Relationship between cell proliferation and transition to elongation in plant roots

Relationship between two main growth processes, cell proliferation and elongation, is reviewed. In literature, meristem and elongation zones are discriminated according to: increase in relative growth rate, change in cell shape, cessation of mitoses, and change in cell structure, vacuolation in particular. Relative growth rate is almost constant along the meristem and increases sharply in the course of cell transition to elongation. The transition of cells to elongation cannot be considered as a continuation of meristematic growth after the cessation of divisions. The most valid criterion of the cell transition to elongation is a sharp rise in relative growth rate, ratherthan change in cell shape (form factor). In the growing roottip, there aretwo regions of more active accumulation of proteins. The first is associated with the fastest cell proliferation while the second corresponds to enhanced cell elongation. The results of experiments with X-irradiation and cytostatic drugs suggest that cell transition to elongation is independent of cell proliferation and is regulated by the processes determining the life-span of cells in the meristem. The rate of cell transition to elongation is controlled by the processes determining both the life-span of cells in the meristem and the rate of cell proliferation. For most meristematic cells, the life-span of most cells in the meristem remains unchanged in treated roots. Thus, if cell proliferation and transition to elongation are regulated independently, any retardation of cell proliferation will automatically result in deceleration of the cell transition to elongation. Cell kinetics in roots is similar to that in some mammalian tissues capable of long-term proliferation.