Involvement of silicon influx transporter OsNIP2;1 in selenite uptake in rice

Cooperation of selenium, iron and phosphorus for simultaneously minimizing cadmium and arsenic concentrations in rice grains

Cadmium (Cd) and arsenic (As), two toxic elements to humans, are ubiquitously coexisting contaminant found in paddy fields. The accumulation of Cd and As in rice, a major food source for many people around the world, can pose a serious threat to food safety and human health. Therefore, it is crucial to be aware of these contaminants and take adequate measures to reduce the accumulation of these two elements in rice. Developing an effective method to simultaneously reduce the accumulation of Cd) and As in rice is challenging. In this study, a pot experiment was conducted to investigate the synergistic effects of selenium (Se), iron (Fe) and phosphorus (P) on the uptake, transport and accumulation of cadmium and arsenic in rice by analyzing the physical and chemical properties of the soil, the elemental concentrations and their interrelationships in the rice tissues, and the composition and morphology of the iron plaque (IP). The results showed that the combined treatments of Se, Fe and P had positive effects on reducing Cd and As accumulation in rice, reducing Cd concentrations in brown rice by 3.86-51.88 % and As concentrations by 25.37-40.81 %. The possible mechanisms for the reduction of As and Cd concentrations in rice grains were: (i) Combined application of Fe, P and Se can effectively reduce the soil available Cd and As concentration. (ii) Combined application significantly improved the formation of IP at the tillering stage and increased the crystalline iron oxides in IP, promoting the deposition of SiO2 in rice roots, thereby effectively inhibiting the uptake of Cd and As by rice roots. (iii) Interplay and interaction between elements facilitated by transporter proteins could contribute to the synergistic mitigation of Cd and As by Se, Fe and P. This study provides a valuable new approach for effective control of Cd and As concentration of rice grown in co-contaminated soil.

Selenium dynamics in plants: Uptake, transport, toxicity, and sustainable management strategies

Selenium (Se) plays crucial roles in human, animal, and plant physiology, but its varied plant functions remain complex and not fully understood. While Se deficiency affects over a billion people worldwide, excessive Se levels can be toxic, presenting substantial risks to ecosystem health and public safety. The delicate balance between Se's beneficial and harmful effects necessitates a deeper understanding of its speciation dynamics and how different organisms within ecosystems respond to Se. Since humans primarily consume Se through Se-rich foods, exploring Se's behavior, uptake, and transport within agroecosystems is critical to creating effective management strategies. Traditional physicochemical methods for Se remediation are often expensive and potentially harmful to the environment, pushing the need for more sustainable solutions. In recent years, phytotechnologies have gained traction as a promising approach to Se management by harnessing plants' natural abilities to absorb, accumulate, metabolize, and volatilize Se. These strategies range from boosting Se uptake and tolerance in plants to releasing Se as less toxic volatile compounds or utilizing it as a biofortified supplement, opening up diverse possibilities for managing Se, offering sustainable pathways to improve crop nutritional quality, and protecting human health in different environmental contexts. However, closing the gaps in our understanding of Se dynamics within agricultural systems calls for a united front of interdisciplinary collaboration from biology to environmental science, agriculture, and public health, which has a crucial role to play. Phytotechnologies offer a sustainable bridge between Se deficiency and toxicity, but further research is needed to optimize these methods and explore their potential in various agricultural and environmental settings. By shedding light on Se's multifaceted roles and refining management strategies, this review contributes to developing cost-effective and eco-friendly approaches for Se management in agroecosystems. It aims to lead the way toward a healthier and more sustainable future by balancing the need to address Se deficiency and mitigate the risks of Se toxicity.

Selenate simultaneously alleviated cadmium and arsenic accumulation in rice (Oryza sativa L.) via regulating transport genes

Cadmium (Cd) and arsenic (As) have contrasting biogeochemical behaviors in paddy soil, which posed an obstacle for reducing their accumulation in rice (Oryza sativa L.) simultaneously. In this study, selenate exhibited a more effective ability than selenite on simultaneous alleviation of Cd and As accumulation in rice under Cd-As co-exposure, and the mechanisms need to be further investigated. The results showed that selenate significantly decreased the root Cd and As contents by 59%-83% and 43%-72% compared to Cd-As compound exposure, respectively. Correspondingly, it significantly down-regulated the expression of uptake-related genes OsNramp5 (87.1%) and OsLsi1 (95.5%) in rice roots. Decreases in Cd (64.5%) and As (16.2%) contents in shoots were also found after selenate addition. Moreover, selenate may promoted the reduction of As(V) to As(Ⅲ) and As(III) efflux to the external medium, resulting in decreased As accumulation and As(Ⅲ) proportion in rice shoots and roots. In addition, selenate could promote the binding of Cd (by 14%-24%) and As (by 9%-15%) in the cell wall, and significantly reduced the oxidative stress by elevating levels of antioxidant enzymes (by 10%-105%) and thiol compounds (by 6%-210%). Additionally, selenate significantly down-regulated the expression of OsNramp1 (49.3%) and OsLsi2 (82.1%) associated with Cd and As transport in rice. These findings suggest selenate has the potential to be an effective material for the simultaneous reduction of Cd and As accumulation in rice under Cd-As co-contamination.

The interaction between selenium and other elements in soil and rice roots shaped by straw and straw biochar regulated the enrichment of selenium in rice grain

The absorption and transport of selenium (Se) in rice depend on the shared transport proteins and channels with other elements. However, the interactions between Se and other elements within the soil-rice system and their relationship with Se-enriched rice are still not well understood. Hence, we conducted pot experiments to investigate the transformation of Se forms in soil and the absorption and enrichment of Se in rice, which varied with other elements influenced by straw and straw biochar returning in Se-rich red paddy soil. Partial least squares path modeling (PLS-PM) analysis was carried out to reveal the interaction between Se and other elements and the crucial processes in Se enrichment in rice grains. The results showed that the incorporation of straw and straw biochar into the fields increased the content of soil-soluble Se (SOL-Se) but significantly decreased the content of iron-manganese oxide-bound Se (FMO-Se) and organic matter-bound Se (OM-Se). Moreover, the rise in the soil-bioavailable Se was mainly attributed to the activation of FMO-Se and OM-Se. Compared with the NPK treatment, the contents of Se in rice grain were increased by 69.22% and 38.09%, under straw and biochar returning, respectively. However, the contents of Se in the leaves decreased. Variation partitioning analysis (VPA) indicated that the migration of Se in rice plants was significantly influenced by differences in rice tissues and their interactions with other nutrients [nitrogen (N), phosphorus (P), potassium (K), and Se], explaining 51.5% and 35.3% of the variations in Se content in different rice parts, respectively. The PLS-PM analysis demonstrated that the absorption of Se by rice roots and its transportation from the leaves to grains were crucial processes affecting Se enrichment in rice. However, these processes were modulated by the interaction between soil properties and root nutrients (N, P, and Se) induced by straw and straw biochar incorporation. The present study provides further understanding of the main factors and key processes in regulating Se absorption and transformation in the soil-rice system to more efficiently utilize Se-rich paddy fields through agricultural management measures.

Metabolic engineering of Escherichia coli for seleno-methylselenocysteine production

Selenium (Se) is an essential trace element for life. Seleno-methylselenocysteine (SeMCys) can serve as a Se supplement with anticarcinogenic activity and can improve cognitive deficits. We engineered Escherichia coli for microbial production of SeMCys. The genes involved in the synthesis of SeMCys were divided into three modules-the SeCys synthesis, methyl donor synthesis and SMT modules-and expressed in plasmids with different copy numbers. The higher copy number of the SeCys synthesis module facilitated SeMCys production. The major routes for SeCys degradation were then modified. Deletion of the cysteine desulfurase gene csdA or sufS improved SeMCys production the most, and the strain that knocked out both genes doubled SeMCys production. The addition of serine in the mid-logarithmic growth phase significantly improved SeMCys synthesis. When the serine synthetic pathway was enhanced, SeMCys production increased by 12.5%. Fed-batch culture for sodium selenite supplementation in the early stationary phase improved SeMCys production to 3.715mg/L. This is the first report of the metabolic engineering of E. coli for the production of SeMCys and provide information on Se metabolism.

