Comparing the influence of selenite (Se4+) and selenate (Se6+) on the inhibition of the mercury (Hg) phytotoxicity to pak choi

Efficient and synergistic treatment of selenium (IV)-contaminated wastewater and mercury (II)-contaminated soil by anaerobic granular sludge: Performance and mechanisms

Wastewater containing selenium (Se) and soil contaminated by mercury (Hg) are two environmental problems, but they are rarely considered for synergistic treatment. In this work, anaerobic granular sludge (AnGS) was used to address both of the aforementioned issues simultaneously. The performance and mechanisms of Se(IV) removal from wastewater and Hg(II) immobilization in soil were investigated using various technologies. The results of the reactor operation indicated that the AnGS efficiently removed Se from wastewater, with a removal rate of 99.94 ± 0.05%. The microbial communities in the AnGS could rapidly reduce Se(IV) to Se0 nanoparticles (SeNPs). However, the AnGS lost the ability to reduce Se(IV) once the Se0 content reached the saturation value of 5.68 g Se/L. The excess sludge of Se0-rich AnGS was applied to remediate soil contaminated with Hg(II). The Se0-rich AnGS largely decreased the percentage of soil Hg in the mobile, extractable phase, with up to 99.1 ± 0.3% immobilization. Soil Hg(II) and Hg0 can react with Se (-II) and Se0, respectively, to form HgSe. The formation of inert HgSe was an important pathway for immobilizing Hg. Subsequently, the pot experiments indicated that soil remediation using Se0-rich AnGS significantly decreased the Hg content in pea plants. Especially, the content of Hg decreased from 555 ± 100 to 24 ± 3 μg/kg in roots after remediation. In summary, AnGS is an efficient and cost-effective material for synergistically treating Se-contaminated wastewater and Hg-contaminated soil.

Selenium-molybdenum interactions reduce chromium toxicity in Nicotiana tabacum L. by promoting chromium chelation on the cell wall

Chromium (Cr) is a hazardous heavy metal that negatively affects animals and plants. The micronutrients selenium (Se) and molybdenum (Mo) have been widely shown to alleviate heavy metal toxicity in plants. However, the molecular mechanism of Cr chelation on the cell wall by combined treatment with Se and Mo has not been reported. Therefore, this study aimed to explore the effects of Se-Mo interactions on the subcellular distribution of Cr (50 µM) and on cell wall composition, structure, functional groups and Cr content, in addition to performing a comprehensive analysis of the transcriptome. Our results showed that the cell walls of shoots and roots accumulated 51.0% and 65.0% of the Cr, respectively. Furthermore, pectin in the cell wall bound 69.5%/90.2% of the Cr in the shoots/roots. Se-Mo interactions upregulated the expression levels of related genes encoding galacturonosyltransferase (GAUT), UTP-glucose-1-phosphate uridylyltransferase (UGP), and UDP-glucose-4-epimerase (GALE), involved in polysaccharide biosynthesis, thereby increasing pectin and cellulose levels. Moreover, combined treatment with Se and Mo increased the lignin content and cell wall thickness by upregulating the expression levels of genes encoding cinnamyl alcohol dehydrogenase (CAD), peroxidase (POX) and phenylalanine amino-lyase (PAL), involved in lignin biosynthesis. Fourier-transform infrared (FTIR) spectroscopy results showed that Se + Mo treatment (in combination) increased the number of carboxylic acid groups (-COOH) groups, thereby enhancing the Cr chelation ability. The results not only elucidate the molecular mechanism of action of Se-Mo interactions in mitigating Cr toxicity but also provide new insights for phytoremediation and food safety.

The alleviating effects and underlying mechanisms of exogenous selenium on both Sb(III) and Sb(V) toxicity in rice seedlings (Oryza sativa L.)

