Synchronous nuclear-envelope breakdown and anaphase onset in plant multinucleate cells.

Multinucleate plant cells with genetically balanced nuclei can be generated by inhibiting cytokinesis in sequential telophases. These cells can be used to relate the effect of changes in the distribution of nuclei in the cytoplasm to the control of the timing of cell cycle transitions. Which mitotic cell cycle events are sensitive to differences in the amount of cytoplasm surrounding each chromosomal complement has not been determined. To address this, we maximized the cell size by transiently inhibiting replication, while cell growth was not affected. The nuclei of 93% of the elongated cells reached prophase asynchronously compared to 46% of normal-sized multinucleate cells. The asynchronous prophases of normal-sized cells became synchronous at the time of nuclear-envelope breakdown, and the ensuing metaphase plate formation and anaphase onset and progression occurred synchronously. The elongated multinucleate cells were also very efficient in synchronizing the prophases at nuclear-envelope breakdown, in the prophase-to-prometaphase transition. However, 2.4% of these cells broke down the nuclear envelope asynchronously, though they became synchronous at the metaphase-to-anaphase transition. The kinetochore-microtubular cycle, responsible for coordinating the metaphase-to-anaphase transition and for the rate of sister segregation to opposite spindle poles during anaphase, remained strictly controlled and synchronous in the different mitoses of a single cell, independently of differences in the amount of cytoplasm surrounding each mitosis or its ploidy. Moreover, the degree of chromosome condensation varied considerably within the different mitotic spindles, being higher in the mitoses with the largest surrounding cytoplasm.