Overexpression of ATP sulfurylase in Indian mustard leads to increased selenate uptake, reduction, and tolerance

In earlier studies, the assimilation of selenate by plants appeared to be l imited by its reduction, a step that is thought to be mediated by ATP sulfu rylase. Here, the Arabidopsis APS1 gene, encoding a plastidic ATP sulfuryla se, was constitutively overexpressed in Indian mustard (Brassica juncea). C ompared with that in untransformed plants, the ATP sulfurylase activity was 2- to 2.5-fold higher in shoots and roots of transgenic seedlings, and 1.5 - to 2-fold higher in shoots but not roots of selenate-supplied mature ATP- sulfurylase-overexpressing (APS) plants. The APS plants showed increased se lenate reduction: x-ray absorption spectroscopy showed that root and shoot tissues of mature APS plants contained mostly organic Se (possibly selenome thionine), whereas wild-type plants accumulated selenate. The APS plants we re not able to reduce selenate when shoots were removed immediately before selenate was supplied. In addition, Se accumulation in APS plants was 2- to 3-fold higher in shoots and 1.5-fold higher in roots compared with wild-ty pe plants, and Se tolerance was higher in both seedlings and mature APS pla nts. These studies show that ATP sulfurylase not only mediates selenate red uction in plants, but is also rate limiting for selenate uptake and assimil ation.