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

Genome-wide analysis of NAC transcription factors and exploration of candidate genes regulating selenium metabolism in Broussonetia papyrifera

MAIN CONCLUSION

Genome-wide identification revealed 79 BpNAC genes belonging to 16 subfamilies, and their gene structures and evolutionary relationships were characterized. Expression analysis highlighted their importance in plant selenium stress responses. Paper mulberry (Broussonetia papyrifera), a deciduous arboreal plant of the Moraceae family, is distinguished by its leaves, which are abundant in proteins, polysaccharides, and flavonoids, positioning it as a novel feedstock. NAC transcription factors, exclusive to plant species, are crucial in regulating growth, development, and response to biotic and abiotic stress. However, extensive characterization of the NAC family within paper mulberry is lacking. In this study, 79 BpNAC genes were identified from the paper mulberry genome, with an uneven distribution across 13 chromosomes. A comprehensive, genome-wide analysis of BpNACs was performed, including investigating gene structures, promoter regions, and chromosomal locations. Phylogenetic tree analysis, alongside comparisons with Arabidopsis thaliana NACs, allowed for categorizing these genes into 16 subfamilies in alignment with gene structure and motif conservation. Collinearity analysis suggested a significant homologous relationship between the NAC genes of paper mulberry and those in Morus notabilis, Ficus hispida, Antiaris toxicaria, and Cannabis sativa. Integrating transcriptome data and Se content revealed that 12 BpNAC genes were associated with selenium biosynthesis. Subsequent RT-qPCR analysis corroborated the correlation between BpNAC59, BpNAC62 with sodium selenate, and BpNAC55 with sodium selenite. Subcellular localization experiments revealed the nuclear functions of BpNAC59 and BpNAC62. This study highlights the potential BpNAC transcription factors involved in selenium metabolism, providing a foundation for strategically breeding selenium-fortified paper mulberry.

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

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

Selenium: a potent regulator of ferroptosis and biomass production

Emerging evidence supports the notion that selenium (Se) plays a beneficial role in plant development for modern crop production and is considered an essential micronutrient and the predominant source of plants. However, the essential role of selenium in plant metabolism remains unclear. When used in moderate concentrations, selenium promotes plant physiological processes such as enhancing plant growth, increasing antioxidant capacity, reducing reactive oxygen species and lipid peroxidation and offering stress resistance by preventing ferroptosis cell death. Ferroptosis, a recently discovered mechanism of regulated cell death (RCD) with unique features such as iron-dependant accumulation of lipid peroxides, is distinctly different from other known forms of cell death. Glutathione peroxidase (GPX) activity plays a significant role in scavenging the toxic by-products of lipid peroxidation in plants. A low level of GPX activity in plants causes high oxidative stress, which leads to ferroptosis. An integrated view of ferroptosis and selenium in plants and the selenium-mediated nanofertilizers (SeNPs) have been discussed in more recent studies. For instance, selenium supplementation enhanced GPX4 expression and increased TFH cell (Follicular helper T) numbers and the gene transcriptional program, which prevent lipid peroxidase and protect cells from ferroptosis. However, though ferroptosis in plants is similar to that in animals, only few studies have focused on plant-specific ferroptosis; the research on ferroptosis in plants is still in its infancy. Understanding the implication of selenium with relevance to ferroptosis is indispensable for plant bioresource technology. In this review, we hypothesize that blocking ferroptosis cell death improves plant immunity and protects plants from abiotic and biotic stresses. We also examine how SeNPs can be the basis for emerging unconventional and advanced technologies for algae/bamboo biomass production. For instance, algae treated with SeNPs accumulate high lipid profile in algal cells that could thence be used for biodiesel production. We also suggest that further studies in the field of SeNPs are essential for the successful application of this technology for the large-scale production of plant biomass.

