Gene expression in chloroplasts is regulated by many nuclear-encoded proteins.In this study,we isolated a rice (Oryza sativa subsp.japonica) mutant osotp51 with significant reduction in photosystem Ⅰ (PSI).The osotp51 is extremely sensitive to light and accumulates a higher level of reactive oxygen species.Its leaves are almost albino when grown at 40 μmol photons/m2 per s.However,grown at 4 μmol photons/m2 per s,osotp51 has a similar phenotype to the wild-type.77K chlorophyll fluorescence analysis showed a blue shift in the highest peak emission from PSI in osotp51.In addition,the level of PSI and PSII dimer is dramatically reduced in osotp51.OSOTP 51 encodes a pentatricopeptide repeats protein,homologous to organelle transcript processing 51 in Arabidopsis.Loss-of-function OSOTP51 affects intron splicing of a number of plastid genes,particularly the ycf3 coding a protein involved in the assembly of PSI complex.OSOTP51 is functionally conserved in higher plants.The mutation of osotp51 indirectly leads to a widespread change in the structure and functions of PSI,results in severe photoinhibition,and finally dies,even when grown under very low light intensity.