DEDIFFERENTIATION AND MICROTUBULE REORGANIZATION IN THE APICAL CELL PROTOPLAST OF SPHACELARIA (PHAEOPHYCEAE)

The apical cell of Sphacelaria, a tip-growing filamentous brown alga, and its protoplast constitute a model for the investigation of the con sequences of cell wall removal on microtubular cytoskeletal organizati on and cell polarity. In the apical cell, the microtubular cytoskeleto n is strongly polarized and, in most cases, extends from two centrosom es to the cortex where it constitutes a fine meshwork. Observations of microtubule dynamics throughout the cell cycle emphasize the coincide nce between orientation of the mitotic axis and cell polarity. Just af ter 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-centro somes is lost in most cases. 12-24 h after protoplast isolation, the c ell shows a more symmetrical organization while a dense cortical micro tubular network spreads out concomitantly with wall reformation. Our d iscussion emphasizes the possible relationship between cell polarity a nd cell totipotency, and the relevance of such a model for higher plan t studies.