SELENIUM IN PLANTS - UPTAKE, FUNCTIONS, AND ENVIRONMENTAL TOXICITY

Selenium (Se) has chemical properties similar to sulfur, but slight di fferences can lead to altered tertiary structure and dysfunction of pr oteins and enzymes, if selenocysteine is incorporated into proteins in place of cysteine. In some areas of California with irrigation agricu lture elevated Se concentration in drainage and shallow groundwaters c aused bioaccumulation of Se in wetlands and Se toxicity to wildlife. A mong higher plants Se accumulators are tolerant to high Se concentrati ons whereas non-accumulators are Se-sensitive. Algae show a requiremen t of Se for growth and development, but no Se essentiality has been de monstrated for higher plants, possibly with the exception of Se accumu lators. Higher plants take up Se preferentially as selenate via the hi gh affinity sulfate permease. Contents of Se in agricultural crops are usually below 1 mg kg-1 DW, and hence such crops are considered safe for human and animal consumption even when grown on moderately high Se soils. Sulfate salinity inhibits uptake of selenate by many plant spe cies. Assimilation of selenate by non-accumulators leads to synthesis of selenocysteine and selenomethionine; Se-cysteine is readily incorpo rated into proteins. High Se can interfere with S and N metabolism in non-accumulators. In contrast, Se accumulators sequester Se mainly in non-protein selenoamino acids. Among several selenoenzymes identified in bacteria and mammals, Se-dependent glutathione peroxidase which cat alyses the reduction of organic peroxides and H2O2 has been demonstrat ed convincingly in algae; in higher plants, however, the experimental evidence regarding its occurrence is controversial. All organisms incl uding higher plants contain Se-cysteyl-tRNAs that decode UGA. Selenocy steine is proposed to function as 21st proteinaceous amino acid and th us is suggested to have a biological role in higher plants. Biogeochem ical cycling of Se involves significant volatilization of methylated s elenides such as dimethyl selenide to the atmosphere from higher plant s as well as freshwater algae, but Se exchange between oceans and the atmosphere appears to proceed as net flux to the oceans.