A point mutation in the pentatricopeptide repeat motif of the AtECB2 protein causes delayed chloroplast development.

AtECB2 encodes a pentatricopeptide repeat (PPR) protein that regulates the editing of the plastid genes accD and ndhF. The ecb2-1 knockout shows an albino phenotype and is seedling lethal. In this study, we isolated an allelic mutant of the AtECB2 gene, ecb2-2, which showed delayed greening phenotype but could complete their life cycle. In this mutant, the Thr(500) is converted to Ile(500) in the 13(th) PPR motif of the AtECB2 protein. Transmission electron microscopy demonstrated that chloroplast development was delayed in both the cotyledons and leaves of the mutant. An investigation of the chloroplast gene expression profile indicated that PEP (plastid-encoded RNA polymerase) activity in ecb2-2 cotyledons was not obviously affected, whereas it was severely impaired in ecb2-1. This result suggests that the PEP activities cause the different phenotypes of the ecb2-1 and ecb2-2 mutants. The editing efficiency of the three editing sites of accD (C794 and C1568) and ndhF (C290) in the mutant was dynamically altered, which was in agreement with the phenotype. This result indicates that the editing efficiency of accD and ndhF in the ecb2-2 mutant is associated with a delayed greening phenotype. As ecb2-2 can survive and set seeds, this mutant can be used for further investigation of RNA editing and chloroplast development in arabidopsis.