Functional Studies On OsCHR4, A Chromatin Remodeling Factor Participating In Chloroplast Early Development In Rice

The chloroplast is the important organ of plant photosynthesis, and is closely related with carbon assimilation. The chloroplasts development has a direct impact on photosynthesis abilities and yields, thus screening abnormal leaf mutants can be used to study chloroplast development as good materials. In this study, we isolated a rice (Oryza sativa L.) mutant showing leaf albino only in the adaxial side, from an EMS genetic mutant library. By transmission electron microscopic images of chloroplasts, the adaxial albino phenotype of the mutant leaves is caused by abnormal formation of chloroplasts. However, the mesophyll cells in abaxial side contained morphologically normal chloroplasts. The mutated gene 0sCHR4 was cloned using the map-based cloning strategy and its gene locus is LOC₀7g31450. This gene was shown to encode a chromatin remodeling factor 4. Transgenic complementation of the mutant verified that OsCHR4 is involved in adaxial chloroplast development. OsCHR4 belongs to the highly conserved CHD family of proteins and its homologous proteins have been identified in vertebrates, Drosophila, yeast, C. elegans and plants.OsCHR4 is targeted to the nucleus and constitutively expressed in all tissues tested. Histochemical staining for GUS activity showed that OsCHR4 is expressed in the active root tissues of mitosis, flowers, all mesophyll cells and vascular bundles of stems. In the end, we further investigated the effect of OsCHR4 mutation on chloroplast development, and the result showed that the transcripts of eight genes associated with chloroplast development were significantly reduced in the adaxial mesophyll of the mutant. On the other hand, KNOX homeobox genes were significantly increased in the mutant which indicates OsCHR4 probably related to the AS1/AS2 pathway for repression of KNOX genes expression. The present results reveal that OsCHR4 is essential for chloroplast biogenesis at the early stage in adaxial mesophyll; this is the first report on the function of a chromatin remodeling factor involved in chloroplast development in plants.