| In cyanobacteria and higher plants, the activity of photosystemâ…¡(PSâ…¡) was commonly inhibited under stressful conditions, such as strong light, thereby affecting their photosynthetic yield. In higher plants, pre-exposure of plants to one form of stress can impact tolerance to other forms of stress. This is called cross-tolerance. Further, cyclic electron transport (CET) around photosystem I in higher plants plays an important role in the mechanism of cross-tolerance. In cyanobacteria, the CET mediated by the photosynthetic NADPH dehydrogenase (NDH-1) is a main cyclic pathway. So far, however, little is known regarding the presence of the cross-tolerance in cyanobacterial cells and, the detailed function role of NDH-CET underlying the cross-tolerance mechanism.To investigate the aforementioned scientific problems, cells of wild type of Synechocystis sp. strain PCC 6803 and its ndh mutants were used as experimental materials. After the moderate heat pretreatment and exposure of cells to the short-time strong light, we measured and analyzed the changes in chlorophyll fluorescence parameters, fluorescence emission spectra, and extra-ATP levels driven by NDH-CET. Our results demonstrated that the short-time moderate heat pretreatment considerably alleviates the inhibition of quantum yield of PSâ…¡caused by the response of cells to strong light preferentially absorbed by phycobilisome such as red light, green light and yellow light and not by photosystemâ… such as blue light. It has been previously reported that the moderate heat treatment partially uncoupled the energy transfer from phycobilisome to the PSâ…¡in cyanobacteria. Our findings of this study further indicated that the NDH-CET is essential for such energetic uncoupling. In conclusion, our findings not only proposed that the strategy of cross-tolerance existed in cyanobacteria, but also indicated a novel and essential role of cyanobacterial NDH-CET underlying the cross-tolerance mechanism.
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