Se Hyperaccumulators Research Articles

Evolution of selenium hyperaccumulation in Stanleya (Brassicaceae) as inferred from phylogeny, physiology and X-ray microprobe analysis.

Past studies have identified herbivory as a likely selection pressure for the evolution of hyperaccumulation, but few have tested the origin(s) of hyperaccumulation in a phylogenetic context. We focused on the evolutionary history of selenium (Se) hyperaccumulation in Stanleya (Brassicaceae). Multiple accessions were collected for all Stanleya taxa and two outgroup species. We sequenced four nuclear gene regions and performed a phylogenetic analysis. Ancestral reconstruction was used to predict the states for Se-related traits in a parsimony framework. Furthermore, we tested the taxa for Se localization and speciation using X-ray microprobe analyses. True hyperaccumulation was found in three taxa within the S. pinnata/bipinnata clade. Tolerance to hyperaccumulator Se concentrations was found in several taxa across the phylogeny, including the hyperaccumulators. X-ray analysis revealed two distinct patterns of leaf Se localization across the genus: marginal and vascular. All taxa accumulated predominantly (65-96%) organic Se with the C-Se-C configuration. These results give insight into the evolution of Se hyperaccumulation in Stanleya and suggest that Se tolerance and the capacity to produce organic Se are likely prerequisites for Se hyperaccumulation in Stanleya.

Characterization of selenium and sulfur accumulation across the genus Stanleya (Brassicaceae): A field survey and common-garden experiment.

• Selenium (Se) hyperaccumulation, the capacity to concentrate the toxic element Se above 1000 mg·kg(-1)·dry mass, is found in relatively few taxa native to seleniferous soils. While Se hyperaccumulation has been shown to likely be an adaptation that protects plants from herbivory, its evolutionary history remains unstudied. Stanleya (Brassicaceae) is a small genus comprising seven species endemic to the western United States. Stanleya pinnata is a hyperaccumulator of selenium (Se). In this study we investigated to what extent other Stanleya taxa accumulate Se both in the field and a greenhouse setting on seleniferous soil.• We collected multiple populations of six of the seven species and all four varieties of S. pinnata We tested leaves, fruit, and soil for in situ Se and sulfur (S) concentrations. The seeds collected in the field were used for a common garden study in a greenhouse.• We found that S. pinnata var. pinnata is the only hyperaccumulator of Se. Within S. pinnata var. pinnata, we found a geographic pattern related to Se hyperaccumulation where the highest accumulating populations are found on the eastern side of the continental divide. We also found differences in genome size within the S. pinnata species complex.• The S. pinnata species complex has a range of physiological properties making it an attractive system to study the evolution of Se hyperaccumulation. Beyond the basic scientific value of understanding the evolution of this fascinating trait, we can potentially use S. pinnata or its genes for environmental cleanup and/or nutrient-enhanced dietary material.

Analysis of selenium accumulation, speciation and tolerance of potential selenium hyperaccumulatorSymphyotrichum ericoides

Symphyotrichum ericoides was shown earlier to contain hyperaccumulator levels of selenium (Se) in the field (>1000 mg kg(-1) dry weight (DW)), but only when growing next to other Se hyperaccumulators. It was also twofold larger next to hyperaccumulators and suffered less herbivory. This raised two questions: whether S. ericoides is capable of hyperaccumulation without neighbor assistance, and whether its Se-derived benefit is merely ecological or also physiological. Here, in a comparative greenhouse study, Se accumulation and tolerance of S. ericoides were analyzed in parallel with hyperaccumulator Astragalus bisulcatus, Se accumulator Brassica juncea and related Asteraceae Machaeranthera tanacetifolia. Symphyotrichum ericoides and M. tanacetifolia accumulated Se up to 3000 and 1500 mg Se kg(-1) DW, respectively. They were completely tolerant to these Se levels and even grew 1.5- to 2.5-fold larger with Se. Symphyotrichum ericoides showed very high leaf Se/sulfur (S) and shoot/root Se concentration ratios, similar to A. bisulcatus and higher than M. tanacetifolia and B. juncea. Se X-ray absorption near-edge structure spectroscopy showed that S. ericoides accumulated Se predominantly (86%) as C-Se-C compounds indistinguishable from methyl-selenocysteine, which may explain its Se tolerance. Machaeranthera tanacetifolia accumulated 55% of its Se as C-Se-C compounds; the remainder was inorganic Se. Thus, in this greenhouse study S. ericoides displayed all of the characteristics of a hyperaccumulator. The larger size of S. ericoides when growing next to hyperaccumulators may be explained by a physiological benefit, in addition to the ecological benefit demonstrated earlier.

