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

Abstract

ABSTRACT In epidermal cells of the plant Nautilocalyx lynchii, induced to divide by explantation, the nucleus undergoes a series of movements, on cytoplasmic strands, leading to construction of a division plane across the vacuole. In the early stage, the nucleus separates from the cortex, occupying an eccentric position in the cell, suspended across the vacuole by few thin strands. In the central stage, the nucleus occupies a central position anchored to the cortex by more numerous, thicker strands. Finally, the phragmosome forms as a coalescence of cytoplasmic strands across the cell, constituting the division plane within which mitosis and cytokinesis take place. The behaviour and alignment of these strands is therefore important since some are precursors of the division plane. In a previous paper (Flanders et al (1990)J. Cell Biol. 110,1111 – 1122), it was pointed out that the alignment of cytoplasmic strands showed features common to a variety of elements under tension. That is, provided they are free to move relative to the cortex, strands radiating from the nucleus should tend to seek short rather than long routes to the cortex. In this way, strands under tension would move away from distant cell corners where two of the cell’s edges make a three-way junction with a neighbouring wall. This provides a basis for the avoidance of four-way junctions and the maintenance of three-way junctions which are a characteristic feature of most plant tissues. In addition, tensile elements such as soap bubble walls contact rigid surfaces perpendicularly. Perpendicular attachment of the cell plate to the side wall is embodied in Sachs’ rule of cell division and the existence of the premitotic strands in a state of tension would provide a basis for this rule. In this study, laser microsurgery has been used to confirm that strands connecting the premitotic nucleus to the cortex are under tension since the severed ends retract immediately upon severance. However,. the response of the nucleus to the breaking of a cytoplasmic strand by laser depends upon the particular stage. The nucleus is most likely to move during the early stage when it is eccentrically placed and has few cytoplasmic strands. In the middle and phragmosomal stages the nucleus is more resistant. Computer-aided image reconstruction of anti-tubulin-labelled confocal sections indicates that the middle stage is characterised by thick microtubule (MT) bundles, which radiate from the nucleus and which seem to account for. the increasing stabilization of the nucleus. Although such strands are more difficult to sever with the laser, their ends still display elastic properties when cut. It is proposed that tension is likely to influence the alignment of strands as well as the position of the nucleus during division plane formation in vacuolated cells.