Synchrotron-based X-ray absorption near-edge spectroscopy imaging for laterally resolved speciation of selenium in fresh roots and leaves of wheat and rice

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.

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.

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.

Effects of Selenium on Growth and Selenium Content Distribution of Virus-Free Sweet Potato Seedlings in Water Culture

Understanding the selenium tolerance of different sweet potato [Dioscorea esculenta (Lour.) Burkill] is essential for simultaneously for breeding of new selenium-tolerant varieties and improving the selenium content in sweet potato. Therefore, a greenhouse experiment was conducted from February to April 2022 to evaluate the effect of sweet potato cultivars and selenium (Na₂SeO₃) concentrations (0-40 mg/L) on plant growth, physiological activities and plant selenium content distribution. The results showed that when the selenium concentration was more than 3 mg/L, the plant growth was significantly affected and the plant height and root length were significantly different compared to the control. While the selenium concentration was 20 and 40 mg/L had the greatest effect on plant growth when the number of internodes and leaves of the plant decreased, the root system stopped growing and the number of internodes of the plant, the number of leaves and the dry-to-fresh weight ratio of the plant a very significant level compared to reached control. The relative amount of chlorophyll in leaves under treatment with a selenium concentration of 1 mg/L was increased, and the relative amount of chlorophyll in 3 mg/L leaves gradually increased with the increase in the selenium concentration. The values of the maximum photochemical efficiency PSII (fv/fm) and the potential activity of PSII (fv/fo) compared to the control under treatment with 40 mg/L selenium concentration and photosynthesis of plants was inhibited. The selenium content in root, stem and leaf increased with the increase in selenium concentration, and the distribution of selenium content in the plant was leaf <stem <root, and the selenium content in root was significantly higher than that in stem and leaf. In summary, the appropriate concentration of selenium tolerance has been determined to be 3 mg/L. The aquatic culture identification method of selenium tolerance of sweet potatoes and growth indices of various selenium tolerant varieties (lines) established in this study will provide a technical basis for selenium tolerant cultivation and mechanism research.

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.

Selenium in cereals: Insight into species of the element from total amount

Selenium (Se) is a trace mineral micronutrient essential for human health. The diet is the main source of Se intake. Se-deficiency is associated with many diseases, and up to 1 billion people suffer from Se-deficiency worldwide. Cereals are considered a good choice for Se intake due to their daily consumption as staple foods. Much attention has been paid to the contents of Se in cereals and other foods. Se-enriched cereals are produced by biofortification. Notably, the gap between the nutritional and toxic levels of Se is fairly narrow. The chemical structures of Se compounds, rather than their total contents, contribute to the bioavailability, bioactivity, and toxicity of Se. Organic Se species show better bioavailability, higher nutritional value, and less toxicity than inorganic species. In this paper, we reviewed the total content of Se in cereals, Se speciation methods, and the biological effects of Se species on human health. Selenomethionine (SeMet) is generally the most prevalent and important Se species in cereal grains. In conclusion, Se species should be considered in addition to the total Se content when evaluating the nutritional and toxic values of foods such as cereals.

Selenium transport and metabolism in plants: Phytoremediation and biofortification implications

The aim of this review is to synthesize current knowledge of selenium (Se) transport and metabolism in plants, with a focus on implications for biofortification and phytoremediation. Selenium is a necessary human micronutrient, and around a billion people worldwide may be Se deficient. This can be ameliorated by Se biofortification of staple crops. Selenium is also a potential toxin at higher concentrations, and multiple environmental disasters over the past 50 years have been caused by Se pollution from agricultural and industrial sources. Phytoremediation by plants able to take up large amounts of Se is an important tool to combat pollution issues. Both biofortification and phytoremediation applications require a thorough understanding of how Se is taken up and metabolized by plants. Selenium uptake and translocation in plants are largely accomplished via sulfur (S) transport proteins. Current understanding of these transporters is reviewed here, and transporters that may be manipulated to improve Se uptake are discussed. Plant Se metabolism also largely follows the S metabolic pathway. This pathway is reviewed here, with special focus on genes that have been, or may be manipulated to reduce the accumulation of toxic metabolites or enhance the accumulation of nontoxic metabolites. Finally, unique aspects of Se transport and metabolism in Se hyperaccumulators are reviewed. Hyperaccumulators, which can accumulate Se at up to 1000 times higher concentrations than normal plants, present interesting specialized systems of Se transport and metabolism. Selenium hyperaccumulation mechanisms and potential applications of these mechanisms to biofortification and phytoremediation are presented.