Effect of inorganic arsenic in paddy soil on the migration and transformation of selenium species in rice plants

Selenium (Se) in paddy rice is one of the significant sources of human Se nutrition. However, the effect of arsenic (As) pollution in soil on the translocation of Se species in rice plants is unclear. In this research, a pot experiment was designed to examine the effect of the addition of 50 mg As/kg soil as arsenite or arsenate on the migration of Se species from soil to indica Minghui 63 and Luyoumingzhan. The results showed that the antagonism between inorganic As and Se was closely related to the rice cultivar and Se oxidation state in soil. Relative to the standalone selenate treatment, arsenite significantly (p < 0.05) decreased the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, sheaths, leaves, brans and kernels of both cultivars by 21.4%-100.0%, 40.0%-100.0%, 41.0%-100%, 5.4%-96.3%, 11.3%-100.0% and 26.2%-39.7% respectively, except for selenocystine in the kernels of indica Minghui 63 and selenomethionine in the leaves of indica Minghui 63 and the stems of indica Luyoumingzhan. Arsenate also decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, brans and kernels of both cultivars by 34.9%-100.0%, 30.2%-100.0%, 11.3%-100.0% and 5.6%-39.6% respectively, except for selenate in the stems of indica Minghui 63. However, relative to the standalone selenite treatment, arsenite and arsenate decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenite only in the roots of indica Minghui 63 by 45.5%-100.0%. Our results suggested that arsenite and arsenate had better antagonism toward Se species in selenate-added soil than that in selenite-added soil; moreover, arsenite had a higher inhibiting effect on the accumulation of Se species than arsenate.

Tissue-specific deposition, speciation and transport of antimony in rice

Rice (Oryza sativa) as a staple food is a potential intake source of antimony (Sb), a toxic metalloid. However, how rice accumulates this element is still poorly understood. Here, we investigated tissue-specific deposition, speciation, and transport of Sb in rice. We found that Sb(III) is the preferential form of Sb uptake in rice, but most Sb accumulates in the roots, resulting in a very low root-to-shoot translocation (less than 2%). Analysis of Sb deposition with laser ablation-inductively coupled plasma-mass spectrometry showed that most Sb deposits at the root exodermis. Furthermore, we found that Sb is mainly present as Sb(III) in the root cell sap after uptake. Further characterization showed that Sb(III) uptake is mediated by Low silicon rice 1 (Lsi1), a Si permeable transporter. Lsi1 showed transport activity for Sb(III) rather than Sb(V) in yeast (Saccharomyces cerevisiae). Knockout of Lsi1 resulted in a significant decrease in Sb accumulation in both roots and shoots. Sb concentration in the root cell sap of two independent lsi1 mutants decreased to less than 3% of that in wild-type rice, indicating that Lsi1 is a major transporter for Sb(III) uptake. Knockout of Lsi1 also enhanced rice tolerance to Sb toxicity. However, knockout of Si efflux transporter genes, including Lsi2 and Lsi3, did not affect Sb accumulation. Taken together, our results showed that Sb(III) is taken up by Lsi1 localized at the root exodermis and is deposited at this cell layer due to lack of Sb efflux transporters in rice.

Genome-wide association study of trace elements in maize kernels

Maize (Zea mays L.), a staple food and significant economic crop, is enriched with riboflavin, micronutrients and other compounds that are beneficial for human health. As emphasis on the nutritional quality of crops increases maize research has expanded to focus on both yield and quality. This study exploreed the genetic factors influencing micronutrient levels in maize kernels through a comprehensive genome-wide association study (GWAS). We utilized a diverse panel of 244 inbred maize lines and approximately 3 million single nucleotide polymorphisms (SNPs) to investigate the accumulation of essential and trace elements including cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), selenium (Se) and zinc (Zn). Our analysis identified 842 quantitative trait loci (QTLs), with 12 QTLs shared across multiple elements and pinpointed 524 potential genes within a 100 kb radius of these QTLs. Notably ZmHMA3 has emerged as a key candidate gene previously reported to influence the Cd accumulation. We highlighted ten pivotal genes associated with trace element transport including those encoding heavy metal ATPases, MYB transcription factors, ABC transporters and other crucial proteins involved in metal handling. Additionally, haplotype analysis revealed that eight inbred linesaccumulated relatively high levels of beneficial elements while harmful elements were minimized. These findings elucidate the genetic mechanisms underlying trace element accumulation in maize kernels and provide a foundation for the breeding of nutritionally enhanced maize varieties.

Overexpression of Wheat Selenium-Binding Protein Gene TaSBP-A Enhances Plant Growth and Grain Selenium Accumulation under Spraying Sodium Selenite

Selenium (Se) is an essential trace element for humans. Low concentrations of Se can promote plant growth and development. Enhancing grain yield and crop Se content is significant, as major food crops generally have low Se content. Studies have shown that Se biofortification can significantly increase Se content in plant tissues. In this study, the genetic transformation of wheat was conducted to evaluate the agronomic traits of non-transgenic control and transgenic wheat before and after Se application. Se content, speciation, and transfer coefficients in wheat grains were detected. Molecular docking simulations and transcriptome data were utilized to explore the effects of selenium-binding protein-A TaSBP-A on wheat growth and grain Se accumulation and transport. The results showed that TaSBP-A gene overexpression significantly increased plant height (by 18.50%), number of spikelets (by 11.74%), and number of grains in a spike (by 35.66%) in wheat. Under normal growth conditions, Se content in transgenic wheat grains did not change significantly, but after applying sodium selenite, Se content in transgenic wheat grains significantly increased. Analysis of Se speciation revealed that organic forms of selenomethionine (SeMet) and selenocysteine (SeCys) predominated in both W48 and transgenic wheat grains. Moreover, TaSBP-A significantly increased the transfer coefficients of Se from solution to roots and from flag leaves to grains. Additionally, it was found that with the increase in TaSBP-A gene overexpression levels in transgenic wheat, the transfer coefficient of Se from flag leaves to grains also increased.

Transcriptome and co-expression network revealed molecular mechanism underlying selenium response of foxtail millet (Setaria italica)

Introduction

Selenium-enriched foxtail millet (Setaria italica) represents a functional cereal with significant health benefits for humans. This study endeavors to examine the impact of foliar application of sodium selenite (Na2SeO4) on foxtail millet, specifically focusing on selenium (Se) accumulation and transportation within various plant tissues.

Methods

To unravel the molecular mechanisms governing selenium accumulation and transportation in foxtail millet, we conducted a comprehensive analysis of selenium content and transcriptome responses in foxtail millet spikelets across different days (3, 5, 7, and 12) under Na2SeO4 treatment (200 μmol/L).

Results

Foxtail millet subjected to selenium fertilizer exhibited significantly elevated selenium levels in each tissue compared to the untreated control. Selenate was observed to be transported and accumulated sequentially in the leaf, stem, and spikes. Transcriptome analysis unveiled a substantial upregulation in the transcription levels of genes associated with selenium metabolism and transport, including sulfate, phosphate, and nitrate transporters, ABC transporters, antioxidants, phytohormone signaling, and transcription factors. These genes demonstrated intricate interactions, both synergistic and antagonistic, forming a complex network that regulated selenate transport mechanisms. Gene co-expression network analysis highlighted three transcription factors in the tan module and three transporters in the turquoise module that significantly correlated with selenium accumulation and transportation. Expression of sulfate transporters (SiSULTR1.2b and SiSULTR3.1a), phosphate transporter (PHT1.3), nitrate transporter 1 (NRT1.1B), glutathione S-transferase genes (GSTs), and ABC transporter (ABCC13) increased with SeO4 2- accumulation. Transcription factors MYB, WRKY, and bHLH were also identified as players in selenium accumulation.

