Improved metal phytoremediation through plant biotechnology

Phytoremediation, i.e; the use of plants for environmental cleanup, offers an attractive approach to remediate metals from mine waste. The goal of this study is to use genetic engineering to increase heavy metal tolerance and accumulation in plants, so as to create better plants for metal phytoremediation. The chosen strategy is to overproduce the heavy metal binding peptides glutathione and phytochelatins. Glutathione (gamma-Glu-Cys-Gly, GSH) plays several important roles in the defense of plants against environmental stresses, and is the precursor for phytochelatins (PCs): heavy metal-binding peptides involved in heavy metal tolerance and sequestration. Glutathione is synthesized in two enzymatic reactions, catalyzed by glutamylcysteine synthetase (ECS) and glutathione synthetase (GS), respectively. To obtain plants with superior metal accumulation and tolerance, we overexpressed the E. coli ECS and GS enzymes in Brassica juncea (Indian mustard), a particularly suitable plant species for heavy metal remediation. The transgenic ECS and GS plants contained higher levels of glutathione and phytochelatins than the wildtype plants. In metal tolerance and accumulation studies using cadmium, the ECS and GS plants accumulated 1.5 to 2-fold more cadmium in their shoots than wildtype plants, and also showed enhanced cadmium tolerance. As a result, the total cadmium accumulation per shoot was similar to 3-fold higher. We conclude that overexpression of the ECS and GS enzymes is a promising strategy for the production of plants with superior heavy metal phytoremediation capacity. Present and future studies include analysis of the ECS/GS plants with other heavy metals, and phytoremediation studies using metal-polluted mine waste.