Abstract

The plastid is an organelle vital to all photosynthetic and some nonphotosynthetic eukaryotes. Although the plastid has its own genome, more than 95% of proteins in this organelle are encoded in the nucleus, representing slightly lower than 10% of all the nuclear-encoded proteins in the model plant Arabidopsis thaliana. Recent functional genomic and molecular genetic studies have shown that ca. 400 nonredundant nuclear genes are required for proper embryo formation of A. thaliana, and that more than one third of them encode plastid-localized proteins. Classification of these genes based on terminal phenotypes of knockout mutants has revealed that nonphotosynthetic metabolic activity of plastids is a prerequisite for the transition of preglobular to globular embryos, and that plastid gene expression becomes significant at or after the globular stage. Functions of the products of 28 out of 105 genes shown to be required for embryo development at the globular and later stages remain unknown. Several approaches to address this issue are discussed.