The element selenium (Se) is chemically similar to sulfur (S). For this reason plants and other organisms mistakenly take up and metabolize Se via S transporters and biochemical pathways. This can cause toxicity due to a combination of (1) oxidative stress caused directly by selenocompounds and (2) replacement of S by Se in proteins and other S compounds, which disrupts their function. On the other hand, Se is an essential trace element for many organisms, including mammals, many bacteria, and certain green algae (Stadtman 1990, 1996, Fu et al. 2002). For higher plants, Se has been reported to be a beneficial nutrient, but it has not been shown to be essential (Cartes et al. 2005, Djanaguiraman et al. 2005, Hartikainen 2005, Lyons et al. 2009, Pilon-Smits et al. 2009). Organisms that require Se produce essential proteins that contain selenocysteine (SeCys) in their active site. To date, no selenoproteins have been confirmed to exist in higher plants (Novoselov et al. 2002). Selenoproteins have antioxidant or other redox functions, which is why Se deficiency often enhances the probability of developing cancers or viral infections; diseases associated with Se deficiency include Keshan disease and male infertility in humans, and white muscle disease in livestock (Whanger 1989, Ellis et al. 2004, Diwadkar-Navsariwala et al. 2006, White and Broadley 2009). There is a relatively narrow window between the amount of Se required as a nutrient and the amount that is toxic, and hence, Se defi- ciency and toxicity are both common problems Worldwide (Terry et al. 2000). As an illustration, daily intake of 50 g Se is recommended for humans, but long-term intake of 10 times higher levels may lead to chronic Se poisoning. The one-time ingestion of plant material containing 1000mg/kg dry weight(dry wt.) or more Se can even lead to acute Se poisoning and death (Draize and Beath 1935, Rosenfeld and Beath 1964, Wilber 1980). Such high Se levels(1,000-10,000mg/kg dry wt.) occur in so-called hyperaccumulator plant spe- cies that are endemic on seleniferous soils in the Western United States and parts of China where Se is naturally present. Human and livestock Se poison- ing, both chronic and acute, are serious problems in these seleniferous areas(Ohlendorf et al. 1986, Harris 1991, Kabata - Pendias 1998, Terry et al. 2000).
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