Metal accumulation by aquacultured seedlings of Indian mustard

Indian mustard (Brassica juncea (L.) Czern) seedlings grown in aerated water (aquacultured) were able to accumulate various metals from artificially contaminated water over a range of environmentally relevant metal concentrations. Seedlings concentrated the divalent cations Pb(ll), Sr(ll), Cd(ll), and Ni(ll) 500-2000 times and concentrated the monovalent Cs(I) and hexavalent Cr(IV) 100-250 times from artificially contaminated water containing the competing ions Ca, Mg, K, SO4, and NO3. Seedlings also removed Cd from artificially contaminated water over a broad Cd concentration range. At the lowest Cd concentration studied, Cd levels were reduced to below 10 ppb (mu g/L). In the absence of competing ions, Cd accumulation in seedlings increased 47-fold. This suggests that a better understanding of the biological processes governing uptake and accumulation of Cd by seedlings should allow the application of modern genetic engineering techniques to improve their selectivity and capacity for Cd removal from waters containing high levels of competing ions. As a first step in this process, we have started to define the tissue and cellular localization of Cd, its accumulation rates and possible uptake mechanisms, and the role of intracellular chelates in Cd detoxification. Intracellular Cd accumulation in seedlings was mediated by saturable transport system(s) and was inhibited competitively in shoots and noncompetitively in roots by Ca2+, Zn2+, and Mn2+. Phytochelatins, the Cd-binding peptides known to be involved in Cd resistance in mature plants, also accumulated in B. juncea seedlings exposed to Cd. Using X-ray absorption spectroscopy, we determined that over time the percentage of Cd bound to phytochelatins in vivo increased from 34% after 6 h of Cd exposure to 60% after 72 h. The remaining intracellular Cd appeared to be octahedrally coordinated by oxygen atoms possibly from organic acids. Our results suggest that the use of aquacultured seedlings of B. juncea could provide a novel approach to the treatment of various metal-contaminated waste streams such as landfill, mining and various industrial runoffs, and leachates.