Cytokinetic callose in unicellular algae

Cytokinesis is the physical separation of the cytoplasm during cell division, giving rise to two daughter cells. While animal and plant cell division are similar in principle, plant cytokinesis involves the formation of a cell plate that develops into the new cell wall. In multicellular plant species, the polysaccharide callose accumulates at the developing cell plate during and contributes to the deposition of the cell wall. Georgia Drakakaki and colleagues (Davis et al., 2020) now show for the first time that cytokinetic callose deposition is not restricted to multicellular plant species but also takes place in the unicellular alga Penium margaritaceum. The authors demonstrate that callose is deposited in a distinct pattern at the division plane, forming a ring-like structure around the nucleus, which persists as daughter nuclei migrate to opposite ends of the cell and division ensues. Callose deposition appears to be essential for cytokinesis, as failure to form the callose ring halts cell division. The failure to complete cytokinesis without callose deposition was attributed to its role in establishing the architecture of the isthmus zone at the cell centre, which in turn affects the overall pattern of polysaccharide deposition during assembly of the new cell wall. These findings may also be relevant for other unicellular streptophyte algae that employ callose deposition during their cytokinesis and could help to obtain a clearer picture of the evolution of land plants.