Conclusion

This study provides preliminary insights into the mechanisms of selenium accumulation and transportation in foxtail millet. The findings hold theoretical significance for the cultivation of selenium-enriched foxtail millet.

Selenium volatilization in plants, microalgae, and microorganisms

The augmented prevalence of Se (Se) pollution can be attributed to various human activities, such as mining, coal combustion, oil extraction and refining, and agricultural irrigation. Although Se is vital for animals, humans, and microorganisms, excessive concentrations of this element can give rise to potential hazards. Consequently, numerous approaches have been devised to mitigate Se pollution, encompassing physicochemical techniques and bioremediation. The recognition of Se volatilization as a potential strategy for mitigating Se pollution in contaminated environments is underscored in this review. This study delves into the volatilization mechanisms in various organisms, including plants, microalgae, and microorganisms. By assessing the efficacy of Se removal and identifying the rate-limiting steps associated with volatilization, this paper provides insightful recommendations for Se mitigation. Constructed wetlands are a cost-effective and environmentally friendly alternative in the treatment of Se volatilization. The fate, behavior, bioavailability, and toxicity of Se within complex environmental systems are comprehensively reviewed. This knowledge forms the basis for developing management plans that aimed at mitigating Se contamination in wetlands and protecting the associated ecosystems.

Absorption and Biotransformation of Selenomethionine and Selenomethionine-Oxide by Wheat Seedlings (Triticum aestivum L.)

An in-depth understanding of Se uptake and metabolism in plants is necessary for developing Se biofortification strategies. Thus, hydroponic experiments were conducted to investigate the associated processes and mechanisms of organic Se (selenomethionine (SeMet) and selenomethionine-oxide (SeOMet)) uptake, translocation, transformation and their interaction in wheat, in comparison to inorganic Se. The results showed that Se uptake by the roots and the root-to-shoot translocation factor under the SeMet treatment were higher than those under the selenite, selenate and SeOMet treatments. The uptake and translocation of SeMet were higher than those of SeOMet within 72 h, although the differences gradually narrowed with time. The uptake of SeMet and SeOMet was also sensitive to the aquaporin inhibitor: AgNO3 addition resulted in 99.5% and 99.9% inhibitions of Se in the root in the SeMet and SeOMet treatments, respectively. Once absorbed by the root, they rapidly assimilated to other Se forms, and SeMet and Se-methyl-selenocysteine (MeSeCys) were the dominant species in SeMet- and SeOMet-treated plants, while notably, an unidentified Se form was also found in the root and xylem sap under the SeMet treatment. In addition, within 16 h, SeOMet inhibited the uptake and translocation of SeMet, while the inhibition was weakened with longer treatment time. Taken together, the present study provides new insights for the uptake and transformation processes of organic Se within plants.

The impact of rainwater-borne H2O2-induced Fenton process on root iron plaque formation and arsenic accumulation in rice

Arsenic (As) contamination is a global concern, especially in paddy fields, as it represents a significant pathway for As reaching in the food chain. This is primarily due to the high accumulation of As in rice grains, which is a staple food for billions of people globally. Here we investigated the effect of synthetic rainwater-borne hydrogen peroxide (H2O2)-induced Fenton oxidation process in paddy soil on As uptake and speciation in rice plants at different growth stages. Results showed that adding Fenton reagents significantly accelerated root iron (Fe) plaque formation, thereby enhancing As retention in soil. Arsenic accumulation in different rice plant parts followed the order: Fe plaque > root > stem > leaf. In rice grains, inorganic As and dimethylarsinic acid (DMA) were the major As species for the first and second-season crops. Notably, that the addition of Fenton reagents to paddy soil led to a significant reduction in As accumulation in rice grains. The synthetic rainwater-borne H2O2-induced Fenton reaction significantly promoted As(V) precipitation and decreased concentration of the dissolved As in soil porewater. The current study highlights that the H2O2-induced Fenton process is an important pathway decreasing As bioavailability in paddy soil and its accumulation in rice grain. The findings have implications for understanding As behavior in paddy fields receiving rainwater-borne H2O2 and for developing cost-effective remediation programs to reduce As accumulation in rice grains.

Agronomic and Genetic Strategies to Enhance Selenium Accumulation in Crops and Their Influence on Quality

Selenium (Se) is an essential trace element that plays a crucial role in maintaining the health of humans, animals, and certain plants. It is extensively present throughout the Earth's crust and is absorbed by crops in the form of selenates and selenite, eventually entering the food chain. Se biofortification is an agricultural process that employs agronomic and genetic strategies. Its goal is to enhance the mechanisms of crop uptake and the accumulation of exogenous Se, resulting in the production of crops enriched with Se. This process ultimately contributes to promoting human health. Agronomic strategies in Se biofortification aim to enhance the availability of exogenous Se in crops. Concurrently, genetic strategies focus on improving a crop's capacity to uptake, transport, and accumulate Se. Early research primarily concentrated on optimizing Se biofortification methods, improving Se fertilizer efficiency, and enhancing Se content in crops. In recent years, there has been a growing realization that Se can effectively enhance crop growth and increase crop yield, thereby contributing to alleviating food shortages. Additionally, Se has been found to promote the accumulation of macro-nutrients, antioxidants, and beneficial mineral elements in crops. The supplementation of Se biofortified foods is gradually emerging as an effective approach for promoting human dietary health and alleviating hidden hunger. Therefore, in this paper, we provide a comprehensive summary of the Se biofortification conducted over the past decade, mainly focusing on Se accumulation in crops and its impact on crop quality. We discuss various Se biofortification strategies, with an emphasis on the impact of Se fertilizer strategies on crop Se accumulation and their underlying mechanisms. Furthermore, we highlight Se's role in enhancing crop quality and offer perspective on Se biofortification in crop improvement, guiding future mechanistic explorations and applications of Se biofortification.

Comparative transcriptomics provides novel insights into the mechanisms of selenium accumulation and transportation in tea cultivars (Camellia sinensis (L.) O. Kuntze)

Tea plants (Camellia sinensis) show discrepancies in selenium accumulation and transportation, the molecular mechanisms of which are not well understood. Hence, we aimed to conduct a systematic investigation of selenium accumulation and transportation mechanisms in different tea cultivars via transcriptome analysis. The Na2SeO3 and Na2SeO4 treatments improved selenium contents in the roots and leaves of three tea cultivars. The high selenium-enrichment ability (HSe) tea cultivars accumulated higher selenium contents in the leaves than did the low selenium-enrichment ability (LSe) tea cultivars. Transcriptome analysis revealed that differentially expressed genes (DEGs) under the Na2SeO3 and Na2SeO4 treatments were enriched in flavonoid biosynthesis in leaves. DEGs under the Na2SeO3 treatment were enriched in glutathione metabolism in the HSe tea cultivar roots compared to those of the LSe tea cultivar. More transporters and transcription factors involved in improving selenium accumulation and transportation were identified in the HSe tea cultivars under the Na2SeO3 treatment than in the Na2SeO4 treatment. In the HSe tea cultivar roots, the expression of sulfate transporter 1;2 (SULTR1;2) and SULTR3;4 increased in response to Na2SeO4 exposure. In contrast, ATP-binding cassette transporter genes (ABCs), glutathione S-transferase genes (GSTs), phosphate transporter 1;3 (PHT1;3), nitrate transporter 1 (NRT1), and 34 transcription factors were upregulated in the presence of Na2SeO3. In the HSe tea cultivar leaves, ATP-binding cassette subfamily B member 11 (ABCB11) and 14 transcription factors were upregulated under the Na2SeO3 treatment. Among them, WRKY75 was explored as a potential transcription factor that regulated the accumulation of Na2SeO3 in the roots of HSe tea cultivars. This study preliminary clarified the mechanism of selenium accumulation and transportation in tea cultivars, and the findings have important theoretical significance for the breeding and cultivation of selenium-enriched tea cultivars.

