ROLE OF AUXIN AND ABSCISIC-ACID IN THE INDUCTION OF CELL-DIVISION IN JERUSALEM ARTICHOKE TUBER TISSUE CULTURED INVITRO

The roles of auxin and abscisic acid in the induction and promotion, respectively, of DNA synthesis in tuber tissue of Jerusalem artichoke (H. tuberosus) was studied. The following was investigated: the need of an auxin during the first few h of the wound response in the tissue, for induction of DNA synthesis and cell division; and the need for the simultaneous presence of NAA [naphthaleneacetic acid] and ABA [abscisic acid] for the promotion of auxin-induced DNA replication. A pre-incubation of the explants in the absence of auxin neither affects the duration of the lag period observed when auxin is provided, nor does it affect the number of cells per explant that respond to auxin at the later stage. If the tuber explants are pre-incubated in the absence of a hormone for 12 h before transfer to medium containing NAA, the synthesis of DNA does not commence until after the tissue has been in contact with the auxin for a time equal to the total duration of the lag period. The peak of DNA synthesis is also delayed by about 12 h in these explants as compared to the controls. While an auxin is required during the whole length of the lag period for the induction of DNA replication, it does not seem to be true for ABA for the promotion of auxin-induced DNA replication. When the explants are pre-incubated in NAA (1 mg/l) for 12 h and then transferred to NAA + ABA, the rates of 3H-thymidine incorporation at different times during the S period are comparable with those grown in a mixture of NAA + ABA from the beginning. Apparently auxin acts on about 60-70% of the cells in the explant to inititdate a sequence of events that can eventually lead to DNA synthesis in these cells. But only about half of these cells (about 30-35% of the total cell population) actually undergo DNA synthesis, while the other half are somewhat inhibited from doing so. ABA, if given from the beginning or just before the initiation of DNA replication, somehow overcomes this inhibition and as a result all of the auxin-activated cells enter the S phase.