Zinc Iodide-Osmium Tetroxide (ZIO) Staining Reveals Differences in the Components of the Endomembrane System of Plant Secretory Cells.

The Zinc Iodide-Osmium Tetroxide method (ZIO) is recognized for its effectiveness in impregnating the endomembrane system, particularly Golgi bodies. Nevertheless, its application in plant cell studies remains limited. This research focuses on Palicourea rigida Kunth, a Rubiaceae species known for its colleters producing a mixed secretion primarily composed of lipophilic substances. The aim of our study was to assess the effectiveness of the ZIO method in investigating ultrastructural variations of epithelial cells during the secretory phase. Samples were fixed for 12 h in a solution containing 2.5% glutaraldehyde and 4% paraformaldehyde in 0.1 mol/L phosphate buffer at pH 7.3. This was followed by a 22-h incubation at 4°C in a solution that included zinc (Zn), resublimed iodine (I), TRIS-aminomethane, and osmium tetroxide (OsO4). Subsequently, the samples underwent dehydration through a graded series of acetone and were embedded in Araldite resin. Ultrathin sections were contrasted with uranyl acetate and lead citrate and analyzed using a transmission electron microscope at 60 kV. While the epithelial cells exhibited morphological similarities, the ZIO method revealed diversity in their endomembrane systems. Some cells were rich in rough endoplasmic reticulum (RER), while others were abundant in smooth endoplasmic reticulum (SER), and some primarily contained Golgi bodies. These findings underscore the division of labor among neighboring secretory cells, which, despite their morphological similarities, show differences in their endomembrane systems during the same secretion stage. Therefore, the ZIO method emerges as a promising tool in plant secretion research, contributing to the understanding of the functional diversity of the endomembrane system in plant glands.