Life-cycle selenium accumulation and its correlations with the rhizobacteria and endophytes in the hyperaccumulating plant Cardamine hupingshanensis

Cardamine hupingshanensis (C. hupingshanensis) is known for its ability to hyperaccumulate selenium (Se). However, the roles of the rhizobacteria or endophytes in Se hyperaccumulation have not been explored in C. hupingshanensis. Here, in-situ-like pot experiments were conducted to investigate the characteristics of Se accumulation throughout C. hupingshanensis growth stages and its correlations with rhizobacteria and endophytes under varying soil Se levels. Results showed that Se levels in roots, stems and leaves increased from the seedling to bolting stage, but remained relatively stable during the flowering and maturity. Leaves exhibited the highest Se levels (736.48 ± 6.51 mg/kg DW), followed by stems (575.39 ± 27.05 mg/kg DW), and lowest in roots (306.62 ± 65.45 mg/kg DW) under high-Se stress. The Se translocation factors from soils to C. hupingshanensis roots was significantly higher (p < 0.05) in low-Se soils compared to medium- and high-Se soils. Rhizobacterial diversity showed significant positive correlations (p < 0.05) with both total and bioavailable soil Se contents. The levels of soil Se and growth stages of C. hupingshanensis were found to have significant effects (p < 0.03) on the compositions of rhizosphere bacteria and C. hupingshanensis endophytes. Low-abundance bacteria (< 5%), including Gemmatimonadetes, Latescibacteria and Nitrospirae, were identified to potentially increase the bioavailable Se levels in the rhizosphere. The Se accumulation significantly decreased (p < 0.05) in C. hupingshanensis grown in sterilized low- (32.4%), medium- (17%) and high-Se (42%) soils. Endophytes in C. hupingshanensis, such as Firmicutes and Proteobacteria, were likely recruited from the rhizobacteria, as evidenced by the isolated bacterial strains, and played an important role in Se hyperaccumulation, particularly during the flowering stage. This study provides new insights into potential mechanism underlying Se hyperaccumulation in C. hupingshanensis.

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.

Transcriptome and physiological determination reveal the effects of selenite on the growth and selenium metabolism in mung bean sprouts

Selenium (Se) biofortification of crops has been studied to substantially improve the Se content in human dietary food intake. In the present study, Vigna radiata (mung bean) seeds were soaked in different concentrations of sodium selenite (Na₂SeO₃). Low concentration of selenite is conducive to seed germination and growth, and can increase the fresh weight (FW) and dry weight (DW) of sprouts. The concentration of Na₂SeO₃ lower than 50 mg/kg resulted in noticeable elongation in the stem and marginal elongation in root. Mung bean seeds soaked with 80 mg/kg Na₂SeO₃ accounted for 93.77% of organic Se after growing for about 5 days. Transcriptome data revealed that Se treatment enhances starch and sugar metabolism, along with the up-regulation of ribosomal protein and DNA synthesis related genes. Further analysis indicated that the mung bean seeds absorbed Na₂SeO₃ through PHT1.1 and NIP2. Se (IV) was transformed into Se (VI) and transported to stems, leaves and roots through cotyledons during the germination of bean sprouts. SULTR3;3 may play an important role in the transit process. Se (VI) or Se (IV) transported to the leaves was catalytically transformed into SeCys through SiR and CS, and SeCys is further converted to MeSeCys through SMT. Most SeCys were transformed into SeHCys through CBL, transported to plastids, and finally transformed into SeMet through Met Synthase.

Selenium in soil-plant system: Transport, detoxification and bioremediation

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. However, high Se doses always exhibit hazardous effects. Recently, Se toxicity in plant-soil system has received increasing attention. This review will summarize (1) Se concentration in soils and its sources, (2) Se bioavailability in soils and influencing factors, (3) mechanisms on Se uptake and translocation in plants, (4) toxicity and detoxification of Se in plants and (5) strategies to remediate Se pollution. High Se concentration mainly results from wastewater discharge and industrial waste dumping. Selenate (Se [VI]) and selenite (Se [IV]) are the two primary forms absorbed by plants. Soil conditions such as pH, redox potential, organic matter and microorganisms will influence Se bioavailability. In plants, excessive Se will interfere with element uptake, depress photosynthetic pigment biosynthesis, generate oxidative damages and cause genotoxicity. Plants employ a series of strategies to detoxify Se, such as activating antioxidant defense systems and sequestrating excessive Se in the vacuole. In order to alleviate Se toxicity to plants, some strategies can be applied, including phytoremediation, OM remediation, microbial remediation, adsorption technique, chemical reduction technology and exogenous substances (such as Methyl jasmonate, Nitric oxide and Melatonin). This review is expected to expand the knowledge of Se toxicity/detoxicity in soil-plant system and offer valuable insights into soils Se pollution remediation strategies.

Polar localization of a rice silicon transporter requires isoleucine at both C- and N-termini as well as positively charged residues

Silicon (Si) is important for stable and high yields in rice (Oryza sativa), a typical Si hyperaccumulator. The high Si accumulation is achieved by the cooperation of 2 Si transporters, LOW SILICON 1 (OsLsi1) and OsLsi2, which are polarly localized in cells of the root exodermis and endodermis. However, the mechanism underlying their polar localization is unknown. Here, we identified amino acid residues critical for the polar localization of OsLsi1. Deletion of both N- and C-terminal regions resulted in the loss of its polar localization. Furthermore, the deletion of the C-terminus inhibited its trafficking from the endoplasmic reticulum to the plasma membrane. Detailed site-directed mutagenesis analysis showed that Ile18 at the N-terminal region and Ile285 at the C-terminal region were essential for the polar localization of OsLsi1. Moreover, a cluster of positively charged residues at the C-terminal region is also required for polar localization. Phosphorylation and Lys modifications of OsLsi1 are unlikely to be involved in its polar localization. Finally, we showed that the polar localization of OsLsi1 is required for the efficient uptake of Si. Our study not only identified critical residues required for the polar localization of OsLsi1, but also provided experimental evidence for the importance of transporter polarity for efficient nutrient uptake.

Selenite reduced uptake/translocation of cadmium via regulation of assembles and interactions of pectins, hemicelluloses, lignins, callose and Casparian strips in rice roots

Selenium (Se) can reduce cadmium (Cd) uptake/translocation via regulating pectins, hemicelluloses and lignins of plant root cell walls, but the detailed molecular mechanisms are not clear. In this study, six hydroponic experiments were set up to explore the relationships of uptake/translocation inhibition of Cd by selenite (Se(IV)) with cell wall component (CWC) synthesis and/or interactions. Cd and Se was supplied (alone or combinedly) at 1.0 mg L^-1 and 0.5 mg L^-1, respectively, with the treatment without Cd and Se as the control. When compared to the Cd1 treatment, the Se0.5Cd1 treatment 1) significantly increased total sugar concentrations in pectins, hemicelluloses and callose, suggesting an enhanced capacity of binding Cd or blocking Cd translocation; 2) stimulated the deposition of Casparian strips (CS) in root endodermis and exodermis to block Cd translocation; 3) stimulated the release of C-O-C (-OH^- or -O^-) and CO (carboxyl, carbonyl, or amide) to combine Cd; 4) regulated differential expression genes (DEGs) and metabolites (DMs) correlated with synthesis and/or interactions of CWSs to affect cell wall net structure to affect root cell division, subsequent root morphology and finally elemental uptake; and 5) stimulated de-methylesterification of pectins via reducing expression abundances of many DMs and DEGs in the Yang Cycle to reduce supply of methyls to homogalacturonan, and regulated gene expressions of pectin methylesterase to release carboxyls to combine Cd; and 6) down-regulated gene expressions associated with Cd uptake/translocation.

