| Cyanobacteria are prokaryotes capable of oxygen-evolving photosynthesis, and are one of the most popular model organisms for photosynthesis research. Cyclic electron transport (CET) around photosystem Ⅰ (PSI) provides the necessary energy in the process of cyanobacterial carbon assimilation, metabolic and resistance to environmental stress by generated ATP. In cyanobacteria, CET consists of two main pathways, the NDH-dependent (NDH-CET) and the PGR5-dependent (PGR5-CET) pathway. NDH-CET is the main pathway of cyanobacteria, plays an important role in the response to a low carbon. high light, high salt stress. PGR5-CET is considered as a secondary, only light sensitive pathway. However, its mechanism, physiological functions, the relationship with the NDH-CET. even localization of the PGR5protein in thylakoid membranes are still unknown. Therefore, in this study, the above questions of some in-depth research and the related results are listed as follows:1.Using△ndhS mutant impaired NDH-CET as material, we found that the ability of CET in AndhS is restored to that in WT under low carbon condition by the methods of chlorophyll fluorescence and P700oxidation reduction kinetics level to monitor the level of CET. Compared to that of WT, the PGR5transcriptional level and PGR5-CET of AndhS mutant were up-regulation under low carbon. Further, the△pgr5and△ndhS/pgr5mutants were constructed and the changes of two CET pathways in WT,△pgr5,△ndhS, and△ndhS/pgr5under low carbon were analysis by reverse genetics and physiological methods, indicating that impaired NDH-CET due to ndhS gene inactivation in low-carbon was compensatory restoration by PGR5-CET. 2.We also found that△ndhS/pgr5double mutant is significantly retarded in low-carbon and lethal even in high light, showed that the compensation of PGR5-CET maintain CO2uptake in Synechocystis6803. and is necessary to the growth of the cells against high light stress.3.BN-PAGE and western blot were used to detect change of the photosystem complex and proteins in WT,△pgr5, AndhS and△ndhS/pgr5under low carbon and high light condition, the results showed that severe photosystem1(PSI) photodamage caused the slow growth of△ndhS/pgr5. Simultaneously, photosynthetic activities of PSI and photosystem Ⅱ (PSⅡ) by adding DCMU or not were detected to determine the PSI photodamage in△ndhS/pgr5caused by excess electrons from the linear electron transfer (LET), indicates that the compensation mechanism of PGR5-CET is participated in PSI photoprotection. And we also found that the protective effect is independent to NPQ mechanism.4.By detecting the state transition capabilities and QA of WT,△pgr5,△ndhS and△ndhS/pgr5in low-carbon and high/induced strength light condition, found that PSI photoprotection in NDH-CET impared mutants were related to state transition and QA redox levels of cells. And the PSI activity after adding DCMU or not under induced strength light in state transition mutant AapcD further confirmed that state transition can alleviate the PSI photodamage caused by deficiency of PGR5-CET compensation in cyanobacteria.5.The△ndhM/pgr5.△ndhD1/D2/pgr5.△ndhB/pgr5and△ndhF1/pgr5mutants were constructed, and PGR5-CET abilities of impaired NDH-CET mutations of△ndhM,△ndhD1/D2,△ndhB and△ndhF1under low carbon were analysis, and the results showed that PGR5-CET compensating function plays roles in AndhM, but not in NDH membrane subunit inactivation mutants,△ndhB,△ndhD1/D2and△ndhF1, revealing the additional compensation function of PGR5-CET depended on NdhF1subunit of the NDH-IL complex in Synechocystis6803.In conclusion, our results not only further improved the function of the cyanobacterial PGR5-CET pathway and its mechanism against high light, but also increased the new understanding of PGR5-CET and NDH-CET pathway by further investigating the regulation of relationship between NDH-CET and PGR5-CET pathways in cyanobacteria. |
PDF Full Text Request