The Relationship Between Programmed Cell Death and the Cell Cycle in the Tobacco BY-2 Cell Line.

Ethylene is an established plant growth regulator linked with programmed cell death (PCD). To investigate the relationship between the cell cycle and PCD, ethylene was used to see if it induced mortality in a cell cycle specific manner. Tobacco BY-2 cultures synchronized with aphidicolin were treated with ethylene. Cell cycle progression and mortality, measured at hourly intervals, showed distinct peaks of mortality at the G2/M boundary and S-phase. In conjunction with this, DNA fragmentation increased at G2/M. Furthermore, ethylene caused a significant reduction in cell size of the cycling population. Simultaneous addition of silver nitrate with ethylene ameliorated ethylene-induced G2/M mortality, although a toxic effect of silver alone was evident. Due to the toxicity of silver, 1-MCP, an alternative chemical for blocking ethylene receptors was used. 1-MCP neither affected the BY-2 cell cycle nor mortality levels. In addition, 1-MCP ameliorated ethylene-induced G2/M mortality. To balance the chemical approaches to blocking ethylene receptors, tobacco BY-2 cells were transformed with Atetr1 that encodes a dominant insensitive form of the Arabidopsis ETR1 ethylene receptor. Atetr1 expression caused a massive perturbation to the tobacco BY-2 cell cycle, especially in S-phase, and resulted in high levels of mortality throughout the cell cycle. Ethylene treatment caused a doubling of G2 duration but did not affect temporal distribution of mortality. However, ethylene treatment generated a peak of mortality in S-phase. These results suggest that ethylene induces PCD at G2/M through the known ethylene signaling pathway. Furthermore, it confirms that 1-MCP and Atetr1 result in ethylene insensitivity.

To examine the G2/M transition, Spcdc25, a positive regulator of G2/M in fission yeast was transformed into the tobacco BY-2 cell line. This resulted in premature entry into mitosis, a shortened cell cycle, and reduced cell size. This was similar to Spcdc25 over-expression in fission yeast and suggests the presence of a CDC25-like phosphatase in plants.