Molecular membrane separation: plants inspire new technologies

Plants draw up their surrounding soil solution to gain water and nutrients required for growth, development and reproduction. Obtaining adequate water and nutrients involves taking up both desired and undesired elements from the soil solution and separating resources from waste. Desirable and undesirable elements in the soil solution can share similar chemical properties, such as size and charge. Plants use membrane separation mechanisms to distinguish between different molecules that have similar chemical properties. Membrane separation enables distribution or retention of resources and efflux or compartmentation of waste. Plants use specialised membrane separation mechanisms to adapt to challenging soil solution compositions and distinguish between resources and waste. Coordination and regulation of these mechanisms between different tissues, cell types and subcellular membranes supports plant nutrition, environmental stress tolerance and energy management. This review considers membrane separation mechanisms in plants that contribute to specialised separation processes and highlights mechanisms of interest for engineering plants with enhanced performance in challenging conditions and for inspiring the development of novel industrial membrane separation technologies. Knowledge gained from studying plant membrane separation mechanisms can be applied to developing precision separation technologies. Separation technologies are needed for harvesting resources from industrial wastes and transitioning to a circular green economy.

The role of selenium and nano selenium on physiological responses in plant: a review

Selenium (Se), being an essential micronutrient, enhances plant growth and development in trace amounts. It also protects plants against different abiotic stresses by acting as an antioxidant or stimulator in a dose-dependent manner. Knowledge of Se uptake, translocation, and accumulation is crucial to achieving the inclusive benefits of Se in plants. Therefore, this review discusses the absorption, translocation, and signaling of Se in plants as well as proteomic and genomic investigations of Se shortage and toxicity. Furthermore, the physiological responses to Se in plants and its ability to mitigate abiotic stress have been included. In this golden age of nanotechnology, scientists are interested in nanostructured materials due to their advantages over bulk ones. Thus, the synthesis of nano-Se or Se nanoparticles (SeNP) and its impact on plants have been studied, highlighting the essential functions of Se NP in plant physiology. In this review, we survey the research literature from the perspective of the role of Se in plant metabolism. We also highlight the outstanding aspects of Se NP that enlighten the knowledge and importance of Se in the plant system.

Graphical abstract

Fast X-ray fluorescence microscopy provides high-throughput phenotyping of element distribution in seeds

The concentration, chemical speciation, and spatial distribution of essential and toxic mineral elements in cereal seeds have important implications for human health. To identify genes responsible for element uptake, translocation, and storage, high-throughput phenotyping methods are needed to visualize element distribution and concentration in seeds. Here, we used X-ray fluorescence microscopy (μ-XRF) as a method for rapid and high-throughput phenotyping of seed libraries and developed an ImageJ-based pipeline to analyze the spatial distribution of elements. Using this method, we nondestructively scanned 4,190 ethyl methanesulfonate (EMS)-mutagenized M1 rice (Oryza sativa) seeds and 533 diverse rice accessions in a genome-wide association study (GWAS) panel to simultaneously measure concentrations and spatial distribution of elements in the embryo, endosperm, and aleurone layer. A total of 692 putative mutants and 65 loci associated with the spatial distribution of elements in rice seed were identified. This powerful method provides a basis for investigating the genetics and molecular mechanisms controlling the accumulation and spatial variations of mineral elements in plant seeds.

Algal polysaccharides-Selenium nanoparticles regulate the uptake and distribution of selenium in rice plants

INTRODUCTION

Selenium (Se) is an essential trace element required for proper human and animal health.

METHODS

In this paper, we investigated the uptake and distribution characteristics of a new Se fertilizer, which comprises algal polysaccharides-selenium nanoparticles (APS-SeNPs), in rice plants in both hydroponic and pot experiments.

RESULTS

The results from the hydroponic experiments revealed that the rice root uptake of APS-SeNPs fitted the Michaelis-Menten equation, with a V _max of 13.54 μg g^-1 root dry weight (DW) per hour, which was 7.69 and 2.23 times those of selenite and selenate treatments, respectively. The root uptake of APS-SeNPs was inhibited by AgNO₃ (64.81%-79.09%) and carbonyl cyanide 3-chlorophenylhydrazone (CCCP; 19.83%-29.03%), indicating that the uptake of APS-SeNPs by rice roots is mainly via aquaporins and is also affected by metabolic activity. Moreover, sulfur deficiency caused rice roots to absorb more APS-SeNPs, but treatment with APS-SeNPs increased the expression of the sulfate transporter OsSULTR1;2 in the roots, suggesting that OsSULTR1;2 is probably involved in the uptake of APS-SeNPs. The application of APS-SeNPs significantly increased the Se content in rice plants and the apparent Se uptake efficiency compared with selenate and selenite treatments. Most of the Se in the roots of rice plants was distributed in the cell wall, while it was primarily located in the cytosol in the shoots when treated with APS-SeNPs. The results from the pot experiments indicated that the application of Se enhanced the Se content of each rice tissue. It is worth noting that the Se content in brown rice under APS-SeNP treatment was higher than that under selenite or selenate treatment and was mainly concentrated in the embryo end, with the Se in organic form.

DISCUSSION

Our findings provide important insights into the uptake mechanism and the distribution of APS-SeNPs in rice plants.

Uptake of Selenite by Rahnella aquatilis HX2 Involves the Aquaporin AqpZ and Na+/H+ Antiporter NhaA

Microbial transformation of selenite [Se(IV)] to elemental selenium nanoparticles (SeNPs) is known to be an important process for removing toxic soluble selenium (Se) oxyanions and recovery of Se from the environment as valuable nanoparticles. However, the mechanism of selenite uptake by microorganisms, the first step through which Se exerts its cellular function, remains not well studied. In this study, the effects of selenite concentration, time, pH, metabolic inhibitors, and anionic analogues on selenite uptake in Rahnella aquatilis HX2 were investigated. Selenite uptake by R. aquatilis HX2 was concentration- and time-dependent, and its transport activity was significantly dependent on pH. In addition, selenite uptake in R. aquatilis HX2 was significantly inhibited by the aquaporin inhibitor AgNO₃ and sulfite (SO₃^2-), and partially inhibited by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 2,4-dinitrophenol (2,4-DNP) treatments. Three mutants with in-frame deletions of aqpZ, glpF, and nhaA genes were constructed. The transport assay showed that the water channel protein AqpZ, and not GlpF, was a key channel of selenite uptake by R. aquatilis HX2, and sulfite and selenite had a common uptake pathway. In addition, the Na⁺/H⁺ antiporter NhaA is also involved in selenite uptake in R. aquatilis HX2.

Illumina RNA and SMRT Sequencing Reveals the Mechanism of Uptake and Transformation of Selenium Nanoparticles in Soybean Seedlings

Selenium (Se) is an essential element for mammals, and its deficiency in the diet is a global problem. Agronomic biofortification through exogenous Se provides a valuable strategy to enhance human Se intake. Selenium nanoparticles (SeNPs) have been regarded to be higher bioavailability and less toxicity in comparison with selenite and selenate. Still, little has been known about the mechanism of their metabolism in plants. Soybean (Glycine max L.) can enrich Se, providing an ideal carrier for Se biofortification. In this study, soybean sprouts were treated with SeNPs, and a combination of next-generation sequencing (NGS) and single-molecule real-time (SMRT) sequencing was applied to clarify the underlying molecular mechanism of SeNPs metabolism. A total of 74,662 nonredundant transcripts were obtained, and 2109 transcription factors, 9687 alternative splice events, and 3309 long non-coding RNAs (lncRNAs) were predicted, respectively. KEGG enrichment analysis of the DEGs revealed that metabolic pathways, biosynthesis of secondary metabolites, and peroxisome were most enriched both in roots and leaves after exposure to SeNPs. A total of 117 transcripts were identified to be putatively involved in SeNPs transport and biotransformation in soybean. The top six hub genes and their closely coexpressed Se metabolism-related genes, such as adenylylsulfate reductase (APR3), methionine-tRNA ligase (SYM), and chloroplastic Nifs-like cysteine desulfurases (CNIF1), were screened by WGCNA and identified to play crucial roles in SeNPs accumulation and tolerance in soybean. Finally, a putative metabolism pathway of SeNPs in soybean was proposed. These findings have provided a theoretical foundation for future elucidation of the mechanism of SeNPs metabolism in plants.