Competition for light induces metal accumulation in a metal hyperaccumulating plant

Plants can respond to competition with a myriad of physiological or morphological changes. Competition has also been shown to affect the foraging decisions of plants belowground. However, a completely unexplored idea is that competition might also affect plants' foraging for specific elements required to inhibit the growth of their competitors. In this study, we examined the effect of simulated competition on root foraging and accumulation of heavy metals in the metal hyperaccumulating perennial plant Arabidopsis halleri, whose metal accumulation has been shown to provide allelopathic ability. A. halleri plants originating from both metalliferous and non-metalliferous soils were grown in a "split-root" setup with one root in a high-metal pot and the other in a low-metal one. The plants were then assigned to either simulated light competition or no-competition (control) treatments, using vertical green or clear plastic filters, respectively. While simulated light competition did not induce greater root allocation into the high-metal pots, it did result in enhanced metal accumulation by A. halleri, particularly in the less metal-tolerant plants, originating from non-metalliferous soils. Interestingly, this accumulation response was particularly enhanced for zinc rather than cadmium. These results provide support to the idea that the accumulation of metals by hyperaccumulating plants can be facultative and change according to their demand following competition.

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.

Selenium Metabolism in Hemp (Cannabis sativa L.)-Potential for Phytoremediation and Biofortification

Selenium (Se) deficiency and toxicity affect over a billion people worldwide. Plants can mitigate both problems, via Se biofortification and phytoremediation. Here we explore the potential of hemp (Cannabis sativa L.) for these phytotechnologies. Field surveys in naturally seleniferous agricultural areas in Colorado, United States, found 15-25 μg of Se/g in seed and 5-10 μg of Se/g dry weight (DW) in flowers and leaves. Thus, 4 g of this hemp seed provides the U.S. recommended daily allowance of 55-75 μg of Se. In controlled greenhouse experiments, hemp seedlings grown in Turface supplied with 40-320 μM selenate showed complete tolerance up to 160 μM and accumulated up to 1300 mg of Se/kg shoot dry weight. Mature hemp grown in Turface supplied with 5-80 μM selenate was completely tolerant up to 40 μM selenate and accumulated up to 200 mg of Se/kg DW in leaves, flowers, and seeds. Synchrotron X-ray fluorescence and X-ray absorption spectroscopies of selenate-supplied hemp showed Se to accumulate mainly in the leaf vasculature and in the seed embryos, with predominant Se speciation in C-Se-C forms (57-75% in leaf and more than 86% in seeds). Aqueous seed extracts were found by liquid chromatography mass spectrometry to contain selenomethionine and methyl-selenocysteine (1:1-3 ratio), both excellent dietary Se sources. Floral concentrations of medicinal cannabidiol (CBD) and terpenoids were not affected by Se. We conclude that hemp has good potential for Se phytoremediation while producing Se-biofortified dietary products.

Distribution and chemical form of selenium in Neptunia amplexicaulis from Central Queensland, Australia

Selenium (Se), a trace element essential for human and animal biological processes, is deficient in many agricultural soils. Some extremely rare plants can naturally accumulate extraordinarily high concentrations of Se. The native legume Neptunia amplexicaulis, endemic to a small area near Richmond and Hughenden in Central Queensland, Australia, is one of the strongest Se hyperaccumulators known on Earth, with foliar concentrations in excess of 4000 μg Se g^-1 previously recorded. Here, we report on the Se distribution at a whole plant level using laboratory micro X-ray Fluorescence Microscopy (μXRF) and scanning electron microscopy (SEM-EDS), as well as on chemical forms of Se in various tissues using liquid chromatography-mass spectrometry (LC-MS) and synchrotron X-ray absorption spectroscopy (XAS). The results show that Se occurs in the forms of methyl-selenocysteine and seleno-methionine in the foliar tissues, with up to 13 600 μg Se g^-1 total in young leaves. Selenium was found to accumulate primarily in the young leaves, flowers, pods and taproot, with lower concentrations present in the fine-roots and stem and the lowest present in the oldest leaves. Trichomes were not found to accumulate Se. We postulate that Se is (re)distributed in this plant via the phloem from older leaves to newer leaves, using the taproot as the main storage organ. High concentrations of Se in the nodes (pulvini) indicate this structure may play an important a role in Se (re)distribution. The overall pattern of Se distribution was similar in a non-Se tolerant closely related species (Neptunia gracilis), although the prevailing Se concentrations were substantially lower than in N. amplexicaulis.

