ABSTRACT
Plastids are the DNA-containing organelles with the widest range of different forms and functions. Interconversion between different plastid types implies marked changes in their architecture and physiology. In vascular plants chloroplast biogenesis from less-differentiated organelle forms occurs: in vegetation tips the differentiation process starts from colorless pro- plastids. The recessive nuclear mutation affects a variety of light-regulated traits, including leaf development, plastid morphology, and gene expression. Plant cells are characterized by three different genomes that have their own machinery for gene expression: the prokaryotic ancestors of plastids and mitochondria lost their autonomy by losing most of their genes during the establishment of endosymbiosis. Cytokinin-induced, as well as light-induced, chloroplast biogenesis is at the molecular level characterized by the accumulation of transcripts from plastid genes. Molecular genetic analysis of the regulation of cytokinin- responsive nuclear genes is scarce, since, up to, no gene expression has been known to be triggered specifically by cytokinin.
