During the growth and development of a seedling plant, the various types of plastids which are found in different cell types develop from those proplastids found in the cells of the shoot and root meristems. These proplastids provide the source for all plastids in all the different types of cells within the plant. The way in which proplastids develop into different plastid types in different types of cells is poorly understood, but by far the best-researched plastid developmental pathway is that of the proplastid developing into a mature chloroplast. This developmental pathway occurs primarily in the mesophyll cells of leaves, resulting in large populations of mature green chloroplasts in these cells, although chloroplast development can also occur to varying degrees in all other green tissues of a plant. The development of a mature chloroplast in a leaf mesophyll cell or in other green tissues requires a huge synthesis of proteins, lipids and metabolites, the vast majority of which are imported from the cytosol into the plastid. In particular, an extensive array of proteins encoded in the nucleus and translated on cytosolic ribosomes are required to be imported using the import mechanisms described in Chapter 5.
Moreover, a major feature of this chloroplast developmental pathway is coordination. Not only is coordination required between the expression of nuclear genes encoding proteins destined for the chloroplast and the expression of those genes encoded on the chloroplast's own genome, but also coordination is required between the expression of these nuclear genes and the developmental status of the chloroplast, as well as its functional status in terms of photosynthetic activity and the degree to which the chloroplast is stressed by environmental conditions.