Molecular Mechanism of Damage and Repair Processes in Photosynthesis

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This research project was focus on the elucidation of molecular mechanism of stress-responses of photosynthetic organisms to wards high intensity of light, active oxygen species and high temperatures. Followings are some examples of major achievements.(1) In vitro random mutagenesis of psbA2 gene of Synechocystis sp.PCC 6803 by PCR with a low fidelity of amplification and subsequent screening at 320 muE・m^<-2>・s^<-1> have provided us 18 distinct mutants tolerant to the high intensity of light, both in terms of pigmentation and growth rate. The molecular mechanisms by which the photo-tolerance has been acquired in these mutants by the substitution of specific amino acid (s) on D1 protein were analyzed.(2) Transgenic tobacco plants enriched or reduced in plastidic glutamine synthetase (GS2), a key enzyme in photorespiration, were constructed. Those transgenic plants having twice the normal amount of GS2 had an improved capacity for photorespiration and an increased tolerance to high-inte … More nsity light, whereas those with a reduced amount of GS2 had a diminished capacity for photorespiration and were photoinhibited more severely by high-intensity of light compared with control plants. From these results, it is concluded that photorespiration protects C_3 plants from photoinhibition.(3) Transgenic tobacco plants with enhanced activities of both glutathione reductase (GR) and superoxide dismutase (SOD) were generated by cross-fertilization between two transgenic lines ; one which had a GR gene from E.coli and another which had a cDNA for cytosolic Cu/Zn SOD from Oryza sativa. Three types of transgenic plants, i.e., GR plants with 2.5-fold higher GR activity, SOD plants with 2.8-fold higher SOD activity, GR-SOD plants with 2.8- and 1.4-fold higher activities of GR and SOD,respectively, than those of control plants, were treated with paraquat. Leaves of the GR-SOD transgenic plants exhibited higher tolerance to paraquat not only more than the control plants but also more than the GR- and the SOD-transgenic plants. These observations confirm that the tolerance of leaves to photooxidative stress caused by paraquat in this study depended on cytosolic activities of both GR and SOD,and that these two enzymes are working together to protect plants against oxidative stress. Less