Replicating potato spindle tuber viroid RNA is accompanied by short RNA fragments that ave characteristic of post-transcriptional gene silencing

The low molecular weight fraction of tomato plants inoculated with potato s pindle tuber viroid (PSTVd) contains a population of short PSTVd-specific R NAs of either polarity. The main constituents were RNAs of 22 and 23 nt rep resenting different domains of the viroid genome. The occurrence of such di stinct RNA species indicated that the nuclear replicating PSTVd RNA induces post-transcriptional gene silencing. The short RNAs were slightly more abu ndant at 30 days post-inoculation than at later stages and were present in plants infected with a mild, severe or lethal isolate of PSTVd. There was n o apparent correlation between the quantity of small PSTVd-specific RNAs an d the degree of virulence of the viroid isolate.