine. Se speciation analysis revealed that fungus was able to change the Se speciation when the lowest Se concentration was applied. Scanning Electron Microscopy showed an alteration of the fungal morphology induced by Se. Considering that the inorganic forms have a higher solubility in water and are cheaper than organic forms, 20 mg kg−1 of Se from selenite can be suggested as the best combination suitable to inhibit F. proliferatum strain. The addition of low concentrations of Se from selenite to conventional fungicides may be a promising alternative approach for the control of Fusarium species.

The First Evidence of the Beneficial Effects of Se-Supplementation on In Vitro Cultivated Olive Tree Explants

Selenium is an essential micronutrient that provides important benefits to plants and humans. At proper concentrations, selenium increases plant growth, pollen vitality, the shelf life of fresh products, and seems to improve stress resistance; these effects can certainly be attributed to its direct and indirect antioxidant capacity. For these reasons, in the present work, the effects of selenium at different dosages on in vitro cultivated olive explants were investigated to observe possible positive effects (in terms of growth and vigor) on the proliferation phase. The work was carried out on four different olive cultivars: "San Felice", "Canino", "Frantoio", and "Moraiolo". The explants were cultured in aseptic conditions on olive medium (OM), with the addition of 4 mg·L^-1 of zeatin, 30 g·L^-1 of sucrose, and 7 g·L^-1 of agar. The experimental scheme included a comparison between explants grown with five different concentrations of Na₂SeO₄ (0, 10, 20, 40, and 80 mg L^-1) added to the medium during three successive subcultures. Interesting information has emerged from the results and all varieties responded to different concentrations of Selenium. The optimal Se dosages varied for each cultivar, but in general, Se concentration between 10 and 40 mg L^-1 increased fresh and dry weight of the explants and shoot lengths. Se treatment induced in all cultivars and for all dosages used an increase in total Se content in proliferated explants. Furthermore, as the subcultures proceeded, the ability of the explants to absorb Se did not diminish. The Se content ranged from 8.55 to 114.21 µg kg^-1 plant DW in 'Frantoio', from 9.83 to 94.85 µg kg^-1 plant DW in 'Moraiolo', from 19.84 to 114.21 µg kg^-1 plant DW in 'Canino', and from 20.97 to 95.54 µg kg^-1 plant DW in 'San Felice'. In general, the effect of selenium tends to decrease with the progress of subcultures and this suggests a sort of "adaptation" effect of the explants to its presence. The present study highlights for the first time the possibility of using in vitro cultures as biotechnological support to study supplementation with selenium and its effects on in vitro olive plant growth.

Impacts of heavy metals and medicinal crops on ecological systems, environmental pollution, cultivation, and production processes in China

Heavy metals are widely distributed in the environment due to the natural processes and anthropogenic human activities. Their migration into no contaminated areas contributing towards pollution of the ecosystems e.g. soils, plants, water and air. It is recognized that heavy metals due to their toxicity, long persistence in nature can accumulate in the trophic chain and cause organism dysfunction. Although the popularity of herbal medicine is rapidly increasing all over the world heavy metal toxicity has a great impact and importance on herbal plants and consequently affects the quality of herbal raw materials, herbal extracts, the safety and marketability of drugs. Effective control of heavy metal content in herbal plants using in pharmaceutical and food industries has become indispensable. Therefore, this review describes various important factors such as ecological and environmental pollution, cultivation and harvest of herbal plants and manufacturing processes which effects on the quality of herbal plants and then on Chinese herbal medicines which influence human health. This review also proposes possible management strategies to recover environmental sustainability and medication safety. About 276 published studies (1988-2021) are reviewed in this paper.

De novo Transcriptome Assembly and Comparative Analysis Highlight the Primary Mechanism Regulating the Response to Selenium Stimuli in Oats (Avena sativa L.)

Selenium is an essential microelement for humans and animals. The specific processing technique of oats can maximize the preservation of its nutrients. In this study, to understand the genetic response of oats in a high-selenium environment, oats were treated with sodium selenate for 24 h, and transcriptome analysis was performed. A total of 211,485,930 clean reads composing 31.30 Gb of clean data were retained for four samples. After assembly, 186,035 unigenes with an average length of 727 bp were generated, and the N50 length was 1,149 bp. Compared with that in the control group, the expression of 7,226 unigenes in the treatment group was upregulated, and 2,618 unigenes were downregulated. Based on the sulfur assimilation pathway and selenocompound metabolic pathway, a total of 27 unigenes related to selenate metabolism were identified. Among them, the expression of both key genes APS (ATP sulfurylase) and APR (adenosine 5'-phosphosulfate reductase) was upregulated more than 1,000-fold under selenate treatment, while that of CBL (cystathionine-β-synthase) was upregulated 3.12-fold. Based on the transcriptome analysis, we suspect that the high-affinity sulfur transporter Sultr1;2 plays a key role in selenate uptake in oats. A preliminary regulatory mechanism explains the oat response to selenate treatment was ultimately proposed based on the transcriptome analysis and previous research.

