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

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.

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.

Symbiotic Modulation as a Driver of Niche Expansion of Coastal Plants in the San Juan Archipelago of Washington State

Modern evolutionary theory and population genetics posit that adaptation and habitat expansion of plants result from processes exclusive to their genomes. Here, we present studies showing that plants can grow across complex habitat gradients by modulating symbiotic associations with Class 2 fungal endophytes. Endophyte analysis of three native (Leymus mollis, Distichlis spicata, and Salicornia pacifica) and one invasive (Spartina anglica) plant growing across adjacent microhabitats in the San Juan Archipelago altered associations with Class 2 fungal endophytes in response to soil salinity levels. At the microhabitat interfaces where the gradation of salinity varied, the plants were colonized by endophytes from both microhabitats. A reciprocal transplant study along a salt gradient demonstrated that Leymus mollis (dunegrass) required endophytes indigenous to each microhabitat for optimal fitness and/or survival. In contrast, when dunegrass and Grindelia integrifolia (gumweed) were found growing in low salinity, but high drought habitats, these plant species had their own unique dominant endophyte association regardless of geographic proximity and conferred drought but not high salt stress tolerance. Modulation of endophyte abundance occurred in planta based on the ability of the symbiont to confer tolerance to the stress imposed on plants. The ability of an endophyte to confer appropriate stress tolerance resulted in a significant increase of in planta fungal abundance. Conversely, the inability of an endophyte to confer stress tolerance resulted in a decrease of in planta fungal abundance. Our studies indicate that Class 2 fungal endophytes can provide a symbiotic mechanism for niche expansion and phenotypic plasticity across environmental gradients.

Hyperaccumulator Stanleya pinnata: In Situ Fitness in Relation to Tissue Selenium Concentration

Earlier studies have shown that Stanleya pinnata benefits from selenium hyperaccumulation through ecological benefits and enhanced growth. However, no investigation has assayed the effects of Se hyperaccumulation on plant fitness in the field. This research aimed to analyze how variation in Se accumulation affects S. pinnata fitness, judged from physiological and biochemical performance parameters and herbivory while growing naturally on two seleniferous sites. Natural variation in Se concentration in vegetative and reproductive tissues was determined, and correlations were explored between Se levels with fitness parameters, herbivory damage, and plant defense compounds. Leaf Se concentration varied between 13- and 55-fold in the two populations, averaging 868 and 2482 mg kg−1 dry weight (DW). Furthermore, 83% and 31% of plants from the two populations showed Se hyperaccumulator levels in leaves (>1000 mg kg−1 DW). In seeds, the Se levels varied 3−4-fold and averaged 3372 and 2267 mg kg−1 DW, well above the hyperaccumulator threshold. Plant size and reproductive parameters were not correlated with Se concentration. There was significant herbivory pressure even on the highest-Se plants, likely from Se-resistant herbivores. We conclude that the variation in Se hyperaccumulation did not appear to enhance or compromise S. pinnata fitness in seleniferous habitats within the observed Se range.

Nanoselenium Enhanced Wheat Resistance to Aphids by Regulating Biosynthesis of DIMBOA and Volatile Components

The mechanism of nanoselenium (nano-Se) improving the resistance induced by plant components to aphids is unclear. In this study, foliar sprayed nano-Se (5.0 mg/L) could significantly reduce the Sitobion avenae number (36%) compared with that in the control. Foliar application of nano-Se enhanced the antioxidant capacity by reducing malondialdehyde (MDA) and increasing GSH-Px, CAT, GSH, Pro, and VE concentrations in wheat seedlings. The phenylpropane pathway was activated by nano-Se biofortification, which increased apigenin and caffeic acid concentrations. The high-level expression of the related genes (TaBx1A, TaBx3A, TaBx4A, TaASMT2, and TaCOMT) induced the promotion of melatonin (88.6%) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) (64.3%). Different ratios of the secondary metabolites to nano-Se were taken to examine the effects on resistance of wheat to S. avenae. The results revealed that the combination of nano-Se and melatonin could achieve the best overall performance by reducing the S. avenae number by 52.2%. The study suggests that the coordinated applications of nano-Se and melatonin could more effectively improve the wheat resistance to aphids via the promotion of volatile organic compound synthesis and modulation in phenylpropane and indole metabolism pathways.