Inoculation of selenium hyperaccumulator Stanleya pinnata and related non‐accumulator Stanleya elata with hyperaccumulator rhizosphere fungi—investigation of effects on Se accumulation and speciation

Little is known about how fungi affect elemental accumulation in hyperaccumulators (HAs). Here, two rhizosphere fungi from selenium (Se) HA Stanleya pinnata, Alternaria seleniiphila (A1) and Aspergillus leporis (AS117), were used to inoculate S. pinnata and related non-HA Stanleya elata. Growth and Se and sulfur (S) accumulation were analyzed. Furthermore, X-ray microprobe analysis was used to investigate elemental distribution and speciation. Growth of S. pinnata was not affected by inoculation or by Se. Stanleya elata growth was negatively affected by AS117 and by Se, but combination of both did not reduce growth. Selenium translocation was reduced in inoculated S. pinnata, and inoculation reduced S translocation in both species. Root Se distribution and speciation were not affected by inoculation in either species; both species accumulated mainly (90%) organic Se. Sulfur, in contrast, was present equally in organic and inorganic forms in S. pinnata roots. Thus, these rhizosphere fungi can affect growth and Se and/or S accumulation, depending on host species. They generally enhanced root accumulation and reduced translocation. These effects cannot be attributed to altered plant Se speciation but may involve altered rhizosphere speciation, as these fungi are known to produce elemental Se. Reduced Se translocation may be useful in applications where toxicity to herbivores and movement of Se into the food chain is a concern. The finding that fungal inoculation can enhance root Se accumulation may be useful in Se biofortification or phytoremediation using root crop species.

Exploring tradeoffs in hyperaccumulator ecology and evolution

Exploring tradeoffs in hyperaccumulator ecology and evolution

Advances in the study of genetic enrichment of selenium in plants

Selenium (Se) is a micronutrient essential for human and animal health. Due to the unbalanced distribution of Se resources in the world, Se deficiency is regarded as a major human health problem. Insufficient ingestion of Se in human caused a series of disease including Keshan disease and Kashin-Beck's disease. While some parts of the world was Se poisoning due to the Se ores on geological stratification or Se pollution. The using Se-enriched and/or hyperaccumulated green plants to treat these problems has acquired more and more attention worldwide from last decade. This paper describes current advances in the efforts of searching Se enriched plant genetic resources, clarifying the Se metabolism and accumulation mechanism in plants, and cloning key genes responsible for Se accumulation or hyper-accumulation and their transformation in plants.

Selenium hyperaccumulator plants Stanleya pinnata and Astragalus bisulcatus are colonized by Se-resistant, Se-excluding wasp and beetle seed herbivores.

