||In Chl biosynthesis, aerobic Mg-protoporphyrin IX monomethyl ester (MPE) cyclase is a key enzyme involved in the synthesis of protochlorophyllide a, and its membrane-bound component is known to be encoded by homologs of CHL27 in photosynthetic bacteria, green algae and plants. Here, we report that the Arabidopsis chl27-t knock-down mutant exhibits retarded growth and chloroplast developmental defects that are caused by damage to PSII reaction centers. The mutant contains a T-DNA insertion within the CHL27 promoter that dramatically reduces the CHL27 mRNA level. chl27-t mutant plants grew slowly with a pale green appearance, suggesting that they are defective in Chl biosynthesis. Chl fluorescence analysis showed significantly low photosynthetic activity in chl27-t mutants, indicating damage in their PSII reaction centers. The chl27-t mutation also conferred severe defects in chloroplast development, including the unstacking of thylakoid membranes. Microarray analysis of the chl27-t mutant showed repression of numerous nuclear genes involved in photosynthesis, including those encoding components of light-harvesting complex I (LHCI) and LHCII, and PSI and PSII, which accounts for the defects in photosynthetic activity and chloroplast development. In addition, the microarray data also revealed the significant repression of genes such as PORA and AtFRO6 for Chl biosynthesis and iron acquisition, respectively, and, furthermore, implied that there is cross-talk in the Chl biosynthetic pathway among the PORA, AtFRO6 and CHL27 proteins.