The relationship between changes in cell wall composition and the establishment of polarity in Fucus embryos

Changes in the appearance and location of fucoidin in the cell walls of Fucus embryos were related to embryo development. Fucoidin was not present in the cell wall until 10–14 hr after fertilization, when the embryos began to incorporate fucoidin preferentially into a localized area of the wall. At this time the site of rhizoid initiation was determined; that is, the embryos had undergone axis commitment. Germination of the single-celled embryo occurred between 12 and 16 hr, after fertilization, with all cell walls from germinated embryos showing fucoidin localization at the rhizoid end of the cell. The percentage of embryos with localized fucoidin at the time of axis fixation equaled the percentage of embryos that subsequently germinated. Culturing the embryos in sea water plus 0.8 M sucrose prevented the outgrowth of the rhizoid, but not the localization of fucoidin in the wall or axis commitment. Cycloheximide, nitroprusside, cytochalasin B, sulfate-free sea water, high levels of Ca²⁺, and a breakdown product of TIBA all prevented rhizoid growth and the specific localization of fucoidin. In addition, axis commitment could not be demonstrated in the presence of these inhibitors. DTNB, PCMBS, TIBA, HgCl₂, Mg²⁺ were ineffective as reversible inhibitors of rhizoid initiation. The authors propose that the fixation of axis commitment is accompanied by localized changes in the cell wall involving the incorporation of fucoidin as a structural component of the wall.