| Summary: | 碩士 === 臺灣大學 === 農業化學研究所 === 98 === Salt stress results in an excessive generation of reactive oxygen species (ROS) that causes oxidative damage to plants. Plants evolve certain strategies to remove salt-induced reactive oxygen species (ROS), such as glutathione-ascorbate cycle (ASH-GSH cycle). Glutathione reductase (GR) plays an important role in ASC-GSH cycle, which converted oxidized GSH (GSSG) to reduced GSH. Three GR genes exist in rice, including a cytosolic (OsGR2) and two chloroplastic isoforms (OsGR1 and OsGR3). Our previous study demonstrated that expression of OsGR3 was increased in response to NaCl and abscisic acid in roots of etiolated rice seedlings. The OsGR3 has been considered to be a non-functional GR isozyme due to the lack of a FAD binding domain and chloroplastic transit sequence (CTS). However, after blasting different rice database, we found a full length OsGR3 open reading frame with complete FAD binding domain and CTS. To investigate the physiological roles of OsGR3 in salt stress and to understand whether the OsGR3 is a functional GR, OsGR3 knockout mutant and overexpression transgenic rice were analyzed in this thesis. In osgr3, 42% GR activity was decreased and salt sensitivity was enhanced. Physiological responses of osgr3 rice mutant revealed that the maximal efficiency of photosystem II and survival rate were notably reduced as compared to WT, and H2O2 was increased after 200 mM NaCl treatment. One of the reasons caused osgr3 mutant intolerant to salt stress is that osgr3 had lower ascorbate content and lower ratio of ascobate/dehydroascorbate than WT rice. Overexpression of OsGR3 in rice plants increased three to six times of GR activity, and also increased transformant''s salt tolerance. Higher ascorbate content in overexpression OsGR3 transgenic rice as compared to WT was also observed. However, overexpression of OsGR3tc, lack of FAD binding domain, diminished GR activity, and transgenic rice were sensitive to salt stress. On the other hand, complementation of osgr3 with OsGR3 (osgr3/OsGR3) increased three to five times of GR activity and restored salt stress tolerance. Meanwhile, leaf discs of osgr3, WT and complementation treated with methyl viologen showed that the degrees of chlorophyll bleaching from osgr3 to WT then osgr3/OsGR3, indicating that OsGR3 is important for oxidative defense system. Our results suggested that OsGR3 is a functional GR, and is involved in salt and oxidative tolerance.
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