Selenium tolerance, accumulation, localization and speciation in a Cardamine hyperaccumulator and a non-hyperaccumulator

Cardamine violifolia (family Brassicaceae) is the first discovered selenium hyperaccumulator from the genus Cardamine with unique properties in terms of selenium accumulation, i.e., high abundance of selenolanthionine. In our study, a fully comprehensive experiment was conducted with the comparison of a non-hyperaccumulator Cardamine species, Cardamine pratensis, covering growth characteristics, chlorophyll fluorescence, spatial selenium/sulfur distribution patterns through elemental analyses (synchrotron-based X-Ray Fluorescence and ICP-OES) and speciation data through selenium K-edge micro X-ray absorption near-edge structure analysis (μXANES) and strong cation exchange (SCX)-ICP-MS. The results revealed remarkable differences in contrast to other selenium hyperaccumulators as neither Cardamine species showed evidence of growth stimulation by selenium. Also, selenite uptake was not inhibited by phosphate for either of the Cardamine species. Sulfate inhibited selenate uptake, but the two Cardamine species did not show any difference in this respect. However, μXRF derived speciation maps and selenium/sulfur uptake characteristics provided results that are similar to other formerly reported hyperaccumulator and non-hyperaccumulator Brassicaceae species. μXANES showed organic selenium, "C-Se-C", in seedlings of both species and also in mature C. violifolia plants. In contrast, selenate-supplied mature C. pratensis contained approximately half "C-Se-C" and half selenate. SCX-ICP-MS data showed evidence of the lack of selenocystine in any of the Cardamine plant extracts. Thus, C. violifolia shows clear selenium-related physiological and biochemical differences compared to C. pratensis and other selenium hyperaccumulators.

A comprehensive review on environmental transformation of selenium: recent advances and research perspectives

Selenium (Se) is an important micronutrient and essential trace element for both humans and animals, which exist in the environment ubiquitously. Selenium deficiency is an important issue worldwide, with various reported cases of its deficiency. Low selenium contents in some specific terrestrial environments have resulted in its deficiency in humans. However, high levels of selenium in the geochemical environment may have harmful influences and can cause a severe toxicity to living things. Due to its extremely narrow deficiency and toxicity limits, selenium is becoming a serious matter of discussion for the scientists who deals with selenium-related environmental and health issues. Based on available relevant literature, this review provides a comprehensive data about Se sources, levels, production and factors affecting selenium bioavailability/speciation in soil, characteristics of Se, biogeochemical cycling, deficiency and toxicity, and its environmental transformation to know the Se distribution in the environment. Further research should focus on thoroughly understanding the concentration, speciation, Se cycling in the environment and food chain to effectively utilize Se resources, remediate Se deficiency/toxicity, and evaluate the Se states and eco-effects on human health.

Simultaneous hyperaccumulation of cadmium and manganese in Celosia argentea Linn

To indentify Mn/Cd co-hyperaccumulatoion in Celosia argentea Linn., 2 pot experiments were conducted using Cd/Mn-amended and real contaminated soils, respectively. The interaction between Cd and Mn with regard to their accumulation in the plants was also assessed. The results indicated that C. argentea can simultaneously hyperaccumulate Cd and Mn. The maximum Cd and Mn concentrations in leaves were 276 and 29,000 mg/kg, respectively. Mn application significantly enhanced the biomass production and Cd accumulation in shoots (p < 0.05). However, Cd addition did not reduce Mn accumulation in the plants. The interactions between Cd and Mn in C. argentea differ from what was previously found in hydroponic experiments. This species grew healthy in soils taken from a Cd/Mn-contaminated site, indicating a high tolerance to Cd and Mn. The transfer and bioaccumulation factors of Cd and Mn were greater than 1, which showed that C. argentea had potential for Cd and Mn phytoextraction. Besides its potential practical benefits, C. argentea is an important resource to study the mechanisms of Cd/Mn hyperaccumulation and tolerance in plants.

