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Multifunctional Role of Selenium for Reducing Arsenic Risk in Crop Plants and Humans
By: Preeti Tripathi** and Rudra Deo Tripathi*
Selenium and arsenic distribution in soils throughout the world
Globally, many areas have been mapped for the soil selenium (Se) and arsenic (As) content. Worldwide soils generally contain 0.01-2 mg kg-1 of
Se, however, global average concentration of As in soil is about 5 mg kg-1. Soil in New Zealand, Australia, Denmark, Finland, Central Siberia,
North-East to South Central China, Turkey, parts of India, Nepal and Bangladesh are deficient in Se. Soils of the great plains of the USA and Canada, the
Ensti region in China, Ireland, Colombia and Venezuela are naturally rich in Se. Due to alluvial origin, high rain fall and flooding upon the soils of
Bangladesh appear to be low in soluble Se.
Arsenic is highly toxic to all forms of life. Widespread chronic inorganic As poisoning is prevailing in the regions of South and
Southeast Asia, South America, China, Vietnam, Taiwan and elsewhere, due to the consumption of contaminated drinking water containing geogenically elevated
level of inorganic As extracted from shallow underground aquifers. The permissible limit of As is 20 mg kg-1 in agricultural soil. Arsenic level
in uncontaminated region ranges below 10 µg kg-1, while in the contaminated soils, As level is beyond the concentration of 30,000 µg
kg-1.
Selenium and arsenic status in crop plants especially rice
The level of Se in crop plants varies between areas and its high level (> 2 ppm) can lead to toxicity. Although, there is no question that consumption
of As-contaminated drinking water is the most important route of As exposure in affected areas, whereas, high availability of As in paddy field makes rice
a significant contributor for As intake in humans. Arsenic also interferes with uptake of nutrient elements viz., P, Si, S, Mn, Zn, Co, Cu, and Mo
also either by direct competition with transport pathway or altering cellular metabolism in plants. Various field trial results demonstrated that As also
perturbed grain Se content genetically in paddy rice, resulting in low grain Se content. Total As concentrations in rice grains of different countries is
0.005 to 0.710 mg kg-1. Se content in Indian rice has been shown to be within the range of 0.005-0.233 mg kg-1.
Rice from the US and India are most enriched in Se, while major rice-producing and consuming countries, such as Egypt (~32 fold lesser Se than their North
American equivalents), China and Thailand produce the rice with lesser Se level. China possessed the greatest variation of 1368 ng g-1 Se. As
and Se concentration in wheat also shows large regional variation in India, China and Egypt as rice cultivars. Surveys of Se concentrations in grain of
ancestral and wild relatives of wheat, wheat landraces and commercial cultivars grown in Australia and Mexico found no significant genotypic variation in
grain Se among the modern wheat cultivars, but diploid wheat and rye had relatively higher grain Se concentrations. Soil Se bioavailability seems to be the
predominant factor in determining grain Se, as was also reported recently in wheat. Genotypic variation in Se concentration might become more prominent
when the Se bioavailability in the medium is high.
Selenium mediated reduction of arsenic toxicity in crop plants especially rice
Essentiality of Se is well proven for human beings, while its presence also considered beneficial for plants. As plants are the important source of dietary
Se, the Se metabolism is important for nutrition of humans and other animals. Inorganic forms of Se, selanate [Se(VI)] and selenite [Se(IV)] are the
predominantly available in the terrestrial environment. Selanate is transported through sulphate transporter, while silicon influx transporter (OsNIP2;1,
gene of aquaporin family) is known for Se(IV) uptake in rice. Inorganic form of As, arsenate [As(V)] is transported through phosphate transporters
(Tripathi et al. 2007), while arsenite [As(III)] through aquaporins such as Nodulin 26 like intrinsic proteins (NIPs) in rice. Thus, As and Se chemistry
and dynamics in paddy fields are complex.
