EFFECT OF ABSCISIC-ACID ON THE CELL-CYCLE IN THE GROWING MAIZE ROOT

The mechanism by which the rate of cell proliferation is regulated in different regions of the root apical meristem is unknown. The cell pop ulations comprising the root cap and meristem cycle at different rates , proliferation being particularly slow in the quiescent centre. In an attempt to detect the control points in the cell cycle of the root ap ical meristem of Zea mays L. (cv. LG 11), quiescent-centre cells were stimulated to synthesise DNA and to enter mitosis either by decapping or by immersing intact roots in an aqueous 3,3-dimethyl-glutaric acid buffer solution. From microdensitometric and flow-cytometric data, we conclude that, upon immersion, the G(2) phase of the cell cycle of int act roots was shortened. However, when 50 mu M abscisic acid (ABA) was added to the immersion buffer, parameters of the cell cycle were rest ored to those characteristic of intact roots held in a moist atmospher e. On the other hand, decapping of primary roots preferentially shorte ned the G(1) phase of the cell cycle in the quiescent centre. When sup plied to decapped roots, ABA reversed this effect. Therefore, in our m odel, applied ABA retarded the completion of the cell cycle and acted upon the exit from either the G(1) or the G(2) phase. Immersion of roo ts in buffer alone seems to trigger cells to more rapid cycling and ma y do so by depleting the root of some ABA-like factor.