Dedifferentiation and microtubule reorganization in the apical cell protoplast ofSphacelaria (Phaeophyceae)

The apical cell ofSphacelaria, a tip-growing filamentous brown alga, and its protoplast constitute a model for the investigation of the consequences of cell wall removal on microtubular cytoskeletal organization and cell polarity. In the apical cell, the microtubular cytoskeleton is strongly polarized and, in most cases, extends from two centrosomes to the cortex where it constitutes a fine meshwork. Observations of microtubule dynamics throughout the cell cycle emphasize the coincidence between orientation of the mitotic axis and cell polarity. Just after protoplast isolation, dramatic alterations of initial polarity are observed, whatever the mitotic stage. In particular, the coincidence between cytoplasmic polarity and polarity of the system nucleus-centrosomes is lost in most cases. 12–24 h after protoplast isolation, the cell shows a more symmetrical organization while a dense cortical microtubular network spreads out concomitantly with wall reformation. Our discussion emphasizes the possible relationship between cell polarity and cell totipotency, and the relevance of such a model for higher plant studies.