Transcription response of Tetranychus cinnabarinus to plant-mediated short-term and long -term selenium treatment

Selenium is a trace element necessary for living organisms. It exists mainly in the form of selenite in nature. In plants, selenium can enhance defenses against pests. In this study, transcriptome sequencing technology was used to analyze the response mechanism of Tetranychus cinnabarinus to plant-mediated selenium treatment. We tested four sodium selenite treatments (5, 20, 50, and 200 μM) that were the same for short (2 d) and long (30 d) treatment durations. The results showed that the number of differentially expressed genes (DEGs) in the short-term treatment was greater than in the long-term treatment. This indicated that the gene expression of spider mites gradually stabilized during the selenium treatment. Regardless of the long-term and short-term conditions, spider mites had the largest response to the 20 μM sodium selenite treatment. The functional annotation classification of DEGs showed no significant difference under different concentrations and treatment durations. A total of 25 genes were differentially expressed in all eight treatments, including four down-regulated cytochrome P450 genes and one up-regulated chitinase gene. We speculate that selenium may have the potential to enhance the activity of chemical acaricides. Transcriptome sequencing of sodium selenite treatment at different concentrations and different times revealed the response mechanism of spider mites under plant-mediated selenium treatment. At the same time, it also provides new clues for the development of methods for preventing and controlling spider mites with selenium.

Effects of selenium hyperaccumulators on soil selenium distribution and vegetation properties

PREMISE

The ecological implications of hyperaccumulation have been investigated at the organismal level, but are poorly understood at the plant community level. Questions addressed here were: Does the presence of selenium (Se) hyperaccumulators affect Se distribution and concentration in their native soil, and do hyperaccumulators affect overall vegetation properties and species composition?

METHODS

Plant survey and soil Se mapping were performed at three seleniferous sites in Colorado. In season one, plots with and without hyperaccumulators were compared for (1) bare ground, canopy cover, and species richness; (2) relative species abundance; (3) soil Se distribution and concentration. In season two, a smaller-scale design was implemented, focusing on areas 3 m in diameter around hyperaccumulators versus nonhyperaccumulators in 44 paired plots on one site.

RESULTS

Plots with hyperaccumulators generally showed more bare ground, less canopy cover, higher species richness, and 2-3-fold higher soil Se levels. These patterns were not consistently significant across all sites; the effects of hyperaccumulators may have been diluted by their low abundance and the relatively large area of survey. In the smaller-scale study, highly significant results were obtained, showing more bare ground, less canopy cover, and higher species richness in plots with hyperaccumulators; soil Se concentration was also higher in plots with hyperaccumulators.

CONCLUSIONS

Hyperaccumulators may significantly affect local soil Se concentration and vegetation over at least a 3 m diameter area, or 4× their canopy. These differences may result from the combined positive and negative allelopathic effects observed earlier at the organismal level.

Identification and physiological comparison of plant species that show positive or negative co-occurrence with selenium hyperaccumulators

Reynolds and coworkers investigated effects of selenium hyperaccumulator plants on local vegetation. Shown is elemental distribution in Alyssum simplex.

