Reduction and coordination of arsenic in Indian mustard

The bioaccumulation of arsenic by plants may provide a means of removing th is element from contaminated soils and waters. However, to optimize this pr ocess it is important to understand the biological mechanisms involved. Usi ng a combination of techniques, including x-ray absorption spectroscopy, we have established the biochemical fate of arsenic taken up by Indian mustar d (Brassica juncea). After arsenate uptake by the roots, possibly via the p hosphate transport mechanism, a small fraction is exported to the shoot via the xylem as the oxyanions arsenate and arsenite. Once in the shoot, the a rsenic is stored as an As-III-tris-thiolate complex. The majority of the ar senic remains in the roots as an As-III-tris-thiolate complex, which is ind istinguishable from that found in the shoots and from As-III-tris-glutathio ne. The thiolate donors are thus probably either glutathione or phytochelat ins. The addition of the dithiol arsenic chelator dimercaptosuccinate to th e hydroponic culture medium caused a 5-fold-increased arsenic level in the leaves, although the total arsenic accumulation was only marginally increas ed. This suggests that the addition of dimercaptosuccinate to arsenic-conta minated soils may provide a way to promote arsenic bioaccumulation in plant shoots, a process that will be essential for the development of an efficie nt phytoremediation strategy for this element.