The genetic control of plastid division in higher plants.

The division of plastids is an important part of plastid differentiation and development and in distinct cell types, such as leaf mesophyll cells, results in large populations of chloroplasts. The morphology and population dynamics of plastid division have been well documented, but the molecular controls underlying plastid division are largely unknown. With the isolation of Arabidopsis mutants in which specific aspects of plastid and proplastid division have been disrupted, the potential exists for a detailed knowledge of how plastids divide and what factors control the rate of division in different cell types. It is likely that knowledge of plant homologues of bacterial cell division genes will be essential for understanding this process in full. The processes of plastid division and expansion appear to be mutually independent processes, which are compensatory when either division or expansion are disrupted genetically. The rate of cell expansion appears to be an important factor in initiating plastid division and several systems involving rapid cell expansion show high levels of plastid division activity. In addition, observation of plastids in different cell types in higher plants shows that cell-specific signals are also important in the overall process in determining not only the differentiation pathway of plastids but also the extent of plastid division. It appears likely that with the exploitation of molecular techniques and mutants, a detailed understanding of the molecular basis of plastid division may soon be a reality.