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.
Keywords: Arabidopsis thaliana, embryogenesis, globular embryo, plastid,
preglobular embryo, SeedGenes.
