The control of the plane of division during stomatal differentiation in Allium

TLDR: In this article, it was shown that the final plane of division is not determined by the orientation of the spindle at metaphase, but instead is established during late anaphase-telophase as a result of directed reorientation movements of spindle-phragmoplast and associated daughter nuclei.

Abstract: Division of the guard mother cell (GMC) in Allium cotyledons has been examined in epidermal slices viewed with Nomarski optics and electron microscopy Special attention has been directed towards elucidating the process by which the dividing cell determines its plane of division In normal development, the cell plate formed during GMC division ultimately lies along the longitudinal axis of the cotyledon, in contrast to the transverse planes formed in other epidermal divisions Our observations reveal that the final plane of division is not determined by the orientation of the spindle at metaphase but instead is established during late anaphase-telophase as a result of directed reorientation movements of the spindle-phragmoplast and associated daughter nuclei The metaphase plate may lie at an oblique angle, even as great as 90°, from the final plane of the plate Thus, daughter chromosomes separate into opposite corners of the cell During late anaphase-telophase, movement of the spindle is activated; the daughter nuclei move along the sides of the cell while the interzone rotates Movement continues until daughter nuclei reach positions opposite each other along the sides of the cell and the midzone or cell plate is positioned in the longitudinal orientation Movement requires 15–20 minutes for completion, is highly directional, and does not overshoot the correct alignment Following movement cytokinesis proceeds to completion forming two young guard cells Possible mechanisms for reorientation are discussed, including one that suggests that interzone microtubules may interact with a cortical site on the plasmalemma adjacent to the end and paradermal walls Such a site may be related to and governed by the same properties which controlled the prior formation of the preprophase band of microtubules in these cells