Acquisition of selenium tolerance by a selenium non-accumulating Astragalus species via selection.

Selection of cultured cells of the selenium sensitive and non-accumulating Astragalus cicer for tolerance to stepwise increasing concentrations of selenite in the medium lead to a variant able to grow at 75 microM selenite. The Se-tolerant culture synthesized a selenocysteine methyltransferase immunologically related but not identical to that of the accumulating A. bisulcatus species and produced Se-methyl-selenocysteine in vivo. Re-cultivation in selenium-free medium lead to breakdown of tolerance and the disappearance of the methyltransferase from cellular proteins. The results prove that the non-accumulating species A. cicer has the cryptic capacity for synthesis of a selenocysteine methyltransferase and also demonstrate that synthesis of the organoselenium compounds in Se-accumulating plants are contributing to selenium tolerance.