LOCALIZED MEMBRANE-WALL ADHESIONS IN PELVETIA ZYGOTES

Membrane-wall adhesions in zygotes of the brown alga Pelvetia were vis ualized following plasmolysis. Strands of cytoplasm remained firmly at tached to the cell wall at discrete adhesion sites during plasmolysis. Adhesion sites were uniformly distributed in ungerminated zygotes, bu t were concentrated in the apical 5 mu m of the elongating rhizoid in germinated zygotes. Few adhesions were detected along the flanks of th e rhizoid or in the thallus region of germinated zygotes. The structur e, physiology and function of apical adhesions in the rhizoid were cha racterized. F-actin was found at adhesion sites in plasmolyzed zygotes labeled with rhodamine phalloidin, and disruption of cortical F-actin reduced the number of adhesions. Manipulation of cytosolic H+ and Ca2 + activities also disrupted adhesions. On the extracellular surface, t he number of adhesions was reduced by inhibition of cellulose synthesi s, protease cleavage of wall proteins, and changes in extracellular H and Ca2+ activities. Chronic treatment with the synthetic peptide RGD S, which prevents cell adhesion in fibroblasts, also reduced adhesion number. The number of adhesions per cell did not correlate with growth rate, but was inversely correlated with the ability to establish new rhizoid growth sites. The results indicate that membrane wall adhesion s containing F-actin on the cytoplasmic face are localized in the grow ing rhizoid apex. The adhesions may be structurally related to focal a dhesions in animal cells.