A critical review of selenium biogeochemical behavior in soil-plant system with an inference to human health

Selenium (Se) is an essential trace element for humans and animals, although controversial for different plant species. There exists a narrow line between essential, beneficial and toxic levels of Se to living organisms which greatly varies with Se speciation, as well as the type of living organisms. Therefore, it is crucial to monitor its solid- and solution-phase speciation, exposure levels and pathways to living organisms. Consumption of Se-laced food (cereals, vegetables, legumes and pulses) is the prime source of Se exposure to humans. Thus, it is imperative to assess the biogeochemical behavior of Se in soil-plant system with respect to applied levels and speciation, which ultimately affect Se status in humans. Based on available relevant literature, this review traces a plausible link among (i) Se levels, sources, speciation, bioavailability, and effect of soil chemical properties on selenium bioavailability/speciation in soil; (ii) role of different protein transporters in soil-root-shoot transfer of Se; and (iii) speciation, metabolism, phytotoxicity and detoxification of Se inside plants. The toxic and beneficial effects of Se to plants have been discussed with respect to speciation and toxic/deficient concentration of Se. We highlight the significance of various enzymatic (catalase, peroxidase, superoxide dismutase, ascorbate peroxidase, glutathione peroxidase) and non-enzymatic (phytochelatins and glutathione) antioxidants which help combat Se-induced overproduction of reactive oxygen species (ROS). The review also delineates Se accumulation in edible plant parts from soils containing low or high Se levels; elucidates associated health disorders or risks due to the consumption of Se-deficient or Se-rich foods; discusses the potential role of Se in different human disorders/diseases.

Transcriptome-wide comparison of selenium hyperaccumulator and nonaccumulator Stanleya species provides new insight into key processes mediating the hyperaccumulation syndrome

To obtain better insight into the mechanisms of selenium hyperaccumulation in Stanleya pinnata, transcriptome-wide differences in root and shoot gene expression levels were investigated in S. pinnata and related nonaccumulator Stanleya elata grown with or without 20 μm selenate. Genes predicted to be involved in sulphate/selenate transport and assimilation or in oxidative stress resistance (glutathione-related genes and peroxidases) were among the most differentially expressed between species; many showed constitutively elevated expression in S. pinnata. A number of defence-related genes predicted to mediate synthesis and signalling of defence hormones jasmonic acid (JA, reported to induce sulphur assimilatory and glutathione biosynthesis genes), salicylic acid (SA) and ethylene were also more expressed in S. pinnata than S. elata. Several upstream signalling genes that up-regulate defence hormone synthesis showed higher expression in S. pinnata than S. elata and might trigger these selenium-mediated defence responses. Thus, selenium hyperaccumulation and hypertolerance in S. pinnata may be mediated by constitutive, up-regulated JA, SA and ethylene-mediated defence systems, associated with elevated expression of genes involved in sulphate/selenate uptake and assimilation or in antioxidant activity. Genes pinpointed in this study may be targets of genetic engineering of plants that may be employed in biofortification or phytoremediation.

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.

The essentiality of selenium for humans, animals, and plants, and the role of selenium in plant metabolism and physiology

After its discovery, selenium was most noted for its harmful effects. Selenium was the first element identified to occur in native vegetation at levels toxic to animals. Poisoning of animals can occur through consumption of plants containing toxic levels of selenium. Livestock consuming excessive amounts of selenized forages are afflicted with “alkali disease” and “blind staggers”. Typical symptoms of these diseases include loss of hair, deformed hooves, blindness, colic, diarrhoea, lethargy, increased heart and respiration rates, and eventually death. On the other hand, selenium deficiency in animal feeds can cause “white muscle disease”, a degenerative disease of the cardiac and skeletal muscles. In this regard, this review paper attempts to summarize the essentiality of selenium for humans, animals, and plants and the role of selenium in plant metabolism and physiology.