Selenium has also been shown to counteract various abiotic stresses induced in plants by cold, drought, high light, water, salinity and heavy metals and
metalloids. Numerous studies have implicated Se in the following mechanisms: the regulation of reactive oxygen species (ROS) and antioxidants, the
inhibition of uptake and translocation of heavy metals, changes in their speciation and finally, rebuilding of the cell membrane and chloroplast structures
and recovery of the photosynthetic system. Selenium plays a significant role for ameliorating As toxicity by acting a cofactor of enzyme glutathione
peroxidase, which is an antioxidant system in plants. The role of Se supplementation on heavy metals (Pb, Cd, Zn, Cu, Al, Sb) uptake has been identified
including As. In some plants such as Pteris, Phaseolus, Oryza, Arabidopsis, Triticum and Medicago etc., Se lowers
As accumulation thus the associated toxicity, while As uptake and associated toxicity were increased sometimes depending on the supplied Se levels. Rice is
a good accumulator of Se and As. The negative correlation between Se and As level in different part of rice viz., root, shoot and grain is evident
in solution culture and field experiments. The studies on interaction of As and Se for reducing As uptake and associated toxicity should be investigated
further in rice plants.
Selenium as an organ protector
The role of Se is identified as organ protector during As stress in mice. Selenium plays hepatoprotective role during As induced liver injury in rats.
Exposure of rats to As caused a significant increase in liver thibarbituric acid reactive substances level, but the co-administration of Se was effective
in reducing its level. Higher dietary Se intake may reduce the risk of As related skin lesions and also reduce the skin cancers risk. Supplementation of Se
prevents cytotoxic effects of As by antimutagenic action of organoselenium, which upregulate the selenoproteins glutathione peroxidase and thioretoxin
reductase which would protect against spontaneous and arsenite induced oxidative DNA damage.
Possible association between modulated Se intake and arsenicosis in human
Daily intake of inorganic As tainted rice grains may comprise an impending As exposure pathway for humans leading to serious health issues such as bladder,
lung, skin and prostate cancer. A survey on As-exposed population of West Bengal found a possible link between increased arsenical skin lesions and low
dietary intake of animal protein, as well as other micronutrients. There is some epidemiological evidence that low Se intake may influence the development
of arsenicosis and the As-linked Blackfoot disease in As-contaminated areas of Taiwan. Similarly, liver biopsies from severely affected arsenicosis people
found high levels of As, while Se was undetectable. Selenium reduces inorganic As toxicity by promoting As methylation and increased content of
dimethylarsenic acid and monomethylarsenic acid and further these species are excreated form the body through urination. A study on 93 pregnant Chilean
women exposed to As found a strong positive correlation between excreted urinary Se and As, as well as increased % dimethylarsenic acid and decreased % of
inorganic As. Similarly, a study on 252 arsenic-exposed Taiwanese people, found that urinary Total Se was correlated with decreased inorganic As and
increased methylated As, suggesting that Se facilitates As methylation.
Outlook
The multifunctional role of Se has been well recognized for prevention of heavy metal toxicity in plants and humans. Lower level of Se (0.01-2ppm) is
expected to reduce the toxicity of As mainly by inhibiting its uptake and/or translocation in plants and partially by its antioxidant properties and
conversion of As to the lesser toxic form through Se-As bonding. Plants used in Se phytoremediation could be used as fortified foods. Studies are already
done to evaluate the impact of feeding Se rich canola meals to cows and sheep. Crops used for Se phytoremediation purposes may be subsequently utilized as
fortified foods especially for people who are exposed to food chain As contamination. Effort is needed to produce Se rich and low grain As crops using
molecular breeding and plant biotechnology methods. Further, the use of Se fertilizers in As contaminated paddy fields may be an effective strategy to
produce low grain As rice cultivars with high nutritional quality. Besides, consumption of Se rich diet may reduce the As risk in humans by protecting from
the cancer risk and increasing the excretion of methylated As from the body. Further, investigations are required to reveal the complex interaction of Se
and As by analyzing their time dependent speciation in soil rhizosphere, within the plant tissue and also in human blood and urine.
*Plant Ecology and Environmental Science Division, CSIR-National Botanical Research Institute, Lucknow
**DST SERB Young Scientist - E-mail:
[email protected]
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