The Root Cap as a Test System for the Evaluation of Golgi Inhibitors: I. STRUCTURE AND DYNAMICS OF THE SECRETORY SYSTEM AND RESPONSE TO SOLVENTS

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Abstract
Maize (Zea mays) seedlings were grown under standard conditions and the time for reformation of a slime droplet of standard size was determined. The structure of the secretory cells was examined by light and electron microscopy. Osmium-zinc iodide (OZI) impregnation was used to provide contrast enhancement of the dictyosome forming-face cisternae and the endoplasmic reticulum (ER). The three-dimensional structure and relationships of these membrane systems were established from examination of 300 nm thick sections. The presence of a marginal cisternal network at the forming-face was confirmed, but no specific orientation of the ER to this face, or connection of ER to any part of the dictyosome could be demonstrated. Neither observation of reforming slime droplets, nor quantification of secretory vesicle numbers gave any support to the suggestion that slime formation is a phasic, or cyclic, process. Dilute solutions of solvents (1% ethanol, 1% and 0.1% DMSO) commonly used to solubilize potential inhibitors of Golgi activity were found to delay significantly the time taken for reformation of a slime droplet of standard size. These solutions also had slight effects on the numerical density of secretory vesicles. Cytochalasin D-induced accumulation of secretory vesicles was used to determine rates of vesicle formation (0.39 dictyosome−1 min−1) and hence turnover times for cisternal (6.5 min) and total dictyosome (26–39 min) membranes and the plasma membrane (9.7 min). The volume contribution of the secretory vesicles to the slime droplet was shown to be exceedingly small (0–1%), throwing doubt on the value of using droplet reformation rates as an indicator of secretory activity.