Figure 1: (A) Overview of division plane selection, maintenance and phragmoplast guidance.(B) Comparison of cellular organization in wild type and mutant root meristems.
A robust early marker of the division plane in most land plants is the cytoskeletal preprophase band, which delineates its periphery at the cell cortex, but disassembles upon metaphase. Work in our laboratory revealed, that this region of the plasma membrane, termed the cortical division zone is continuously occupied by kinesin-12 class proteins PHRAGMOPLAST ORIENTING KINESIN (POK)1 and POK2. These are required for the maintenance of the specific cortical division zone identity markers, TANGLED, RanGAP1 and PHGAPs (Lipka et al., 2014, Stöckle et al., 2016, Herrmann et al., 2018). Loss of POK and TANGLED function results in phragmoplast mis-guidance and oblique cell plate insertion, whereas loss of PHGAPs, regulators of Rho of Plants, affects division plane selection (Figure 1B). POK2 serves additional functions in phragmoplast expansion and stabilization of the phragmoplast midzone via interaction with the microtubule cross-linker MAP65-3/PLEIADE (Herrmann et al., 2018).
In our current work we focus on the functional characterization of POK kinesins, PHGAP involved in cell polarity establishment and on the identification of novel regulators of cortical division zone maintenance.
Movies: Microtubule arrays during mitosis in the Arabidopsis root meristem of wild type (to the left) and pok1 pok2 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 pok1 pok2 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.