The tobacco plastid accD gene is essential and is required for leaf development

Angiosperm plastid genomes typically encode approximately 80 polypeptides, mainly specifying plastid-localized functions such as photosynthesis and gene expression. Plastid protein synthesis and expression of the plastid clpP1 gene are essential for development in tobacco, indicating the presence of one or more plastid genes whose influence extends beyond the plastid compartment. The plastid accD gene encodes the beta-carboxyl transferase subunit of acetyl-CoA carboxylase and is present in the plastids of most flowering plants, including non-photosynthetic parasitic plants. We replaced the wild-type accD gene with an aadA-disrupted mutant allele using homologous recombination. Persistent heteroplasmy in the presence of antibiotics indicated that the wild-type accD allele was essential. The phenotype of the accD knockout was revealed in plastid transformants grown in the absence of antibiotics. Leaves contained pale green sectors and lacked part or all of the leaf lamina due to arrested division or loss of cells. Abnormal structures were present in plastids found in mutant plants, indicating that accD might be required to maintain the plastid compartment. Loss of the plastid compartment would be expected to be lethal. These results provide genetic evidence showing the essential role of plastid ACCase in the pathway leading to the synthesis of products required for the extra-plastidic processes needed for leaf development.