Relationship between tracheary element differentiation and the cell cycle in single cells isolated from the mesophyll of Zinnia elegans

A relationship between tracheary element differentiation and the cell cycle was studied in single cells isolated from the mesophyll of Zinnia elegans L. cv. Canary bird. Almost all nuclei of isolated mesophyll cells were at the 2 C level of DNA, indicating that almost all cells were initially in the G 1 phase and that somatic polyploidy was absent. Cultured cells underwent partially synchronous DNA replication at 42 h and mitosis at 54 h of culture, and the first cell cycle time was approximately 58 h. The occurrence and timing of DNA replication and mitosis during cytodifferentiation to tracheary elements were investigated using microspectrophotometry, microfluorometry, tritiated thymidine autoradiography, and serial observation. More than 55% of the nuclei of the immature tracheary elements were at the 2 C level of DNA and were not labeled by continuous feeding with tritiated thymidine, providing clear evidence that these cells differentiated without interventing DNA replication. Some tracheary elements (approximately 30%) were formed after one round of the cell cycle, and others (less than 5%) were formed after passing through the S phase, but without intervening mitosis. All types of tracheary elements appeared simultaneously after 58 h of culture, and their patterns of increase in number were similar. From the results, we propose a hypothesis concerning the relationship between cytodifferentiation and the cell cycle.