Cadmium uptake and translocation: selenium and silicon roles in Cd detoxification for the production of low Cd crops: a critical review

Cadmium (Cd) is a primary contaminant in agricultural soils of the world. The ability of Cd uptake, transport, detoxification, and accumulation varies among different plant species and genotypes. Cd is translocated from soil to root by different transporters which are used for essential plant nutrient uptake. A number of strategies have been suggested for decreasing Cd toxicity in Cd contaminated soils. Recently, a lot of research have been carried out on minimizing Cd uptake through selenium (Se) and silicon (Si) applications. Both Se and Si have been reported to mitigate Cd toxicity in different crops. Vacuolar sequestration, formation of phytochelatins, and cell wall adsorption have been reported as effective mechanisms for Cd detoxification. The present review discussed past and current knowledge of literature to better understand Cd toxicity and its mitigation by adopting different feasible and practical approaches.

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

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

Exploring the new dimensions of selenium research to understand the underlying mechanism of its uptake, translocation, and accumulation

Selenium (Se) is a vital mineral for both plants and animals. It is widely distributed on the earth's crust and is taken up by the plants as selenite or selenate. Plants substantially vary in their physiological response to Se. The amount of Se in edible plants is genetically controlled. Its availability can be determined by measuring its phytoavailability in soil. The low concentration of Se in plants can help them in combating stress, whereas higher concentrations can be detrimental to plant health and in most cases it is toxic. Thus, solving the double-edged sword problem of nutritional Se deficiency and its elevated concentrations in environment requires a better understanding of Se uptake and metabolism in plants. The studies on Se uptake and metabolism can help in genetic biofortification of Se in plants and also assist in phytoremediation. Moreover, Se uptake and transport, especially biochemical pathways of assimilation and incorporation into proteins, offers striking mechanisms of toxicity and tolerance. These developments have led to a revival of Se research in higher plants with significant break throughs being made in the previous years. This review explores the new dimensions of Se research with major emphasis on key research events related to Se undertaken in last few years. Further, we also discussed future possibilities in Se research for crop improvement.

Transcriptome, proteome, and metabolome reveal the mechanism of tolerance to selenate toxicity in Cardamine violifolia

Cardamine violifolia was found here to accumulate selenium (Se) to over 9000 mg kg^-1 dry weight. To investigate the mechanism of Se accumulation and tolerance in C. violifolia, metabolome, transcriptome, and proteome technologies were applied to C. violifolia seedlings treated with selenate. Several sulfate transporter (Sultr) genes (Sultr1;1, Sultr1;2, and Sultr2;1) and sulfur assimilatory enzyme genes showed high expression levels in response to selenate. Many calcium protein and cysteine-rich kinase genes of C. violifolia were downregulated, whereas selenium-binding protein 1 (SBP1) and protein sulfur deficiency-induced 2 (SDI2) of C. violifolia were upregulated by selenate. The expression of genes involved in the ribosome and posttranslational modifications and chaperones in C. violifolia were also detected in response to selenate. Based on the results of this study and previous findings, we suggest that the downregulated expression of calcium proteins and cysteine-rich kinases, and the upregulated expression of SBP1 and SDI2, were important contributors to the Se tolerance of C. violifolia. The downregulation of cysteine-rich kinases and calcium proteins would enhance Se tolerance of C. violifolia is a novel proposition that has not been reported on other Se hyperaccumulators. This study provides us novel insights to understand Se accumulation and tolerance in plants.

Peanut selenium distribution, concentration, speciation, and effects on proteins after exogenous selenium biofortification

Fortification of Se is vital importance for both nutritional demand and prevention of Se-deficiency-related diseases. To better understand t selenium distribution, concentration, speciation, its effects on proteins, and cytotoxic activity, the biofortification of exogenous Se in peanut was conducted in this study. Our data have shown that foliar spraying of Se-riched fertilizer was more efficient for biotransformation of inorganic Se to organic Se by peanut plant. Besides, the Se content in peanut was increased in a dose-dependent manner. Our present study also confirmed that SeCys₂, MeSeCys, and SeMet were the main Se speciation within peanut proteins. Moreover, the secondary structure and thermostability of peanut protein were altered as a result of the Se treatments, and these alterations could be attributed to the replacements of cysteine and methionine by selenocysteine and selenomethionine, respectively. The Se-enriched peanut protein could significantly inhibit the growth of Caco-2 and HepG2 in a concentration-dependent manner.

