Linxi Yuan

Biofortification and phytoremediation of selenium in China

Selenium (Se) is an essential trace element for humans and animals but at high concentrations, Se becomes toxic to organisms due to Se replacing sulfur in proteins. Selenium biofortification is an agricultural process that increases the accumulation of Se in crops, through plant breeding, genetic engineering, or use of Se fertilizers. Selenium phytoremediation is a green biotechnology to clean up Se-contaminated environments, primarily through phytoextraction and phytovolatilization. By integrating Se phytoremediation and biofortification technologies, Se-enriched plant materials harvested from Se phytoremediation can be used as Se-enriched green manures or other supplementary sources of Se for producing Se-biofortified agricultural products. Earlier studies primarily aimed at enhancing efficacy of phytoremediation and biofortification of Se based on natural variation in progenitor or identification of unique plant species. In this review, we discuss promising approaches to improve biofortification and phytoremediation of Se using knowledge acquired from model crops. We also explored the feasibility of applying biotechnologies such as inoculation of microbial strains for improving the efficiency of biofortification and phytoremediation of Se. The key research and practical challenges that remain in improving biofortification and phytoremediation of Se have been highlighted, and the future development and uses of Se-biofortified agricultural products in China has also been discussed.

A Novel Selenocystine-Accumulating Plant in Selenium-Mine Drainage Area in Enshi, China

Plant samples of Cardamine hupingshanesis (Brassicaceae), Ligulariafischeri (Ledeb.) turcz (Steraceae) and their underlying top sediments were collected from selenium (Se) mine drainage areas in Enshi, China. Concentrations of total Se were measured using Hydride Generation-Atomic Fluorescence Spectrometry (HG-AFS) and Se speciation were determined using liquid chromatography/UV irradiation-hydride generation-atomic fluorescence spectrometry (LC-UV-HG-AFS). The results showed that C. hupingshanesis could accumulate Se to 239±201 mg/kg DW in roots, 316±184 mg/kg DW in stems, and 380±323 mg/kg DW in leaves, which identifies it as Se secondary accumulator. Particularly, it could accumulate Se up to 1965±271 mg/kg DW in leaves, 1787±167 mg/kg DW in stem and 4414±3446 mg/kg DW in roots, living near Se mine tailing. Moreover, over 70% of the total Se accumulated in C. hupingshanesis were in the form of selenocystine (SeCys2), increasing with increased total Se concentration in plant, in contrast to selenomethionine (SeMet) in non-accumulators (eg. Arabidopsis) and secondary accumulators (eg. Brassica juncea), and selenomethylcysteine (SeMeCys) in hyperaccumulators (eg. Stanleya pinnata). There is no convincing explanation on SeCys2 accumulation in C. hupingshanesis based on current Se metabolism theory in higher plants, and further study will be needed.

Selenium in Plants and Soils, and Selenosis in Enshi, China: Implications for Selenium Biofortification

The total selenium (Se) content of soils in Enshi, China, the so-called “World Capital of Selenium”, is concentrated in a range of 20–60 mg/kg DW which is approximately 150–500 times greater than the average Se content (0.125 mg/kg DW) in Se-deficient areas and approximately 50–150 times greater than that (0.40 mg/kg DW) in Se-enriched areas in China, respectively. However, the distribution of Se in soils is greatly uneven with some exceptionally high contents of more than 100 mg/kg DW, which is very likely caused by the micro-topographical features and leaching conditions. Among the 14 plant species in Enshi, Adenocaulon himalaicum has the highest contents of Se from 299 to 2,278 (mean 760) mg/kg DW in the leaf, from 268 to 1,612 (mean 580) mg/kg DW in the stem, from 227 to 8,391 (mean 1,744) mg/kg DW in the root, and therefore was identified as a secondary Se-accumulating plant. Furthermore, the SeCys2 fraction was predominant in the tissues with a proportion of 70–98 %, which is quite different from other Se-accumulating plants, e.g., garlic, onion, and broccoli. Although the Se concentration in resident foods and the daily Se intake decreased significantly from 1963 to 2010 in Enshi, the present daily Se intake (575 μg/d) is still above the recommended maximum safe intake of 550 μg/d, which indicates that there may be potential risk for selenosis in Enshi. Both Se distributions in soils and plants and human daily Se intakes obviously indicate that Enshi, China should be Se-phytoremediated to decrease the risk for selenosis there. Fortunately, Se-biofortification was taken as an effective method to overcome this problem. Hopefully, Enshi, China is moving on a natural field-scale trial for integration of Se-phytoremediation and Se-biofortification.

Indications of Selenium Protection against Cadmium and Lead Toxicity in Oilseed Rape (Brassica napus L.)

The present study investigated the beneficial role of selenium (Se) in protecting oilseed rape (Brassica napus L.) plants from cadmium (Cd+2) and lead (Pb+2) toxicity. Exogenous Se markedly reduced Cd and Pb concentration in both roots and shoots. Supplementation of the medium with Se (5, 10, and 15 mg kg-1) alleviated the negative effect of Cd and Pb on growth and led to a decrease in oxidative damages caused by Cd and Pb. Furthermore, Se-enhanced superoxide free radicals (O2•¯), hydrogen peroxide (H2O2), and lipid peroxidation, as indicated by malondialdehyde accumulation, but decreased superoxide dismutase and glutathione peroxidase activities. Meanwhile, the presence of Cd and Pb in the medium affected Se speciation in shoots. The results suggest that Se could alleviate Cd and Pb toxicity by preventing oxidative stress in oilseed rape plant.

