| Abstract: | An analysis was undertaken of the effects of a toxic metal,zinc, on plant cell suspension cultures of the TBY–2 cellline of tobacco (Nicotiana tabacum cv. Bright Yellow 2) in orderto determine whether Zn acts in a cell cycle-specific manner.In the control treatment (0 Zn), following a 24 h synchronizationwith aphidicolin and 7 h after the release from the inhibitor,the mitotic index peaked at 45%. The inclusion of Zn in the24 h aphidicolin treatment (100, 200 or 300 µM Zn) resultedin a concentration–dependent decrease in the mitotic peakto 30%, 22% and 10%, respectively, but did not affect the timingof the peak. Hence, despite high concentrations of Zn, cellstraversed from S–phase to mitosis, albeit in smaller proportions,at the same rate as the controls. Cells treated with 0, 100or 200 µM Zn during synchronization and then releasedinto Zn–free media showed successive peaks in mitoticindex at 7 h and 21 h following release, i.e. Zn-treated cellsprogressed through a complete cell cycle at the same rate asthe controls. Synchronization and subsequent release into Zn–containingmedium (100 µM) examined the effect of the metal on predominantlylate G1 cells. In this treatment, the mitotic index peaked at7 h and 19 h, indicating a slightly faster cell cycle (12 h)compared with the control (14 h). Continuous exposure to 100µM Zn through both synchronization and release resultedin a cell cycle of 11 h and a differential effect on the componentphases: M–phase lengthened (1.5 h to 3.5 h) and G1 shortened(6 h to 1 h) compared with the control treatment. Vital staining (Evans Blue) revealed that cell mortality increasedfrom 2.7% (0 Zn) to 6.1% and 6.5% at 100 and 200 µM Zn,respectively. The Zn content of cells increased 40–lfoldfrom 0 to 100 µM Zn. The data are consistent with theeffects of Zn reducing the cycling cell population primarilythrough cell arrest rather than cell death, but also revealthat a substantial population of TBY–2 cells progressedthrough the cell cycle despite accumulating Zn. In particular,the duration of G2 and S-phase was remarkably invariant, clearlyindicating that once plant cells meet the requirements of lateG1 check-points, they are committed to divide, even in the presenceof toxic concentrations of Zn. The synchronous TBY–2 cell suspension, which lacks theheterogeneity and developmental constraints of plant meristems,is an excellent system to study the effects of known toxic metals,and indeed other environmental factors, on the plant cell cycle. Key words: Cell cycle, plant cell suspensions, Nicotiana tabacum, zinc, toxicity |
