Overexpression of glutathione synthetase in Indian mustard enhances cadmium accumulation and tolerance

An important pathway by which plants detoxify heavy metals is through seque stration with heavy-metal-binding peptides called phytochelatins or their p recursor, glutathione. To identify limiting factors for heavy-metal accumul ation and tolerance, and to develop transgenic plants with an increased cap acity to accumulate and/or tolerate heavy metals, the Escherichia coli gshl l gene encoding glutathione synthetase (CS) was overexpressed in the cytoso l of Indian mustard (Brassica juncea). The transgenic CS plants accumulated significantly more Cd than the wild type: shoot Cd concentrations were up to 25% higher and total Cd accumulation per shoot was up to 3-fold higher. Moreover, the CS plants showed enhanced tolerance to Cd at both the seedlin g and mature-plant stages. Cd accumulation and tolerance were correlated wi th the gshll expression level. Cd-treated GS plants had higher concentratio ns of glutathione, phytochelatin, thiol, S, and Ca than wild-type plants. W e conclude that in the presence of Cd, the CS enzyme is rate limiting for t he biosynthesis of glutathione and phytochelatins, and that overexpression of CS offers a promising strategy for the production of plants with superio r heavy-metal phytoremediation capacity.