We have examined the cytological effects of microinjecting recombinant birch profilin in dividing and interphase stamen hair cells of Tradescantia virginiana. Microinjection of profilin at anaphase and telophase led to a marked effect on cytokinesis; cell plate formation was often delayed, blocked, or completely inhibited. In addition, the initial appearance of the cell plate was wrinkled, thin, and sometimes fragmented. Injection of profilin at interphase caused a thinning or the collapse of cytoplasmic strands and a retardation or inhibition of cytoplasmic streaming in a dose-dependent manner. Confocal laser scanning microscopy of rhodamine-phalloidin staining in vivo revealed that high levels of microinjected profilin induced a degradation of the actin cytoskeleton in the phragmoplast, the perinuclear zone, and the cytoplasmic strands. However, some cortical actin filaments remained intact. The data demonstrate that profilin has the ability to act as a regulator of actin-dependent events and that centrally located actin filaments are more sensitive to microinjected profilin than are cortical actin filaments. These results add new evidence supporting the hypothesis that actin filaments play a crucial role in the formation of the cell plate and provide mechanical support for the cytoplasmic strands in interphase cells.
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