Concluding Remarks
The mechanism of cell division has undergone several adjustments during the transition from streptophyte algae to land plants. These alterations occurred stepwise and some transitional stages can be seen in extant plants, for example cell division in the liverworts, which involves PPBs and centrosome-like polar organizers. Mapping cell biological data onto the recently established phylogenetic tree suggests that centrosomes were lost repeatedly in the phragmoplastophyta. The phragmoplast as a device for dividing the cell was lost or reduced in the Zygnematophyceae, many of which returned to cleavage mechanisms secondarily. Because the phragmoplast was established first, and the PPB of microtubules was introduced later, the interesting question arises of how early phragmoplasts became aligned (e.g., in Chara or Coleochaete)? It is reasonable to speculate that an early ‘cryptic’ alignment mechanism successively transformed into the PPB, for example by engaging cortical microtubules in cell division. Interestingly, conspicuous microtubule rings have been found in Zygnematophyceae preparing for cell division, supporting that the PPB mechanism was first established by algae rather than by bryophytes as previously thought (see also Outstanding Questions). Taken together, the PPB of higher plants functionally replaces the centrosome with regard to cell division plane alignment and, specifically, the PPB compensates for the lack of cytoplasmic dynein in the green lineage. The PPB/phragmoplast mechanism may have allowed the ancestor of today's land plants to perform sudden changes in division plane alignment, thereby forming the basis of 3D development.