ANALYSIS OF CELL-DIVISION AND ELONGATION UNDERLYING THE DEVELOPMENTALACCELERATION OF ROOT-GROWTH IN ARABIDOPSIS-THALIANA

To investigate the relation between cell division and expansion in the regulation of organ growth rate, we used Arabidopsis thaliana primary roots grown vertically at 20 degrees C with an elongation rate that i ncreased steadily during the first 14 d after germination. We measured spatial profiles of longitudinal velocity and cell length and calcula ted parameters of cell expansion and division, including rates of loca l cell production (cells mm(-1) h(-1)) and cell division (cells cell(- 1) h(-1)). Data were obtained for the root cortex and also for the two types of epidermal cell, trichoblasts and atrichoblasts. Accelerating root elongation was caused by an increasingly longer growth zone, whi le maximal strain rates remained unchanged. The enlargement of the gro wth zone and, hence, the accelerating root elongation rate, were accom panied by a nearly proportionally increased cell production. This incr eased production was caused by increasingly numerous dividing cells, w hereas their rates of division remained approximately constant. Additi onally, the spatial profile of cell division rate was essentially cons tant. The meristem was longer than generally assumed, extending well i nto the region where cells elongated rapidly. In the two epidermal cel l types, meristem length and cell division rate were both very similar to that of cortical cells, and differences in cell length between the two epidermal cell types originated at the apex of the meristem. Thes e results highlight the importance of controlling the number of dividi ng cells, both to generate tissues with different cell lengths and to regulate the rate of organ enlargement.