Nucleus-associated microtubules help determine the division plane of plant epidermal cells: avoidance of four-way junctions and the role of cell geometry.

Abstract

Abstract. To investigate the spatial relationship between the nucleus and the cortical division site, epidermal cells were selected in which the separation between these two areas is large. Avoiding enzyme treatment and air drying, Datura stramonium cells were labeled with antitubulin antibodies and the threedimensional aspect of the cytoskeletons was reconstructed using computer-aided optical sectioning.In vacuolated cells preparing for division, the nucleus migrates into the center of the cell, suspended by transvacuolar strands. These strands are now shown to contain continuous bundles of microtubules which bridge the nucleus to the cortex. These nucleus-radiating microtubules adopt different configurations in cells of different shape. In elongated cells with more or less parallel side walls, oblique strands radiating from the nucleus to the long side walls are presumably unstable, for they are progressively realigned into a transverse disc (the phragmosome) as broad, cortical, preprophase bands (PPBs) become tighter. The phragmosome and the PPB are both known predictors of the division plane and our observations indicate that they align simultaneously in elongated epidermal cells. These observations suggest another hypothesis: that the PPB may contain microtubules polymerized from the nuclear surface. In elongated cells, the majority of the radiating microtubules, therefore, come to anchor the nucleus in the transverse plane, consistent with the observed tendency of such cells to divide perpendicular to the long axis.In nonrectangular isodiametric epidermal cells, which approximate regular hexagons in section, the radial microtubular strands emanating from the nucleus tend to remain associated with the middle of each subtending cell wall. The strands are not reorganized into a single dominant transverse bar, but remain as a starlike array until mitosis. PPBs in these cells are not as tight; they may only be a sparse accumulation of microtubules, even forming along nondiametrical radii. This arrangement is consistent with the irregular division patterns observed in epidermal mosaics of isodiametric D. stramonium cells.The various conformations of the radial strands can be modeled by springs held in two-dimensional hexagonal frames, and by soap bubbles in three-dimensional hexagonal frames, suggesting that the division plane may, by analogy, be selected by minimal path criteria. Such behavior offers a cytoplasmic explanation of long-standing empirically derived "rules" which state that the new cell wall tends to meet the maternal wall at right angles. The radial premitotic strands and their analogues avoid taking the longer path to the vertex of an angle where a cross wall is already present between neighboring cells. The resultant tendency to form three-rayed vertices rather than intercrossing septa helps explain the hexagonal packing of plant cells.

Universal rule for the symmetric division of plant cells

“…The known interactions between cytoplasmic strands, their associated microtubules and actin filaments, and cortical microtubules during PPB positioning can explain these two observations. Lloyd and coworkers (21,24) compared the tense cytoplasmic strands to springs anchoring the nucleus to the cell surface and argued that they must reorganize themselves along a finite number of spatial configurations-each strand connecting the nucleus to an edge by spanning the intervening space with the shortest distance possible (Fig. 5 A and B).…”

“…Given the central role played by the PPB in guiding the cell plate, events leading to its positioning are of particular interest for understanding the selection of the division plane. Observations of premitotic cells have established that cytoplasmic strands populated by microtubules and actin filaments span the space between the nucleus and the cell surface (18)(19)(20)(21). Laser-ablation experiments demonstrate that these strands are under tension (22,23) and are likely responsible for maintaining the nucleus in a central position.…”

“…Laser-ablation experiments demonstrate that these strands are under tension (22,23) and are likely responsible for maintaining the nucleus in a central position. The tensional forces would also explain the tendency of the strands to span the shortest distance between the nucleus and the cell surface (21,24). During preprophase, these strands stabilize cortical microtubules recruited for the PPB and ultimately coalesce into a cytoskeletal structure known as the phragmosome that bridges the nucleus and the cell cortex (18,19).…”

Analysis of cell division patterns in the Arabidopsis shoot apical meristem

“…Division in these cells is anticlinal such that each layer remains one cell thick. The underlying mechanism determining the location of new cell walls is unknown but the qualitative properties of meristematic cell division are well documented (4)(5)(6)(7)(8). Perhaps the best known summary is Errera's rule, derived following observations of soap bubble formation.…”

“…Other observations are that new cell walls form in a plane perpendicular to existing cell walls (6), that cell walls tend to avoid four-way junctions (7), and that cell division planes tend to be staggered, like bricks in a wall (8). Because chemical signals can be induced by physical interactions such as mechanical stress and strain it is conceivable that these geometric indicators are merely emergent properties of the underlying physicochemical interaction processes that drives cell division.…”