Functional Analysis Of Cyanobacterial NDH-CET Under Heat Stress Conditions

The high temperature stress casued by human activities, such as the greenhouseeffect, influences the photosynthetic efficiency of higher plants and cyanobacterialcells, and may even cause death. To acclimatize to high temperature stress conditions,a variety of strategies against heat have been developed in higher plants andcyanobacteria, including the enhancement of cyclic electron transport aroundphotosystem I （CET）. Further, in higher plants, the increase in CET not only producedthe extra ATP that can be used to repair the key enzymes of carbon assimilation, butalso helped in building the proton gradient across the thylakoid membrane to increaseheat dissipation, thereby alleviating the damage caused by heat stress. However, littleis known regarding the functional roles of CET against heat stress in cyanobacteria.In cyanobacteria cells, the main route of CET is NADPH dehydrogenase（NDH-1）-mediated one. This thesis, therefore, measured the chlorophyll content,NDH-CET activity, intracellular H₂O₂content, and linear electron transfer activity inthe unicellular cyanobacterium wild-type Synechocystis sp. strain PCC6803（hereafter Synechocystis6803） and its NDH-CET defective mutant M55during theswitches between growth condition （30°C） and high temperature （45°C）. In wild-typecells, the chlorophyll content rose rapidly in a short period of time, and then slowlydecreased, while intracellular H₂O₂content gradually increased after a plateau periodwhen cells were shifted from growth temperature to heat. By contrast, a moreremarkable increasing or decreasing trend of various parameters occurred inNDH-CET-defective mutant M55; this was further confirmed by the treatment ofrotenone, a specific inhibitor of NDH-CET activity. Also, the heat-retarded growthphyeotype was quickly recovered and the amounts of H₂O₂also gradually decreasedin wild-type cells with a high activity of NDH-CET when cells were shifted from heatto growth temperature. However, the growth recovery phenomena did not occur inNDH-CET-defective mutant M55cells. We therefore conclude that cyanobacterialNDH-CET has an ability to alleviate the degradation of chlorophyll molecules causedby heat stress. Furether, the decreasing levels of carbon assimilation were found to be slowerthan that in M55after the transfer of cells from growth temperture to heat. The resultssuggested that in the cyanobacterium Synechocytsis6803, heat stress significantlyincreased the activity of NDH-CET to produce additional ATP that was used to repairthe key enzymes of carbon assimilation, thereby reducing the production of reactiveoxygen species such as H₂O₂, and alleviating the degradation of chlorophyllmolecules.