ABSCISIC-ACID ANTAGONIZES THE EFFECT OF CYTOKININ ON DNA-REPLICATION ORIGINS

In previous studies (Houssa et al., 1990, 1994) we observed that cytokinins stimulate the cell division process in vegetative and reproductive shoot meristems of monocotyledonous and dicotyledonous species by activating latent DNA-replication origins. Here we report that abscisic acid antagonizes this effect in the shoot meristem of Sinapis alba L. Abscisic acid reduces DNA synthesis by inactivating some DNA-replication origins resulting in a lengthening of the replicon size. It is hypothesized that the balance between abscisic acid and cytokinin levels is one of the major factors controlling the rate of DNA replication, and ultimately the rate of cell division, in shoot meristems.