The Arabidopsis chromatin-modifying nuclear siRNA pathway involves a nucleolar RNA processing center

RNA fluorescence in situ hybridization (RNA-FISH) together with protein immunolocalization is used to present evidence for the Arabidopsis chromatin-modifying small interfering RNS (siRNA) processing centers associated with nucleolus. The results have shown that maintenance of the heterochromatic state involves locus-specific nuclear RNA polymerase IV (Pol IVa) transcription followed by siRNA production and assembly of ARGONAUTE4 (AGO-4) and the largest-subunit genes (NRPD1b)-containing silencing complexes within nucleolar processing centers.