Grain Accumulation of Selenium Species in Rice (Oryza sativa L.) Anne-Marie Carey, † Kirk G. Scheckel, ‡ Enzo Lombi, § Matt Newville, ∥ Yongseong Choi, ∥ Gareth J. Norton, † Adam H. Price, † and Andrew A. Meharg* ,† † Institute of Biological and Environmental Sciences, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen AB24 3UU, United Kingdom ‡ National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 5995 Centre Hill Avenue, Cincinnati, Ohio 45224, United States § Centre for Environmental Risk Assessment and Remediation, University of South Australia, Building X, Mawson Lakes Campus, Mawson Lakes, South Australia SA-5095, Australia ∥ Centre for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, United States * S Supporting Information ABSTRACT: Efficient Se biofortification programs require a thorough understanding of the accumulation and distribution of Se species within the rice grain. Therefore, the translocation of Se species to the filling grain and their spatial unloading were investigated. Se species were supplied via cut flag leaves of intact plants and excised panicle stems subjected to a ± stem- girdling treatment during grain fill. Total Se concentrations in the flag leaves and grain were quantified by inductively coupled plasma mass spectrometry. Spatial accumulation was investigated using synchrotron X-ray fluorescence microtomography. Selenomethionine (SeMet) and selenomethylcysteine (SeMeSeCys) were transported to the grain more efficiently than selenite and selenate. SeMet and SeMeSeCys were translocated exclusively via the phloem, while inorganic Se was transported via both the phloem and xylem. For SeMet- and SeMeSeCys- fed grain, Se dispersed throughout the external grain layers and into the endosperm and, for SeMeSeCys, into the embryo. Selenite was retained at the point of grain entry. These results demonstrate that the organic Se species SeMet and SeMeSeCys are rapidly loaded into the phloem and transported to the grain far more efficiently than inorganic species. Organic Se species are distributed more readily, and extensively, throughout the grain than selenite. ■ INTRODUCTION Se is an essential micronutrient in which up to 1 billion people worldwide are deficient, 1 causing a range of health disorders and potentially an increased risk of certain cancers. 2−7 In a recent global survey of Se content in rice, concentrations were too low in the majority of samples to meet the nutritional requirements of populations depending on rice consumption for their dietary Se intake. 5 There is, therefore, considerable interest in fortifying rice and other grain crops with Se through the application of Se fertilizers to crops or by genetically engineering cultivars to accumulate high concentrations of Se. 8 Understanding the mechanisms of Se accumulation in the filling rice grain would help improve the efficiency of biofortification programs and direct the breeding of high Se rice cultivars. 8 Organic Se species are better assimilated by the human body, and selenomethionine (SeMet) and selenomethylcysteine (SeMeSeCys) are more effective anticarcinogens than inorganic Se, with SeMeSeCys the most potent. 2,3,9 Therefore, when the mechanisms of Se accumulation in the grain are investigated, Se speciation must be considered. Selenate and selenite are the main Se species that plants absorb from the soil, 2 although they can also take up organic species such as SeMet. 10,11 While selenate and selenite are both water-soluble, selenite’saffinity for iron oxyhydroxides means that selenate is relatively more bioavailable to plant roots. 8 The chemical similarity shared by selenate and sulfate enables selenate to enter the plant via sulfate transporters, although the affinity that sulfate trans- porters have for selenate appears to vary among plant species. 8 Selenite uptake into the plant may occur via phosphate transporters. 12 Once in the plant root, selenate is widely thought to be transported via the sulfur assimilation pathway and ultimately converted to selenite. 11,13 Selenite is converted to selenide by glutathione and, via a series of steps, is assimilated into organic Se species. 2,3,13 The organic Se species selenocysteine (SeCys) and SeMet can be incorporated into proteins, replacing cysteine (Cys) and methionine (Met), respectively, which can result in toxicity in plants. 10,14,15 SeMeSeCys, a nonprotein selenoamino acid, is produced by Se hyperaccumulators and to a lesser degree by other plants, thereby limiting Se toxicity. 10,15 Within the plant, methylation of SeMet to dimethyl selenide, and of SeMeSeCys to dimethyl diselenide, can lead to substantial volatilization. 2,8−10 Received: October 31, 2011 Revised: March 21, 2012 Accepted: April 13, 2012 Published: April 13, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 5557 dx.doi.org/10.1021/es203871j | Environ. Sci. Technol. 2012, 46, 5557−5564