Selenium species transforming along soil-plant continuum and their beneficial roles for horticultural crops

Selenium (Se) acquirement from daily diet can help reduce the risk of many diseases. The edible parts of crop plants are the main source of dietary Se, while the Se content in crops is determined by Se bioavailability in soil. We summarize recent research on the biogeochemical cycle of Se driven by specific microorganisms and emphasize the oxidizing process in the Se cycle. Moreover, we discuss how plant root exudates and rhizosphere microorganisms affect soil Se availability. Finally, we cover beneficial microorganisms, including endophytes, that promote crop quality and improve crop tolerance to environmental stresses. Se availability to plants depends on the balance between adsorption and desorption, reduction, methylation and oxidation, which are determined by interactions among soil properties, microbial communities and plants. Reduction and methylation processes governed by bacteria or fungi lead to declined Se availability, while Se oxidation regulated by Se-oxidizing microorganisms increases Se availability to plants. Despite a much lower rate of Se oxidization compared to reduction and methylation, the potential roles of microbial communities in increasing Se bioavailability are probably largely underestimated. Enhancing Se oxidation and Se desorption are crucial for the promotion of Se bioavailability and uptake, particularly in Se-deficient soils. Beneficial roles of Se are reported in terms of improved crop growth and quality, and enhanced protection against fungal diseases and abiotic stress through improved photosynthetic traits, increased sugar and amino acid contents, and promoted defense systems. Understanding Se transformation along the plant-soil continuum is crucial for agricultural production and even for human health.

Effect of Nano-Selenium on Nutritional Quality of Cowpea and Response of ABCC Transporter Family

It is an important way for healthy Selenium (Se) supplement to transform exogenous Se into organic Se through crops. In the present study, Vigna unguiculata was selected as a test material and sprayed with biological nano selenium (SeNPs) and Na₂SeO₃, and its nutrient composition, antioxidant capacity, total Se and organic Se content were determined, respectively. Further, the response of ABC transporter family members in cowpea to different exogenous Se treatments was analyzed by transcriptome sequencing combined with different Se forms. The results show that the soluble protein content of cowpea increased after twice Se treatment. SeNPs treatment increased the content of cellulose in cowpea pods. Na₂SeO₃ treatment increased the content of vitamin C (Vc) in cowpea pods. Se treatments could significantly increase the activities of Peroxidase (POD), polyphenol oxidase (PPO) and catalase (CAT) in cowpea pods and effectively maintain the activity of Superoxide dismutase (SOD). SeNPs can reduce the content of malondialdehyde (MDA) in pods. After Se treatment, cowpea pods showed a dose-effect relationship on the absorption and accumulation of total Se, and Na₂SeO₃ treatment had a better effect on the increase of total Se content in cowpea pods. After treatment with SeNPs and Na₂SeO₃, the Se species detected in cowpea pods was mainly SeMet, followed by MeSeCys. Inorganic Se can only be detected in the high concentration treatment group. Analysis of transcriptome data of cowpea treated with Se showed that ABC transporters could play an active role in response to Se stress and Se absorption, among which ABCB, ABCC and ABCG subfamilies played a major role in Se absorption and transportation in cowpea. Further analysis by weighted gene co-expression network analysis (WGCNA) showed that the content of organic Se in cowpea treated with high concentration of SeNPs was significantly and positively correlated with the expression level of three transporters ABCC11, ABCC13 and ABCC10, which means that the ABCC subfamily may be more involved in the transmembrane transport of organic Se in cells.

Synergy of Selenium and Silicon to Mitigate Abiotic Stresses: a Review

It is evident the increase in the occurrence of different stresses that impact agriculture and so there has been an increase in research to study stress mitigators including silicon (Si) and selenium (Se). However, the great challenge to be answered would be to assess whether it is possible to maximize these benefits by combining these two elements. Therefore, this review focused on discussing the feasibility of combining Se and Si in mitigating abiotic stresses and also measuring gains in yield and quality of agricultural products. These are the main challenges of plant mineral nutrition with these two elements for sustainable cultivation, ensuring food security with the possibility of improving human health. As the mode of application of an element can change absorption and assimilation processes and consequently the plant's response, it is important to consider research with supply of these elements via the foliar and root route. Thus, we highlighted the potential of the combined application of Se and Si and whether or not they are relevant to overcome the individual application in stress mitigation or even in plants without stress. In addition, we pointed out new directions for research on this topic in order to reinforce the combined use of stress relievers and their potential benefit to crop plants.

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

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

Selenium Regulates Antioxidant, Photosynthesis, and Cell Permeability in Plants under Various Abiotic Stresses: A Review

Plant growth is affected by various abiotic stresses, including water, temperature, light, salt, and heavy metals. Selenium (Se) is not an essential nutrient for plants but plays important roles in alleviating the abiotic stresses suffered by plants. This article summarizes the Se uptake and metabolic processes in plants and the functions of Se in response to water, temperature, light, salt, and heavy metal stresses in plants. Se promotes the uptake of beneficial substances, maintains the stability of plasma membranes, and enhances the activity of various antioxidant enzymes, thus alleviating adverse effects in plants under abiotic stresses. Future research directions on the relationship between Se and abiotic stresses in plants are proposed. This article will further deepen our understanding of the relationship between Se and plants.

Interaction between selenium and essential micronutrient elements in plants: A systematic review

Nutrient imbalance (i.e., deficiency and toxicity) of microelements is an outstanding environmental issue that influences each aspect of ecosystems. Although the crucial roles of microelements in entire lifecycle of plants have been widely acknowledged, the effective control of microelements is still neglected due to the narrow safe margins. Selenium (Se) is an essential element for humans and animals. Although it is not believed to be indispensable for plants, many literatures have reported the significance of Se in terms of the uptake, accumulation, and detoxification of essential microelements in plants. However, most papers only concerned on the antagonistic effect of Se on metal elements in plants and ignored the underlying mechanisms. There is still a lack of systematic review articles to summarize the comprehensive knowledge on the connections between Se and microelements in plants. In this review, we conclude the bidirectional effects of Se on micronutrients in plants, including iron, zinc, copper, manganese, nickel, molybdenum, sodium, chlorine, and boron. The regulatory mechanisms of Se on these micronutrients are also analyzed. Moreover, we further emphasize the role of Se in alleviating element toxicity and adjusting the concentration of micronutrients in plants by altering the soil conditions (e.g., adsorption, pH, and organic matter), promoting microbial activity, participating in vital physiological and metabolic processes, generating element competition, stimulating metal chelation, organelle compartmentalization, and sequestration, improving the antioxidant defense system, and controlling related genes involved in transportation and tolerance. Based on the current understanding of the interaction between Se and these essential elements, future directions for research are suggested.

The combined impacts of selenium and phosphorus on the fate of arsenic in rice seedlings (Oryza sativa L.)

