Microtubule arrays during mitosis in monoplastidic root tip cells of Isoetes

Mitosis in monoplastidic root tip cells of Isoetes was studied by fluorescence (confocal laser scanning microscopy) and transmission electron microscopy. The two major components of division polarity, spindle axis and division site, are established more or less simultaneously in preprophase. Morphogenetic plastid migration results in positioning of a daughter plastid at each pole of the future spindle. Concomitant with establishment of the spindle axis, the division site is marked by a girdling band of mirotubules (preprophase band). Endoplasmic microtubules interconnect the polar plastids and preprophase band suggesting a mechanism for communication and final alignment of spindle axis and division site. As in other monoplastidic cells, the spindle appears to emanate from the plastids. In telophase, the focus of microtubules shifts to the reforming nuclei as the phragmoplast is initiated. Microtubules which continue to emanate from plastids are incorporated into the phragmoplast as it expands beyond the interzonal region. Cortical microtubules are restored in a random fashion before assuming a transverse arrangement in interphase. © 1992 Springer-Verlag.