OVEREXPRESSION OF GLUTATHIONE-REDUCTASE BUT NOT GLUTATHIONE SYNTHETASE LEADS TO INCREASES IN ANTIOXIDANT CAPACITY AND RESISTANCE TO PHOTOINHIBITION IN POPLAR TREES

A poplar hybrid, Populus tremula x Populus alba, was transformed with the bacterial genes for either glutathione reductase (GR) (gor) or glu tathione synthetase (CS) (gshll). When the gor gene was targeted to th e chloroplasts, leaf CR activities were up to 1000 times greater than in all other lines. In contrast, targeting to the cytosol resulted in 2 to 10 times the CR activity. CR mRNA, protein, and activity levels s uggest that bacterial CR is more stable in the chloroplast. When the g shll gene was expressed in the cytosol, GS activities were up to 100 t imes greater than in other lines. Overexpression of CR or GS in the cy tosol had no effect on glutathione levels, but chloroplastic-GR expres sion caused a doubling of leaf glutathione and an increase in reductio n state. The high-chloroplastic-CR expressors showed increased resista nce to photoinhibition. The herbicide methyl viologen inhibited CO2 as similation in all lines, but the increased leaf levels of glutathione and ascorbate in the high-chloroplastic-CR expressors persisted despit e this treatment. These results suggest that overexpression of CR in t he chloroplast increases the antioxidant capacity of the leaves and th at this improves the capacity to withstand oxidative stress.