Potato spindle tuber viroid as inducer of RNA silencing in infected tomato

Potato spindle tuber viroid (PSTVd), an RNA plant pathogen encoding no know n proteins, induces systemic symptoms on tomato plants. We report detection of small RNAs of approximately 25 nucleotides with sequence specificity to PSTVd in infected plants: an indication of the presence of RNA silencing. RNA silencing, however, did not appear to be responsible for the differing symptoms induced by a mild and a severe strain of PSTVd. The unique structu ral and biological features of viroids make them attractive experimental to ols to investigate mechanisms of RNA silencing and pathogen counterdefense.