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The presence of sulfate in the rhizosphere inhibits selenate uptake and accumulation, suggesting direct competition between selenate and sulfate for transport or repression of transcription of sulfate transporter genes by sulfate and its metabolites (Vidmar et al. 2000; White et al. 2004). In contrast to this antagonistic relation a synergistic relationship between S and Se has been reported. Studies in onions, rice and wheat have shown that low concentrations of Se enhanced S uptake and accumulation (Mikkelsen & Wan 1990; Kopsell & Randle 1997). Furthermore, the presence of abundant sulphate can ameliorate the phytotoxic effects of excessive Se and prevent yield reduction (Mikkelsen & Wan 1990).
Data suggest that during phosphorus starvation selenite uptake is increased, suggesting a role of the phosphate transport pathway in selenite uptake (Li et al. 2008). An antagonistic effect between phosphorus and Se has been noticed (Hopper & Parker 1999; Wu & Huang 2004; Li et al. 2008). Ten fold more phosphate than the normal concentration in the soil causes decreases in Se concentration of about 50% in both roots and shoots in ryegrass and 20% in roots of strawberry clover (Hopper & Parker 1999). Li et al. (2008) showed that phosphorus starvation resulted in 60% increase in selenite uptake by wheat, possibly because phosphorus starvation up-regulate the expression of the phosphate transporter genes.
Organic matter affects Se adsorption in the soil and subsequently Se availability and uptake by plants. Cattle slurry added together with selenate increased Se concentration in wheat grains at the higher pH level in both peat and loam soils (Falk Øgaard et al. 2006). A trend for lower Se concentrations in wheat was observed when Se-rich fish silage was added compared to the control (Sogn et al. 2007). The incorporation of catch crop plant material, grown in non seleniferous soil, decreased Se concentration in Indian mustards plants compared to unamended soil (Stavridou et al. 2011). Similar results were found by Ajwa et al. (1998), the addition of crop residues or animal manure in selenate treated soils considerably reduced Se uptake by canola and tall fescue.
Selenium concentrations in vegetables and its bioavailability to humans
Vegetables usually contain less than 0.1 mg Se kg-1, however when grown in seleniferous soil can contain up to 6 mg kg-1 (Rayman 2008). In Denmark Se concentrations in vegetables vary from 0.05 to 6.5 Î¼g per 100 g of fresh weight of the edible part. It is interesting to note, that mushrooms contain the higher Se concentrations followed by Cruciferae and Allium species (2008). In response to the need for Se to support human health Se has become a focus of functional food development. Selenium enriched broccoli, garlic, onions, celery, Brassica sprouts produced by various Se fertilizations can contain several hundred mg Se kg-1 of dry weight (Kopsell & Randle 1997; Pyrzynska 2009).
The bioavailability and benefits to human health of dietary Se depend not only on the amount but also the chemical forms of Se supplied. The dominant organoselenium compounds differ between plant species; some vegetables contain high concentrations of organoselenium compounds that are particularly beneficial to human health. Selenium displays anti-carcinogenic potentials through its incorporation into various selenoenzymes, which function to reduce free radical injury to cells (Irion 1999). Many Allium (A. cepa L., A. sativum L., A. schoenoprasum L., etc.) and Cruciferae species (Brassica juncea and B. oleracea) are able to incorporate high quantities of Se and to produce selenoamino acids, which are potentially bioactive for nutrition purposes and phytoremediation and normally implicated the S pathways (Arnault & Auger 2006; Pedrero et al. 2006).
