Abstract
In plant cells, cytoplasm is partitioned by a cell plate during cytokinesis. The cell plate expands by centrifugal expansion of the phragmoplast, which transports cell wall materials. Because assembly of microtubules drives phragmoplast expansion, molecular mechanism of the microtubule assembly in the phragmoplast has been analyzed. We have shown that microtubules nucleate on existing microtubules as branches in interphase cortical arrays of microtubules (Murata et al. 2005). In the present study we tested if microtubules nucleate on existing microtubules during phragmoplast development by direct observation of microtubules in living cells. We found that microtubules were extended from periphery of the phragmoplast and then incorporated into the outermost sides of the phragmoplast. These microtubules seemed to be nucleated on phragmoplast microtubules. We propose that the phragmoplast is expanded by addition of newly formed microtubules, formed on existing microtubules as branches, onto the sides of the phragmoplast.
