Cell wall and rhizoid polarity in Pelvetia embryos

The developmental potentials of the rhizoid and thallus cells of two-celled Pelvetia embryos were investigated. Ablation of either the thallus or the rhizoid cell was accomplished by puncture with a micropipette. Thallus cells continued to divide repeatedly after rhizoid ablation, and in no case did the thallus initiate new rhizoid growth. In the reciprocal experiment, rhizoid cells elongated and divided in the normal transverse orientation after ablation of the thallus cell. The rhizoid did not appear to initiate a new thallus. Similar results were obtained when laser irradiation was applied to arrest division in one of the two cells. Thus, neither thallus nor rhizoid cell compensated for ablation or arrest of its sibling cell. The role of the cell wall-plasma membrane connections in organizing polar growth in the rhizoid cell was investigated by separating the wall from the protoplast. This was accomplished in two ways, by plasmolysis and by enzymatic wall digestion. Wall digestion yielded rhizoid cell protoplasts capable of wall regeneration and division, but the polar growth habit was irreversibly lost. Loss of polar growth correlated with loss of polarity in the microtubule cytoskeleton as visualized by indirect immunofluorescence using confocal microscopy. Transient plasmolysis of rhizoid cells resulted in abandonment of the preexisting apex and the initiation of a new rhizoid tip after rehydration. We suggest that interaction between the cell wall and the plasma membrane is involved in organizing polar growth.