Molecular Control of Division Site Maintenance
Plant cells are surrounded by rigid cell walls which confine their shape and determine their location within tissues. Thus the position of a new cell wall formed during cytokinesis contributes to the shape of the cell and consequently to the overall morphology of the plant.
In plants the positional information for the future division site is specified early, at G2/M transition by the formation of the preprophase band. This transient cytoskeletal array forms at the cell cortex and demarcates the future division plane, but disassembles upon entry into mitosis. Subsequently, the division plane is identified by kinesin-12 class proteins PHRAGMOPLAST ORIENTING KINESIN (POK)1 and POK2, which are required for the maintenance of the specific cortical division site marker TANGLED (Fig. 1, Müller et al., 2009). Loss of POK function results in phragmoplast mis-guidance and oblique cell plate insertion.
Figure 1: Establishment, maintenance and recognition of the division plane requires kinesin-12 proteins.
The preprophase band aids in the recruitment of markers which identify the plane of cell division throughout mitosis. Maintenance of the identity factor TANGLED (TAN) requires the function of kinesin-12 class motor proteins PHRAGMOPLAST ORIENTING KINESIN (POK)1 and POK2. Together, POKs and TAN are necessary for phragmoplast guidance and accurate cell plate positioning.
pok1pok2 double mutants exhibit pronounced cell wall positioning defects as a consequence of misguided phragmoplasts during cytokinesis (Movie). Genetic and in vitro evidence suggest that POK kinesins are required for the maintenance of TANGLED and RanGAP1 at the cortical division site from metaphase onward (Walker et al., 2007, Xu et al., 2008).
Movies: Microtubule arrays during mitosis in the Arabidopsis root meristem of wild type (to the left) and pok1pok2 double mutant (to the right). The cytoskeleton is a major player in the execution of cell division. Even before a cell begins to divide, the plane of division is highlighted by a plant specific cytoskeletal array, the preprophase band (yellow arrow heads) . The predictive band disassembles upon entry into mitosis. Nevertheless, later in mitosis the cytokinetic apparatus or phragmopast directs the formation of the separating cell plate in the plane of cell division as forcast by the preprophase band. In pok1pok2 double mutants the position of the preprophase band (yellow arrow heads) and the position of the division plane (white arrows) do not match, indicative of phragmoplast guidance defects.Microtubules are visualized with the GFP-MAP4 reporter gene (green, Marc et al., 1998, Plant Cell 10, 1927-1940) and cell walls are stained with propidium iodide (magenta). Time intervals are 2.5 min at 1.5 fps.
In our current work we focus on the functional characterization of POK kinesins and on the identification of novel regulators of division site establishment.