Getting to the Root of Selenium Hyperaccumulation—Localization and Speciation of Root Selenium and Its Effects on Nematodes

Elemental hyperaccumulation protects plants from many aboveground herbivores. Little is known about effects of hyperaccumulation on belowground herbivores or their ecological interactions. To examine effects of plant selenium (Se) hyperaccumulation on nematode root herbivory, we investigated spatial distribution and speciation of Se in hyperaccumulator roots using X-ray microprobe analysis, and effects of root Se concentration on root-associated nematode communities. Perennial hyperaccumulators Stanleya pinnata and Astragalus bisulcatus, collected from a natural seleniferous grassland contained 100–1500 mg Se kg−1 root dry weight (DW). Selenium was concentrated in the cortex and epidermis of hyperaccumulator roots, with lower levels in the stele. The accumulated Se consisted of organic (C-Se-C) compounds, indistinguishable from methyl-selenocysteine. The field-collected roots yielded 5–400 nematodes g−1 DW in Baermann funnel extraction, with no correlation between root Se concentration and nematode densities. Even roots containing > 1000 mg Se kg−1 DW yielded herbivorous nematodes. However, greenhouse-grown S. pinnata plants treated with Se had fewer total nematodes than those without Se. Thus, while root Se hyperaccumulation may protect plants from non-specialist herbivorous nematodes, Se-resistant nematode taxa appear to associate with hyperaccumulators in seleniferous habitats, and may utilize high-Se hyperaccumulator roots as food source. These findings give new insight into the ecological implications of plant Se (hyper)accumulation.

On the Ecology of Selenium Accumulation in Plants

Plants accumulate and tolerate Se to varying degrees, up to 15,000 mg Se/kg dry weight for Se hyperaccumulators. Plant Se accumulation may exert positive or negative effects on other species in the community. The movement of plant Se into ecological partners may benefit them at low concentrations, but cause toxicity at high concentrations. Thus, Se accumulation can protect plants against Se-sensitive herbivores and pathogens (elemental defense) and reduce surrounding vegetation cover via high-Se litter deposition (elemental allelopathy). While hyperaccumulators negatively impact Se-sensitive ecological partners, they offer a niche for Se-tolerant partners, including beneficial microbial and pollinator symbionts as well as detrimental herbivores, pathogens, and competing plant species. These ecological effects of plant Se accumulation may facilitate the evolution of Se resistance in symbionts. Conversely, Se hyperaccumulation may evolve driven by increasing Se resistance in herbivores, pathogens, or plant neighbors; Se resistance also evolves in mutualist symbionts, minimizing the plant's ecological cost. Interesting topics to address in future research are whether the ecological impacts of plant Se accumulation may affect species composition across trophic levels (favoring Se resistant taxa), and to what extent Se hyperaccumulators form a portal for Se into the local food chain and are important for Se cycling in the local ecosystem.

Fungal Endophyte Alternaria tenuissima Can Affect Growth and Selenium Accumulation in Its Hyperaccumulator Host Astragalus bisulcatus

Endophytes can enhance plant stress tolerance by promoting growth and affecting elemental accumulation, which may be useful in phytoremediation. In earlier studies, up to 35% elemental selenium (Se0) was found in Se hyperaccumulator Astragalus bisulcatus. Since Se0 can be produced by microbes, the plant Se0 was hypothesized to be microbe-derived. Here we characterize a fungal endophyte of A. bisulcatus named A2. It is common in seeds from natural seleniferous habitat containing 1,000-10,000 mg kg-1 Se. We identified A2 as Alternaria tenuissima via 18S rRNA sequence analysis and morphological characterization. X-ray microprobe analysis of A. bisulcatus seeds that did or did not harbor Alternaria, showed that both contained >90% organic seleno-compounds with C-Se-C configuration, likely methylselenocysteine and glutamyl-methylselenocysteine. The seed Se was concentrated in the embryo, not the seed coat. X-ray microprobe analysis of A2 in pure culture showed the fungus produced Se0 when supplied with selenite, but accumulated mainly organic C-Se-C compounds when supplied with selenate. A2 was completely resistant to selenate up to 300 mg L-1, moderately resistant to selenite (50% inhibition at ∼50 mg Se L-1), but relatively sensitive to methylselenocysteine and to Se extracted from A. bisulcatus (50% inhibition at 25 mg Se L-1). Four-week old A. bisulcatus seedlings derived from surface-sterilized seeds containing endophytic Alternaria were up to threefold larger than seeds obtained from seeds not showing evidence of fungal colonization. When supplied with Se, the Alternaria-colonized seedlings had lower shoot Se and sulfur levels than seedlings from uncolonized seeds. In conclusion, A. tenuissima may contribute to the Se0 observed earlier in A. bisulcatus, and affect host growth and Se accumulation. A2 is sensitive to the Se levels found in its host's tissues, but may avoid Se toxicity by occupying low-Se areas (seed coat, apoplast) and converting plant Se to non-toxic Se0. These findings illustrate the potential for hyperaccumulator endophytes to affect plant properties relevant for phytoremediation. Facultative endophytes may also be applicable in bioremediation and biofortification, owing to their capacity to turn toxic inorganic forms of Se into non-toxic or even beneficial, organic forms with anticarcinogenic properties.

