Selenium tolerance in Astragalus chrysochlorus: identification of a cDNA fragment encoding a putative Selenocysteine methyltransferase

Abstract

Selenium (Se) plays an indispensable role in human nutrition and has been implicated to have important health benefits, including being a cancer preventative agent. Selected members of the genus Astragalus (Fabaceae) are known for their ability to accumulate high levels of selenium, mainly in the form of methyl-selenocysteine (MeSeCys). The Se-hyperaccumulator Astragalus bisulcatus metabolizes > 90% of the accumulated Se into MeSeCys in young shoot tissue. Selenocysteine methyltransferase (SMT) catalyzes the methylation of SeCys to yield MeSeCys. In this study, we aimed to investigate selenium accumulation ability of Astragalus chrysochlorus. For this reason, A. chrysochlorus plants were cultured in Murashige and Skoog medium containing 1, 5, 25 or 75 ppm sodium selenate. Both shoot and root length decreased significantly when plants exposed more than 5 ppm sodium selenate. Dried plant materials were analysed with ICP-MS in the terms of Se and S accumulation. Regarding the calculated discrimination coefficients (DC (i) ) value (0.95) A. chrysochlorus was evaluated as a secondary Se accumulator plant. Putative SMT fragments were amplified by using the primers which were designed according to conserved region of Astragalus bisulcatus, Camelia sinensis and Brassica oleracea SMT genes. Reverse transcription PCR demonstrated that 595 bp fragment was expressed (Accession no: GQ844862), and a database search indicated that the similarity between A. bisulcatus SMT nucleotide sequence and the putative AcSMT fragment was 92%. The putative AcSMT gene represents a single copy sequence in Astragalus chrysoclorus genome.