Transition of G1 to early S phase may be required for zinnia mesophyll cells to trans-differentiate to tracheary elements

We have used the Zinnia elegans mesophyll cell system, in which single isolated leaf mesophyll cells can be induced to trans-differentiate into tracheary elements in vitro, to study the relationship between the cell division cycle and cell differentiation. Almost all cells go through several rounds of division before characteristic features of tracheary element formation are observed. The addition of aphidicolin, a DNA synthesis inhibitor, blocks cell division but not cell differentiation in the zinnia system. Low concentrations of aphidicolin, which possibly delay cells in the early S phase, can significantly enhance levels of tracheary element formation. In contrast, roscovitine, an inhibitor of cyclin-dependent kinase activity, decelerates the cell division cycle and inhibits tracheary element formation with similar dose responses. Cells blocked in S phase and then transferred to roscovitine-containing medium can divide once, indicating that roscovitine may target the G1/S transition, but do not differentiate. Cells inhibited in G1/S in roscovitine-containing medium that are subsequently blocked in S phase by transfer to aphidicolin-containing medium, do not divide but do differentiate. Taken together, our results indicate that cells may be required to transit the G1/S checkpoint and enter early S phase to acquire competence to trans-differentiate to tracheary elements.