Mechanisms of selenium hyperaccumulation in plants: A survey of molecular, biochemical and ecological cues

BACKGROUND

Selenium (Se) is a micronutrient required for many life forms, but toxic at higher concentration. Plants do not have a Se requirement, but can benefit from Se via enhanced antioxidant activity. Some plant species can accumulate Se to concentrations above 0.1% of dry weight and seem to possess mechanisms that distinguish Se from its analog sulfur (S). Research on these so-called Se hyperaccumulators aims to identify key genes for this remarkable trait and to understand ecological implications.

SCOPE OF REVIEW

This review gives a broad overview of the current knowledge about Se uptake and metabolism in plants, with a special emphasis on hypothesized mechanisms of Se hyperaccumulation. The role of Se in plant defense responses and the associated ecological implications are discussed.

MAJOR CONCLUSIONS

Hyperaccumulators have enhanced expression of S transport and assimilation genes, and may possess transporters with higher specificity for selenate over sulfate. Genes involved in antioxidant reactions and biotic stress resistance are also upregulated. Key regulators in these processes appear to be the growth regulators jasmonic acid, salicylic acid and ethylene. Hyperaccumulation may have evolved owing to associated ecological benefits, particularly protection against pathogens and herbivores, and as a form of elemental allelopathy.

GENERAL SIGNIFICANCE

Understanding plant Se uptake and metabolism in hyperaccumulators has broad relevance for the environment, agriculture and human and animal nutrition and may help generate crops with selenate-specific uptake and high capacity to convert selenate to less toxic, anticarcinogenic, organic Se compounds.

Developmental selenium exposure and health risk in daily foodstuffs: A systematic review and meta-analysis

Selenium (Se) is a trace mineral and an essential nutrient of vital importance to human health in trace amounts. It acts as an antioxidant in both humans and animals, immunomodulator and also involved in the control of specific endocrine pathways. The aim of this work is to provide a brief knowledge on selenium content in daily used various foodstuffs, nutritional requirement and its various health consequences. In general, fruits and vegetables contain low content of selenium, with some exceptions. Selenium level in meat, eggs, poultry and seafood is usually high. For most countries, cereals, legumes, and derivatives are the major donors to the dietary selenium intake. Low level of selenium has been related with higher mortality risk, dysfunction of an immune system, and mental failure. Selenium supplementation or higher selenium content has antiviral outcomes and is necessary for effective reproduction of male and female, also decreases the threat of chronic disease (autoimmune thyroid). Generally, some advantages of higher content of selenium have been shown in various potential studies regarding lung, colorectal, prostate and bladder cancers risk, nevertheless results depicted from different trials have been diverse, which perhaps indicates the evidence that supplementation will merely grant advantage if the intakes of a nutrient is deficient. In conclusion, the over-all people should be advised against the usage of Se supplements for prevention of cardiovascular, hepatopathies, or cancer diseases, as advantages of Se supplements are still ambiguous, and their haphazard usage could result in an increased Se toxicity risk. The associations among Se intake/status and health, or disease risk, are complicated and need exposition to notify medical practice, to improve dietary recommendations, and to develop adequate communal health guidelines.