Effects of foliar selenium application on growth and rhizospheric soil micro-ecological environment of Atractylodes macrocephala Koidz

Atractylodes macrocephala (A. macrocephala), a famous medicinal herb in China, is widely cultivated and consumed in China with various beneficial effects. Numerous studies have shown that selenium (Se) plays an important role in promoting plant growth, although Se has not been considered an essential element for higher plants. The objectives of this research were to determine the effects of foliar Se application (0, 2.5, 5.0, 10.0 and 20.0 mg m^-2 Se in sodium selenite, sprayed monthly from May to August) on the growth and rhizospheric soil micro-ecological environment of A. macrocephala, and explore the possible mechanisms underlying plant response to foliar Se application through a field experiment. The results were: The foliar application of 5.0 mg m^-2 Se significantly increased the survival rate of A. macrocephala compared to the control. The yield of A. macrocephala was increased when the Se level maintained belowed 10.0 mg m^-2 but decreased when Se level reached 20.0 mg m^-2. The Se content in the rhizome of A. macrocephala showed a significant positive correlation with the Se level, while the insect attack rate was significantly negatively correlated with the Se level. However, foliar Se application hardly affected the concentration of bioactive compound atractylenolide in the rhizome of A. macrocephala. Notably, the application of foliar Se changed the content of partial soil nutrients, microbial diversity and composition in the rhizosphere soil of A. macrocephala. Bacterial diversity was positively correlated with A. macrocephala growth whereas fungal diversity was negatively correlated, suggesting that microbial diversity in the rhizosphere soils is closely related to plant growth. Moreover, correlation analysis showed that available potassium, Burkholderia and Cupriavidus in rhizospheric soil might be critical factors for promoting the growth of A. macrocephala. Overall, the foliar application of Se at moderate concentration was beneficial for the growth of A. macrocephala, and 5.0-10.0 mg m^-2 Se level was the optimum. Our findings revealed novel insights into the response of A. macrocephala to foliar Se application from plant growth, rhizospheric soil nutrient and microbial community composition .

Methyl jasmonate mitigates high selenium damage of rice via altering antioxidant capacity, selenium transportation and gene expression

Beneficial effects of methyl jasmonate (MeJA) on plants under different abiotic conditions have long been demonstrated. This study aimed to figure out how exogenous MeJA mitigated high-Se toxicity in rice from plant physiology and gene express perspective to provide the theory and technique for safe production of Se-rich rice. The results showed that low concentrations of MeJA at 0.1-1.0 μM inhibited high-Se induced nonreversible toxicity by enhancing antioxidant-system and reducing H₂O₂ and MDA content in rice seedlings. In comparison with control, addition of low concentrations of MeJA at 0.1-1.0 μM reduced the Se content in roots by 13.6-48.8% and in shoots by 52.6-59.9%. Besides, lower concentrations of MeJA decreased the Se(IV) transformation to SeCys and SeMet. The qRT-PCR analysis showed that application of low concentration of MeJA down-regulated the gene expression of OsNIP2;1, and OsPT2 in roots and OsNIP2;1, OsPT2, OsSBP1, and OsCS in shoots, which inhibited Se absorption. However, high concentrations of MeJA at 2.5-5.0 μM decreased antioxidant capacity and increased H₂O₂ and MDA content in rice seedlings. The results suggested that MeJA at 0.1-1.0 μM can be used to mitigate high-Se toxicity in rice production. This research augments the knowledge for future utilization of MeJA in down-regulating Se levels in crops.

The fate of arsenic in rice plants (Oryza sativa L.): Influence of different forms of selenium

As contamination of rice plants has aroused worldwide concern because of the threats posed to human health through its accumulation in the food chain. However, no data are currently available on the effect of Se nanoparticles (SeNPs) on the fate of As in higher plants, and previously reported relationships between As and Se are inconsistent. Therefore, in this study, the possible mediating roles of SeNPs or selenite on the uptake, translocation, subcellular distribution, and transformation of arsenite and arsenate in rice seedlings (Oryza sativa L.) were investigated through hydroponic experiments. The results showed that, when supplied as arsenite and arsenate, selenite significantly increased root As uptake by 71.7% and 45.9% but decreased shoot As content by 48.9% and 52.4%, respectively. In comparison, the reducing effect of SeNPs on shoot As content (37.1%) was only significant in arsenite-treated rice plants. Furthermore, selenite significantly reduced and increased the As content of different shoot and root subcellular fractions, respectively; and SeNPs also led to a dramatic decrease in the As content of the different shoot subcellular fractions of arsenite-treated rice plants. Moreover, As(III) and As(V) content was reduced in rice shoots while enhanced in rice roots by selenite. Generally, neither As(III) nor As(V) content in rice tissues was dramatically changed by SeNPs. Our results indicate that both SeNPs and selenite are effective in mitigating As toxicity in rice plants, although selenite showed a stronger inhibiting effect on As translocation.