Selenium (Se) hyperaccumulator plants can concentrate the toxic element Se up to 1% of shoot (DW) which is known to protect hyperaccumulator plants from generalist herbivores. There is evidence for Se-resistant insect herbivores capable of feeding upon hyperaccumulators. In this study, resistance to Se was investigated in seed chalcids and seed beetles found consuming seeds inside pods of Se-hyperaccumulator species Astragalus bisulcatus and Stanleya pinnata. Selenium accumulation, localization and speciation were determined in seeds collected from hyperaccumulators in a seleniferous habitat and in seed herbivores. Astragalus bisulcatus seeds were consumed by seed beetle larvae (Acanthoscelides fraterculus Horn, Coleoptera: Bruchidae) and seed chalcid larvae (Bruchophagus mexicanus, Hymenoptera: Eurytomidae). Stanleya pinnata seeds were consumed by an unidentified seed chalcid larva. Micro X-ray absorption near-edge structure (µXANES) and micro-X-Ray Fluorescence mapping (µXRF) demonstrated Se was mostly organic C-Se-C forms in seeds of both hyperaccumulators, and S. pinnata seeds contained ∼24% elemental Se. Liquid chromatography–mass spectrometry of Se-compounds in S. pinnata seeds detected the C-Se-C compound seleno-cystathionine while previous studies of A. bisulcatus seeds detected the C-Se-C compounds methyl-selenocysteine and γ-glutamyl-methyl-selenocysteine. Micro-XRF and µXANES revealed Se ingested from hyperaccumulator seeds redistributed throughout seed herbivore tissues, and portions of seed C-Se-C were biotransformed into selenocysteine, selenocystine, selenodiglutathione, selenate and selenite. Astragalus bisulcatus seeds contained on average 5,750 µg Se g−1, however adult beetles and adult chalcid wasps emerging from A. bisulcatus seed pods contained 4–6 µg Se g−1. Stanleya pinnata seeds contained 1,329 µg Se g−1 on average; however chalcid wasp larvae and adults emerging from S. pinnata seed pods contained 9 and 47 µg Se g−1. The results suggest Se resistant seed herbivores exclude Se, greatly reducing tissue accumulation; this explains their ability to consume high-Se seeds without suffering toxicity, allowing them to occupy the unique niche offered by Se hyperaccumulator plants.

Selenium hyperaccumulation by Astragalus (Fabaceae) does not inhibit root nodule symbiosis

A survey of the root-nodule symbiosis in Astragalus and its interaction with selenium (Se) has not been conducted before. Such studies can provide insight into how edaphic conditions modify symbiotic interactions and influence partner coevolution. In this paper plant-organ Se concentration ([Se]) was investigated to assess potential Se exposure to endophytes. • Selenium distribution and molecular speciation of root nodules from Se-hyperaccumulators Astragalus bisulcatus, A. praelongus, and A. racemosus was determined by Se K-edge x-ray absorption spectroscopy. A series of greenhouse experiments were conducted to characterize the response of root-nodule symbiosis in Se-hyperaccumulators and nonhyperaccumulators. • Nodules in three Se-hyperaccumulators (Astragalus crotalariae, A. praelongus, and A. preussii) are reported for the first time. Leaves, flowers, and fruits from Se-hyperaccumulators were routinely above the hyperaccumulator threshold (1,000 µg Se g(-1) DW), but root samples rarely contained that amount, and nodules never exceeded 110 µg Se g(-1) DW. Nodules from A. bisulcatus, A. praelongus, and A. racemosus had Se throughout, with a majority stored in C-Se-C form. Finally, an evaluation of nodulation in Se-hyperaccumulators and nonhyperaccumulators indicated that there was no nodulation inhibition because of plant Se tolerance. Rather, we found that in Se-hyperaccumulators higher levels of Se treatment (up to 100 µM Se) corresponded with higher nodule counts, indicating a potential role for dinitrogen fixation in Se-hyperaccumulation. The effect was not found in nonhyperaccumulators. • As the evolution of Se hyperaccumulation in Astragalus developed, root-nodule symbiosis may have played an integral role.

Selenium distribution and speciation in the hyperaccumulator Astragalus bisulcatus and associated ecological partners.

The goal of this study was to investigate how plant selenium (Se) hyperaccumulation may affect ecological interactions and whether associated partners may affect Se hyperaccumulation. The Se hyperaccumulator Astragalus bisulcatus was collected in its natural seleniferous habitat, and x-ray fluorescence mapping and x-ray absorption near-edge structure spectroscopy were used to characterize Se distribution and speciation in all organs as well as in encountered microbial symbionts and herbivores. Se was present at high levels (704-4,661 mg kg(-1) dry weight) in all organs, mainly as organic C-Se-C compounds (i.e. Se bonded to two carbon atoms, e.g. methylselenocysteine). In nodule, root, and stem, up to 34% of Se was found as elemental Se, which was potentially due to microbial activity. In addition to a nitrogen-fixing symbiont, the plants harbored an endophytic fungus that produced elemental Se. Furthermore, two Se-resistant herbivorous moths were discovered on A. bisulcatus, one of which was parasitized by a wasp. Adult moths, larvae, and wasps all accumulated predominantly C-Se-C compounds. In conclusion, hyperaccumulators live in association with a variety of Se-resistant ecological partners. Among these partners, microbial endosymbionts may affect Se speciation in hyperaccumulators. Hyperaccumulators have been shown earlier to negatively affect Se-sensitive ecological partners while apparently offering a niche for Se-resistant partners. Through their positive and negative effects on different ecological partners, hyperaccumulators may influence species composition and Se cycling in seleniferous ecosystems.

