Effects of centrifugation on preprophase-band formation in Adiantum protonemata.

Protonemata of Adiantum capillus-veneris L., grown in red light and consisting of a single elongate cell, were centrifuged at various times after or just before the induction of synchronous cell division by irradiation with blue light (BL), and formation and location of preprophase bands (PPBs) of microtubules (MTs) were observed by immunofluorescence microscopy. The cells divided approx. 14-20 h after the onset of BL irradiation, irrespective of the timing of centrifugation. Basipetal centrifugation caused basipetal displacement (about 100 μm) of the nucleus. The nucleus did not return to its original position and cell division occurred at the new nuclear position. A PPB was formed in the region of the displaced nucleus except in some cells which presumably had been centrifuged just before cell division. Microtubule behavior in the apical region, where cell division occurred in non-centrifuged cells, depended on the time of centrifugation relative to the cell cycle. When protonemata were centrifuged before PPB formation, no PPB was formed in the apical region of most cells but, in some cells, another MT band indistinguishable from a PPB (we consider it PPB) was formed at this site after the displacement of the nucleus. When protonemata were centrifuged during or after PPB formation, the PPB, which had already formed in the apical region, did not disappear until the sedimented nucleus entered telophase or early interphase, while a new PPB was formed in most cells in the region of the displaced nucleus. These cells had thus two PPBs: one in the apical region, and another in the region where the displaced nucleus was located. Double centrifugation to displace the nucleus twice showed that once a PPB formed, irrespective of its intracellular position, it remained after the displacement of the nucleus. However, no PPBs were observed in the region where the nucleus was located after the first centrifugation if the nucleus was again displaced before a PPB had formed in this region. The positioning of the cell plate was also examined. When centrifugation was performed just before cell division, PPB formation around the displaced nucleus decreased and oblique cell plates were observed at an increased frequency. In conclusion, preprophase nuclei may have the ability to induce PPB formation in the neighboring cell cortex, and PPBs, in turn, may have a role in governing the orientation of cell plates.