Selenium (Se) has been used to detoxify various heavy metals in plants. However, the effects and underlying mechanisms of exogenous Se application on the toxicity of antimonite [Sb(III)] and antimonate [Sb(V)] in crops are still poorly understood. Therefore, the potential alleviating roles of Se on the plant growth, antioxidant system, uptake and subcellular distribution of Sb, and expression of Sb-related genes were comprehensively investigated in rice seedlings (Oryza sativa L.) under both Sb(III) and Sb(V) stress conditions. The results showed that high concentrations of Sb(III) (100 µM) and Sb(V) (300 µM) caused a significant decrease in plant growth parameters, photosynthetic pigments and relative water content in rice seedlings. In contrast, the addition of Se (20 or 2 µM) improved rice growth, decreased Sb accumulation, and reduced oxidative stress in rice seedlings when exposed to 100 µM Sb(III) and 300 µM Sb(V), respectively. Furthermore, Se application could effectively improve the physiological adaptability of rice seedlings under Sb(III) and Sb(V) stress by regulating enzymatic and non-enzymatic antioxidant systems, Sb subcellular distribution and transcription levels of Sb-related genes, including in antioxidant response (OsCuZnSOD2, OsCATA and OsGSH1), detoxification (OsPCS1, OsPCS2 and OsABCC1) and Sb transport and sequestration (OsLsi1 and OsWAK11). Moreover, we also discovered that the mitigation effect of Se was dose-dependent and depended on Sb valence states. Thus, these findings contribute to our understanding of the mechanisms underlying Se-Sb antagonism in rice, offering a potentially useful method for producing both safe and Se-rich crops.

A quantitative review of the effects of Se application on the reduction of Hg concentration in plant: a meta-analysis

Mercury (Hg) is a highly toxic heavy metal entering the human body through the food chain after absorption by plant. Exogenous selenium (Se) has been suggested as a potential solution to reduce Hg concentration in plants. However, the literature does not provide a consistent picture of the performance of Se on the accumulation of Hg in plant. To obtain a more conclusive answer on the interactions of Se and Hg, 1,193 data records were collected from 38 publications for this meta-analysis, and we tested the effects of different factors on Hg accumulation by meta-subgroup analysis and meta-regression model. The results highlighted a significant dose-dependent effect of Se/Hg molar ratio on the reduction of Hg concentration in plants, and the optimum condition for inhibiting Hg accumulation in plants is at a Se/Hg ratio of 1-3. Exogenous Se significantly reduced Hg concentrations in the overall plant species, rice grains, and non-rice species by 24.22%, 25.26%, and 28.04%, respectively. Both Se(IV) and Se(VI) significantly reduced Hg accumulation in plants, but Se(VI) had a stronger inhibiting effect than Se(IV). Se significantly decreased the BAF_Grain in rice, which indicated that other physiological processes in rice may be involved in restricting uptake from soil to rice grain. Therefore, Se can effectively reduce Hg accumulation in rice grain, which provides a strategy for effectively alleviating the transfer of Hg to the human body through the food chain.

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

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

Selenium/sulfur-modified montmorillonite materials mitigate mercury pollution in farmland

Selenium (Se) amendment could reduce mercury (Hg) bioaccumulation in crops, but sometimes it could cause excessive Se accumulation in crops and potential Se exposure risks for humans. In this study, we designed and synthesized selenium and sulfur-modified montmorillonite materials (Se/S-Mont) to effectively reduce mercury levels and avoid excessive Se enrichment in plants. The results of pot experiments (1 g Se/S-Mont/100 g soil) and field microplot trials (0.3 g Se/S-Mont/100 g soil, 8 t/ha) showed that Se/S-Mont amendments significantly reduced the Hg concentrations in water spinach and hybrid Pennisetum by 28-68% and 57%-92% (P < 0.05), respectively, while they did not lead to excessive Se bioaccumulation in the plants. Se/S-Mont was more efficient in mitigating soil Hg pollution than adding raw materials (e.g., NaSeO₃) and their combinations, and they significantly reduced the available Se fraction in the soil and the Se levels in the plants (P < 0.05). The potential mechanisms revealed by X-ray absorption near-edge spectra (XANES) and pot experiments were the adsorption and slow release of Hg, S, and Se by Se/S-Mont, the high affinity between Hg and Se, competition between Se and S, and the formation of stable complexes containing Se-S-Hg. The Se/S-Mont immobilizer was easy to prepare and required the application of small amounts, and the remediation effect was relatively stable and exhibited few negative effects; therefore, the approach showed high environmental and economic potentials.