Selenium Deficiency—From Soil to Thyroid Cancer

Selenium (Se) is an essential micronutrient present in human diet, entering in the composition of selenoproteins as selenocysteine (Se-Cys) amino acid. At the thyroid level, these proteins play an important role as antioxidant and in hormone metabolism. Selenoproteins are essential for the balance of redox homeostasis and antioxidant defense of mammalian organisms, while the corresponding imbalance is now recognized as the cause of many diseases including cancer. The food chain is the main source of Se in human body. Dietary intake is strongly correlated with Se content in soil and varies according to several factors such as geology and atmospheric input. Both Se deficiency and toxicity have been associated with adverse health effects. This review synthesizes recent data on the transfer of Se from soil to humans, Se U-shaped deficiency and toxicity uptake effects and particularly the impact of Se deficiency on thyroid cancer.

Uptake, translocation and biotransformation of selenium nanoparticles in rice seedlings (Oryza sativa L.)

Background

Selenium (Se) in soil mainly consists of selenite, selenate, and elemental Se. However, little is known about the mechanism involved in the uptake and biotransformation of elemental Se by plants.

Results

In this study, the uptake, translocation, subcellular distribution and biotransformation of selenium nanoparticles (SeNPs) in rice (Oryza sativa L.), and a comparison with selenite and selenate, were investigated through hydroponic experiments. The study revealed that SeNPs could be absorbed by rice plants; and aquaporin inhibitor was responsible for a 60.4% inhibition of SeNP influx, while metabolic inhibitor was ineffective. However, the SeNPs uptake rate of rice roots was approximately 1.7 times slower than that of selenite or selenate. Under the SeNPs or selenite treatment, Se was primarily accumulated in roots rather than in shoots, whereas an opposite trend was observed with selenate treatment. Additionally, most of the absorbed Se was distributed in cell wall of the SeNPs or selenite treated-rice plants, while its proportion was the highest in soluble cytosol of the selenate treated-rice plants. The absorbed SeNPs or selenite was rapidly assimilated to organic forms, with SeMet being the most predominant species in both shoots and roots of the rice plants. However, following selenate treatment, Se(VI) remained as the most predominant species, and only a small amount of it was converted to organic forms.

Conclusion

Therefore, this study provides a deeper understanding of the mechanisms associated SeNPs uptake and biotransformation within plants.

Methods to Visualize Elements in Plants

Understanding the distribution of elements in plants is important for researchers across a broad range of fields, including plant molecular biology, agronomy, plant physiology, plant nutrition, and ionomics. However, it is often challenging to evaluate the applicability of the wide range of techniques available, with each having its own strengths and limitations. Here, we compare scanning/transmission electron microscopy-based energy-dispersive x-ray spectroscopy, x-ray fluorescence microscopy, particle-induced x-ray emission, laser ablation inductively coupled plasma-mass spectrometry, nanoscale secondary ion mass spectroscopy, autoradiography, and confocal microscopy with fluorophores. For these various techniques, we compare their accessibility, their ability to analyze hydrated tissues (without sample preparation) and suitability for in vivo analyses, as well as examining their most important analytical merits, such as resolution, sensitivity, depth of analysis, and the range of elements that can be analyzed. We hope that this information will assist other researchers to select, access, and evaluate the approach that is most useful in their particular research program or application.

Development of XANES nanoscopy on BL7C at PLS-II

X-ray absorption near-edge structure (XANES) imaging is a powerful tool to visualize the chemical state distribution of transition-metal-based materials at synchrotron radiation facilities. In recent years, the electrochemical working rechargeable battery has been the most studied material in XANES imaging owing to the large increase of portable electronics and electric vehicles. This work acknowledges the importance of battery analysis and has developed the XANES imaging system on BL7C at Pohang Light Source-II (PLS-II). BL7C employs an undulator taper configuration to obtain an energy band >130 eV near the K-absorption edge of the target element with a minimum energy interval >0.2 eV. While measuring energy-dependent images, the zone plate translation maintains the best focus, and then various data processes such as background correction, image registration and clustering allow single XANES spectrum extraction and chemical distribution mapping. Here, the XANES imaging process is described, the XANES spectrum quality is identified and the chemical states of the partially charged cathode material used in lithium-ion batteries as an application example are examined.

