THE GENETIC-CONTROL OF PLASTID DIVISION IN HIGHER-PLANTS

The division of plastids is an important part of plastid differentiati on and development and in distinct cell types, such as leaf mesophyll cells, results in large populations of chloroplasts. The morphology an d population dynamics of plastid division have been well documented, b ut the molecular controls underlying plastid division are largely unkn own. With the isolation of Arabidopsis mutants in which specific aspec ts of plastid and proplastid division have been disrupted, the potenti al exists for a detailed knowledge of how plastids divide and what fac tors control the rate of division in different cell types. It is likel y that knowledge of plant homologues of bacterial cell division genes will be essential for understanding this process in full. The processe s of plastid division and expansion appear to be mutually independent processes, which are compensatory when either division or expansion ar e disrupted genetically. The rate of cell expansion appears to be an i mportant factor in initiating plastid division and several systems inv olving rapid cell expansion show high levels of plastid division activ ity. In addition, observation of plastids in different cell types in h igher plants shows that cell-specific signals are also important in th e 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 det ailed understanding of the molecular basis of plastid division may soo n be a reality.