MICROTUBULE-ASSOCIATED PROTEIN65 is essential for maintenance of phragmoplast bipolarity and formation of the cell plate in Physcomitrella patens.

The phragmoplast, a plant-specific apparatus that mediates cytokinesis, mainly consists of microtubules (MTs) arranged in a bipolar fashion, such that their plus ends interdigitate at the equator. Membrane vesicles are thought to move along the MTs toward the equator and fuse to form the cell plate. Although several genes required for phragmoplast MT organization have been identified, the mechanisms that maintain the bipolarity of phragmoplasts remain poorly understood. Here, we show that engaging phragmoplast MTs in a bipolar fashion in protonemal cells of the moss Physcomitrella patens requires the conserved MT cross-linking protein MICROTUBULE-ASSOCIATED PROTEIN65 (MAP65). Simultaneous knockdown of the three MAP65s expressed in those cells severely compromised MT interdigitation at the phragmoplast equator after anaphase onset, resulting in the collapse of the phragmoplast in telophase. Cytokinetic vesicles initially localized to the anaphase midzone as normal but failed to further accumulate in the next several minutes, although the bipolarity of the MT array was preserved. Our data indicate that the presence of bipolar MT arrays is insufficient for vesicle accumulation at the equator and further suggest that MAP65-mediated MT interdigitation is a prerequisite for maintenance of bipolarity of the phragmoplast and accumulation and/or fusion of cell plate-destined vesicles at the equatorial plane.