Selenomethionine induces oxidative stress and modifies growth in rice (Oryza sativa L.) seedlings through effects on hormone biosynthesis and primary metabolism

Although the chemical characteristics of selenomethionine (SeMet) are similar to those of methionine (Met), the physiological activity of SeMet apparently differs in its ability to stimulate ethylene production in plant tissues. Since selenium alters root architecture of rice seedlings by modifying ethylene production, the investigation of the effect of SeMet and Met on rice growth would be a step forward towards unraveling factors that underlie selenium toxicity. Here, we report that SeMet increased concentrations of reactive oxygen species (ROS), inhibiting auxin and increasing ethylene production in rice seedlings. The effect of SeMet on seedlings was mediated by the inhibition of the abundance of transcripts encoding auxin transport and cell expansion proteins. Moreover, SeMet led to increased seedling respiration, which was positively correlated with organic acids consumption, but negatively with sugars consumption, thereby decreasing seedling growth. In contrast with SeMet treatment, Met did not affect ROS production, hormone biosynthesis and seedling growth, indicating an exclusive selenium effect. The singlet oxygen scavenger, 1,4-diazabicyclooctane, overrode the repressive effect of SeMet in seedling growth. Our results demonstrate a phytotoxic effect of SeMet for rice seedlings and reveal a relationship between reactive oxygen species, hormone homeostasis and carbon availability, which regulates growth responses.

An Overview of Hazardous Impacts of Soil Salinity in Crops, Tolerance Mechanisms, and Amelioration through Selenium Supplementation

Soil salinization is one of the major environmental stressors hampering the growth and yield of crops all over the world. A wide spectrum of physiological and biochemical alterations of plants are induced by salinity, which causes lowered water potential in the soil solution, ionic disequilibrium, specific ion effects, and a higher accumulation of reactive oxygen species (ROS). For many years, numerous investigations have been made into salinity stresses and attempts to minimize the losses of plant productivity, including the effects of phytohormones, osmoprotectants, antioxidants, polyamines, and trace elements. One of the protectants, selenium (Se), has been found to be effective in improving growth and inducing tolerance against excessive soil salinity. However, the in-depth mechanisms of Se-induced salinity tolerance are still unclear. This review refines the knowledge involved in Se-mediated improvements of plant growth when subjected to salinity and suggests future perspectives as well as several research limitations in this field.

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

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

Zinc and lead encapsulated in amorphous ferric cements within hardpans in situ formed from sulfidic Cu-Pb-Zn tailings

Hardpans are massively indurated layers formed at the top layer of sulfidic tailings dams, which develop cementation structures and result in heavy metal immobilization. However, the micro-structural and complex forms of the cementing materials are not fully understood, as well as the mechanisms by which Zn and Pb are stabilized in the hardpans. The present study deployed synchrotron-based X-ray fluorescence microscopy (XFM) to have characterized the cementing structures, examined the distribution of Fe, Zn and Pb, and obtained laterally-resolved speciation of Zn within the hardpans using fluorescence X-ray absorption near-edge structure (XANES) imaging. The XFM analyses revealed that the Fe-rich cement layers consisted of Fe (oxyhydr)oxides coupled with amorphous Si materials, immobilizing Zn and Pb. Through laterally-resolved XANES imaging analyses, Zn-ferrihydrite-like precipitates were predicted to account for >76% of the total Zn within the Fe-rich cement layers. In contrast, outside of the cement layers, 9-63% of the Zn was estimated as labile ZnSO₄^.7H₂O, with the remainder in the form of Zn-sulfide. These findings demonstrated that the Fe-rich cement layers were critical in immobilizing Zn and Pb within hardpans via mineral passivation and encapsulation, as the basis for long-term geochemical stability in the hardpan layer of sulfidic mine tailings.

Recent advances in analysis of trace elements in environmental samples by X-ray based techniques (IUPAC Technical Report)

Trace elements analysis is a fundamental challenge in environmental sciences. Scientists measure trace elements in environmental media in order to assess the quality and safety of ecosystems and to quantify the burden of anthropogenic pollution. Among the available analytical techniques, X-ray based methods are particularly powerful, as they can quantify trace elements in situ. Chemical extraction is not required, as is the case for many other analytical techniques. In the last few years, the potential for X-ray techniques to be applied in the environmental sciences has dramatically increased due to developments in laboratory instruments and synchrotron radiation facilities with improved sensitivity and spatial resolution. In this report, we summarize the principles of the X-ray based analytical techniques most frequently employed to study trace elements in environmental samples. We report on the most recent developments in laboratory and synchrotron techniques, as well as advances in instrumentation, with a special attention on X-ray sources, detectors, and optics. Lastly, we inform readers on recent applications of X-ray based analysis to different environmental matrices, such as soil, sediments, waters, wastes, living organisms, geological samples, and atmospheric particulate, and we report examples of sample preparation.

