Plant growth is dependent on oriented cell divisions that employ the microtubular preprophase band (PPB) to position the cell plate. It has been intriguing how this transient cytoskeletal array imprints the spatial information to be read by the cytokinetic phragmoplast at later stages of mitotic cell division. InArabidopsis thaliana, we discovered that the PPB recruited the Myosin XI motor MYA1 to the cortical division site where it joined microtubule-associated proteins and motors to form a ring of prominent cytoskeletal assemblies which received the expanding phragmoplast. This regulatory function of MYA1 in phragmoplast guidance is dependent on intact actin filaments. The discovery of these assemblies revealed the mechanism underlying how two dynamic cytoskeletal networks govern PPB-dependent division plane orientation during vegetative growth in flowering plants.

Microtubule (MT) motors in the Kinesin-14 subfamily proliferated in photosynthetic organisms and they often incorporated sequences bearing novel structural features. To gain insights into the functions of diversified Kinesin-14 motors from an evolutionary perspective, we performed phylogenetic analyses across different eukaryotic kingdoms. Compared to fungi that have a single class of Kinesin-14, the early divergent protist Giardia possesses two classes and the motile green alga Chlamydomonas produces four classes (Kinesin-14A to Kinesin-14D). The fifth class Kinesin-14E first appeared among immotile green algae and the sixth Kinesin-14F emerged in mosses, concomitantly with the display of 3D growth. The conservation of Kinesin-14D from green algae prompted us to investigate its function in Arabidopsis in which three such motors functioned in cell cycle-dependent manners. They localized on selective spindle MTs and/or sometimes kinetochore-like structures, and later all became conspicuous on MT bundles in the spindle midzone following sister chromatid segregation. Genetic dissection of Kinesin-14D1 showed that its loss led to hypersensitivity to low doses of the MT-depolymerizing herbicide oryzalin. Kinesin-14D1 association with the midzone MTs in both prophase and mitotic spindles. The oryzalin treatment left behind discrete kinetochore fibers attached to randomly positioned chromosomes in the mitotic kinesin-14d1 cells but prevented the pole convergence of bipolar mitotic spindles. This function of Kinesin-14D1 in the spindle midzone is likely dependent on an MT-binding domain at the C-terminus to the catalytic motor domain. Therefore, our results revealed a novel Kinesin-14D-dependent mechanism that regulates the formation of bipolar spindle apparatus with converged acentrosomal poles.

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