Abstract
Selenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.
Highlights
The trace element selenium (Se) is essential for human and animal health, while no Se requirement has been shown for higher plants [1,2,3,4]
Se-amended inorganic fertilizers can be beneficial for producing Se-biofortified food and animal feed products [18], disadvantages include low Se recovery rates in edible portions of crops [28] or excessive Se accumulation in the soil with long-term application of Se fertilizer that might become toxic for nearby ecosystems [32]
Selenate and selenite uptake is enhanced by low S and P concentrations in soils, respectively [175] causing plants to increase the expression of sulfate and phosphate transporter genes in order to compensate for the lower availability of these nutrients
Summary
The trace element selenium (Se) is essential for human and animal health, while no Se requirement has been shown for higher plants [1,2,3,4]. The role of Se in physiology is mainly derived from its presence in the Se-containing amino acids selenomethionine (SeMet) and selenocysteine (SeCys) The latter has been termed the 21st amino acid since it is needed in a small set of selenoproteins that serve redox functions [5]. To prevent Se toxicity and to provide adequate Se in the human diet, it is of great importance to obtain insight into the processes that govern the distribution and speciation of Se in agricultural soils and plants. This review is aimed at summarizing the current knowledge of Se cycling across the soil-plant-atmosphere interfaces These interfaces are of major importance as they control the local-scale mobility of Se in agricultural and natural systems and potentially play an important role in the global Se distribution. Biofortification and Se hyperaccumulation, this review identifies knowledge gaps and missing links in/between the different areas of research
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