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.

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.

Development of an algal treatment system for Se removal: Effects of light regimes, nutrients, sulfate and hypersalinity

Selenium (Se) exposure poses potential risks to wildlife at the Salton Sea. Our previous research suggests Chlorella sp. be highly efficient at absorbing and volatilizing Se. In developing an algal treatment system for Se removal, this study further evaluated the performance under the conditions to be encountered in the field using Chlorella pyrenoidosa and Chlorella vulgaris. The results show the algal Se removal efficiency was little affected by photoperiod, yet volatilization became relatively greater in dark/light cycles over a longer term. The rates of Se absorption and volatilization by C. vulgaris were 88% and 77% more, respectively, in the DI water, while C. pyrenoidosa acted oppositely, indicating C. vulgaris will perform better in Se removal if nutrient levels are reduced in advance. The presence of sulfate reduced biomass Se, especially through volatilization, by 8% for C. vulgaris, lessening potential ecotoxicity. Finally, C. vulgaris released biomass Se back to the water column under hypersaline conditions, leading to a 6% increase in water Se concentrations. These results suggest C. vulgaris be the best alga for the treatment of Se laden river water in the Salton Sea area, yet a filtering system is required to prevent Se containing algae from entering food chains.

Effect of copper tolerant Elsholtzia splendens on bacterial community associated with Commelina communis on a copper mine spoil

Facilitation, or positive plant-plant interaction, has received increasing concern from ecologists over the last two decades. Facilitation may occur through direct mitigation of severe environments or indirect mediation by a third participant from the same or different trophic levels. The copper (Cu) tolerant species Elsholtzia splendens facilitates the establishment and growth of co-occurring Commelina communis through indirect enrichment of microbial activity. However, whether and how E. splendens impacts the microbial community that is associated with C. communis is less known. We characterized the soil bacterial community in the rhizosphere of C. communis in the absence and presence of E. splendens using PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) and sequencing. The result showed that the richness of the bacterial community increased, but diversity and evenness remained similar, in the presence of E. splendens. Chloroflexi, Acidobacteria and Proteobacteria were the most dominant bacteria. The relative abundance of dominant and minor bacterial groups showed distinctly different responses to E. splendens. Principal component analysis and redundancy analysis indicated that variation of the bacterial community was determined by multiple factors and might be driven by the tested soil parameters collectively, or alternatively changed through plant root exudates or other microorganisms. Our results enhance the understanding of how the bacterial community associated with a beneficiary plant responds to a benefactor plant and suggests that the changes of bacterial community composition may have far-reaching influence on plant-soil feedback and the aboveground plant community in the long run.

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.

Selenium hyperaccumulators harbor a diverse endophytic bacterial community characterized by high selenium resistance and plant growth promoting properties

Selenium (Se)-rich plants may be used to provide dietary Se to humans and livestock, and also to clean up Se-polluted soils or waters. This study focused on endophytic bacteria of plants that hyperaccumulate selenium (Se) to 0.5-1% of dry weight. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to compare the diversity of endophytic bacteria of hyperaccumulators Stanleya pinnata (Brassicaceae) and Astragalus bisulcatus (Fabaceae) with those from related non-accumulators Physaria bellii (Brassicaceae) and Medicago sativa (Fabaceae) collected on the same, seleniferous site. Hyperaccumulators and non-accumulators showed equal T-RF diversity. Parsimony analysis showed that T-RFs from individuals of the same species were more similar to each other than to those from other species, regardless of plant Se content or spatial proximity. Cultivable endophytes from hyperaccumulators S. pinnata and A. bisulcatus were further identified and characterized. The 66 bacterial morphotypes were shown by MS MALDI-TOF Biotyper analysis and 16S rRNA gene sequencing to include strains of Bacillus, Pseudomonas, Pantoea, Staphylococcus, Paenibacillus, Advenella, Arthrobacter, and Variovorax. Most isolates were highly resistant to selenate and selenite (up to 200 mM) and all could reduce selenite to red elemental Se, reduce nitrite and produce siderophores. Seven isolates were selected for plant inoculation and found to have plant growth promoting properties, both in pure culture and when co-cultivated with crop species Brassica juncea (Brassicaceae) or M. sativa. There were no effects on plant Se accumulation. We conclude that Se hyperaccumulators harbor an endophytic bacterial community in their natural seleniferous habitat that is equally diverse to that of comparable non-accumulators. The hyperaccumulator endophytes are characterized by high Se resistance, capacity to produce elemental Se and plant growth promoting properties.