Interactions of selenium hyperaccumulators and nonaccumulators during cocultivation on seleniferous or nonseleniferous soil – the importance of having good neighbors

• This study investigated how selenium (Se) affects relationships between Se hyperaccumulator and nonaccumulator species, particularly how plants influence their neighbors' Se accumulation and growth. • Hyperaccumulators Astragalus bisulcatus and Stanleya pinnata and nonaccumulators Astragalus drummondii and Stanleya elata were cocultivated on seleniferous or nonseleniferous soil, or on gravel supplied with different selenate concentrations. The plants were analyzed for growth, Se accumulation and Se speciation. Also, root exudates were analyzed for Se concentration. • The hyperaccumulators showed 2.5-fold better growth on seleniferous than on nonseleniferous soil, and up to fourfold better growth with increasing Se supply; the nonaccumulators showed the opposite results. Both hyperaccumulators and nonaccumulators could affect growth (up to threefold) and Se accumulation (up to sixfold) of neighboring plants. Nonaccumulators S.elata and A.drummondii accumulated predominantly (88-95%) organic C-Se-C; the remainder was selenate. S.elata accumulated relatively more C-Se-C and less selenate when growing adjacent to S.pinnata. Both hyperaccumulators released selenocompounds from their roots. A.bisulcatus exudate contained predominantly C-Se-C compounds; no speciation data could be obtained for S.pinnata. • Thus, plants can affect Se accumulation in neighbors, and soil Se affects competition and facilitation between plants. This helps to explain why hyperaccumulators are found predominantly on seleniferous soils.

Selenometabolomics Explored by Speciation

Selenium (Se) belongs to the same group as sulfur in the periodic table but possesses certain chemical properties characteristic of a metal. It is an essential element in animals but becomes severely toxic when the amount ingested exceeds the required level. On the other hand, Se is not essential in plants although some plants are Se hyperaccumulators. Se changes into several chemical forms when metabolized. Thus, the identification of selenometabolites would enable us to formulate a metabolic chart of Se. Recently, speciation analysis by hyphenated techniques has contributed immensely to the study of selenometabolomes, i.e., the entirety of selenometabolites. Indeed, speciation has unveiled some unique selenometabolites in biological samples. The aim of this review is to present newly identified selenometabolites in animals and plants by speciation using hyphenated techniques and to delineate the perspectives of Se biology and toxicology from the viewpoint of speciation.

Ecological aspects of plant selenium hyperaccumulation

Hyperaccumulators are plants that accumulate toxic elements to extraordinary levels. Selenium (Se) hyperaccumulators can contain 0.1-1.5% of their dry weight as Se, levels toxic to most other organisms. In this review we summarise what is known about the ecological functions and implications of Se (hyper)accumulation by plants. Selenium promotes hyperaccumulator growth and also offers a plant several ecological advantages through negative effects on Se-sensitive partners. High tissue Se levels reduce herbivory and pathogen infection, and high-Se litter deposition can inhibit neighbouring plants. There is no evidence for a cost of hyperaccumulation in terms of reproductive functions or pollinator visitation. Hyperaccumulators offer a niche for Se-tolerant herbivores, pollinators, microbes and neighbouring plants. They may even facilitate these partners through Se enrichment: neighbouring plants with elevated Se levels enjoy enhanced growth and reduced herbivory. Through combined negative and positive effects on ecological partners, Se hyperaccumulators likely affect local plant, microbial and animal species composition and richness, favouring Se-tolerant species at different trophic levels. By locally concentrating Se and altering its chemical form, Se hyperaccumulators likely play an important role in Se entry into, and Se cycling through, seleniferous ecosystems. These findings are of significance since they provide insight into the ecological reverberations of Se hyperaccumulation, and shed light on the possible selection pressures that have led to the evolution of this fascinating phenomenon. Better ecological insight will also help in the management of seleniferous areas and the agricultural production of Se-rich crops for phytoremediation or biofortification.

