Figure 1.

PSTVd and RNA silencing. Upon inoculation, mature PSTVd molecules enter the plant cell nucleus. During the asymmetric replication cycle, POLII transcribes oligomeric RNA of (−)-orientation from the circular viroid RNA. In the nucleoplasm, this RNA is transcribed into oligomeric (+) RNA, again by POLII. Subsequently to its transport into the nucleolus, a type III-like RNase and a DNA ligase are recruited to assemble the mature viroid molecule. Oligomeric (+) and (−) replication intermediates may form dsRNA molecules. Alternatively, dsRNA molecules may be produced by RDR6-mediated transcription of mature viroid RNA and/or of oligomeric (−) in the nucleoplasm. In addition, it cannot be excluded that in the nucleolus, dsRNA is synthesized by RDR2 transcription of oligomeric (+) RNA. All four DCLs produce vd-siRNAs. However, it is not clear if only dsRNAs or, due to the high capacity of viroid RNA to form intermolecular secondary structures, also single stranded viroid RNA, (mature viroid or oligomeric intermediates) is DCL-processed. Both polarities of vd-siRNAs could be loaded onto several AGOs and initiate PTGS in the cytoplasm and RdDM in the nucleus.