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.

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.

Effects of selenium biofortification on crop nutritional quality

Selenium (Se) at very low doses has crucial functions in humans and animals. Since plants represent the main dietary source of this element, Se-containing crops may be used as a means to deliver Se to consumers (biofortification). Several strategies have been exploited to increase plant Se content. Selenium assimilation in plants affects both sulfur (S) and nitrogen (N) metabolic pathways, which is why recent research has also focused on the effect of Se fertilization on the production of S- and N- secondary metabolites with putative health benefits. In this review we discuss the function of Se in plant and human nutrition and the progress in the genetic engineering of Se metabolism to increase the levels and bioavailability of this element in food crops. Particular attention is paid to Se biofortification and the synthesis of compounds with beneficial effects on health.

Exploring the importance of sulfate transporters and ATP sulphurylases for selenium hyperaccumulation-a comparison of Stanleya pinnata and Brassica juncea (Brassicaceae)

Selenium (Se) hyperaccumulation, the capacity of some species to concentrate Se to levels upwards of 0.1% of dry weight, is an intriguing phenomenon that is only partially understood. Questions that remain to be answered are: do hyperaccumulators have one or more Se-specific transporters? How are these regulated by Se and sulfur (S)? In this study, hyperaccumulator Stanleya pinnata was compared with related non-hyperaccumulator Brassica juncea with respect to S-dependent selenate uptake and translocation, as well as for the expression levels of three sulfate/selenate transporters (Sultr) and three ATP sulphurylases (APS). Selenium accumulation went down ~10-fold with increasing sulfate supply in B. juncea, while S. pinnata only had a 2-3-fold difference in Se uptake between the highest (5 mM) and lowest sulfate (0 mM) treatments. The Se/S ratio was generally higher in the hyperaccumulator than the non-hyperaccumulator, and while tissue Se/S ratio in B. juncea largely reflected the ratio in the growth medium, S. pinnata enriched itself up to 5-fold with Se relative to S. The transcript levels of Sultr1;2 and 2;1 and APS1, 2, and 4 were generally much higher in S. pinnata than B. juncea, and the species showed differential transcript responses to S and Se supply. These results indicate that S. pinnata has at least one transporter with significant selenate specificity over sulfate. Also, the hyperaccumulator has elevated expression levels of several sulfate/selenate transporters and APS enzymes, which likely contribute to the Se hyperaccumulation and hypertolerance phenotype.

Roles of rhizobial symbionts in selenium hyperaccumulation in Astragalus (Fabaceae)

PREMISE OF THE STUDY

Are there dimensions of symbiotic root interactions that are overlooked because plant mineral nutrition is the foundation and, perhaps too often, the sole explanation through which we view these relationships? In this paper we investigate how the root nodule symbiosis in selenium (Se) hyperaccumulator and nonaccumulator Astragalus species influences plant selenium (Se) accumulation.

METHODS

In greenhouse studies, Se was added to nodulated and nonnodulated hyperaccumulator and nonaccumulator Astragalus plants, followed by investigation of nitrogen (N)-Se relationships. Selenium speciation was also investigated, using x-ray microprobe analysis and liquid chromatography-mass spectrometry (LC-MS).

