Iron superoxide dismutases (FeSODs) play an important role in preventing the oxidative damage associated with photosynthesis. To investigate the mechanisms of FeSOD in protection against photooxidative stress, we obtained transgenic tobacco ( Nicotiana tabacum ) plants with severely decreased FeSOD by using a gene encoding tobacco chloroplastic FeSOD for the RNAi construct. Transgenic plants were highly sensitive to photooxidative stress and accumulated increased levels of O 2 •− under normal light conditions. Spectroscopic analysis and electron transport measurements showed that PSII activity was significantly reduced in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed that there was a slow electron transfer between Q A and Q B and decreased redox potential of Q B in transgenic plants, whereas the donor side function of PSII was not affected. Immunoblot and blue native gel analyses showed that PSII protein accumulation was also decreased in transgenic plants. PSII photodamage and D1 protein degradation under high light treatment was increased in transgenic plants, whereas the PSII repair was not affected, indicating that the stability of the PSII complex was decreased in transgenic plants. The results in this study suggest that FeSOD plays an important role in maintaining PSII function by stabilizing PSII complexes in tobacco plants.
