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.

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

In these studies we identified and compared the properties of plant species that showed positive or negative co-occurrence with selenium (Se) hyperaccumulators in their natural habitat. The main questions addressed were: which species are most abundant directly adjacent to hyperaccumulators, and which are absent? How do Se accumulation and tolerance compare in species found to positively or negatively co-occur with hyperaccumulators? Approaches included field surveys, X-ray microprobe analysis of field samples, and a lab Se tolerance and accumulation study. When 54 hyperaccumulators across two naturally seleniferous sites were surveyed for their five nearest neighboring species, and the relative abundance of these species around hyperaccumulators compared to that in the overall vegetation, some species were identified to positively or negatively co-occur with hyperaccumulators. Several positively co-occurring species showed high Se accumulation capability (up to 900 mg Se per kg dry weight), which may reflect Se tolerance. Leaf X-ray microprobe analysis found relatively more organic forms of Se in two positively co-occurring species than in a negatively co-occurring one. There were elevated soil Se levels around Se hyperaccumulators, and neighbors of Se hyperaccumulators had a higher tissue Se concentration as compared to when the same species grew elsewhere in the area. The elevated soil Se levels around Se hyperaccumulators - likely resulting from litter deposition- may significantly affect the local plant community, facilitating Se-tolerant plant community members but lowering the fitness of Se-sensitive members.

Selenium Accumulation, Speciation and Localization in Brazil Nuts (Bertholletia excelsa H.B.K.).

More than a billion people worldwide may be selenium (Se) deficient, and supplementation with Se-rich Brazil nuts may be a good strategy to prevent deficiency. Since different forms of Se have different nutritional value, and Se is toxic at elevated levels, careful seed characterization is important. Variation in Se concentration and correlations of this element with other nutrients were found in two batches of commercially available nuts. Selenium tissue localization and speciation were further determined. Mean Se levels were between 28 and 49 mg kg-1, with up to 8-fold seed-to-seed variation (n = 13) within batches. Brazil nut Se was mainly in organic form. While present throughout the seed, Se was most concentrated in a ring 1 to 2 mm below the surface. While healthy, Brazil nuts should be consumed in moderation. Consumption of one seed (5 g) from a high-Se area meets its recommended daily allowance; the recommended serving size of 30 g may exceed the allowable daily intake (400 µg) or even its toxicity threshold (1200 µg). Based on these findings, the recommended serving size may be re-evaluated, consumers should be warned not to exceed the serving size and the seed may be sold as part of mixed nuts, to avoid excess Se intake.

Comparison of ATP sulfurylase 2 from selenium hyperaccumulator Stanleya pinnata and non-accumulator Stanleya elata reveals differential intracellular localization and enzyme activity levels.

BACKGROUND: The plant Stanleya pinnata hyperaccumulates Se up to 0.5% of its dry weight in organic forms, whereas the closely related Stanleya elata does not hyperaccumulate Se. ATP sulfurylase (ATPS) can catalyze the formation of adenosine 5'-phosphoselenate (APSe) from ATP and selenate. We investigated the S. pinnata ATPS2 isoform (SpATPS2) to assess its possible role in Se hyperaccumulation. METHODS: ATPS expression and activity was compared in the two Stanleya species. The ATPS2 protein sequences were modeled. Sub-cellular locations were analyzed using GFP fusions. Enzyme activity of purified recombinant SpATPS2 was measured. RESULTS: ATPS2 transcript levels were six-fold higher in roots of S. pinnata relative to S. elata. Overall root ATPS enzyme activity was two-fold elevated in S. pinnata. Cloning and sequencing of SpATPS2 and S. elata ATPS2 (SeATPS2) showed the predicted SeATPS2 to be canonical, while SpATPS2, although very similar in its core structure, has unique features, including an interrupted plastid targeting signal due to a stop codon in the 5' region of the coding sequence. Indeed GFP fusions revealed that SpATPS2 had exclusive cytosolic localization, while SeATPS2 showed dual localization in plastids and cytosol. SpATPS2 activity was inhibited by both sulfate and selenate, indicating that the enzyme acts on both substrates. CONCLUSIONS: The ATPS2 from S. pinnata differs from non-accumulator ATPS2 in its elevated expression and sub-cellular localization. It likely acts on both selente and sulfate substrates. GENERAL SIGNIFICANCE: These observations shed new light on the role of ATPS2 in the evolution of Se hyperaccumulation in plants. This article is part of a Special Issue entitled Selenium research in biochemistry and biophysics - 200 year anniversary issue, edited by Dr. Elias Arnér and Dr. Regina Brigelius-Flohe.

Characterization of Selenium Accumulation, Localization and Speciation in Buckwheat-Implications for Biofortification.

Buckwheat is an important crop species in areas of selenium (Se) deficiency. To obtain better insight into their Se metabolic properties, common buckwheat (Fagopyrum esculentum) and tartary buckwheat (F. tataricum) were supplied with different concentrations of Se, supplied as selenate, selenite, or Astragalus bisulcatus plant extract (methyl-selenocysteine). Se was supplied at different developmental stages, with different durations, and in the presence or absence of potentially competing ions, sulfate, and phosphate. The plants were analyzed for growth, Se uptake, translocation, accumulation, as well as for Se localization and chemical speciation in the seed. Plants of both buckwheat species were supplied with 20 µM of either of the three forms of Se twice over their growth period. Both species accumulated 15-40 mg Se kg-1 DW in seeds, leaves and stems, from all three selenocompounds. X-ray microprobe analysis showed that the Se in seeds was localized in the embryo, in organic C-Se-C form(s) resembling selenomethionine, methyl-selenocysteine, and γ-glutamyl-methylselenocysteine standards. In short-term (2 and 24 h) Se uptake studies, both buckwheat species showed higher Se uptake rate and shoot Se accumulation when supplied with plant extract (methyl-selenocysteine), compared to selenite or selenate. In long-term (7 days) uptake studies, both species were resistant to selenite up to 50 µM. Tartary buckwheat was also resistant to selenate up to 75 µM Se, but >30 µM selenate inhibited common buckwheat growth. Selenium accumulation was similar in both species. When selenite was supplied, Se levels were 10-20-fold higher in root (up to 900 mg Se kg-1 DW) than shoot, but 4-fold higher in shoot (up to 1,200 mg Se kg-1 DW) than root for selenate-supplied plants. Additionally, sulfate and phosphate supply affected Se uptake, and conversely selenate enhanced S and P accumulation in both species. These findings have relevance for crop Se biofortification applications.