CELL-SHAPE, CHROMOSOME ORIENTATION AND THE POSITION OF THE PLANE OF DIVISION IN VICIA-FABA ROOT CORTEX CELLS

Three-dimensional chromosome orientation was studied in thick sections of Vicia faba root meristem, using confocal microscopy and digital image analysis techniques. In the proliferative part of the root meristem, where the cells are organized in longitudinal files, it was expected to find dividing cells with a spindle axis parallel to the file axis and, occasionally, perpendicular to the file axis (resulting in a local file bifurcation). However, we observed a large number of oblique spindle axes. From metaphase to telophase there was a progressive increase in the rotation of the spindle axis. A 90-degrees turn of the metaphase equator plane was never observed. Three-dimensional measurements of both the space occupied by the ana- and telophase chromosome configurations, and the size of the corresponding cortex cells, showed that most cells were too flat for an orientation of the spindle parallel to the file axis. Apparently, cell size limitations forced the spindle to rotate during mitosis. Consequently, the nuclei in the daughter cells were positioned diagonally in opposite directions, instead of on top of each other. In the majority of these cells, a transverse plane of division would intersect the nuclei. Therefore, the new cell wall was sigmoid shaped or oblique. Most daughter cells remained within the original cell file but, occasionally, in extremely flat cells the position of the daughter nuclei forced the cell to set a plane of division parallel to the file axis. This resulted in file bifurcation. It has been concluded that cell shape, the extent of spindle rotation and the position of the division plane are related.