The initial assumption was that the active Se compound against cancer was selenomethionine, the main Se compound in cereals. Recently, studies has demonstrated that Se-methylselenocysteine, Î³-glutamyl-Se-methylselenocysteine and methylselenic acid are anti-cancer agents with similar action mechanism (Abdulah et al. 2005). Stable methylated Se compounds such as selenobetaine or Se-methylselenocysteine serve as precursors and release methylselenol or methylselenenic acid through the action of cysteine conjugate Î²-lyase or related lysases. The monomethylated Se compounds are effective in vitro at very low concentrations to have chemopreventive effects (apoptosis and cell cycle arrest) in transformed cells (Keck & Finley 2004; Abdulah et al. 2005). Selenium enriched garlic contains Se-methylselenocysteine and Î³-glutamyl-Se-methylselenocysteine that inhibit tumerogenesis. Furthermore, broccoli, onion and radish grow in soils with high Se concentration convert much of the Se into the amino acids selenomethionine, Se-methylselenocysteine and selenocysteine (Irion 1999; Finley 2003; Abdulah et al. 2005; Arnault & Auger 2006; Pedrero et al. 2006).
Selenium essential for humans
Selenium is an essential nutrient for animals, humans and microorganisms. Originally, selenium was only considered for its toxic capabilities but the potential health benefits of some Se compounds have resulted in the increased study of Se (Ellis & Salt 2003).
Selenium is an essential component of more than 30 mammalian selenoproteins or selenoenzymes. At least fifteen selenoproteins have been characterized for their biological functions. Selenoproteins can be subdivided into groups based on the location of selenocysteine in the selenoprotein polypeptides such as glutathione peroxidases (GSHPx) and thioredoxin reductases, which are involved in controlling tissue concentrations of highly reactive oxygen-containing metabolites, and iodothyronine deiodnases types I, II, III that are involved in the production of active thyroid hormones, (Abdulah et al. 2005; Hawkesford & Zhao 2007). Selenium is associated with the cardiovascular disease risk, the optimal function of the immune system, the male fertility, the progression of AIDS and a number of other diseases (Rayman 2000). Increasing evidence points to an anti-carcinogenic potential of Se-compounds (Se-methylselenocysteine and Î³-glutamyl-Se-methylselenocysteine), which have been shown to provide chemo protective effects against certain types of cancer in humans (Rayman 2000; National Academy of Sciences.Institute of Medicine.Food and Nutrition Board. 2000; Abdulah et al. 2005; Arnault & Auger 2006).
The first report of selenium deficiency in humans occurred in China. Keshan disease is a cardiomyopathy of children and young women of child-bearing age. Another Se-responsive disease, reported in children in China and less extensively in south-east Siberia, is Kaschin-Beck disease, which is an osteoarthropathy, characterized by joint necrosis epiphyseal degeneration of the arm and leg joints resulting in structural shortening of the fingers and long bones with consequent growth retardation and stunting (Tinggi 2003).
There is a narrow margin between the harmful and the beneficial effects of Se in humans. Selenium toxicity in humans is rare; however, the effects of Se toxicity have been reported to cause hair loss, skin lesions, vomiting, nausea, abnormalities in the beds of the fingernails and fingernail loss, hypo chronic anaemia and leucopenia (Tinggi 2003).
Selenium human intake
Geographic differences in the content and availability of selenium from soils to food crops and animal products have a marked effect on the selenium status of entire communities (Combs Jr 2001). The levels of Se in blood and blood plasma and the activities of GSHPx in blood plasma are common biomarkers used to assess Se status in humans. The American Recommended Dietary Allowances (RDA), which is based on Se levels considered to be necessary to achieve plateau concentrations of plasma GSHPx and maximize GSHPx activity, is 55 Î¼g Se day-1 for both women and men (National Academy of Sciences.Institute of Medicine.Food and Nutrition Board. 2000). In several EU countries the RDA differs, e.g. in Nordic countries is 40 and 50 Î¼g Se day-1 while in UK it is 60 and 70 Î¼g Se day-1 for females and males, respectively (Nordic Council of Ministers 2004; Broadley et al. 2006). However, there is growing evidence for further cancer prevention of Se at even higher intake. Clark et al. (1996) demonstrated that dietary supplementation of 200 Î¼g of Se day-1 significantly decreased the incidences of non-skin cancers, carcinomas, prostate, colorectal and lung cancers, as well as mortality due to lung and total cancers.