Selenium Application Enhances the Accumulation of Flavones and Anthocyanins in Bread Wheat (Triticum aestivum L.) Grains

Selenium (Se) biofortification in wheat reduces the risk of Se deficiency in humans. Se biofortification increases the concentration of Se and anthocyanins in wheat grains. However, it is unknown whether Se biofortification can enhance flavonoids other than anthocyanins and the mechanism underlying flavonoid accumulation in wheat grains. Here, foliar application of selenite solution in wheat was conducted 10 days after flowering. Metabolite profiling and transcriptome sequencing were performed in Se-treated grains. A significant increase in the total contents of Se, anthocyanins, and flavonoids was observed in Se-treated mature grains. Twenty-seven significantly increased flavonoids were identified in Se-treated immature grains. The significant accumulation of flavones (tricin, tricin derivatives, and chrysoeriol derivatives) was detected, and six anthocyanins, dihydroquercetin (the precursor for anthocyanin biosynthesis) and catechins were also increased. Integrated analysis of metabolites and transcriptome revealed that Se application enhanced the biosynthesis of flavones, dihydroquercetin, anthocyanins, and catechins by increasing the expression levels of seven key structural genes in flavonoid biosynthesis (two TaF3Hs, two TaDFRs, one TaF3'5'H, one TaOMT, and one TaANR). Our findings shed new light on the molecular mechanism underlying the enhancement in flavonoid accumulation by Se supplementation and pave the way for further enhancing the nutritional value of wheat grains.

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.

Influence of arbuscular mycorrhizal fungi on selenium uptake by winter wheat depends on the level of selenate spiked in soil

Selenium (Se) deficiency has been a public health concern for years. Arbuscular mycorrhizal fungi (AMF) play an essential role in improving Se uptake in crops, but related mechanisms still remain unclear. To explore the influence of AMF on uptake of Se in winter wheat, a pot experiment was conducted to inoculate wheat with Funneliformis mosseae (F.m) or not under different levels of selenate in soil. The present results indicated that inoculation of F.m significantly (p < 0.05) increased Se concentration in shoots and roots of wheat under low level of selenate (≤5.0 mg kg^-1) treatments, while the contrary pattern was recorded under high level of selenate (15 and 20 mg kg^-1) treatments. Moreover, inoculation of F.m significantly increased concentration of available Se in soil by 4.68-34.05%. Under selenate ≤5 mg kg^-1 treatments, the expression of TaeSultr1;1 and TaeSultr1;3 in roots of mycorrhizal wheat was significantly up-regulated by 3.06-5.53 and 0.63-5.12 times, while reached saturation under selenate >5 mg kg^-1 treatments. In addition, partial least squares path modeling (PLS-PM) showed that inoculation of AMF directly affected the expression of sulfate transporter and that both sulfate transporter and soil Se fractions played a significant positive effect on plant Se content. The present study indicated that AMF on Se concentration in winter wheat depends on the level of selenate spiked in soil and added to our understanding of the functions and applications of AMF on crop Se absorption.

Detoxification difference of cadmium between the application of selenate and selenite in native cadmium-contaminated soil

Cadmium (Cd) has strong mobility and could cause toxicity to plants, and selenium (Se) can effectively detoxify Cd stress. However, differences in the detoxification effects of different species and dosages of exogenous Se on Cd and its mechanism are still unclear. In this study, a pot experiment was conducted to determine the effects of different rates of selenite and selenate application on radish growth, the uptake and translocation of Cd, and the fractions of Cd transformation in native Cd-contaminated soil. Results indicated that the decrease in radish biomass in selenate treatment was significantly greater than that in selenite treatment at a high Se application rate (2.5 mg·kg^-1) (p < 0.05). In contrast to selenite treatments, the application of selenate significantly increased the translocation of Cd from radish roots to shoots (p < 0.05). Cadmium concentration and its bioaccumulation factor in radish decreased gradually with increasing selenite application rates, while these values decreased at low Se rate (1 mg·kg^-1) and increased at high Se rate for selenate treatment. Different Se application rates resulted in Cd fractions distributions to change in soil. Therefore, the application of selenite treatment had a greater detoxification effect on Cd in soil than that in selenate treatment, and the double toxic effect was observed between Se and Cd in high selenate treatment (2.5 mg·kg^-1). Combined with human health risk asseeement, the application of 2.5 mg·kg^-1 selenite could be a good approach for detoxification in native Cd-contaminated soil used in this study.