Biofortification of Tomato (Solanum lycopersicum) Fruit with the Anticancer Compound Methylselenocysteine Using a Selenocysteine Methyltransferase from a Selenium Hyperaccumulator

Methylselenocysteine (MeSeCys) is an amino acid derivative that possesses potent anticancer activity in animals. Plants that can tolerate growth on soils with high Se content, known as Se hyperaccumulators, do so by converting inorganic Se to MeSeCys by the enzyme selenocysteine methyltransferase (SMT). A cDNA encoding the SMT from a Se hyperaccumulator was overexpressed in tomato (Solanum lycopersicum). Transgenic plants were provided with selenite or selenate to the roots during fruit development, and liquid chromatography-mass spectrometry was used to show that MeSeCys accumulated in the fruit but not in the leaves. Depending on the transgenic line and Se treatment, up to 16% of the total Se in the fruit was present as MeSeCys. MeSeCys was produced more effectively from selenite on a percentage conversion basis, but greater accumulation of MeSeCys could be achieved from selenate due to its better translocation from the roots. MeSeCys was heat stable and survived processing of the fruit to tomato juice.

Selenium Hyperaccumulators Facilitate Selenium-Tolerant Neighbors via Phytoenrichment and Reduced Herbivory

Soil surrounding selenium (Se) hyperaccumulator plants was shown earlier to be enriched in Se, impairing the growth of Se-sensitive plant species. Because Se levels in neighbors of hyperaccumulators were higher and Se has been shown to protect plants from herbivory, we investigate here the potential facilitating effect of Se hyperaccumulators on Se-tolerant neighboring species in the field. We measured growth and herbivory of Artemisia ludoviciana and Symphyotrichum ericoides as a function of their Se concentration and proximity to hyperaccumulators Astragalus bisulcatus and Stanleya pinnata. When growing next to hyperaccumulators, A.ludoviciana and S.ericoides contained 10- to 20-fold higher Se levels (800-2,000mg kg(-1) DW) than when growing next to nonaccumulators. The roots of both species were predominantly (70%-90%) directed toward hyperaccumulator neighbors, not toward other neighbors. Moreover, neighbors of hyperaccumulators were 2-fold bigger, showed 2-fold less herbivory damage, and harbored 3- to 4-fold fewer arthropods. When used in laboratory choice and nonchoice grasshopper herbivory experiments, Se-rich neighbors of hyperaccumulators experienced less herbivory and caused higher grasshopper Se accumulation (10-fold) and mortality (4-fold). Enhanced soil Se levels around hyperaccumulators can facilitate growth of Se-tolerant plant species through reduced herbivory and enhanced growth. This study is the firstto show facilitation via enrichment with a nonessential element. It is interesting that Se enrichment of hyperaccumulator neighbors may affect competition in two ways, by reducing growth of Se-sensitive neighbors while facilitating Se-tolerant neighbors. Via these competitive and facilitating effects, Se hyperaccumulators may affect plant community composition and, consequently, higher trophic levels.

Effects of selenium accumulation on reproductive functions in Brassica juncea and Stanleya pinnata.

Selenium (Se) is an essential micronutrient for many organisms, but is also a toxin and environmental pollutant at elevated levels. Due to its chemical similarity to sulphur, most plants readily take up and assimilate Se. Se accumulators such as Brassica juncea can accumulate Se between 0.01% and 0.1% of dry weight (DW), and Se hyperaccumulators such as Stanleya pinnata (Brassicaeae) contain between 0.1% and 1.5% DW of Se. While Se accumulation offers the plant a variety of ecological benefits, particularly protection from herbivory, its potential costs are still unexplored. This study examines the effects of plant Se levels on reproductive functions. In B. juncea, Se concentrations >0.05–0.1% caused decreases in biomass, pollen germination, individual seed and total seed weight, number of seeds produced, and seed germination. In S. pinnata there was no negative effect of increased Se concentration on pollen germination. In cross-pollination of B. juncea plants with different Se levels, both the maternal and paternal Se level affected reproduction, but the maternal Se concentration had the most pronounced effect. Interestingly, high-Se maternal plants were most efficiently pollinated by Se-treated paternal plants. These data provide novel insights into the potential reproductive costs of Se accumulation, interactive effects of Se in pollen grains and in the pistil, and the apparent evolution of physiological tolerance mechanisms in hyperaccumulators to avoid reproductive repercussions.

