Kinetics of Golgi apparatus membrane flux following monensin treatment of embryogenic carrot cells.
Ultrastructural changes resulting from treatment with the sodium selective ionophore, monensin, were studied in embryogenic suspension cultures of carrot, Daucus carota (L.) in the presence of 10 microM monensin, an early change in the Golgi apparatus was an increase in the number of cisternae per stack (dictyosome). An average of one additional cisterna per stack was formed within the first 2 to 4 min of monensin treatment; in some experiments a second cisterna was formed within about 8 min. Thereafter, large vacuoles began to appear in the cytoplasm adjacent to the Golgi apparatus with a return of the number of cisternae per dictyosomal stack to the control number of about 5. Cells treated comparable but in the absence of monensin showed no ultrastructural changes during the entire observation period. By 1 h of monensin treatment, the regions of the cells containing dictyosomes were populated by large number of vacuoles (up to 20 or more per electron microscope section). These vacuoles were interpreted as swollen dictyosome cisternae that separated from the stack but had not migrated from the Golgi apparatus zone in the monensin-treated cells. The results permitted an estimation of the average time for formation of a new dictyosome cisterna of 2 to 4 min. This range of values agreed with estimates for mammalian cells from short time labeling and turnover experiments of 3 to 4 min assuming a dynamic model for Golgi apparatus function in which cisternae are released from a maturing face and new cisternae are built up at an opposite or forming face.