Stability and plasticity during chloroplast development.

Chloroplast development occurs during cellular development. In non-limiting conditions chloroplast development is a highly conserved process, it is also complex and involves the continuous interaction of both chloroplast and nuclear genomes. In the first part of the paper the sequential and structural changes characteristic of chloroplast division and development in angiosperms are described. The synthesis of the major chloroplast components including chlorophylls a and b, lipids, nucleic acids and the major soluble and membrane proteins are then described. Chloroplast development in biochemical terms is a quantitative accretion of additional functional units. In development from proplastid to fully mature chloroplast the molecular changes are almost exclusively quantitative and the youngest plastids that can be analysed are already photochemically fully competent. In the second part of the paper the dominant role of the nuclear genome in chloroplast development is discussed. Recent work in the author's laboratory on the synthesis and accumulation of ribulose bisphosphate carboxylase-oxygenase in the developing chloroplasts of young wheat leaves is cited to illustrate the stable genomic and genotypic differences that can be recognized. In comparisons of wheat species of differing ploidy, in hexaploid cultivars and in artificially processed genetic lines, several genomic and genotypic effects have been detected. The possibilities for future investigation are discussed.