Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan.
Department of Plant Biotechnology and Bioinformatics, Ghent University, 9052 Ghent, Belgium.
Proc Natl Acad Sci U S A. 2017 Oct 17;114(42):E8847-E8854. doi: 10.1073/pnas.1713925114. Epub 2017 Oct 2.
Proper orientation of the cell division axis is critical for asymmetric cell divisions that underpin cell differentiation. In animals, centrosomes are the dominant microtubule organizing centers (MTOC) and play a pivotal role in axis determination by orienting the mitotic spindle. In land plants that lack centrosomes, a critical role of a microtubular ring structure, the preprophase band (PPB), has been observed in this process; the PPB is required for orienting (before prophase) and guiding (in telophase) the mitotic apparatus. However, plants must possess additional mechanisms to control the division axis, as certain cell types or mutants do not form PPBs. Here, using live imaging of the gametophore of the moss , we identified acentrosomal MTOCs, which we termed "gametosomes," appearing de novo and transiently in the prophase cytoplasm independent of PPB formation. We show that gametosomes are dispensable for spindle formation but required for metaphase spindle orientation. In some cells, gametosomes appeared reminiscent of the bipolar MT "polar cap" structure that forms transiently around the prophase nucleus in angiosperms. Specific disruption of the polar caps in tobacco cells misoriented the metaphase spindles and frequently altered the final division plane, indicating that they are functionally analogous to the gametosomes. These results suggest a broad use of transient MTOC structures as the spindle orientation machinery in plants, compensating for the evolutionary loss of centrosomes, to secure the initial orientation of the spindle in a spatial window that allows subsequent fine-tuning of the division plane axis by the guidance machinery.
细胞分裂轴的正确取向对于支持细胞分化的不对称细胞分裂至关重要。在动物中，中心体是主要的微管组织中心（MTOC），通过定向有丝分裂纺锤体在轴确定中起着关键作用。在缺乏中心体的陆地植物中，观察到一个微管环结构，即前期带（PPB），在这个过程中起着关键作用；PPB 是定向（在前期）和引导（在末期）有丝分裂装置所必需的。然而，植物必须拥有额外的机制来控制分裂轴，因为某些细胞类型或突变体不会形成 PPB。在这里，我们使用藓类配子体的活细胞成像，鉴定出无中心体的 MTOC，我们称之为“配子体”，它在前期细胞质中独立于 PPB 形成而新出现并短暂存在。我们表明，配子体对于纺锤体的形成不是必需的，但对于中期纺锤体的取向是必需的。在某些细胞中，配子体类似于在被子植物前期核周围短暂形成的双极 MT“极帽”结构。烟草细胞中极帽的特异性破坏使中期纺锤体错位，并经常改变最终分裂面，表明它们在功能上类似于配子体。这些结果表明，在植物中，瞬态 MTOC 结构作为纺锤体取向机制被广泛使用，补偿了中心体的进化缺失，以确保纺锤体在允许后续通过导向机制对分裂面轴进行微调的空间窗口中的初始取向。