Effects of sulfur application on selenium uptake and seed selenium speciation in soybean (Glycine max L.) grown in different soil types

AIMS

The objective of the present study was to elucidate the effects of sulfur (S) application on selenium (Se) uptake and seed Se speciation in high-protein soybean (Glycine max L.) grown in different soil types.

METHODS

Pot experiments were conducted with soybean plants grown in yellow-brown soil (pH 5.68) and in calcareous alluvial soil (pH 7.87). Sodium selenate (Na2SeO4, 2 mg kg-1) was applied to soil with or without S fertilizer (S, 100 mg kg-1).

RESULTS

Soybean grain yield and total biomass in calcareous alluvial soil were both approximately 1.3-fold the levels in yellow-brown soil. Following Se application, seed Se concentration in calcareous alluvial soil was 3.2-fold the concentration in yellow-brown soil, although additional S application reduced the corresponding seed Se concentrations by 55.6% and 38.6%, respectively. Generally, Se application facilitated Se translocation and enrichment in soybean seeds. Organic Se accounted for 92% of seed total Se and Se-methionine (>90%) was always the major Se species. Available Se (soluble and exchangeable fractions) accounted for 50.7% (yellow-brown soil) and 70.1% (calcareous alluvial soil) of soil total Se under Se treatment, while additional S application decreased the corresponding proportion of soluble Se by 12.6% and 14.4%.

CONCLUSIONS

The bioavailability of selenate in calcareous alluvial soil was higher than the bioavailability in yellow-brown soil and was more negatively affected by S application. Although S application inhibited Se uptake in soybean plants in both soil types, it did not influence seed Se speciation and Se-methionine was the major Se species.

Application of Natural Deep Eutectic Solvents for the metal nanoparticles extraction from plant tissue

The study aimed to use Natural Deep Eutectic Solvents (NADES) as an extractant of metal oxide NPs from plant material. The plant chosen for the study was radish after exposure, growing on media containing: copper(II) oxide, cerium(IV) oxide, and titanium(IV) oxide. The first step of the study was to investigate the influence of NADES on NPs. In the second step, selected NADES solvents were used as extractants of NPs present in radish after exposure. Single-particle Inductively Coupled Plasma Mass Spectrometry technique (SP-ICP-MS) was used to determine the number and size of NPs. As a result of the research, it was found that copper(II) oxide NPs, regardless of the solvent used, is not present in the extract. Only the ionic form of the element was present in the solution. Higher sized cerium(IV) oxide NPs were accumulated in the root, while smaller sized ones were found in radish leaves. The titanium(IV) oxide NPs were agglomerated and were present in a small amount in radish leaves, accumulating mainly in the root. Finally, it can be concluded that NADES allows the extraction of metal oxide NPs from the plant material.

Underlying mechanisms responsible for restriction of uptake and translocation of heavy metals (metalloids) by selenium via root application in plants

Since selenium (Se) was shown to be an essential element for humans in 1957, the biofortification of Se to crops via foliar spraying or soil fertilization has been performed for several decades to satisfy the daily nutritional need of humans. Appropriate doses of Se were found to counteract a number of abiotic and biotic stresses, such as exposure to heavy metals (metalloids) (HMs), via influencing the regulation of antioxidant systems, by stimulation of photosynthesis, by repair of damaged cell structures and functions, by regulating the metabolism of some substances and the rebalancing of essential elements in plant tissues. However, few concerns were paid on why and how Se could reduce the uptake of a variety of HMs. This review will mainly address the migration and transformation of HMs regulated by Se in the soil-plant system in order to present a hypothesis of why and how Se can reduce the uptake of HMs in plants. The following aspects will be examined in greater detail, including 1) how the soil characteristics influences the ability of Se to reduce the bioavailability of HMs in soils and their subsequent uptake by plants, which include soil Se speciation, pH, water regime, competing ions and microbes; 2) how the plant root system influenced by Se affects the uptake or the sequestration of HMs, such as root morphology, root iron plaques and root cell wall; 3) how Se combines with HMs and then sequesters them in plant cells; 4) how Se competes with arsenic (As) and thereby reduces As uptake in plants; 5) how Se regulates the expression of genes encoding functions involved in uptake, translocation and sequestration of HMs by Se in plants.

