Laser microsurgery demonstrates that cytoplasmic strands anchoring the nucleus across the vacuole of premitotic plant cells are under tension. Implications for division plane alignment

In epidermal cells of the plant Nautilocalyx lynchii, induced to divide by explantation, the nucleus undergoes a series of movements, on cytoplasmic leading to construction of a division plane across the vacuole. In the stage, the nucleus separates from the cortex, occupying an eccentric the cell, suspended across the vacuole by few thin strands. In the central the nucleus occupies a central position anchored to the cortex by more thicker strands. Finally, the phragmosome forms as a coalescence of strands across the cell, constituting the division plane within which cytokinesis take place. The behaviour and alignment of these strands is important since some are precursors of the division plane. In a previous (Flanders et al (1990) J. Cell Biol. 110, 1111-1122), it was pointed out alignment of cytoplasmic strands showed features common to a variety of under tension. That is, provided they are free to move relative to the strands radiating from the nucleus should tend to seek short rather than routes to the cortex. In this way, strands under tension would move away distant cell corners where two of the cell's edges make a three-way a neighbouring wall. This provides a basis for the avoidance of four-way and the maintenance of three-way junctions which are a characteristic most plant tissues. In addition, tensile elements such as soap bubble contact rigid surfaces perpendicularly. Perpendicular attachment of the to the side wall is embodied in Sachs' rule of cell division and the the premitotic strands in a state of tension would provide a basis for In this study, laser microsurgery has been used to confirm that strands the premitotic nucleus to the cortex are under tension since the severed retract immediately upon severance. However, the response of the nucleus breaking of a cytoplasmic strand by laser depends upon the particular nucleus is most likely to move during the early stage when it is placed and has few cytoplasmic strands. In the middle and phragmosomal nucleus is more resistant. Computer-aided image reconstruction of anti-
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