Regulation of cycle progression in plant cells.

Induction of polynucleate cells in onion root meristems by inhibition of two sequential cytokineses is used to study controls operating in cell cycle progression. Triggering of both replication and metaphase occurs synchronously in nuclei sharing a common cytoplasm, independent of their ploidy or intracellular position. The replication rate appears to be activated by the simultaneous intracytoplasm presence of other replicating nuclei. On the other hand, central position of a nucleus in the cell as well as increase in ploidy leads to slowing down of replication rate. The relative advance and lag of S period in the different nuclei in a common cytoplasm is partially counter-balanced by differential times of G2. Moreover prophase lengthening in the fast interphase nuclei points ot a negative control exerted by the lagging nuclei mediated by cytoplasm. Finally, it could be worth emphasizing similarities in the control mechanism operating in cycle progression both in animal and plant cells.