The proteolytic degradation of the photodamaged D1 core subunit during the photosystem Ⅱ (PSⅡ) repair cycle is well understood,but chlorophyll turnover during D1 degradation remains unclear.Here,we report thatArabidopsis thaliana CHLOROPHYLLASE 1 (CLH1) plays important roles in the PSⅡ repair process.The abundance of CLH1 and CLH2 peaks in young leaves and is induced by high-light exposure.Seedlings of clh1 single and clh1-1/2-2 double mutants display increased photoinhibition after long-term high-light exposure,whereas seedlings overexpressing CLH1 have enhanced light tolerance compared with the wild type.CLH1 is localized in the developing chloroplasts of young leaves and associates with the PSⅡ-dismantling complexes RCC1 and RC47,with a preference for the latter upon exposure to high light,Furthermore,degradation of damaged D1 protein is retarded in young clh1-1/2-2 leaves after 18-h high-light exposure but is rescued by the addition of recombinant CLH1 in vitro.Moreover,overexpression of CLH1 in a variegated mutant (var2-2) that lacks thylakoid protease FtsH2,with which CLH1 interacts,sup-presses the variegation and restores D1 degradation.A var2-2 clh1-1/2-2 triple mutant shows more severe variegation and seedling death.Taken together,these results establish CLH1 as a long-sought chlorophyll dephytylation enzyme that is involved in PSⅡ repair and functions in long-term adaptation of young leaves to high-light exposure by facilitating FtsH-mediated D1 degradation.