Alleviation of photoinhibition by coordination of chlororespiration and cyclic electron flow mediated by NDH under heat stressed condition in tobacco

With increase of temperature, Fo gradually rose in both WT and the mutant inactivated in the type-1 NAD(P)H dehydrogenase (NDH), a double mutant disrupted the genes of ndhJ and ndhK (∆ndhJK) or a triple mutant disrupted the genes of ndhC, ndhJ and ndhK (∆ndhCJK). The temperature threshold of Fo rise was about 3-5 C lower in the mutants than in WT, indicating ∆ndhJK and ∆ndhCJK were more sensitive to elevated temperature. The Fo rise after the threshold was slower and the reached maximal level was lower in the mutants than in WT, implying the chlororespiratory pathway was suppressed when NDH was inactivated. Meanwhile, the maximum quantum efficiency of photosystem II (PS II) (Fv/Fm) decreased to a similar extent below 50C in WT and mutants. However, the decline was sharper in WT when temperature rose above 55C, indicating a down regulation of PS II photochemical activity by the chlororespiraory pathway in response to elevated temperature. On the other hand, in the presence of n-propyl gallate, one of the inhibitors of plastid terminal oxidase (PTOX ), the less evident increase in Fo while the more decrease in Fv/Fm in ∆ndhCJK than in WT after incubation at 50C for 6h suggest the increased sensitivity to heat stress when both NDH and chlororespiratory pathways are suppressed. Moreover, the net photosynthetic rate and photo-efficiency decreased more significantly in ΔndhJK than in WT under the heat stressed conditions. Compared to the light-oxidation of P700, the difference in the dark-reduction of P700+ between WT and ndhJK disruptant was much less under the heat stressed conditions, implying significantly enhanced cyclic electron flow in light and the competition for electron from PQ between PTOX and photosystem I in the dark at elevated temperature. Heat-stimulated expression of both NdhK and PTOX significantly increased in WT, while the expression of PTOX was less in ΔndhJK than in WT. Meanwhile, the amount of active form of Rubisco activase decreased much more in the mutant. The results suggest that chlororespiration and cyclic electron flow mediated by NDH may coordinate to alleviate the over-reduction of stroma, thus to keep operation of CO2 assimilation a

Oscillation kinetics of post-illumination increase in Chl fluorescence in Cyanobacterium Synechocystis PCC 6803

After termination of longer-illumination (more than 30 s), the wild type of Synechocystis PCC 6803 showed the oscillation kinetics of post-illumination increase in Chl fluorescence: a fast phase followed by one or two slow phases. Unlike the wild type, ndh-B defective mutant M55 did not show any post-illumination increase under the same conditions, indicating that not only the fast phase, but also the slow phases were related to the NDH-mediated cyclic electron flow around photosystem I (PS I) to plastoquinone (PQ).The fast phase was stimulated by dark incubation or in the presence of Calvin cycle inhibitor, iodoacetamid (IA) or cyclic photophosphorylation cofactor, phenazinemethosulphate (PMS), implying the redox changes of PQ by electrons generated at PS I reduced side, probably NAD(P)H or ferredoxin (Fd). In contrast, the slow phases disappeared after dark starvation or in the presence of IA or PMS, and reappeared by longer re-illumination, suggesting that they are related to the redox changes of PQ by the electrons from the photoreductants produced in carbon assimilation process. Both the fast phase and slow phases were stimulated at high temperature and the slow phase (P2) was promoted by response to high concentration of NaCl. The mutant M55 without both phases could not survive under the stressed conditions