Membrane recycling occurs during asymmetric tip growth and cell plate formation in Fucus distichus zygotes

Zygotes of the brown alga Fucus distichus undergo a series of intracellular changes resulting in the establishment of a polar growth axis prior to the first embryonic cell division. In order to examine the dynamics of membran e recycling which occur in the zygote during polar growth of the rhizoid, w e probed living Fucus zygotes with the vital stain FM4-64, N-(3-triethylamm oniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide. In newly fertilized, spherical zygotes, FM4-64 staining is symmetric and p redominantly in the perinuclear region which is rich in endoplasmic reticul um, Golgi, and vacuolar membranes. As rhizoid or tip growth is initiated, t his population of stained membranes becomes asymmetrically redistributed, c oncentrating at the rhizoid tip and extending centrally to the perinuclear region. This asymmetric localization is maintained in the zygote throughout polar growth of the rhizoid and during karyokinesis. Subsequently FM4-64 s taining also begins to accumulate in a central location between the daughte r nuclei. As cytokinesis proceeds, this region of stain expands laterally f rom this central location, perpendicular to the plane of polar rhizoid outg rowth. The staining pattern thus delineates the formation of a cell plate, similar spatially to the accumulation of nascent plate membranes of higher plants. Treatment of Fucus zygotes with brefeldin-A inhibits both asymmetri c growth of the rhizoid and formation of a new cell plate. These data sugge st that in F. distichus FM4-64 is labeling a Golgi-derived membrane fractio n that appears to be recycling between the site of tip growth, perinuclear region, and new cell plate.