Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Abstract

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 degrees C for 2 h with light at an intensity of 2,500 micromol photons m(-2) s(-1), the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.

Application of low temperatures during photoinhibition allows characterization of individual steps in photodamage and the repair of photosystem II

“…However, during subsequent incubation under weak light, levels of preD1-1 and preD1-2 remained unchanged (Fig. 2), suggesting that the rigidification of membrane lipids had decreased the ability of PSII to process preD1 (Allakhverdiev et al 2005b). It is noteworthy that this non-repairable photoinhibition had no significant effect on the level of the D2 protein in PSII.…”

“…Photoinhibition is observed when the rate of photodamage to PSII exceeds the rate of repair of PSII. Moreover, whereas the rate of photodamage is proportional to light intensity and is unaffected by environmental factors Nishiyama et al 2004;Murata et al in press), repair is very sensitive to various external factors (Nishiyama et al 2001(Nishiyama et al , 2004Allakhverdiev et al 2002, 2005a, 2005b; for reviews, see Nishiyama et al 2005Nishiyama et al , 2006Murata et al in press).…”

“…It is noteworthy that this non-repairable photoinhibition had no significant effect on the level of the D2 protein in PSII. Moreover, the non-repairable photoinhibition had no effect on the levels of psbA transcripts (Allakhverdiev et al 2005b). As stated above, the repair of PSII involves a series of steps.…”

“…The rigidification of membrane lipids in such mutant cells can be visualized by a variety of physical techniques, such as FT-IR (Fourier-transformed infrared resonance) spectrometry (Szalontai et al 2000). The difference in temperature that was equivalent to the change in fluidity of membranes in the two types of cell was 7-10°C (Allakhverdiev et al 2005b). The desA -/ desD -mutant cells were significantly less able than wildtype cells to recover their PSII activity after photodamaging treatment, even at 25°C, and non-repairable photoinhibition became still more significant with decreases in temperature.…”

“…More recent extensive investigations of non-repairable photoinhibition in Synechocystis cells were facilitated by the fact that experimental conditions can be defined more precisely in such cells than in leaves. When Synechocystis cells were exposed to strong light (approximately 2,500 lmol photons m -2 s -1 ) at 25°C and then transferred back to weak light (approximately 50 lmol photons m -2 s -1 ), the normal photodamage/repair cycle of PSII could be monitored in terms of oxygenevolving activity (Allakhverdiev et al 2005b). However, non-repairable photoinhibition became evident and was enhanced at low temperatures.…”