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showed no chlorosis, and their chlorophyll fluorescence To determine the importance of glutathione reductase (GR, EC 1.6.4.2) for heavy metal accumulation and tolerance, a parameters F v /F m and photochemical quenching were higher. bacterial GR was expressed in Indian mustard (Brassica Cadmium tolerance at the whole-plant level (plant growth) juncea L.), targeted to the cytosol or the plastids. GR activ-was not affected. The lower cadmium stress experienced by the cpGR chloroplasts may be the result of reduced cad-ity in the cytosolic transgenics (cytGR) was about two times higher compared to wild-type plants; in the plastidic trans-mium uptake and/or translocation: cadmium levels in shoots genics (cpGR) the activity was up to 50 times higher. When of cpGR plants were half as high as those in wild-type treated with 100 mM CdSO 4 , cytGR plants did not differ shoots. These differences in cadmium tolerance and accumulation may result from increased root glutathione levels, from wild type in cadmium tolerance or accumulation. CpGR plants, however, showed enhanced cadmium tolerance which were up to two times higher in cpGR plants than in the wild type. at the chloroplast level: in contrast to wild-type plants they make chloroplasts a major source of active oxygen intermediates in photosynthetic tissues of plants (Allen 1995, Prasad, 1995. Heavy metals like Cd are effective inhibitors of photosynthesis (Prasad, 1995). Cadmium exposure was shown to result in a reduction in chlorophyll content (Ouzounidou et al. 1997), inhibition of photosystem II (Atal et al. 1993), and a negative effect on chlorophyll fluorescence (Horvath et al. 1996). The hypothesis tested in this study is that overexpression of GR will reduce the damage that Cd has on plant function, based on the assumptions that Cd causes oxidative stress and GR activity alleviates oxidative stress. To test this hypothesis the E. coli gor gene encoding GR was introduced in Indian mustard, targeted either to the cytosol or to the plastids. The transgenic plants were characterized with respect to GR activity profiles and GSH levels, and the Cd tolerance of the transgenic GR plants was compared with that of wild-type plants, in particular with respect to chloroplast function.

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To determine the importance of glutathione reductase (GR, EC 1.6.4.2) for heavy metal accumulation and tolerance, a bacterial GR was expressed in Indian mustard (Brassica juncea L.), targeted to the cytosol or the plastids. GR activity in the cytosolic transgenics (cytGR) was about two times higher compared to wild‐type plants; in the plastidic transgenics (cpGR) the activity was up to 50 times higher. When treated with 100 μM CdSO4, cytGR plants did not differ from wild type in cadmium tolerance or accumulation. CpGR plants, however, showed enhanced cadmium tolerance at the chloroplast level: in contrast to wild‐type plants they showed no chlorosis, and their chlorophyll fluorescence parameters Fv/Fm and photochemical quenching were higher. Cadmium tolerance at the whole‐plant level (plant growth) was not affected. The lower cadmium stress experienced by the cpGR chloroplasts may be the result of reduced cadmium uptake and/or translocation: cadmium levels in shoots of cpGR plants were half as high as those in wild‐type shoots. These differences in cadmium tolerance and accumulation may result from increased root glutathione levels, which were up to two times higher in cpGR plants than in the wild type.