KEY RESULTS

Nodulation enhanced biomass production and Se to S ratio in both hyperaccumulator and nonaccumulator plants. The hyperaccumulator contained more Se when nodulated, while the nonaccumulator contained less S when nodulated. Shoot [Se] was positively correlated with shoot N in Se-hyperaccumulator species, but not in nonhyperaccumulator species. The x-ray microprobe analysis showed that hyperaccumulators contain significantly higher amounts of organic Se than nonhyperaccumulators. LC-MS of A. bisulcatus leaves revealed that nodulated plants contained more γ-glutamyl-methylselenocysteine (γ-Glu-MeSeCys) than nonnodulated plants, while MeSeCys levels were similar.

CONCLUSIONS

Root nodule mutualism positively affects Se hyperaccumulation in Astragalus. The microbial N supply particularly appears to contribute glutamate for the formation of γ-Glu-MeSeCys. Our results provide insight into the significance of symbiotic interactions in plant adaptation to edaphic conditions. Specifically, our findings illustrate that the importance of these relationships are not limited to alleviating macronutrient deficiencies.

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.

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.

Sulfur-selenium-molybdenum interactions distinguish selenium hyperaccumulator Stanleya pinnata from non-hyperaccumulator Brassica juncea (Brassicaceae)

Long-term sulfate, selenate and molybdate accumulation and translocation were investigated in two ecotypes of Stanleya pinnata and non-hyperaccumulator Brassica juncea under different levels of applied sulfate and selenate. Morphological differences were observed between the ecotypes of S. pinnata, but few differences in selenium (Se) and sulfur (S) accumulation were measured. Se-to-S ratios were nearly identical between the ecotypes under all treatments. When compared with B. juncea, several unique trends were observed in the hyperaccumulators. While both S. pinnata ecotypes showed no significant effect on Se content of young leaves when the supplied sulfate in the growth medium was increased tenfold (from 0.5 to 5 mM), the Se levels in B. juncea decreased 4- to 12-fold with increased sulfate in the growth medium. Furthermore, S. pinnata’s S levels decreased slightly with high levels of supplied Se, suggesting competitive inhibition of uptake, while B. juncea showed higher S levels with increasing Se, possibly due to up-regulation of sulfate transporters. Both ecotypes of S. pinnata showed much larger Se concentrations in young leaves, while B. juncea showed slightly higher levels of Se in older leaves relative to young. Molybdenum (Mo) levels significantly decreased in S. pinnata with increasing sulfate and selenate in the medium; B. juncea did not show the same trends. These findings support the hypothesis that S. pinnata contains a modified sulfate transporter with a higher specificity for selenate.

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.

Inoculation of Astragalus racemosus and Astragalus convallarius with selenium-hyperaccumulator rhizosphere fungi affects growth and selenium accumulation

Little is known about how fungi affect plant selenium (Se) accumulation. Here we investigate the effects of two fungi on Se accumulation, translocation, and chemical speciation in the hyperaccumulator Astragalus racemosus and the non-accumulator Astragalus convallarius. The fungi, Alternaria astragali (A3) and Fusarium acuminatum (F30), were previously isolated from Astragalus hyperaccumulator rhizosphere. A3-inoculation enhanced growth of A. racemosus yet inhibited growth of A. convallarius. Selenium treatment negated these effects. F30 reduced shoot-to-root Se translocation in A. racemosus. X-ray microprobe analysis showed no differences in Se speciation between inoculation groups. The Astragalus species differed in Se localization and speciation. A. racemosus root-Se was distributed throughout the taproot and lateral root and was 90 % organic in the lateral root. The related element sulfur (S) was present as a mixture of organic and inorganic forms in the hyperaccumulator. Astragalus convallarius root-Se was concentrated in the extreme periphery of the taproot. In the lateral root, Se was exclusively in the vascular core and was only 49 % organic. These findings indicate differences in Se assimilation between the two species and differences between Se and S speciation in the hyperaccumulator. The finding that fungi can affect translocation may have applications in phytoremediation and biofortification.