The cortical cytoskeletal network and cell-wall dynamics in the unicellular charophycean green alga Penium margaritaceum

Background and Aims

Penium margaritaceum is a unicellular charophycean green alga with a unique bi-directional polar expansion mechanism that occurs at the central isthmus zone prior to cell division. This entails the focused deposition of cell-wall polymers coordinated by the activities of components of the endomembrane system and cytoskeletal networks. The goal of this study was to elucidate the structural organization of the cortical cytoskeletal network during the cell cycle and identify its specific functional roles during key cell-wall developmental events: pre-division expansion and cell division.

Methods

Microtubules and actin filaments were labelled during various cell cycle phases with an anti-tubulin antibody and rhodamine phalloidin, respectively. Chemically induced disruption of the cytoskeleton was used to elucidate specific functional roles of microtubules and actin during cell expansion and division. Correlation of cytoskeletal dynamics with cell-wall development included live cell labelling with wall polymer-specific antibodies and electron microscopy.

Key Results

The cortical cytoplasm of Penium is highlighted by a band of microtubules found at the cell isthmus, i.e. the site of pre-division wall expansion. This band, along with an associated, transient band of actin filaments, probably acts to direct the deposition of new wall material and to mark the plane of the future cell division. Two additional bands of microtubules, which we identify as satellite bands, arise from the isthmus microtubular band at the onset of expansion and displace toward the poles during expansion, ultimately marking the isthmus of future daughter cells. Treatment with microtubule and actin perturbation agents reversibly stops cell division.

Conclusions

The cortical cytoplasm of Penium contains distinct bands of microtubules and actin filaments that persist through the cell cycle. One of these bands, termed the isthmus microtubule band, or IMB, marks the site of both pre-division wall expansion and the zone where a cross wall will form during cytokinesis. This suggests that prior to the evolution of land plants, a dynamic, cortical cytoskeletal array similar to a pre-prophase band had evolved in the charophytes. However, an interesting variation on the cortical band theme is present in Penium, where two satellite microtubule bands are produced at the onset of cell expansion, each of which is destined to become an IMB in the two daughter cells after cytokinesis. These unique cytoskeletal components demonstrate the close temporal control and highly coordinated cytoskeletal dynamics of cellular development in Penium.