Photoinhibition of photosystem II in tobacco plants overexpressing glutathione reductase and poplars overexpressing superoxide dismutase

We studied photoinhibition in two cultivars of tobacco (Nicotiana tabacum L ,) expressing the bacterial gor gene In the cytosol and in four lines of po plar (Populus tremula x P, alba) expressing the FeSOD gene of Arabidopsis t haliana in the chloroplast, The respective total activities of glutathione reductase (EC 1.6.4.2) in leaves of gor tobaccos and superoxide dismutase ( EC 1.15.1.1) in the FeSOD poplars were 5-8 times higher than in the respect ive untransformed control plants. Leaves of control and transformed plants were subjected to high-light stress at 20 degrees C, and photoinhibition of photosystem Il (PSII) was measured by oxygen evolution and chlorophyll flu orescence, The leaves were illuminated both in the presence and absence of lincomycin, which inhibits chloroplast protein synthesis. In both cases, th e time course of loss of PSII activity was identical in plants overproducin g superoxide dismutase (SOD) and in the untransformed controls, suggesting that the ability to convert superoxide to hydrogen peroxide is not a limiti ng factor in protection against photoinhibition, or in the repair of photoi nhibitory damage or that the site of O-2(-) production is not accessible to the transgene product. The rate constant of photoinhibition, measured in l incomycin-treated leaves, was smaller in glutathione reductase (GR) overpro ducing tobacco cv, Samsun than in the respective wild-type, but this differ ence was not seen in cv, Eel W3, The steady-state level of PSII activity me asured when the PSII repair cycle was allowed to equilibrate with photoinhi bitory damage under high light was not higher in the GR overproducing cv, S amsun, suggesting that the repair of photoinhibitory damage was not enhance d in plants overproducing GR in the cytosol.