Transformations of the Pleiomorphic Plant MTOC during Sporogenesis in the Hepatic Marchantia polymorpha

Sporogenesis in the hepatic Marchantia polymorpha L. provides an outstanding example of the pleiomorphic nature of the plant microtubule organizing center (MTOC). Microtubules are nucleated from γ-tubulin in MTOCs that change form during mitosis and meiosis. Following entry of cells into the reproductive pathway of sporogenesis, successive rounds of mitosis give rise to packets of 4-16 sporocytes. Mitotic spindles are organized at discrete polar organizers (POs), a type of MTOC that is unique to this group of early divergent land plants. An abrupt and radical transformation in microtubule organization occurs when sporocytes enter meiosis: POs are lost and γ-tubulin is closely associated with surfaces of two large elongated plastids that subsequently divide into four. Migration of the four plastid MTOCs into a tetrahedral arrangement establishes the future spore domains and the division polarity of meiosis. As is typical of many bryophytes,cones of microtubules from the four plastid MTOCs initiate a quadripolar microtubule system (QMS) in meiotic prophase.At this point a transformation in the organization of the MTOCs occurs. The γ-tubulin detaches from plastids and forms a diffuse spheroidal pole in each of the spore domains. The plastids, which are no longer MTOCs, continue to divide. The diffuse MTOCs continue to nucleate cones of microtubules during transformation of the QMS to a bipolar spindle. Following meiosis I, γ-tubulin is associated with nuclear envelopes, and the spindles of meiosis Ⅱ are organized from diffuse MTOCs at the tetrad poles. At simultaneous cytokinesis, radial microtubule systems are organized at nuclear envelope MTOCs in each of the tetrad members.