Hormonal control of gene expression during reactivation of the cell cycle in tobacco mesophyll protoplasts

The roles of auxin and cytokinin in cell cycle reactivation were studied during the first 48 h of culture of mesophyll protoplasts of Nicotiana tabacum. Using hormone delay and withdrawal studies we found that auxin was required by 0-4 h of culture, whereas cytokinin was not required until hour 10-12, which is 6-10 h before S phase. Cycloheximide blocks division, indicating that protein synthesis is required. In an effort to detect a molecular response to either hormone, we examined the expression of the cell cycle marker, cdc2. Cdc2 expression was detected by 12 h of culture, coincident with the timing of the cytokinin requirement and well before the entry into S. However, cdc2 was partially induced by either auxin or cytokinin alone, suggesting that cdc2 expression is not the primary target of either hormone. Our hormone delay experiments suggest that there are separate signal transduction pathways leading from auxin and from cytokinin to reactivation of the cell cycle and that these pathways converge before S. The underlying mechanisms for these distinct pathways remain to be elucidated.