Phragmoplasts in the absence of nuclear division

All land plants (embryophytes) use a phragmoplast for cytokinesis. Phragmop lasts are distinctive cytoskeletal structures that are instrumental in the deposition of new walls in both vegetative and reproductive phases of the l ife cycle. In meristems, the phragmoplast is initiated among remaining nonk inetochore spindle fibers between sister nuclei and expands to join parenta l walls at the site previously marked by the preprophase band of microtubul es (PPB). The microtubule cycle and cell cycle are closely coordinated: the hoop-like cortical microtubules of interphase are replaced by the PPB just prior to prophase, the PPB disappears as the spindle forms, and the phragm oplast mediates cell plate deposition after nuclear division. In the reprod uctive phase, however, cortical microtubules and PPBs are absent and cytoki nesis may be uncoupled from the cell cycle resulting in multinucleate cells (syncytia). Minisyncytia of 4 nuclei occur in microsporocytes and several (typically 8) nuclei occur in the developing megagametophyte. Macrosyncytia with thousands of nuclei may occur in the nuclear type endosperm. Cellular ization of syncytia involves formation of adventitious phragmoplasts at bou ndaries of nuclear-cytoplasmic domains (NCDs) defined by radial microtubule systems (RMSs) emanating from non-sister nuclei. Once initiated in the reg ion of microtubule overlap at interfaces of opposing RMSs, the adventitious phragmoplasts appear structurally identical to interzonal phragmoplasts. P hragmoplasts are constructed of multiple opposing arrays similar to what ha ve been termed microtubule converging centers. The individual phragmoplast units are distinctive fusiform bundles of anti-parallel microtubules bisect ed by a dark mid-zone where vesicles accumulate and fuse into a cell plate.