The NACK-PQR MAP Kinase Cascade Controls Plant Cytokinesis

Abstract

Cell division involves three essential processes for the proper distribution of genetic information to daughter cells: replication of the sets of chromosomes, separation of the chromosomal pairs, and division of the cytoplasm. The final step is called cytokinesis, and it is the critical step for guaranteeing precise cell division and proper cell differentiation. In contrast to the first two steps of cell division, the mode of cytokinesis in plants appears to be different from the modes in yeast and animals. In animal cells, cytokinesis is achieved by constriction of the cell membrane from the outside to the inside (Field et al. 1999; Fig. 1A), whereas cytokinesis in plant cells occurs through the formation of new cell walls. This cell wall formation occurs from the interior to the periphery of the cell and is mediated by a plant-specific apparatus called the phragmoplast (Nishihama and Machida 2001; Fig. 1B). Regardless of the opposite directions of cytokinesis in plant and animal cells, the cytoskeletal structures are largely conserved. During late anaphase and telophase, animal cells develop a well-defined network of microtubules (MTs) between the two daughter nuclei as they migrate to the opposite poles of the cell. This MT-based architecture, called the central spindle or midbody, plays an essential role in cytokinesis. Plant cells construct a phragmoplast, which is also composed of MTs that lie mainly between the two daughter nuclei at late anaphase. The expansion of the phragmoplast is essential for the formation of new cell plates and for complete cytokinesis. The central spindle and the phragmoplast both contain two bundles of antiparallel MTs that are interdigitated at their plus ends. The minus ends of these MTs face the chromosomes separating to the opposite cell poles. The similaritiesof these cytokinetic structures imply that thereareaspectsof the regulation of cytokinesis that are conserved between animals and plants. Recently, XMAP215 and PRC1, which are involved in the regulation of MT structures during cell division in animal cells, were found to be conserved in higher plants andnecessary for organizingMTs and for completing cytokinesis (Whittington et al. 2001; Twell et al. 2002;Muller et al. 2004). This suggests that, although thedirectionof cell divisionhasdiverged, similarmechanismsexist in different species for controllingcytokinesis via regulationofMTdynamics.Our