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.

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.

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 Biofortification and Interaction With Other Elements in Plants: A Review

Selenium (Se) is an essential element for humans and animals and its deficiency in the diet is a global problem. Crop plants are the main source of Se for consumers. Therefore, there is much interest in understanding the factors that govern the accumulation and distribution of Se in the tissues of crop plants and the mechanisms of interaction of Se absorption and accumulation with other elements, especially with a view toward optimizing Se biofortification. An ideal crop for human consumption is rich in essential nutrient elements such as Se, while showing reduced accumulation of toxic elements in its edible parts. This review focuses on (a) summarizing the nutritional functions of Se and the current understanding of Se uptake by plant roots, translocation of Se from roots to shoots, and accumulation of Se in grains; and (b) discussing the influence of nitrogen (N), phosphorus (P), and sulfur (S) on the biofortification of Se. In addition, we discuss interactions of Se with major toxicant metals (Hg, As, and Cd) frequently present in soil. We highlight key challenges in the quest to improve Se biofortification, with a focus on both agronomic practice and human health.

Effects of selenium combined with zinc amendment on zinc fractions and bioavailability in calcareous soil

Selenium (Se) and zinc (Zn) are two important trace elements for human being and animals. The interaction between Se and Zn on the bioavailability of Zn in soil is still unclear. Therefore, pot experiments exposed to different dosages of zinc sulfate (ZnSO₄) (0, 20, and 50 mg/kg soil) and sodium selenite (Na₂SeO₃) (0, 0.5, 1.0, and 2.5 mg/kg soil) were conducted to investigate the effects of selenite application on Zn bioavailability in calcareous soil and its related mechanisms. The total Zn content of different tissues (roots and shoots) of pak choi (Brassica chinensis L.) and the changes in Zn fraction distribution in soil before planting and after harvest were determined, and the mobility factor (M_F) and distribution index (D_I) of Zn in soils were calculated. In addition, the Pearson correlation and path analysis were conducted to clarify the relationships between Zn fractions in soil and the Zn uptake of pak choi. Results showed that Se amendment elevated soil Zn bioavailability at appropriate levels of Se and Zn. When 1.0 and 2.5 mg/kg of Se and 20 mg/kg of Zn were applied in soil, the proportion of exchangeable Zn (Ex-Zn) and Zn weakly bound to organic matter (Wbo-Zn) to the total content of Zn was significantly increased by 28.14%-82.52% compared with that of the corresponding single Zn treatment. Therefore, the Zn concentration in the shoots of pak choi was significantly increased by 27.2%-31.1%. High Zn (50 mg/kg) and Se co-amended treatments showed no significantly beneficial effect on the bioavailability of Zn. In addition, the potential available Zn content in soil (weakly bound to organic matter and carbonate bound Zn) and M_F and D_I values were all positively correlated with the Zn concentrations in pak choi, indicating that these indexes can be used to predict the bioavailability of Zn in soil. This study can provide a good reference for Se and Zn biofortification of plants in calcareous soil.

Mitigation of rice cadmium (Cd) accumulation by joint application of organic amendments and selenium (Se) in high-Cd-contaminated soils

A pot experiment was conducted to investigate the possible mediatory effect of organic amendments (vermicompost and biochar) and selenium (Se) on Cd bioaccumulation in both rice cultivars (high-Cd accumulation rice: Yuzhenxiang (YZX) and low-Cd accumulation rice: Changliangyou772 (CLY)) in high-Cd-contaminated soils. The results showed that Cd sensitivity and tolerance were cultivar-dependent, and grain Cd contents for CLY accorded with the Chinese national food safety standards (0.2 mg kg^-1), whereas grain Cd levels for YZX were 1.4-5.8 times higher than those for CLY. Soil applications of amendments decreased grain Cd levels by 3.5%-36.9% for YZX and 36.1%-74.4% for CLY. Moreover, vermicompost (VC) was more effective in reducing Cd bioaccumulation than biochar (BC). A combination of Se and organic amendments could significantly increase grain Se contents and help further reduce grain Cd levels by 5.8%-20.8%, compared to the single organic amendments. This mitigation progress could be attributed to the changes of Cd translocation and distribution among rice tissues and the inhibition of Cd bioavailability in soil through the alteration in soil properties. Organic amendments, especially high dose (5%), increased soil pH and organic matter contents, and correspondingly decreased soil Cd bioavailability. A sequential extraction analysis suggested that organic amendments and Se facilitated the transformation of soil Cd from the bioavailable form to the immobilized Cd form, and thus decreased grain Cd levels. Hence, co-applications of organic amendments and Se in combination with low-Cd accumulation cultivar could be an effective strategy for both Se needs of humans and safe utilization of Cd polluted soil.

