Identification of the plant nucleolonema using silver staining of serial ultrathin sections

A simple silver staining technique was applied to serial ultrathin sections of Vicia faba root tip meristems to investigate the structural organization of the nucleoli. The procedure consists of three steps: hydrophilic Lowicryl resin embedding, preparation of ribbons of serial ultrathin sections, and silver impregnation. This technique provided good contrast for a filamentous structure, previously called the "nucleolonema", in nucleoli at the light microscope level. Silver appears to react with some acidic proteins involved with ribosomal RNA transcription, since the major constituents of the nucleolonema are believed to be active rRNA transcriptional units. Tracing the nucleolonema through serial ultrathin sections strongly suggests that the nucleolonema is meandering and partly coiling within the nucleolus. The present technique permits identification of the nucleolonema in other plant species and investigation of its three-dimensional structural organization.