Selenium-Containing Proteins/Peptides from Plants: A Review on the Structures and Functions

Selenium is an essential microelement required for biological processes. Traditional selenium supplements (selenite and selenomethionine mainly) remain concerns due to toxicity and bioavailability. In recent decades, biofortification strategies have been applied to produce selenium-enriched edible plants to address the challenges of superior nutritional quality requirements. Plant-derived selenium-containing proteins/peptides offer potential health benefits beyond the basic nutritional requirements of Se. Highly nucleophilic seleno-amino acids, special peptide sequences, and favorable bioavailability contribute to the biological activities of selenium-containing proteins/peptides, such as antioxidant, antihypertensive, anti-inflammatory, and immunomodulatory effects. However, their applications on a commercial scale are insufficient owing to the complexity of purification and identification techniques and the sparse information on bioavailability and metabolism. In this review, selenium status, structural features, bioactivities, structure-activity relationships, and bioavailability, as well as the mechanisms underlying the bioactivities and metabolism of plant-derived selenium-containing proteins/peptides, are summarized and discussed for their nutraceutical use.

Combined use of arbuscular mycorrhizal fungus and selenium fertilizer shapes microbial community structure and enhances organic selenium accumulation in rice grain

Selenium (Se) deficiency is a public health concern that is mainly caused by inadequate intake of Se from staple crops. The purpose of this study is to investigate the effects of inoculation with different arbuscular mycorrhizal fungus (AMF) strains, including Funneliformis mosseae (Fm) and Glomus versiforme (Gv), and fertilization with selenite or selenate on the accumulation and speciation of Se in rice. The results showed that using both AMF inoculation and Se fertilization could promote organic Se accumulation in rice grain than using only Se fertilization. Moreover, grain of rice inoculated with Fm and grown in soil fertilized with selenate had the highest accumulation of Se, of which selenomethionine was the dominant Se species. The AMF inoculation also led to high content of available Se and high relative abundance of Firmicutes in soil. The high concentration of available Se in soil suggests that the AMF inoculation may modify the microbial community, which then causes the Se uptake of rice to increase, in turn causing the amount of organic Se accumulated in rice to increase. Based on these results, using AMF inoculation combined with Se fertilization can be a promising strategy for Se biofortification in rice.

1Selenium supply alters the subcellular distribution and chemical forms of cadmium and the expression of transporter genes involved in cadmium uptake and translocation in winter wheat (Triticum aestivum)

BACKGROUND

Cadmium (Cd) accumulation in crops affects the yield and quality of crops and harms human health. The application of selenium (Se) can reduce the absorption and transport of Cd in winter wheat.

RESULTS

The results showed that increasing Se supply significantly decreased Cd concentration and accumulation in the shoot and root of winter wheat and the root-to-shoot translocation of Cd. Se application increased the root length, surface area and root volume but decreased the average root diameter. Increasing Se supply significantly decreased Cd concentration in the cell wall, soluble fraction and cell organelles in root and shoot. An increase in Se supply inhibited Cd distribution in the organelles of shoot and root but enhanced Cd distribution in the soluble fraction of shoot and the cell wall of root. The Se supply also decreased the proportion of active Cd (ethanol-extractable (FE) Cd and deionized water-extractable (FW) Cd) in root. In addition, the expression of TaNramp5-a, TaNramp5-b, TaHMA3-a, TaHMA3-b and TaHMA2 significantly increased with increasing Cd concentration in root, and the expression of TaNramp5-a, TaNramp5-b and TaHMA2 in root was downregulated by increasing Se supply, regardless of Se supply or Cd stress. The expression of TaHMA3-b in root was significantly downregulated by 10 μM Se at both the 5 μM and 25 μM Cd level but upregulated by 5 μM Se at the 25 μM Cd level. The expression of TaNramp5-a, TaNramp5-b, TaHMA3-a, TaHMA3-b and TaHMA2 in shoot was downregulated by increasing Se supply at 5 μM Cd level, and 5 μM Se upregulated the expression of those genes in shoot at 25 μM Cd level.

CONCLUSIONS

The results confirm that Se application limits Cd accumulation in wheat by regulating the subcellular distribution and chemical forms of Cd in winter wheat tissues, as well as the expression of TaNramp5-a, TaNramp5-b and TaHMA2 in root.