Although the single role of selenium (Se) or phosphorus (P) in regulating the As contamination of rice plants has been reported in some studies, the combined impacts of Se and P on the fate of As and the underlying mechanisms are poorly understood. To address this knowledge gap, the uptake, translocation, and biotransformation of As mediated by Se were investigated in rice (Oryza sativa L.) seedlings hydroponically cultured with P-normal and P-deficient conditions. The results showed Se addition stimulated the uptake of arsenite and arsenate by 15.6% and 30.7%, respectively in P-normal condition, and such effect was more profound in P-deficient condition with the value of 43.8% and 70.8%. However, regardless of Se addition, P-deficiency elevated the As uptake by 47.0%-92.1% for arsenate but had no obvious effects for arsenite. Accompanying with the As transfer factor_Shoot/Root reduced by 74.5%-80.2% and 71.1%-85.7%, Se addition decreased the shoot As content by 65.8%-69.7% and 59.6%-73.1%, respectively, in the arsenite- and arsenate-treated rice plants. Relative to the corresponding treatments of P-normal condition, P-deficiency reduced the As transfer factor_Shoot/Root by 38.9%-52.5% and thus decreasing the shoot As content by 35.2%-42.5% in the arsenite-treated plants; while the opposite impacts were observed in the arsenate-treated plants, in which the shoot As content was increased by 22.4%-83.7%. The analysis results of As species showed As(III) was dominant in both shoots (68.9%-75.1%) and roots (94.9%-97.2%), and neither Se addition nor P-deficiency had obvious impacts on the interconversion between As(III) and As(V). Our results demonstrate the regulating roles of Se in As accumulation mainly depend on P regimes and the specific rice tissues, but the effects of P-deficiency on the fate of As were influenced by the form of As added to the culture.

Selenite and selenate showed contrasting impacts on the fate of arsenic in rice (Oryza sativa L.) regardless of the formation of iron plaque

The different effects of selenite and selenate on the fate of As and the function of iron plaque in the interaction between Se and As are poorly understood. Rice seedlings (Oryza sativa L.) were selected as experimental plants in this study, the hydroponic experiments were conducted to investigate the possible regulatory roles of selenite and selenate on the uptake, translocation, and transformation of arsenite or arsenate accompanied by iron plaque. In arsenite- and arsenate-treated rice, the Fe30 treatments stimulated root uptake by 12.4-39.8% and 18.6-37.0%, respectively, but inhibited the movement of As from iron plaque to the roots, resulting in the absorption of a considerable amount of As on iron plaque. Regardless of the iron plaque formation, selenite (selenate) significantly increased (decreased) the root uptake of arsenite and arsenate by 28.1-53.0% and 40.0%-61.7%, respectively (45.6-56.3% and 42.5-47.7%, respectively). Interestingly, the supply of selenite significantly reduced root-to-shoot As translocation by 71.9-77.3% and 66.2-67.7%, respectively, in arsenite- and arsenate-treated rice seedlings; however, a significant increase (90.5-122.9%) was induced by selenate was found only in the arsenate-treated plants. Furthermore, the translocation of As from iron plaque to the roots was significantly increased (decreased) by selenite (selenate). As and Fe in iron plaque were significantly positively correlated in all As-treated rice plants, and this correlation was more profound than that in the shoots and roots. However, neither Fe treatments nor inorganic Se addition affected the interconversion between As(III) and As(V) obviously; and As(III) was the dominant species in both shoots (68.3-84.9%) and roots (90.7-98.2%). Our results indicate selenite and selenate are effective in reducing the As accumulation in an opposite way, and the presence of iron plaque had no obvious impact on the interaction between Se and As in rice plants.

Advances in Research on the Involvement of Selenium in Regulating Plant Ecosystems

Selenium is an essential trace element which plays an important role in human immune regulation and disease prevention. Plants absorb inorganic selenium (selenite or selenate) from the soil and convert it into various organic selenides (such as seleno amino acids, selenoproteins, and volatile selenides) via the sulfur metabolic pathway. These organic selenides are important sources of dietary selenium supplementation for humans. Organoselenides can promote plant growth, improve nutritional quality, and play an important regulatory function in plant ecosystems. The release of selenium-containing compounds into the soil by Se hyperaccumulators can promote the growth of Se accumulators but inhibit the growth and distribution of non-Se accumulators. Volatile selenides with specific odors have a deterrent effect on herbivores, reducing their feeding on plants. Soil microorganisms can effectively promote the uptake and transformation of selenium in plants, and organic selenides in plants can improve the tolerance of plants to pathogenic bacteria. Although selenium is not an essential trace element for plants, the right amount of selenium has important physiological and ecological benefits for them. This review summarizes recent research related to the functions of selenium in plant ecosystems to provide a deeper understanding of the significance of this element in plant physiology and ecosystems and to serve as a theoretical basis and technical support for the full exploitation and rational application of the ecological functions of selenium-accumulating plants.

Can As concentration in crop be controlled by Se fertilization? A meta-analysis and outline of As sequestration mechanisms

Arsenic (As) is a pollutant with a strong toxic effect on animals, plants and human beings. Exogenous selenium (Se) has been suggested to reduce the accumulation of As in crops, but contradictory results were found in the published literature. In order to clarify the possible processes, we collected the literature that reports on the effects of Se application on As uptake and accumulation in crops, analyzed the data by meta-analysis, and tested the effects of different factors on As accumulation by meta-regression model and subgroup analysis. The results highlighted a significant dose-dependent reduction of As content in crops after Se addition. Exogenous Se can significantly reduce As concentrations in grains by 18.76%. The reduction was dose-dependent for rice grains under aerobic soil conditions but not for rice grains under anoxic soil conditions. Se-enriched soils (greater than 0.5 mg kg^-1) significantly reduced As concentrations in grains. Selenium significantly decreased the transfer factor of As from root to shoot. Moreover, selenite had a stronger inhibiting effect on the transport of As from root to shoot than selenate. The inhibition of selenium fertilization on As concentrations seems to take place in root and soil, while physiological processes in rice may be involved in restricting uptake and transport from root to shoot. These findings provide new ideas for effectively alleviating the transfer of As to the human body through the food chain.

Transcriptome analysis revealed accumulation-assimilation of selenium and physio-biochemical changes in alfalfa (Medicago sativa L.) leaves

BACKGROUND

Selenium (Se) is an increasing concern for investigators predominantly because of its consumption in the human body mainly from crops. As the fourth largest plant crop globally, alfalfa is one of the most important forages. Alfalfa was fertilized with selenium(IV) (Se(IV)) under field conditions to study the accumulation and assimilation of Se(IV) and to assess the impact of Se fertilization.

RESULTS

It was analyzed that the physio-biochemistry, Se species, combined with transcriptome after spraying Se(IV) at different times (0, 12, and 48 h). 9402 and 12 607 differentially expressed genes (DEGs) were identified at 12 h (versus 0 h) and 48 h (versus 12 h). DEG functional enrichments proposed two time-specific biological processes: Se(IV) accumulation was the primary process at 0-12 h, and its assimilation mainly occurred during 12-48 h. This was further proved by the separation of various Se speciation at different times. It showed that Se-supplementation also affected the soluble protein, soluble sugar, pigment contents and antioxidant capacity. Selenium-biofortification could improve the stress resistance of alfalfa by enhancing antioxidant system to scavenge reactive oxygen species (e.g. hydrogen peroxide) and boosting carbohydrate metabolism.

CONCLUSION

By integrating physio-biochemistry, Se-related metabolites, and transcriptome under Se(IV) treatment, this study provides data to guide further work on Se-fortification in alfalfa. © 2022 Society of Chemical Industry.

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.

Selenium Uptake, Transport, Metabolism, Reutilization, and Biofortification in Rice

Selenium (Se) is an essential trace element for humans and other animals. The human body mainly acquires Se from plant foods, especially cereal grains. Rice is the staple food for more than half of the world's population. Increasing the Se concentration of rice grains can increase the average human dietary Se intake. This review summarizes recent advances in the molecular mechanisms of Se uptake, transport, subcellular distribution, retranslocation, volatilization, and Se-containing protein degradation in plants, especially rice. The strategies for improving Se concentration in rice grains by increasing Se accumulation, reducing Se volatilization, and optimizing Se form were proposed, which provide new insight into Se biofortification in rice by improving the utilization efficiency of Se.