Phenotypical, physiological and biochemical analyses provide insight into selenium-induced phytotoxicity in rice plants

The present study investigated the phenotypical, physiological and biochemical changes of rice plants exposed to high selenium (Se) concentrations to gain an insight into Se-induced phytotoxicity. Results showed that exposure of rice plants to excessive Se resulted in growth retardation and biomass reduction in connection with the decreased levels of chlorophyll, carotenoids and soluble proteins. The reduced water status and an associated increase in sugar and proline levels indicated Se-induced osmotic stress in rice plants. Measurements of Se contents in roots, leaf sheaths and leaves revealed that Se was highly accumulated in leaves followed by leaf sheaths and roots. Se also potentiated its toxicity by impairing oxidative metabolism, as evidenced by enhanced accumulation of hydrogen peroxide, superoxide and lipid peroxidation product. Se toxicity also displayed a desynchronized antioxidant system by elevating the level of glutathione and the activities of superoxide dismutase, glutathione-S-transferase and glutathione peroxidase, whereas decreasing the level of ascorbic acid and the activities of catalase, glutathione reductase and dehydroascorbate reductase. Furthermore, Se triggered methylglyoxal toxicity by inhibiting the activities of glyoxalases I and II, particularly at higher concentrations of Se. Collectively, our results suggest that excessive Se caused phytotoxic effects on rice plants by inducing chlorosis, reducing sugar, protein and antioxidant contents, and exacerbating oxidative stress and methylglyoxal toxicity. Accumulation levels of Se, proline and glutathione could be considered as efficient biomarkers to indicate degrees of Se-induced phytotoxicity in rice, and perhaps in other crops.

The fascinating facets of plant selenium accumulation - biochemistry, physiology, evolution and ecology

Contents 1582 I. 1582 II. 1583 III. 1588 IV. 1590 V. 1592 1592 References 1592 SUMMARY: The importance of selenium (Se) for medicine, industry and the environment is increasingly apparent. Se is essential for many species, including humans, but toxic at elevated concentrations. Plant Se accumulation and volatilization may be applied in crop biofortification and phytoremediation. Topics covered here include beneficial and toxic effects of Se on plants, mechanisms of Se accumulation and tolerance in plants and algae, Se hyperaccumulation, and ecological and evolutionary aspects of these processes. Plant species differ in the concentration and forms of Se accumulated, Se partitioning at the whole-plant and tissue levels, and the capacity to distinguish Se from sulfur. Mechanisms of Se hyperaccumulation and its adaptive significance appear to involve constitutive up-regulation of sulfate/selenate uptake and assimilation, associated with elevated concentrations of defense-related hormones. Hyperaccumulation has evolved independently in at least three plant families, probably as an elemental defense mechanism and perhaps mediating elemental allelopathy. Elevated plant Se protects plants from generalist herbivores and pathogens, but also gives rise to the evolution of Se-resistant specialists. Plant Se accumulation affects ecological interactions with herbivores, pollinators, neighboring plants, and microbes. Hyperaccumulation tends to negatively affect Se-sensitive ecological partners while facilitating Se-resistant partners, potentially affecting species composition and Se cycling in seleniferous ecosystems.

Intraspecific genetic variation and species coexistence in plant communities

Many studies report that intraspecific genetic variation in plants can affect community composition and coexistence. However, less is known about which traits are responsible and the mechanisms by which variation in these traits affect the associated community. Focusing on plant-plant interactions, we review empirical studies exemplifying how intraspecific genetic variation in functional traits impacts plant coexistence. Intraspecific variation in chemical and architectural traits promotes species coexistence, by both increasing habitat heterogeneity and altering competitive hierarchies. Decomposing species interactions into interactions between genotypes shows that genotype × genotype interactions are often intransitive. The outcome of plant-plant interactions varies with local adaptation to the environment and with dominant neighbour genotypes, and some plants can recognize the genetic identity of neighbour plants if they have a common history of coexistence. Taken together, this reveals a very dynamic nature of coexistence. We outline how more traits mediating plant-plant interactions may be identified, and how future studies could use population genetic surveys of genotype distribution in nature and methods from trait-based ecology to better quantify the impact of intraspecific genetic variation on plant coexistence.