Selenium mitigates the chromium toxicity in Brassicca napus L. by ameliorating nutrients uptake, amino acids metabolism and antioxidant defense system

The phytotoxicity of chromium (Cr) makes it obligatory for the researchers to develop strategies that seek to hinder its accumulation in food chains. While, protective role of selenium (Se) has not been discussed in detail under adverse conditions in oilseed rape. Here, our aim was to investigate the potential use of Se (0, 5 and 10 μM) in alleviating the Cr toxicity (0, 100 and 200 μM) in Brassica napus L. Results delineated that Se-supplementation notably recovered the Cr-phytotoxicity by reducing the Cr accumulation in plant tissues and boosted the inhibition in plant growth and biomass. Under Cr stress, the exogenously applied Se significantly recovered the impairment in photosynthesis related parameters (chlorophyll a, chlorophyll b, carotenoids, net photosynthetic rate, stomatal conductance, and photochemical efficiency of photosystem II), and counteracted the reduction in nutrients uptake and improved the essential amino acids (EAAs) levels. In addition, Se activated the antioxidants enzymes included in AsA-GSH cycle (SOD, CAT, APX, GR, DHAR, MDHAR, GSH, and AsA) and glyoxalase (Gly) system (Gly I and Gly II) and minimized the excessive generation of reactive oxygen species (ROS) and methylglyoxal (MG) contents in response to Cr stress. In a nutshell, Se (more effective at 5 μM) alleviated the Cr and MG induced phytotoxicity and oxidative damages by minimizing their (Cr and MG) accumulation and enhanced the plant growth, nutrients element level, nutrition quality by improving EAAs, antioxidant and Gly system. By considering the above-mentioned biomarkers, the addition of exogenous Se in Cr polluted soils might be effective approach to decrease the Cr uptake and its linked phytotoxicity in B. napus.

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.

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.

Detoxification of mercury in soil by selenite and related mechanisms

A better understanding of the benefits of selenium (Se) fertilization to alleviate the toxicity of mercury (Hg) on plants and of the underlying mechanisms involved in Hg stress is important for the remediation of soils contaminated by Hg. This study is aimed to explore the effects of the application of selenite to alleviate the toxicity of Hg in soils to plants and related mechanisms involved in this process. The chemical (Hg uptake of pak choi), biological (root and shoot length, root and shoot weight) and physiological effects (antioxidant enzyme activities, non-enzymatic antioxidant contents (proline) and lipid peroxidation products (malondialdehyde)) produced over plants by the application of different doses of Hg and Se to soil has been investigated through a pot experiment, which was conducted with exposure to different dosages of mercuric chloride (0, 1.0, 2.0, and 3.0 mg/kg soil) and sodium selenite (0, 0.5, 1.0, and 2.5 mg/kg soil). Results indicated that single high Hg treatment (3.0 mg/kg Hg) resulted in significantly increase in Hg uptake by plants (P < 0.01), thus the growth of pak choi was inhibited. However, the Se application at 1.0 and 2.5 mg/kg led to significantly alleviated Hg uptake by plants (P < 0.05). Meanwhile, the low Se (at 0.5 and 1.0 mg/kg) applied to soil induced significantly improvement the growth of pak choi (P < 0.05) by elevating the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione peroxidase (GSH-Px) enzymes and the content of chlorophyll (SPAD value) as well as suppressed the lipid peroxidation products contents (MDA) and proline. Results collectively indicated that applied Se played an important role in promoting the detoxification of Hg and growth of pak choi under oxidative stress. Notably, this role may only be significant when Se application at the appropriate concentration (≤ 1.0 mg/kg).

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

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