Short interfering RNAs specific for potato spindle tuber viroid are found in the cytoplasm but not in the nucleus

Short interfering (si) and micro (mi) RNAs influence gene expression at post-transcriptional level. In plants, different classes of DICER-LIKE (DCL) enzymes are responsible for the generation of these small regulatory RNAs from different precursors. To characterize the cellular site of their generation and accumulation, we purified nuclei from tomato plants infected with potato spindle tuber viroid (PSTVd) RNA, which is known to replicate in the nucleus via double-stranded (ds) RNA intermediates. We could detect PSTVd-specific siRNAs in the cytoplasmic fraction, but not in the nuclear fraction. To correlate the localization of the PSTVd-specific siRNAs with that of similarly sized small RNAs, we studied the compartmentalization of a naturally occurring miRNA. We could detect the precursor of miR167 in the nucleus, but the mature miRNA was found only in the cytoplasmic fraction. We discuss the consequences of this finding for the model of viroid replication and heterochromatin formation.