RESUMEN
The distribution of ribosomal transcripts in the plant nucleolus has been studied by non-isotopic in situ hybridization in ultrathin Lowicryl K4M sections and by high-resolution autoradiography after labelling with tritiated uridine. In parallel, cytochemical techniques were applied to localize RNA on different plant nucleolar components of Allium cepa L. root meristematic cells and Capsicum annuum L. pollen grains. For RNA/RNA in situ hybridization, several biotinylated single-stranded ribosomal RNA probes were used for mapping different fragments of the 18 S and the 25 S rRNA gene transcribed regions. Ribosomal RNAs (from pre-rRNAs to mature 18 and 25 S RNAs) were found in the nucleolus, in the dense fibrillar (DFC) and granular components (GC). Hybridization signal was found at the periphery of some fibrillar centres (FCs) with probes recognizing both 18 and 25 S rRNA sequences. A quantitative study was performed to analyze the significance of this labelling. Incorporation of tritiated uridine into roots was carried out and, later, after a long time-exposure, autoradiography revealed the presence of newly synthesized RNA mainly in the DFC and at the periphery of the FCs. The presence of RNA in these areas was also confirmed by the cytochemical techniques used in this study. Taken together, these data favour the hypothesis that transcription can begin at the periphery of the FCs, although we cannot exclude the possibility that the DFC plays a role in this process.