Synchrotron-Based X-Ray Fluorescence Microscopy as a Technique for Imaging of Elements in Plants

Understanding the distribution of elements within plant tissues is important across a range of fields in plant science. In this review, we examine synchrotron-based x-ray fluorescence microscopy (XFM) as an elemental imaging technique in plant sciences, considering both its historical and current uses as well as discussing emerging approaches. XFM offers several unique capabilities of interest to plant scientists, including in vivo analyses at room temperature and pressure, good detection limits (approximately 1-100 mg kg-1), and excellent resolution (down to 50 nm). This has permitted its use in a range of studies, including for functional characterization in molecular biology, examining the distribution of nutrients in food products, understanding the movement of foliar fertilizers, investigating the behavior of engineered nanoparticles, elucidating the toxic effects of metal(loid)s in agronomic plant species, and studying the unique properties of hyperaccumulating plants. We anticipate that continuing technological advances at XFM beamlines also will provide new opportunities moving into the future, such as for high-throughput screening in molecular biology, the use of exotic metal tags for protein localization, and enabling time-resolved, in vivo analyses of living plants. By examining current and potential future applications, we hope to encourage further XFM studies in plant sciences by highlighting the versatility of this approach.

Synchrotron-based X-Ray Approaches for Examining Toxic Trace Metal(loid)s in Soil-Plant Systems

Elevated levels of trace metal(loid)s reduce plant growth, both in soils contaminated by industrial activities and in acid agricultural soils. Although the adverse effects of trace metal(loid)s have long been recognized, there remains much unknown both about their behavior in soils, their toxicity to plants, and the mechanisms that plants use to tolerate elevated concentrations. Synchrotron-based approaches are being utilized increasingly in soil-plant systems to examine toxic metal(loid)s. In the present review, brief consideration is given to the theory of synchrotron radiation. Thereafter, we review the use of synchrotron-based approaches for the examination of various trace metal(loid)s in soil-plant systems, including aluminum, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, and cadmium. Within the context of this review, X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (μ-XRF) are of particular interest. These techniques can provide in situ analyses of the distribution and speciation of metal(loid)s in soil-plant systems. The information presented here serves not only to understand the behavior of trace metals in soil-plant systems, but also to provide examples of the potential applications of synchrotron radiation that can be used to advantage in other studies.

Impact of Surface Charge on Cerium Oxide Nanoparticle Uptake and Translocation by Wheat (Triticum aestivum)

Nanoparticle (NP) physiochemical properties, including surface charge, affect cellular uptake, translocation, and tissue localization. To evaluate the influence of surface charge on NP uptake by plants, wheat seedlings were hydroponically exposed to 20 mg/L of ∼4 nm CeO₂ NPs functionalized with positively charged, negatively charged, and neutral dextran coatings. Fresh, hydrated roots and leaves were analyzed at various time points over 34 h using fluorescence X-ray absorption near-edge spectroscopy to provide laterally resolved spatial distribution and speciation of Ce. A 15-20% reduction from Ce(IV) to Ce(III) was observed in both roots and leaves, independent of NP surface charge. Because of its higher affinity with negatively charged cell walls, CeO₂(+) NPs adhered to the plant roots the strongest. After 34 h, CeO₂(-), and CeO₂(0) NP exposed plants had higher Ce leaf concentrations than the plants exposed to CeO₂(+) NPs. Whereas Ce was found mostly in the leaf veins of the CeO₂(-) NP exposed plant, Ce was found in clusters in the nonvascular leaf tissue of the CeO₂(0) NP exposed plant. These results provide important information for understanding mechanisms responsible for plant uptake, transformation, and translocation of NPs, and suggest that NP coatings can be designed to target NPs to specific parts of plants.

