Cytoplasmic MTOCs control spindle orientation for asymmetric cell division in plants

Abstract

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 Physcomitrella patens, 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.H ow do plants set their spindle division axis without centrosomes? In many angiosperm cell types, the cortical microtubules (MTs) are reorganized in G2 phase into a MT-based ring structure, the preprophase band (PPB), which encircles the nucleus at the cell cortex and represents a unique and key structure for oriented divisions in plants (1, 2). The PPB gradually degenerates during prophase. However, it defines the future division zone by recruiting a specific set of proteins to the cortex (3-8). During telophase, the phragmoplast, the postanaphase mitotic apparatus that recruits membrane and cell plate material for cytokinesis, centrifugally expands toward the cell cortex and strikingly precisely reaches the zone that the PPB formerly occupied. In addition to this "phragmoplast guidance" function, the PPB also sets initial spindle orientation. During degeneration of the PPB, nuclear envelope (NE)-associated MT structures, called polar caps, are formed on the opposite sides of the nucleus, perpendicular to the plane of the PPB (this structure is also called prospindle or prophase spindle) (9-12). Arabidopsis cell lines that do not assemble PPBs fail to establish these caps, indicating a critical role of the PPB in initial spindle orientation (9). However, whether initial spindle orientation by the PPB is critical for division plane determination i...

Microtubule nucleation for the assembly of acentrosomal microtubule arrays in plant cells

“…Therefore, these findings have further strengthened an earlier conclusion that mitosis mechanisms in plants and animals are not as divided as previously thought (Lloyd & Chan, ). The polar cap in angiosperm cells also influence the division plane in cytokinesis, like the gametosome function in moss cells (Kosetsu et al ., ). This PPB‐independent mechanism of spatial regulation of cell division is consistent with a recent finding that the PPB warrants the efficiency but may be dispensable for the process of cytokinesis (Schaefer et al ., ).…”

“…How are MT nucleation activities coupled with MT polymerization regulated by MAPs like EB1, MOR1 (Microtubule Organization 1), and CLASP (CLIP-Associated Protein) that act primarily at MT plus ends? Spatiotemporally coordinated nucleation and polymerization are critical for cells to establish new MT arrays that underlie physiological events like asymmetric cell division (Kosetsu et al, 2017).…”

An overview of plant division‐plane orientation

“…In the next sections, we focus on PPB form and function, but note that not all plant cells require a PPB for division-plane orientation. Examples of PPB-independent divisions include meiocytes (Otegui & Staehelin, 2004), endosperm (Brown & Lemmon, 2001) and some moss cells (Doonan et al, 1987;Kosetsu et al, 2017). Many PPB-independent divisions occur in invariant locations suggesting strong positioning cues.…”

“…Interestingly, in early gametophytic moss cells that do not make PPBs, spindle bipolarity is still anticipated by bipolar accumulation of cytoplasmic microtubule organizing centers to promote proper division-plane orientation (Kosetsu et al, 2017), similar to cytoplasmic microtubule organizing centers that accumulate before PPB formation in Marchantia polymorpha (Buschmann et al, 2016). Although altered spindle positioning may lead to divisionplane defects, spindle rotation and other defects occur in many cells without division-plane defects (Rasmussen et al, 2013).…”

“…When the final division occurs outside the PPB location, division-plane maintenance is defective. If the cell naturally does not form a PPB, division-plane maintenance defects can be inferred by comparing developmentally matched WT and mutant cell division patterns (Wu & Bezanilla, 2014;Kosetsu et al, 2017). In moss cells with no PPBs, the myosin VIII mutant has division-plane defects.…”