EMB2738, which encodes a putative plastid-targeted GTP-binding protein, is essential for embryogenesis and chloroplast development in higher plants

In higher plants, chloroplasts carry out many important functions, and normal chloroplast development is required for embryogenesis. Numerous chloroplast-targeted proteins involved in embryogenesis have been identified. Nevertheless, their functions remain unclear. In this study, a chloroplast-localized protein, EMB2738, was reported to be involved in Arabidopsis embryogenesis. EMB2738 knockout led to defective embryos, and the embryo development in emb2738 was interrupted after the globular stage. Complementation experiments identified the AT3G12080 locus as EMB2738. Cellular observation indicated that severely impaired chloroplast development was observed in these aborted embryos. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis showed that chloroplast-encoded photosynthetic genes, which are transcribed by plastid-encoded RNA polymerase (PEP), are predominantly decreased in defective embryogenesis, compared with those in the wild-type. In contrast, genes encoding PEP core subunits, which are transcribed by nucleus-encoded RNA polymerase (NEP), were increased. These results suggested that the knockout of EMB2738 strongly blocked chloroplast-encoded photosynthesis gene expression in embryos. Silencing of the EMB2738 orthologue in tobacco through a virus-induced genome silencing technique resulted in an albinism phenotype, vacuolated chloroplasts and decreased PEP-dependent plastid transcription. These results suggested that NtEMB2738 might be involved in plastid gene expression. Nevertheless, genetic analysis showed that the NtEMB2738 coding sequence could not fully rescue the defective embryogenesis of the emb2738 mutant, which suggested functional divergence between NtEMB2738 and EMB2738 in embryogenesis. Taken together, these results indicated that both EMB2738 and NtEMB2738 are involved in the expression of plastid genes in higher plants, and there is a functional divergence between NtEMB2738 and EMB2738 in embryogenesis.