In-vivo observations of a spherical aggregate of endoplasmic reticulum and of Golgi vesicles in the tip of fast-growing Chara rhizoids.

In-vivo differential interference contrast microscopy was used to detect individual Golgi vesicles and a new structure in the tip of fast-growing rhizoids of Chara fragilis Desvaux. This structure is a spherical clear zone which is free of Golgi vesicles, has a diameter of 5 μm and is positioned in the center of the apical Golgi-vesicle accumulation ("Spitzenkörper"). After glutaraldehyde fixation and osmium tetroxide-potassium ferricyanide staining of the rhizoid, followed by serial sectioning and three-dimensional reconstruction, the spherical zone shows a tight accumulation of anastomosing endoplasmic reticulum (ER) membranes. The ER membranes radiate from this aggregate towards the apical plasmalemma and to the membranes of the statolith compartments. Upon gravistimulation the ER aggregate changes its position according to the new growth direction, indicating its participation in growth determination. After treatment of the rhizoid with cytochalasin B or phalloidin the ER aggregate disappears and the statoliths sediment. It is concluded that the integrity of the ER aggregate is actin-dependent and that it is related to the polar organisation of the gravitropically growing cell tip.