DELAYED GREENING 238, a Nuclear-Encoded Chloroplast Nucleoid Protein, Is Involved in the Regulation of Early Chloroplast Development and Plastid Gene Expression in Arabidopsis thaliana

Chloroplast development is an essential process for plant growth that is regulated by numerous proteins. Plastid-encoded plastid RNA polymerase (PEP) is a large complex that regulates plastid gene transcription and chloroplast development. However, many proteins in this complex remain to be identified. Here, through large-scale screening of Arabidopsis mutants by Chl fluorescence imaging, we identified a novel protein, DELAYED GREENING 238 (DG238), which is involved in regulating chloroplast development and plastid gene expression. Loss of DG238 retards plant growth, delays young leaf greening, affects chloroplast development and lowers photosynthetic efficiency. Moreover, blue-native PAGE (BN-PAGE) and Western blot analysis indicated that PSII and PSI protein levels are reduced in dg238 mutants. DG238 is mainly expressed in young tissues and is regulated by light signals. Subcellular localization analysis showed that DG238 is a nuclear-encoded chloroplast nucleoid protein. More interestingly, DG238 was co-expressed with FLN1, which encodes an essential subunit of the PEP complex. Bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP) assays showed that DG238 can also interact with FLN1. Taken together, these results suggest that DG238 may function as a component of the PEP complex that is important for the early stage of chloroplast development and helps regulate PEP-dependent plastid gene expression.