| Toxic heavy metal contamination of soils, aqueous waste streams and groundwater poses a major environmental and human health problem. Phytoremediation, the use of plants to clean up pollutants, has been proposed as an innovative and cost-effective way of remediating contaminated soil and water. Living plants can be compared to solar-driven pumps that extract and concentrate heavy metals from their environments; however, excessive accumulation of these heavy metals may become toxic to plants.; Glutathione and phytochelatins play important roles in detoxifying and sequestering heavy metals. The objectives of present study were to investigate rate limiting factors for glutathione and phytochelatin production, and to develop transgenic plants with superior abilities to tolerate and/or accumulate heavy metals. In order to carry out these objectives, Escherichia coli genes, gshI, encoding gamma-glutamylcysteine synthetase (gamma-ECS), and gshII, encoding glutathione synthetase (GS), were each transferred to Indian mustard (Brassica juncea). Transgenic Indian mustard plants overexpressing either gamma-ECS or GS exhibited enhanced Cd tolerance and accumulation compared to untransformed (wildtype, WT) plants. The transgenic plants also showed increased biosynthesis of glutathione and/or phytochelatins under Cd stress.; The conclusions of the study are: (1) GS is rate limiting for GSH/phytochelatin synthesis under Cd-stressed conditions, but is not rate limiting under unstressed (no Cd treatment) conditions; (2) gamma-ECS is rate limiting for GSH/phytochelatin synthesis under both Cd-stressed and unstressed conditions; (3) phytochelatin synthesis can be enhanced by overexpressing gamma-ECS or GS; and (4) both Cd tolerance and accumulation in Indian mustard can be enhanced simultaneously by overexpressing gamma-ECS or GS.; In addition, the application of these promising transgenic plants for heavy metal phytoremediation is discussed. Furthermore, I propose a possible new pathway for PC synthesis, and that Cd-treated leaves can synthesize PCs as well as roots, which are currently considered to be the major site for PC synthesis in the whole plant. |