Ferric minerals and organic matter change arsenic speciation in copper mine tailings

Arsenic (As) is commonly associated with Cu ore minerals, with the resultant risk that As can be released offsite from mine tailings. We used synchrotron-based fluorescence X-ray absorption near-edge spectroscopy (XANES) imaging to provide in situ, laterally-resolved speciation of As within tailings which differed in magnetite content (5-12%) and organic matter content (0-5%). Although the total As content was lower in tailings with low magnetite (LM), the soluble (pore water) As was actually 7-times higher in LM tailings than in high magnetite (HM) tailings. Additionally, amendment with 5% sugarcane mulch residues (SMR) (for revegetation) further increased soluble As due to the dissolution and oxidation of arsenopyrite or orpiment. Indeed, in HM tailings, arsenopyrite and orpiment initially accounted for 88% of the total As, which decreased to 48% upon the addition of SMR - this being associated with an increase in As^V-ferrihydrite from 12% to 52%. In LM tailings, the pattern of As distribution and speciation was similar, with As as As^V-ferrihydrite increasing from 57% to 75% upon the addition of SMR. These findings indicate that changes in ore processing technology, such as the recovery of magnetite could have significant environmental consequences regarding the As mobilisation and transformation in mine tailings.

Growth inhibition by selenium is associated with changes in primary metabolism and nutrient levels in Arabidopsis thaliana

Although Selenium (Se) stress is relatively well known for causing growth inhibition, its effects on primary metabolism remain rather unclear. Here, we characterized both the modulation of the expression of specific genes and the metabolic adjustments in Arabidopsis thaliana in response to changes in Se level in the soil. Se treatment culminated with strong inhibition of both shoot and root growth. Notably, growth inhibition in Se-treated plants was associated with an incomplete mobilization of starch during the night. Minor changes in amino acids levels were observed in shoots and roots of plants treated with Se whereas the pool size of tricarboxylic acid (TCA) cycle intermediates in root was not altered in response to Se. By contrast, decreased levels of organic acids involved in the first part of the TCA cycle were observed in shoots of Se-treated plants. Furthermore, decreased expression levels of expansins and endotransglucosylases/endohydrolases (XHTs) genes were observed after Se treatment, coupled with a significant decrease in the levels of essential elements. Collectively, our results revealed an exquisite interaction between energy metabolism and Se-mediated control of growth in Arabidopsis thaliana to coordinate cell wall extension, starch turnover and the levels of a few essential nutrients.

Element distribution and iron speciation in mature wheat grains (Triticum aestivum L.) using synchrotron X-ray fluorescence microscopy mapping and X-ray absorption near-edge structure (XANES) imaging

Several studies have suggested that the majority of iron (Fe) and zinc (Zn) in wheat grains are associated with phytate, but a nuanced approach to unravel important tissue-level variation in element speciation within the grain is lacking. Here, we present spatially resolved Fe-speciation data obtained directly from different grain tissues using the newly developed synchrotron-based technique of X-ray absorption near-edge spectroscopy imaging, coupling this with high-definition μ-X-ray fluorescence microscopy to map the co-localization of essential elements. In the aleurone, phosphorus (P) is co-localized with Fe and Zn, and X-ray absorption near-edge structure imaging confirmed that Fe is chelated by phytate in this tissue layer. In the crease tissues, Zn is also positively related to P distribution, albeit less so than in the aleurone. Speciation analysis suggests that Fe is bound to nicotianamine rather than phytate in the nucellar projection, and that more complex Fe structures may also be present. In the embryo, high Zn concentrations are present in the root and shoot primordium, co-occurring with sulfur and presumably bound to thiol groups. Overall, Fe is mainly concentrated in the scutellum and co-localized with P. This high resolution imaging and speciation analysis reveals the complexity of the physiological processes responsible for element accumulation and bioaccessibility.

Tracking Se Assimilation and Speciation through the Rice Plant - Nutrient Competition, Toxicity and Distribution

Up to 1 billion people are affected by low intakes of the essential nutrient selenium (Se) due to low concentrations in crops. Biofortification of this micronutrient in plants is an attractive way of increasing dietary Se levels. We investigated a promising method of Se biofortification of rice seedlings, as rice is the primary staple for 3 billion people, but naturally contains low Se concentrations. We studied hydroponic Se uptake for 0-2500 ppb Se, potential phyto-toxicological effects of Se and the speciation of Se along the shoots and roots as a function of added Se species, concentrations and other nutrients supplied. We found that rice germinating directly in a Se environment increased plant-Se by factor 2-16, but that nutrient supplementation is required to prevent phyto-toxicity. XANES data showed that selenite uptake mainly resulted in the accumulation of organic Se in roots, but that selenate uptake resulted in accumulation of selenate in the higher part of the shoot, which is an essential requirement for Se to be transported to the grain. The amount of organic Se in the plant was positively correlated with applied Se concentration. Our results indicate that biofortification of seedlings with selenate is a successful method to increase Se levels in rice.