Plastid division. The origin of replication

The fact that chlorophyll-containing chloroplasts are remarkably abundant organelles in plants is obvious to anyone contemplating the verdant green expanses of most global ecosystems. For those who have viewed leaf mesophyll cells by peering down a microscope, it is also apparent that these specialized cells can contain large numbers of chloroplasts—up to 200 in some cells—and that chloroplasts can cover up to 70% of the mesophyll cell surface. Presumably, as leaves developed to become the major photosynthetic structure in plants, the accumulation of chloroplasts in mesophyll cells was evolutionarily favored because increases in chloroplast number would correlate with increases in photosynthetic capacity. Just as all the cells in a plant are derived from meristematic cells, all the plastids within the various cells of that plant are derived from proplastids that reside in the meristem cells. These proplastids must divide to ensure that, following cell division, both daughter cells contain proplastids. Similarly, post-meristematic cellular differentiation and maturation events are paralleled by events that define the development and differentiation of proplastids. For example, during the differentiation and development of mesophyll cells, chloroplasts differentiate from proplastids and they divide several times to give rise to the large populations of chloroplasts that are present within individual mesophyll cells. Both proplastid and chloroplast division seems to take place primarily by a process of binary fission, and the morphological changes that occur during this process have been well characterized (Pyke, 1997). Nevertheless, the only plastid division–specific structure that has been observed in several plant species by electron microscopy is an electron-dense torus around the conPlastid Division: The Origin of Replication