From what has been discussed in the preceding chapters, it should be clear that the plastid is a highly dynamic organelle in terms of its biochemistry, its molecular biological systems, its photochemistry and its molecular interaction with the nucleus. In addition to these aspects of plastid biology, one also needs to consider how the plastid resides within the cytoplasm of the cell, in many cases as a large closely packed population of individual organelles resulting from extensive divisions, and how the individual organelles move about in the cell and physically interact with other cellular organelles. Classic images of plastids from sectioned leaves and electron micrographs (see Figs. 2.4, 2.5) give the impression of a static organelle in which little changes physically with time. Modern imaging techniques, together with developments in the use of visible molecular markers that can be artificially incorporated into plastids, reveal a very different view of plastids: that of a highly dynamic organelle capable of rapid changes in morphology and movement within the cell. In this chapter, some of these dynamic aspects of plastid biology are described. In addition, we consider how different types of plastids differentiate in different types of cells and how they might influence plant development itself.
All plastids in a plant originate from proplastids in the cells of the meristem, which in turn are generally derived from the few proplastids in the egg cell in the flower. The fact that plastids need to divide to establish large populations of plastids in the large number of cells that make up a plant is obvious.