OsMTOPVIB is required for meiotic bipolar spindle assembly.

Abstract

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, <i>Drosophila melanogaster</i>, and <i>Xenopus</i> oocytes; however, the mechanism of bipolar spindle assembly in plant meiosis remains elusive. According to our observations of microtubule assembly in <i>Oryza sativa</i>, <i>Zea mays</i>, <i>Arabidopsis thaliana</i>, and <i>Solanum lycopersicum</i>, 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 <i>OsmtopVIB</i> with the defect in double-strand break (DSB) formation. However, bipolar spindles were normally assembled in several other mutants lacking DSB formation, such as <i>Osspo11-1</i>, <i>pair2</i>, and <i>crc1</i>, indicating that bipolar spindle assembly is independent of DSB formation. We further revealed that the mono-orientation of sister kinetochores was prevalent in <i>OsmtopVIB</i>, whereas biorientation of sister kinetochores was frequently observed in <i>Osspo11-1</i>, <i>pair2</i>, and <i>crc1</i> In addition, mutations of the cohesion subunit OsREC8 resulted in biorientation of sister kinetochores as well as bipolar spindles even in the background of <i>OsmtopVIB</i> Therefore, we propose that biorientation of the kinetochore is required for bipolar spindle assembly in the absence of homologous recombination.