Cell Division in Nautilocalyx Explants III. Effects of 2,6-dichlorobenzonitrile on Phragmosome, Band of Microtubules and Cytokinesis.

In the highly vacuolated epidermis cells of leaf explants of Nautilocalyx we investigated whether inhibition of cytokinesis by 2,6-dichlorobenzonitrile (DCB) could be caused by a suppression of the formation of phragmosome and band of microtubules (BMT), two structures which are probably involved in cytokinesis and in the determination of the plane of cell division. Interference contrast microscopy showed that DCB (116 μM) did not interfere with the formation of phragmosome and BMT in the expected plane of cell division. Also the positioning of the nucleus and the nuclear division proceeded normally. The phragmoplast was formed, and cell plate formation started, but in most cases the cell plate was not completed. After some time the part of the cell plate already formed shrank: folds appeared, and sometimes tears. The phragmoplast remained present for a long time after this premature end of cell plate growth. Electron microscopical studies showed a shortage of small Golgi vesicles with electron-dense contents in the plane of cell division where very large vesicles with little electron-dense material were present. Furthermore a dilatation of the endoplasmic reticulum in the microtubule zone of the phragmoplast was observed. These results indicate that the DCB-inhibition of cytokinesis does not result from interference with phragmosome and BMT. It seems likely that cytokinesis stops because a weak cell plate is formed that does not mature to a firm cell wall.