Attenuation mechanisms of arsenic induced toxicity and its accumulation in plants by engineered nanoparticles: A review

The excessive arsenic (As) accumulation in plant tissues enforced toxic impacts on growth indices. So, the utilization of As-contaminated food leads to risks associated with human health. For the reduction of As concentrations in foods, it is obligatory to fully apprehend the take up, accretion, transportation and toxicity mechanisms of As within plant parts. This metalloid impairs the plant functions by disturbing the metabolic pathways at physio-biochemical, cellular and molecular levels. Though several approaches were utilized to reduce the As-accumulation and toxicity in soil-plant systems. Recently, engineered nanoparticles (ENPs) such a zinc oxide (ZnO), silicon dioxide or silica (SiO₂), iron oxide (FeO) and copper oxide (CuO) have emerged new technology to reduce the As-accumulation or phytotoxicity. But, the mechanistic approaches with systematic explanation are missing. By knowing these facts, our prime focus was to disclose the mechanisms behind the As toxicity and its mitigation by ENPs in higher plants. ENPs relives As toxicity and its oxidative damages by regulating the transporter or defense genes, modifying the cell wall composition, stimulating the antioxidants defense, phytochelatins biosynthesis, nutrients uptake, regulating the metabolic processes, growth improvement, and thus reduction in As-accumulation or toxicity. Yet, As-detoxification by ENPs depends upon the type and dose of ENPs or As, exposure method, plant species and experimental conditions. We have discussed the recent advances and highlight the knowledge or research gaps in earlier studies along with recommendations. This review may help scientific community to develop strategies such as applications of nano-based fertilizers to limit the As-accumulation and toxicity, thus healthy food production. These outcomes may govern sustainable application of ENPs in agriculture.

Comparative study of the effects of selenium yeast and sodium selenite on selenium content and nutrient quality in broccoli florets (Brassica oleracea L. var. italica)

BACKGROUND

Approximately 0.5-1 billion people worldwide face the risk of selenium (Se) deficiency because of the low Se concentration in their diets. Broccoli can accumulate Se and comprises a source of daily Se supplement for humans. Se biofortification is an effective strategy for enhancing Se content in crops. In the present study, the effects of Se yeast and selenite application on the Se content and nutrient quality of broccoli were investigated.

RESULTS

Broccoli growth was promoted by Se yeast but inhibited by selenite. The total Se content of broccoli florets remarkably increased with increasing exogenous Se fertilizer concentrations. The main Se species in broccoli florets were methyl-selenocysteine and selenomethionine, and their contents were significantly higher under Se yeast treatments than under selenite treatments. Se(VI) was detected only under selenite treatments. Se yeast and selenite had different influences on soluble sugar, soluble protein, vitamin C and free amino acid contents in broccoli florets. The total phenolic acid and glucosinolate contents were substantially increased by Se yeast and selenite, although the total flavonoid content was reduced by Se yeast. Tests on antioxidant enzyme activities revealed that several antioxidant enzymes (catalase, peroxidase, superoxide dismutase and glutathione peroxidase) responded to Se yeast and selenite treatments.

CONCLUSION

Se yeast is preferred over selenite for maximizing Se uptake and nutrient accumulation in Se-rich broccoli cultivation. However, an extremely high Se content in broccoli florets cannot be directly consumed by humans, although they can be processed into Se supplements. © 2021 Society of Chemical Industry.

Integrative Transcriptome and Proteome Analysis Reveals the Absorption and Metabolism of Selenium in Tea Plants [Camellia sinensis (L.) O. Kuntze]

Certain tea plants (Camellia sinensis) have the ability to accumulate selenium. In plants, the predominant forms of bioavailable Se are selenite (SeO3 2-) and selenate (SeO4 2-). We applied transcriptomics and proteomics to hydroponically grown plants treated with selenite or selenate for 48 h in the attempt to elucidate the selenium absorption and assimilation mechanisms in tea. A total of 1,844 differentially expressed genes (DEGs) and 691 differentially expressed proteins (DEPs) were obtained by comparing the Na2SeO3 and Na2SeO4 treatments against the control. A GO analysis showed that the genes related to amino acid and protein metabolism and redox reaction were strongly upregulated in the plants under the Na2SeO3 treatment. A KEGG pathway analysis revealed that numerous genes involved in amino acid and glutathione metabolism were upregulated, genes and proteins associated with glutathione metabolism and ubiquinone and terpenoid-quinone biosynthesis were highly expressed. Genes participating in DNA and RNA metabolism were identified and proteins related to glutathione metabolism were detected in tea plants supplemented with Na2SeO4. ABC, nitrate and sugar transporter genes were differentially expressed in response to selenite and selenate. Phosphate transporter (PHT3;1a, PHT1;3b, and PHT1;8) and aquaporin (NIP2;1) genes were upregulated in the presence of selenite. Sulfate transporter (SULTR1;1 and SULTR2;1) expression increased in response to selenate exposure. The results of the present study have clarified Se absorption and metabolism in tea plants, and play an important theoretical reference significance for the breeding and cultivation of selenium-enriched tea varieties.

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.

Genome-wide identification of SULTR genes in tea plant and analysis of their expression in response to sulfur and selenium

Sulfur (S) is an essential macronutrient required by plants. Plants absorb and transport S through sulfate transporters (SULTRs). In this study, we cloned 8 SULTR genes (CsSULTR1;1/1;2/2;1/3;1/3;2/3;3/3;5/4;1) from tea plant (Camellia sinensis), all of which contain a typical sulfate transporter and antisigma factor antagonist (STAS) conserved domain. Phylogenetic tree analysis further divided the CsSULTRs into four main groups. Many cis-acting elements related to hormones and environmental stresses were found within the promoter sequence of CsSULTRs. Subcellular localization results showed that CsSULTR4;1 localized in the vacuolar membrane and that other CsSULTRs localized to the cellular membrane. The tissue-specific expression of the 8 CsSULTR genes showed different expression patterns during the active growing period and dormancy period. In particular, the expression of CsSULTR1;1 was highest in the roots, but that of CsSULTR1;2 was lowest in the dormancy period. The expression of CsSULTR1;1/1;2/2;1/3;2 was stimulated under different concentrations of selenium (Se) and S; moreover, CsSULTR1;2/2;1/3;3/3;5 was upregulated in response to different valences of Se.

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.

The roles of selectivity filters in determining aluminum transport by AtNIP1;2

Aquaporins (AQPs) are channel proteins involved in transporting a variety of substrates. It has been proposed that the constriction regions in the central pores of the AQP channels play a crucial role in determining transport substrates and activities of AQPs. Our previous results suggest that AtNIP1;2, a member of the AQP superfamily in Arabidopsis, facilitates aluminum transport across the plasma membrane. However, the functions of the constriction regions in AtNIP1;2-mediated transport activities are unclear. This study reports that residue substitutions of the constriction regions affect AtNIP1;2-mediated aluminum uptake, demonstrating the critical roles of the constriction regions for transport activities. Furthermore, a constriction region that partially or wholly mimics AtNIP5;1, a demonstrated boric-acid transporter, could not render the boric-acid transport activity to AtNIP1;2. Therefore, besides the constriction regions, other structural features are also involved in determining the nature of AtNIP1;2's transport activities.Abbreviations: AIAR: alanine-isoleucine-alanine-arginine; AIGR: alanine-isoleucine-glycine- arginine; AQP: aquaporin; Al-Mal: aluminum-malate; ar/R: aromatic/arginine; AVAR: alanine-valine-alanine-arginine; CK: control; H: helical domain; ICP-MS: inductively coupled plasma mass spectrometry; LA - LE: inter-helical loops A to E; NIP: nodulin 26-like intrinsic protein; NPA: asparagine-proline-alanine; NPG: asparagine-proline- glycine; NPS: asparagine-proline-Serine; NPV: asparagine-proline-valine; ORF: open reading frame; PIP: plasma membrane intrinsic proteins; SIP: small basic intrinsic proteins; TM: transmembrane helices; WIAR: tryptophan-isoleucine-alanine-arginine; WVAR: tryptophan-valine-alanine-arginine; WVGR: tryptophan-valine-glycine- arginine.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

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.