Ethylene Induces Cell Death at Particular Phases of the Cell Cycle in the Tobacco, TBY-2 Cell Line.

It was examined whether ethylene induces program¬med cell death in a cell cycle-specific manner. Following synchronization of the tobacco TBY -2 cell line with aphidicolin and its subsequent removal, ethylene was injected into the head space of 300 cm3 culture flasks at 0 h or 3.5 h later and cells were sampled for 26 h. There were significant increases in cell mortality at G2/M in both the 0 h and 3.5 h ethylene treatments, and for the latter treatment, another peak in S-phase. The effect at G2/M was greater in the 3.5 h treatment, but was ameliorated by the simultaneous addition of silver nitrate (1.2 uM). In addition, the 3.5 h ethylene treatment resulted in a 1 h delay in the characteristic rise in the mitotic index following aphidicolin-induced synchrony. The addition of silver nitrate alone (1.2 uM), also delayed the entry of cells into mitosis but had no effect on cell cycle length compared with the controls (14 h throughout all treatments) but it induced a peak of mortality 2.5 h after its addition. Nuclear shrinkage was also a characteristic feature of dying cells at G2/M. Using Apoptag®), an in situ apoptosis detection kit, nuclear DNA fragmentation was observed in the TBY-2 cells which were often isolated on the end of a filament of normal cells. In the 3.5 h ethylene treat¬ment, a marked increase was noted in the percentage of such cells at the G2/M transition compared with the controls. Hence, the data show cell death occur¬ring at a major phase transition of the cell cycle and the observations of nuclear shrinkage, isolation of dying cells and nuclear DNA fragmentation suggest a programmed mechanism of cell death exacerbated by ethylene treatment.