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OsMTOPVIB is required for meiotic bipolar spindle assembly

Significance The organization of microtubules into a bipolar spindle is essential for chromosome segregation. Both centrosome and chromatin-dependent spindle assembly mechanisms are well studied in animals; however, the mechanism of bipolar spindle assembly in plant meiosis remains unclear. We found the key step of plant bipolar spindle assembly is the correction of multipolar spindles into bipolar spindles at metaphase I. The multipolar spindles failed to transition into bipolar spindles in OsmtopVIB. However, the absence of cohesion subunit OsREC8 resulted in bipolar spindles in the background of OsmtopVIB. Thus, OsMTOPVIB plays a crucial role in meiotic spindle assembly. Biorientation of sister kinetochores in a univalent is essential for bipolar spindle formation when homologous recombination is absent. The organization of microtubules into a bipolar spindle is essential for chromosome segregation. Both centrosome and chromatin-dependent spindle assembly mechanisms are well studied in mouse, Drosophila melanogaster, and Xenopus oocytes; however, the mechanism of bipolar spindle assembly in plant meiosis remains elusive. According to our observations of microtubule assembly in Oryza sativa, Zea mays, Arabidopsis thaliana, and Solanum lycopersicum, we propose that a key step of plant bipolar spindle assembly is the correction of the multipolar spindle into a bipolar spindle at metaphase I. The multipolar spindles failed to transition into bipolar ones in OsmtopVIB with the defect in double-strand break (DSB) formation. However, bipolar spindles were normally assembled in several other mutants lacking DSB formation, such as Osspo11-1, pair2, and crc1, indicating that bipolar spindle assembly is independent of DSB formation. We further revealed that the mono-orientation of sister kinetochores was prevalent in OsmtopVIB, whereas biorientation of sister kinetochores was frequently observed in Osspo11-1, pair2, and crc1. In addition, mutations of the cohesion subunit OsREC8 resulted in biorientation of sister kinetochores as well as bipolar spindles even in the background of OsmtopVIB. Therefore, we propose that biorientation of the kinetochore is required for bipolar spindle assembly in the absence of homologous recombination.