The effects of applied electric fields on Micrasterias. II. The distributions of cytoplasmic and plasma membrane components.

The accompanying paper describes the effects of applied electric fields on the morphogenesis and patterns of wall deposition of growing cells of Micrasterias denticulata. This paper details the effects of electric fields (approximately 14 V cm-1) on the subcellular components of Micrasterias, including a description of the plasma membrane of growing semi-cells as visualized by freeze-fracturing. There are no gross cytoplasmic abnormalities or asymmetrics in the distributions of cytoplasmic organelles caused by the fields. In particular, neither the Large Vesicles nor Dark Vesicles are concentrated in the cathode-facing (CF) halves of lobes oriented perpendicular to the fields, where extra deposition of wall material has been shown to occur. In freeze-fracture replicas, there are about twice as many plasma membrane particles near the tips of growing lobes as there are in proximal regions of the lobes. Additionally, rosettes, consisting of 6 membrane particles, are seen predominantly in the distal parts of the lobes, and these rosettes are believed to be important in the synthesis of cell wall microfibrils. The applied fields cause a large asymmetry in the distributions of membrane particles, with larger numbers being found on the CF sides of lobes oriented perpendicular to the fields. We were not able to detect a specific effect on any class of particles. Taken all together, the data support the hypothesis that some of the factors responsible for growth localization in Micrasterias reside in the plasma membrane.