Effect of Selenium on Control of Postharvest Gray Mold of Tomato Fruit and the Possible Mechanisms Involved

Selenium (Se) has important benefits for crop growth and stress tolerance at low concentrations. However, there is very little information on antimicrobial effect of Se against the economically important fungus Botrytis cinerea. In the present study, using sodium selenite as Se source, we investigated the effect of Se salts on spore germination and mycelial growth of the fungal pathogen in vitro and gray mold control in harvested tomato fruit. Se treatment at 24 mg/L significantly inhibited spore germination of the fungal pathogen and effectively controlled gray mold in harvested tomato fruit. Se treatment at 24 mg/L seems to induce the generation of intracellular reactive oxygen species in the fungal spores. The membrane integrity damage was observed with fluorescence microscopy following staining with propidium iodide after treatment of the spores with Se. These results suggest that Se has the potential for controlling gray mold rot of tomato fruits and might be useful in integrated control against gray mold disease of postharvest fruits and vegetables caused by B. cinerea. The mechanisms by which Se decreased gray mold decay of tomato fruit may be directly related to the severe damage to the conidia plasma membrane and loss of cytoplasmic materials from the hyphae.

Phytoremediation and Biofortification: Two Sides of One Coin

Phytoremediation is a biotechnology to clean the contaminated sites by toxic elements (e.g. Cd, Cu, Zn, As, Se, Fe) via plant breeding, plant extracting, and plant volatilizing. Biofortification is an agricultural process that increases the uptake and accumulation of trace mineral nutrients (Fe, I, Cu, Zn, Mn, Co, Cr, Se, Mo, F, Sn, Si, and V) in staple crops through plant breeding, genetic engineering, or manipulation of agricultural practices. However, these two biotechnologies could be connected closely just like two sides of one coin. Actually, plant materials produced from phytoremediation could be used as supplementary sources for foods, animal feedstuff for fortified meat, or green fertilizers for fortified agricultural products. Furthermore, the transgenic technology will substantially increase their accumulation of micronutrient elements in plants or staple crops, which could be used for phytoremediation and biofortification, respectively. Future work will be needed to phytoremediate and biofortify multiple micronutrients, and then integrate both.

Phytoremediation of the Metalloid Selenium in Soil and Water

Toxic heavy metal selenium (Se), is constantly released into the environment. There is an urgent need to develop low-cost, effective, and sustainable methods for Se removal. Plant-based approaches, such as phytoremediation, are relatively inexpensive since they are performed in situ and are solar-driven. In this review, we discuss specific advances in plant-based approaches for the remediation of Se-contaminated water and soil. Dilute concentrations of Se contaminants can be removed from large volumes of wastewater by constructed wetlands. We discuss the potential of constructed wetlands for use in remediating Se in agricultural drainage water and industrial effluent, as well as concerns over their potential ecotoxicity. In upland ecosystems, plants may be used to accumulate Se in their harvestable biomass (phytoextraction). Plants can also convert and release Se in a volatile form (phytovolatilization). We discuss how genetic engineering has been used to develop plants with enhanced efficiencies for phytoextraction and phytovolatilization. For example, Se-hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate, and detoxify Se represent an important reservoir of unique genes that could be transferred to fast-growing plant species for enhanced Se of phytoremediation. There is also a need to develop new strategies to improve the acceptability of using genetically engineered plants for Se of phytoremediation.

Phytoremediation of Cadmium and Copper Contaminated Soils

With the development of modern industry and agriculture, the Cadium (Cd) and Copper (Cu) contents in soil have significantly increased. Pollution of Cd and Cu is more serious in a soil–plant environment, so that the remediation of the contaminated environment has been paid more attention. Phytoremediation is to use living green plants to reduce, remove, degrade, or immobilize toxins from contaminated soil, which is an emerging cost-competitive environmental-friendly technology. Recent researches on Cd and Cu hyperaccumulators will be reviewed in this chapter. Future research on the screening Cd and Cu hyperaccumulators, and their molecular mechanisms are necessary for developing phytoremediation.

Selenium Accumulation, Antioxidant Enzyme Levels, and Amino Acids Composition in Chinese Mitten Crab (Eriocheir sinensis) Fed Selenium-Biofortified Corn.

The effects of selenium (Se)-biofortified corn on the total Se contents, the antioxidant enzyme levels, and the amino acids composition in Chinese mitten crab (Eriocheir sinensis) during the stage of the fifth shelling to maturity were investigated in the present study. The culture density of crabs was 600 per 667 m2, and they were continuously fed 120.4 mg Se from Se-biofortified corn per 667 m2 every two days for 90 days. The results showed that the total muscle Se levels in the crabs were significantly increased (p < 0.05). Activities of hemolymph supernatant enzymes including alkaline phosphatase (AKP), lysozyme (LZM), glutathione peroxidase (GPx), and superoxide dismutase (SOD) were also enhanced (p < 0.05). The protein and crude fat levels at maturity were higher than those at the fourth molt. The levels of total essential amino acids (∑EAAs) and total delicious amino acids (∑DAAs) were significantly increased (p < 0.05). We demonstrate that the feeding of Se-biofortified corn could significantly improve total muscle Se concentrations and hemolymph supernatant antioxidant enzyme activities in Chinese mitten crab, and slow down the rapid decline of ∑EAAs and ∑DAAs at maturity, thus improving the nutritional quality of Chinese mitten crab.

Element Case Studies: Selenium

Selenium hyperaccumulator plants such as Stanleya pinnata, Astragalus bisulcatus, and the newly discovered Cardamine hupingshanensis may play an important role in the Se cycle from soil to plant to human, especially in China. Se-hyperaccumulators can be used for agromining or for phytoremediation of Se, as well as for applications to Se-deficient soils in Se-biofortification strategies.