Selenium accumulation in flowers and its effects on pollination

• Selenium (Se) hyperaccumulation has a profound effect on plant-arthropod interactions. Here, we investigated floral Se distribution and speciation in flowers and the effects of floral Se on pollen quality and plant-pollinator interactions. • Floral Se distribution and speciation were compared in Stanleya pinnata, an Se hyperaccumulator, and Brassica juncea, a comparable nonhyperaccumulator. Pollen germination was measured from plants grown with varying concentrations of Se and floral visitation was compared between plants with high and low Se. • Stanleya pinnata preferentially allocated Se to flowers, as nontoxic methyl-selenocysteine (MeSeCys). Brassica juncea had higher Se concentrations in leaves than flowers, and a lower fraction of MeSeCys. For B. juncea, high floral Se concentration impaired pollen germination; in S. pinnata Se had no effect on pollen germination. Floral visitors collected from Se-rich S. pinnata contained up to 270 μg g(-1), concentrations toxic to many herbivores. Indeed, floral visitors showed no visitation preference between high- and low-Se plants. Honey from seleniferous areas contained 0.4-1 μg Se g(-1), concentrations that could provide human health benefits. • This study is the first to shed light on the possible evolutionary cost, through decreased pollen germination in B. juncea, of Se accumulation and has implications for the management of seleniferous areas.

Characterization of rhizosphere fungi from selenium hyperaccumulator and nonhyperaccumulator plants along the eastern Rocky Mountain Front Range

Selenium-hyperaccumulator plants can store over 1% (dry mass) Se in their tissues, despite the toxicity of this element at high concentrations across eukaryotes. These levels of Se can have widespread effects on the plant's ecological partners, including herbivores and pathogens. Still other partners seem to have coevolved Se tolerance. This is the first known study addressing the rhizosphere mycoflora of Se hyperaccumulators and aims to evaluate the rhizospheric fungal diversity and Se tolerance to further the knowledge of how these organisms interact with their host plants and survive in these extreme habitats. Rhizosphere fungi were isolated from Se-hyperaccumulator and nonaccumulator plant species collected from five sites in Colorado and Wyoming; four seleniferous sites and one nonseleniferous site. 259 isolates were identified to genus or species and evaluated for Se tolerance. Among the 24 represented genera, 11 comprised 86% of the isolates. The majority of isolates from the seleniferous sites were unaffected by 10 mg·L(-1) Se, irrespective of host plant (hyperaccumulator vs. nonaccumulator), while rhizosphere fungi from a control, nonseleniferous site were highly sensitive to Se at 10 mg·L(-1) and as a group were significantly less (α = 0.05) tolerant than the isolates from the seleniferous sites. Even though Se is a commonly used antifungal agent, these results suggest that rhizosphere fungi from seleniferous habitats have widespread Se tolerance, likely an adaptive advantage in their Se-rich habitat.

Selenium accumulation in the floral tissues of two Brassicaceae species and its impact on floral traits and plant performance

