Function of ROC4 in the Efficient Repair of Photodamaged Photosystem II in Arabidopsis

ROC4 is the only cyclophilin in the chloroplast stroma. Here, we used the T-DNA knockout mutant of roc4 to study the physiological role of ROC4 in vivo in Arabidopsis thaliana. Our results showed that ROC4 is not required for the biogenesis and functional operation of photosystem II (PSII). However, growth in greenhouse and PSII activity, as detected by photoinhibition measurements showed increased sensitivity to high light irradiance in the mutant. In the presence of chloroplast protein synthesis inhibitor lincomycin, which blocks de novo protein synthesis and thus the repair of PSII, wild-type and mutant plants showed a similar extent of inactivation of PSII activity. The recovery of PSII activity in roc4 leaves from photoinhibition is also impaired compared with that of wild-type plants. Immunoblot analysis showed that the degradation of PSII reaction center proteins occurred at a similar rate in the presence of lincomycin in wild-type and mutant plants. Thus, these results suggest that ROC4 functions in the repair of photodamaged PSII.