According to World Health Organization (WHO) the Tolerable Upper Intake Level for selenium pertains to selenium intake from food and supplements is 400 Î¼g day-1 for adults (National Academy of Sciences.Institute of Medicine.Food and Nutrition Board. 2000). Toxic effects of selenium occurred in people with a blood selenium concentration greater than 12,7 Î¼mol L-1, corresponding to a selenium intake above 850 Î¼g day-1 (National Academy of Sciences.Institute of Medicine.Food and Nutrition Board. 2000).
Selenium intake among humans in Sweden and Denmark is below Nordic Nutrition Recommendations 2004 (Nordic Council of Ministers 2004; Rayman 2008). In Finland in mid-1970s, daily selenium intake was 25 Î¼g day-1 however since the introduction of a nationwide Se fertilization policy Se reached a plateau of 110-120 Î¼g day-1 (Varo 1993).
Strategies to increase Se human intake
Increased human Se intake may be achieved in several ways, and strategies include increased consumption of foods that naturally contain much Se. Brazil nuts, offal, fish or shellfish are naturally rich food sources of Se, but the content is highly variable. However, consumers should be aware that Brazil nuts also contain high amounts of barium. Moreover, in Western countries, Se supplements are available in both inorganic and organic forms. However, studies suggest that dietary sources of Se or supplements in organic forms are more bioavailable and more effective than inorganic supplements (Rayman 2008).
Direct fortification of the food during processing with Se is a resource-saving way to human Se intake. Both inorganic and organic Se forms might be used as food supplements. Direct selenium supplementation of livestock with inorganic Se or via Se rich pastures will secure the Se requirement for the animal itself, preventing it from Se deficiency disease (Haug et al. 2007).
Selenium enriched fertilizers is a commonly used practice in order to increase Se concentrations in plants. The best example of agronomic biofortification of crops comes from Finland. The use of Se enriched multielement fertilizers in Finland has been mandatory since 1984. The addition of Se to fertilizers raised the Se content in crops and subsequently the Finn's Se intake (Varo 1993).
Exploiting the genetic variability in crop plants for Se accumulation may be an effective method to improve Se intake in humans (Lyons et al. 2005; Broadley et al. 2006). Breeding plant and crop varieties with enhanced Se-accumulation characteristics to increase the Se concentration of the human diet may be an alternative to the Se fertilization.
In temperate climatic zones during the autumn after the harvest of the main crop, temperature and light conditions allow some plant growth, though not enough to produce commercial crops. Many attempts have been made to use this period to grow plants, which prevent nutrient leaching, affect nutrient availability, increase soil biological activity, affect soil water content, influence the appearance of pests, pathogens and weeds, and improve soil physical properties (Thorup-Kristensen et al. 2003).
Most recent research in catch crops has focused on their effects on N. It has been demonstrated that catch crops take up N from the soil and thereby reduce N leaching losses from the soil. Catch crops then after being incorporating into the soil, increases N availability for the succeeding crops (Thorup-Kristensen 1994). However, in order to maximize the effects of catch crops the local climate, soil type, main and catch crop species and the farming system must be considered (Thorup-Kristensen 1994; Thorup-Kristensen 1999; Thorup-Kristensen 2001; Thorup-Kristensen et al. 2003).
Eriksen and Thorup-Kristensen (2002) demonstrated that catch crops may influence soil sulphate distribution and reduce sulphate leaching as for N. It has been found that Brassica species, which usually have a high plant S concentration, can take up 22-36 kg S ha-1, while Italian ryegrass took up only 8 kg S ha-1 (Eriksen & Thorup-Kristensen 2002). This is also confirmed in the S availability effect on the succeeding crop. The S mineralization rates were higher for Brassicas compared to legumes (Eriksen & Thorup-Kristensen 2002; Eriksen et al. 2004). Selenium behaves very similar to sulphate in the soil system, and it can easily be lost by leaching in the form of selenate. Catch crops may also exert a significant influence on Se availability, through their influence in Se leaching or Se availability for the succeeding crop (Stavridou et al. 2011).