A Task for Molecular Motors: Maintaining the Cortical Division Zone in Arabidopsis thaliana

Abstract

Plant cells are confined and immobilized by rigid cell walls, which determine their shape and location within tissues. Therefore plant cells demand specific spatial control over cell division and have evolved unique cytoskeletal structures, which aid in coping with these spatial demands. Prior to mitosis a prominent but transient band of microtubules and actin filaments, called the preprophase band (PPB) marks the site where the future cell plate will fuse with the parental plasma membrane. Thus, the PPB is a faithful predictor of the division plane. PPBs spatial information is preserved after its disassembly by the cortical division zone (CDZ), which provides guidance for localized cell plate fusion. Cell plate formation by vesicle fusion initiates in the center of the cell and is aided by the plant specific cytokinetic apparatus, the phragmoplast. Yet only little is known about the molecular identity of the CDZ and how it attracts the phragmoplast and cell plate. A pair of kinesin-12 class motor proteins, PHRAGMOPLAST ORIENTING KINESIN 1 (POK1) and POK2, is essential for the spatial control of cytokinesis. Here we report that a functional full length POK1 fusion protein (YFP-POK1) is dynamically recruited to the PPB and permanently resides at the CDZ until cell plate fusion takes place. In vivo analysis of phragmoplast expansion in pok1 pok2 double mutants revealed a PPB – phragmoplast misalignment caused by phragmoplast tilting, which results in cell wall mis-positioning. Moreover, maintenance of the CDZ identity marker TANGLED is dependent on POK function, suggesting that POK motor proteins might act as a scaffold to retain CDZ identity markers at the plasma membrane, thus maintaining the molecular memory of the division plane. The second project intends to elucidate the effects of a specific posttranslational tubulin modification (PTM) on microtubule organization and CDZ establishment in Arabidopsis thaliana. We demonstrate, that low concentrations of NO2Tyr, which is incorporated into the C-terminus of α-tubulin, are not detrimental for plant health. However the NO2Tyr treatment affects the organization of the cortical microtubule array, resulting in non-polar cell expansion and induces oblique cell wall integration. The results indicate that PTM of α-tubulin might be important for microtubule organization and for division plane establishment during plant development.