The cytoskeleton and spatial control of cytokinesis in the plant life cycle

One of the intriguing aspects of development in plants is the precise contr ol of division plane and subsequent placement of walls resulting in the spe cific architecture of tissues and organs. The placement of walls can be dir ected by either of two microtubule cycles, The better known microtubule cyc le is associated with control of the future division plane in meristematic growth where new cells become part of tissues. The future daughter domains are determined before the nucleus enters prophase and the future site of cy tokinesis is marked by a preprophase band (PPB) of cortical microtubules. T he spindle axis is then organized in accordance with the PPB and, following chromosome movement, a phragmoplast is initiated in the interzone and expa nds to join with parental walls at the site previously occupied by the PPB. The alternative microtubule cycle lacks both the hooplike cortical microtu bules of interphase and the PPB. Wall placement is determined by a radial m icrotubule system that defines a domain of cytoplasm either containing a nu cleus or destined to contain a nucleus (the nuclear cytoplasmic domain) and controls wall placement at its perimeter. This more flexible system allows for cytoplasmic polarization and migration of nuclei in coenocytes prior t o cellularization. The uncoupling of cytokinesis from karyokinesis is a reg ular feature of the reproductive phase in plants and results in specific, o ften unusual, patterns of cells which reflect the position of nuclei at the time of cellularization (e.g., the arrangement of spores in a tetrad, cell s of the male and female gametophytes of angiosperms, and the distinctive c ellularization of endosperm). Thus, both microtubule cycles are required fo r completion of plant life cycles from bryophytes to angiosperms. In angios perm seed development, the two methods of determining the boundaries of dom ains where walls will be deposited are operative side by side. Whereas the PPB cycle drives embryo development, the radial-microtubule-system cycle dr ives the common nuclear type of endosperm development from the syncytial st age through cellularization. However, a switch to the PPB cycle can occur i n endosperm, as it does in barley, when peripheral cells divide to produce a multilayered aleurone. The triggers for the switch between microtubule cy cles, which are currently unknown, are key to understanding plant developme nt.