Certain species belonging to the Astragalus genus (Fabaceae) have the extraordinary ability to hyperaccumulate up to 0.6% of their shoot dry weight as selenium (Se). Given that the biochemical properties of Se parallel those of sulfur (S), we probed enzymes involved in Se/S metabolism in these plants and identified metabolic factors that influence their ability to hyperaccumulate Se. We demonstrate that Se hyperaccumulation in Astragalus species is linked with over production of S-methylcysteine (MeCys) Se-methylselenocysteine (MeSeCys) through elevation of selenocysteine methyltransferase (SMT) activity, and we show that this unique enzyme is structurally and functionally related to enzymes that belong to the family of homocysteine methyltransferases. Furthermore, key S assimilatory enzymes ATP sulfurylase (APS) and APS reductase (APR) were found to be major contributors to selenate reduction in planta. However, total enzymatic capacities of these key enzymes are not altered in Se hyperaccumulators compared to non-accumulators. We conclude that Se hyperaccumulation in Astragalus is not driven by an overall increase in the total capacity of these Se/S assimilatory enzymes, but rather by an increased Se flux through the S/Se assimilatory pathway, generated by the SMT-dependent production of the sink metabolites MeCys or MeSeCys. Consistent with this model, the hyperaccumulator SMT enzyme was found to be localized in the chloroplast, the major organelle associated with S/Se reduction.

Intriguingly, both Se hyperaccumulating and non-accumulating Astragalus species were found to produce an SMT-like enzyme. The primary structure of these SMT enzymes are very similar, however they do contain conserved amino acid residues that can be used to classify the SMTs from hyperaccumulator and non-accumulator species. Enzymatic analysis of both cell-free extracts and recombinant enzymes revealed that the SMT enzyme from the non-accumulators has a very low specific activity for methylating selenocysteine and cysteine, explaining their inability to synthesize either MeCys or MeSeCys.