Selenium Biofortification and Phytoremediation Phytotechnologies: A Review

The element selenium (Se) is both essential and toxic for most life forms, with a narrow margin between deficiency and toxicity. Phytotechnologies using plants and their associated microbes can address both of these problems. To prevent Se toxicity due to excess environmental Se, plants may be used to phytoremediate Se from soil or water. To alleviate Se deficiency in humans or livestock, crops may be biofortified with Se. These two technologies may also be combined: Se-enriched plant material from phytoremediation could be used as green fertilizer or as fortified food. Plants may also be used to "mine" Se from seleniferous soils. The efficiency of Se phytoremediation and biofortification may be further optimized. Research in the past decades has provided a wealth of knowledge regarding the mechanisms by which plants take up, metabolize, accumulate, and volatilize Se and the role plant-associated microbes play in these processes. Furthermore, ecological studies have revealed important effects of plant Se on interactions with herbivores, detrivores, pollinators, neighboring vegetation, and the plant microbiome. All this knowledge can be exploited in phytotechnology programs to optimize plant Se accumulation, transformation, volatilization, and/or tolerance via plant breeding, genetic engineering, and tailored agronomic practices.

Symphyotrichum ericoides populations from seleniferous and nonseleniferous soil display striking variation in selenium accumulation

Symphyotrichum ericoides (Asteraceae) from naturally seleniferous habitat (Pine Ridge) was shown previously to have selenium (Se) hyperaccumulator properties in field and glasshouse studies, and to benefit from Se through protection from herbivory. To investigate whether Se hyperaccumulation is ubiquitous in S. ericoides or restricted to seleniferous soils, the S. ericoides Pine Ridge (PR) population was compared with the nearby Cloudy Pass (CP) population from nonseleniferous soil. The S. ericoidesPR and CP populations were strikingly physiologically different: in a common garden experiment, PR plants accumulated up to 40-fold higher Se concentrations than CP plants and had 10-fold higher Se : sulfur (S) ratios. Moreover, roots of S. ericoidesPR plants showed directional growth toward selenate, while CP roots did not. Growth of both accessions responded positively to Se. Each accession grew best on its own soil. Rhizosphere soil inoculum from the S. ericoidesPR population stimulated plant growth and Se accumulation in both S. ericoidesPR and S. ericoidesCP plants, on both PR and CP soils. While the S. ericoidesPR population hyperaccumulates Se, the nearby CP population does not. The capacity of S. ericoidesPR plants to hyperaccumulate Se appears to be a local phenomenon that is restricted to seleniferous soil. Mutualistic rhizosphere microbes of the S. ericoidesPR population may contribute to the hyperaccumulation phenotype.

Effect of Co-planted Purslane (Portulaca Oleracea L.) on Cd Accumulation by Sunflower in Different Levels of Cd Contamination and Salinity: A Pot Study

Heavy metal bioaccumulation can be affected by various crop-weed interactions that potentially exist in agroecosystems. A pot experiment was conducted to evaluate the role of rhizosphere interaction of sunflower and purslane (Portulaca oleracea L.) weed on cadmium (Cd) uptake and its allocation to sunflower grains. The experimental treatments consisted of two cropping systems (mono and mixed culture), two adjusted salinity levels (0 and 0.5% NaCl) and three artificial levels of Cd in soil (Control, 3 and 6 mg kg(-1)). The results showed that the growth of sunflower in the presence of purslane in comparison to mono culture of sunflower led to change of total Cd content and Cd allocated to grains only in saline conditions. Promoting effects of salinity on Cd concentration of grain were alleviated where sunflower was co-planted with purslane. Besides, supply of Zn in grains of co-planted sunflower was strongly affected by salinity. Results of this study revealed that although co-planted purslane could alter conditions in the shared rhizosphere, it had no effect on enhancing Cd uptake by neighboring sunflower directly.

