NUCLEAR REPLICATION ACTIVITIES DURING IMBIBITION OF ABSCISIC ACID-DEFICIENT AND GIBBERELLIN-DEFICIENT TOMATO (LYCOPERSICON-ESCULENTUM MILL)SEEDS

The role of cis-abscisic acid (ABA) and gibberellins (GAs) in the indu ction of cell-cycle activities has been studied during imbibition and subsequent germination of tomato seeds. Using flow cytometry, nuclear replication activity was investigated in embryo root tips isolated fro m seeds of the ABA-deficient mutant sit(w), the GA-deficient mutant gi b-1, and the wild-type (MM) tomato (Lycopersicon esculentum Mill, cv. Moneymaker) upon imbibition in water, 10 mu M GA(4+7), 5 mu M ABA or 5 mu M ABA + 10 mu M GA(4+7). The nuclei of fully matured dry MM, sit(w ) and gib-1 seeds predominantly showed 2C DNA signals, indicating that the cell-cycle activity of most root-tip cells had been arrested at t he G(1) phase of nuclear division. However, ABA-deficient sit(w) seeds contained a significantly higher amount of G(2) cells (4C DNA) compar ed with the other genotypes, suggesting that, during maturation, cell- cycle activity in sit(w) seeds is less efficiently arrested in G(1). U pon imbibition in water, an induction of the 4C signal, indicating nuc lear replication, was observed in the root tip cells of both MM and si t(w) embroys. The augmentation in the 4C signal occurred before visibl e germination. Gib-1 seeds did not show cell-cycle activity and did no t germinate in water. Upon imbibition in GA(4+7), both cell-cycle acti vity and subsequent germination were enhanced in MM and sit(w) seeds, and were induced in gib-1 In ABA, the germination of MM and sit(w) see ds was inhibited while nuclear replication of these seeds was not affe cted. It is concluded that GA influences germination by acting upon pr ocesses that precede cell-cycle activation, while ABA affects growth b y acting upon processes that follow cell-cycle activation.