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 populations 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 apical 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 G₂ phase of the cell cycle of intact roots was shortened. However, when 50 M abscisic acid (ABA) was added to the immersion buffer, parameters of the cell cycle were restored to those characteristic of intact roots held in a moist atmosphere. On the other hand, decapping of primary roots preferentially shortened the G₁ phase of the cell cycle in the quiescent centre. When supplied to decapped roots, ABA reversed this effect. Therefore, in our model, applied ABA retarded the completion of the cell cycle and acted upon the exit from either the G₁ or the G₂ phase. Immersion of roots in buffer alone seems to trigger cells to more rapid cycling and may do so by depleting the root of some ABA-like factor.Abbreviations ABA cis-abscisic acid - DGA 3,3-dimethyl-glutaric acid - DAPI 4,6-diamidino-2-phenylindole - LI labelling index We thank Pierre Zaech of the Ludwig Institute, Epalinges, Switzerland, for expert assistance in flow cytometry and Dr. Jean-Marcel Ribaut of our Institute for providing data on exodiffusion and metabolism of ABA.