Selenium speciation in wild-type and genetically modified Se accumulating plants with HPLC separation and ICP-MS/ES-MS detection

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Abstract
Some plants have the ability not only to grow in the presence of potential environmental contaminants, such as Se, but also to accumulate them (Se accumulators). As such, some of these plant species are excellent candidates for Se phytoremediation. One of the accumulation mechanisms for selenium tolerant plants is the formation of organoselenium compounds that cannot be incorporated into proteins, thereby avoiding toxicity. Brassica juncea (Indian mustard) accumulated selenium when grown hydroponically in the presence of selenite, selenate, and Se-methionine. Additionally, genetically modified Brassica juncea overexpressing the enzyme Se-cysteine methyltransferase (SMT) was grown under the same set of conditions as the wild type. Se speciation for the three different Se regimens was performed by ion-pairing reversed phase liquid chromatography using ICP-MS as the detector (RP-HPLC-ICP-MS). The overexpression of SMT leads to the formation of Se-methylselenocysteine, a non-protein amino acid that corresponds with increased Se tolerance in the transgenic plants. The total amount of Se accumulated and the species formed were determined by matching the observed retention times with available standards using HPLC-ICP-MS. Full characterization of Se species is done by ES-QTOF. The capability of ICP-MS to prescreen particular Se species as potential targets for ES-MS is demonstrated. ES-MS followed by CID gives strong evidence for the species proposed: Se-methylselenocysteine, Se-homocysteine and Se-cystathionine.