Do selenium hyperaccumulators affect selenium speciation in neighboring plants and soil? An X-Ray Microprobe Analysis

Neighbors of Se hyperaccumulators Stanleya pinnata and Astragalus bisulcatus were found earlier to have elevated Se levels. Here we investigate whether Se hyperaccumulators affect Se localization and speciation in surrounding soil and neighboring plants. X-ray fluorescence mapping and X-ray absorption near-edge structure spectroscopy were used to analyze Se localization and speciation in leaves of Artemisia ludoviciana, Symphyotrichum ericoides and Chenopodium album growing next to Se hyperaccumulators or non-accumulators at a seleniferous site. Regardless of neighbors, A. ludoviciana, S. ericoides and C. album accumulated predominantly (73-92%) reduced selenocompounds with XANES spectra similar to the C-Se-C compounds selenomethionine and methyl-selenocysteine. Preliminary data indicate that the largest Se fraction (65-75%), both in soil next to hyperaccumulator S. pinnata and next to nonaccumulator species was reduced Se with spectra similar to C-Se-C standards. These same C-Se-C forms are found in hyperaccumulators. Thus, hyperaccumulator litter may be a source of organic soil Se, but soil microorganisms may also contribute. These findings are relevant for phytoremediation and biofortification since organic Se is more readily accumulated by plants, and more effective for dietary Se supplementation.

Analysis of selenium accumulation, speciation and tolerance of potential selenium hyperaccumulator Symphyotrichum 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.

Facilitation allows plant coexistence in Cuban serpentine soils

Serpentine soils represent stressful habitats where plants have to cope with heavy metals, moisture limitation and low nutrient availability. We propose that facilitation is an important mechanism structuring plant communities under such stressful conditions. Facilitation has been shown to generate the spatial association of species, forming discrete vegetation patches of phylogenetically distant species. We measured these spatial and phylogenetic signatures left by facilitation in a serpentine plant community of central Cuba. Our results show that seedlings preferentially grow under plants of different species, and that adults are significantly aggregated into vegetation patches. In these patches, adults tend to co-occur with distant relatives, ultimately forming phylogenetically diverse neighbourhoods. We discuss possible mechanisms explaining how species adapted to serpentine areas may be acting as nurses, reducing the stressful conditions for the establishment of other species.

Evolutionary aspects of elemental hyperaccumulation

Hyperaccumulation is the uptake of one or more metal/metalloids to concentrations greater than 50-100× those of the surrounding vegetation or 100-10,000 mg/kg dry weight depending on the element. Hyperaccumulation has been documented in at least 515 taxa of angiosperms. By mapping the occurrence of hyperaccumulators onto the angiosperm phylogeny, we show hyperaccumulation has had multiple origins across the angiosperms. Even within a given order, family or genus, there are typically multiple origins of hyperaccumulation, either for the same or different elements. We address which selective pressures may have led to the evolution of hyperaccumulation and whether there is evidence for co-evolution with ecological partners. Considerable evidence supports the elemental-defense hypothesis, which states that hyperaccumulated elements protect the plants from herbivores and pathogens. There is also evidence that hyperaccumulation can result in drought stress protection, allelopathic effects or physiological benefits. In many instances, ecological partners of hyperaccumulators have evolved resistance to the hyperaccumulated element, indicating co-evolution. Studies on the molecular evolution of hyperaccumulation have pinpointed gene duplication as a common cause of increased metal transporter abundance. Hypertolerance to the hyperaccumulated element often relies upon chelating agents, such as organic acids (e.g., malate, citrate) or peptide/protein chelators that can facilitate transport and sequestration. We conclude the review with a summary and suggested future directions for hyperaccumulator research.

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.

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.