Selenium (Se) is a metalloid that can occur naturally in soils from the Cretaceous shale deposits of a prehistoric inland sea in the western United States. Agricultural irrigation and runoff solubilizes Se from these shales, causing buildups of toxic levels of selenate (SeO 4 2−) in water and soil. Our main objective was to investigate the accumulation of Se in two Brassicaceae species chosen for their potential as phytoremediators of Se contaminated soils. We tested the hypothesis that Se will accumulate in the pollen and nectar of two plant species and negatively affect floral traits and plant reproduction. Certain species of Brassicaceae can accumulate high concentrations of Se in their leaf tissues. In this study Se accumulation in plant tissues was investigated under greenhouse conditions. Se accumulator ( Brassica juncea) and Se hyperaccumulator ( Stanleya pinnata) plants were irrigated in sand culture with 0 μM selenate (control), 8 μM selenate, and 13 μM selenate. Nectar and pollen in S. pinnata contained up to 150 μg Se mL −1 wet weight and 12900 μg Se g −1 dry weight when irrigated with 8 μM selenate. Se levels in nectar (110 μg Se mL −1 wet weight) and pollen (1700 μg Se g −1 dry weight) were not as high in B. juncea. Floral display width, petal area and seed pod length were significantly reduced in the 13 μM selenate Se treatment in B. juncea. S. pinnata floral traits and seeds were unaffected by the Se treatments. This study provides crucial information about where some of the highest concentrations of Se are found in two phytoremediators, and may shed light on the potential risks pollinators may face when foraging upon these accumulating plants. In the field, duration of the plant's exposure, Se soil and water concentrations as well as other environmental factors may also play important roles in determining how much Se is accumulated into the leaf and floral tissues. Our greenhouse study shed light on two species’ ability to accumulate Se, as well as determined the specific plant tissues where Se concentrations are highest.

Effects of selenium hyperaccumulation on plant–plant interactions: evidence for elemental allelopathy?

• Few studies have investigated plant-plant interactions involving hyperaccumulator plants. Here, we investigated the effect of selenium (Se) hyperaccumulation on neighboring plants. • Soil and litter Se concentrations were determined around the hyperaccumulators Astragalus bisulcatus and Stanleya pinnata and around the nonhyperaccumulators Medicago sativa and Helianthus pumilus. We also compared surrounding vegetative cover, species composition and Se concentration in two plant species (Artemisia ludoviciana and Symphyotrichum ericoides) growing either close to or far from Se hyperaccumulators. Then, Arabidopsis thaliana germination and growth were compared on soils collected next to the hyperaccumulators and the nonhyperaccumulators. • Soil collected around hyperaccumulators contained more Se (up to 266 mg Se kg(-1) ) than soil collected around nonhyperaccumulators. Vegetative ground cover was 10% lower around Se hyperaccumulators compared with nonhyperaccumulators. The Se concentration was higher in neighboring species A. ludoviciana and S. ericoides when growing close to, compared with far from, Se hyperaccumulators. A. thaliana showed reduced germination and growth, and higher Se accumulation, when grown on soil collected around Se hyperaccumulators compared with soil collected around nonaccumulators. • In conclusion, Se hyperaccumulators may increase the surrounding soil Se concentration (phytoenrichment). The enhanced soil Se contents around hyperaccumulators can impair the growth of Se-sensitive plant species, pointing to a possible role of Se hyperaccumulation in elemental allelopathy.

Selenium Accumulation in Plants—Phytotechnological Applications and Ecological Implications

Selenium (Se) is an essential trace element for many organisms including humans, yet toxic at higher levels. Both Se deficiency and toxicity are problems worldwide. Since plants readily accumulate and volatilize Se, they may be used both as a source of dietary Se and for removing excess Se from the environment. Plant species differ in their capacity to metabolize and accumulate Se, from non-Se accumulators (<100 mg Se/kg DW), to Se-accumulators (100–1000 mg Se/kg DW) to Se hyperaccumulators (>1,000 mg Se/kg DW). Here we review plant mechanisms of Se metabolism in these various plant types. We also summarize results from genetic engineering that have led to enhanced plant Se accumulation, volatilization, and/or tolerance, including field studies. Before using Se-accumulating plants at a large scale we need to evaluate the ecological implications. Research so far indicates that plant Se accumulation significantly affects the plant's ecological interactions below and above ground. Selenium can protect plants from fungal pathogens and from a variety of invertebrate and vertebrate herbivores, due to both deterrence and toxicity. However, specialist (Se-tolerant herbivores), detritivores and endophytes appear to utilize Se hyperaccumulator plants as a resource. These findings are relevant for managing phytoremediation of Se and similar elements.