Xuebin Yin

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.

Daily Dietary Selenium Intake in a High Selenium Area of Enshi, China

Enshi is a high selenium (Se) region in Hubei, China, where human selenosis was observed between 1958 and 1963. This study investigated the daily dietary Se intake of residents in Shadi, a town located 72 km northeast of Enshi City, to assess the risk of human selenosis in the high Se area. Foods consumed typically by the local residents and their hair samples were analyzed for total Se concentration. Concentrations of Se in different diet categories were as follows: cereals: 0.96 ± 0.90 mg kg−1 DW in rice and 0.43 ± 0.55 mg kg−1 DW in corn; tuber: 0.28 ± 0.56 mg kg−1 in potato and 0.36 ± 0.12 mg kg−1 in sweet potato; vegetables: ranging from 0.23 ± 1.00 mg kg−1 in carrot to 1.57 ± 1.06 mg kg−1 in kidney bean; animal proteins: 1.99 ± 1.11 mg kg−1 in chicken and egg. Based on the food Se concentrations and the daily per-capita consumption, the estimated daily Se intake in Shadi was 550 ± 307 µg per capita. Moreover, the Se concentrations in the hairs of local adult residents were 3.13 ± 1.91 mg kg−1 (n = 122) and 2.21 ± 1.14 mg kg−1 (n = 122) for females and males, respectively, suggesting that females might be exposed to higher levels of Se from daily cooking. Although there was no human selenosis occurrence in recent years, the high level of the daily Se intake suggested that the potential risk of selenosis for local residents, especially females, might be a matter of concern.

Nitrogen and oxygen isotopic compositions of water-soluble nitrate in Taihu Lake water system, China: implication for nitrate sources and biogeochemical process

The stable isotope nitrogen-15 (15N) is a robust indicator of nitrogen (N) source, and the joint use of δ15N and δ18O–NO3− values can provide more useful information about nitrate source discrimination and N cycle process. The δ15N and δ18O–NO3− values, as well as major ion tracers, from Taihu Lake in east China were investigated to identify the primary nitrate sources and assess nitrate biogeochemical process in the present study. The results show that the nitrate concentration in West Taihu Lake (WTL) was generally higher than those in East Taihu Lake (ETL) and its upstream inflow rivers. The NO3−/Cl− value combined with mapping of δ15N–NO3− and NO3− concentration suggest that the mixing process should play a major effect in WTL, and denitrification was the dominant N transformation process in WTL. A linear relationship of close to ~1: 2 was observed between δ15N–NO3− and δ18O–NO3− values in WTL, confirming the occurrence of denitrification in WTL. The δ15N–NO3− data imply that sewage and manure were the principal nitrate sources in WTL and its feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and atmospheric deposition. The δ18O–NO3− data indicate most of nitrate from microbial nitrification of organic nitrogen matter possibly make a significant contribution to the lake.

Identification of nitrate sources in Taihu Lake and its major inflow rivers in China, using δ15N-NO3− and δ18O-NO3− values

In the present study, δ(15)N and δ(18)O-NO(3)(-) values, as well as concentrations of some major ion tracers were determined in seasonal water samples from Taihu Lake and major watersheds to investigate the temporal and spatial variations of nitrate sources and assess the underlying nitrogen (N) biogeochemistry process. The results lead to the conclusion that the nitrate concentrations in Taihu Lake are lower in summer than that in winter due to the dilution effect of wet deposition. In winter, sewage and manure were the primary nitrate sources in major inflow rivers and North Taihu Lake (NTL), while nitrate sources in East Taihu Lake (ETL) probably derived from soil organic N. In summer, atmospheric deposition and sewage/manure inputs appear to play an important role in controlling the distribution of nitrates in the whole lake. The δ(18)O-NO(3)(-) values suggest that the nitrate produced from microbial nitrification is another major nitrate source during both winter and summer months. The variations in isotopic values in nitrate suggest denitrification enriched the heavier isotopes of nitrate in NTL in winter and in ETL in summer.

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.

Biofortification to Struggle Against Iron Deficiency

Iron is an essential micronutrient for human beings, but it is not readily available. Consequently, iron deficiency is a major threat to the health and development of the human populations in the world with more than 2 billion people suffering from iron-deficiency anemia. To alleviate iron deficiency, dietary modification or diversification, iron supplementation or food fortification, and crops biofortification have been adopted. Crops biofortification, achieved through three approaches: agronomic intervention, plant breeding, and genetic engineering, could provide a sustainable and cost-effective solution for iron deficiency in food. Agronomic intervention is a traditional approach to increase the yield and quality of crops. Some researches indicate that the application of nitrogen fertilizer and intercropping, such as maize/peanut, guava/sorghum or maize and chickpea/wheat, can facilitate iron uptake by crops. Plant breeding could improve the level and bioavailability of minerals in staple crops through their natural genetic variation. The variation in iron concentration of wheat, maize, and rice suggests that selective breeding might increase the iron content of these staple foods. The transgenic approach for iron biofortification focuses on improving iron accumulation and bioavailability, or decreasing anti-nutrient contents in crops. Expressing ferritin, an iron storage protein, has achieved great success in enhancing iron concentration in seeds. Studies have shown that cysteine could enhance iron absorption in human bodies and thus, greater iron availability is expected by increasing the amount of cysteine residues in crop tissues. Reducing antinutrients such as phytic acid can also increase the bioavailability of iron.