Relationships between transcription, silver staining, and chromatin organization of nucleolar organizers in Secale cereale

The nucleolar organizer regions (NORs) are composed of hundreds of rRNA genes, typically spanning several megabases. Cytologically, NORs include regions that are highly condensed and regions that are decondensed, the latter corresponding to regions at which associated proteins stain intensively with silver (Ag-NORs) and where active rRNA gene transcription is thought to occur. To test the relationship between rRNA gene activity, NOR silver staining, and rDNA (genes coding for rRNA) chromatin condensation, we used the DNA methyl-transferase inhibitor 5-azacytidine to evaluate the correlation between the epigenetic regulation of rRNA genes and NOR silver staining in the plant Secale cereale. Following 5-azacytidine treatment, we observed an increase in rRNA gene transcription as well as a reduction in the number of cells showing a significant difference in the size of the silver-stained domains in the two NORs. These transcriptional changes occurred concomitantly with an increase in nuclear and nucleolar size and were associated with the reallocation of most of the rDNA from perinucleolar heterochromatin into the nucleolus. Collectively, these results suggest that rRNA gene transcription, silver staining, and NOR decondensation are interrelated in S. cereale.