Viroid-specific small RNA in plant disease.

Viroids are the smallest autonomous infectious nucleic acids known today. They are non-coding, unencapsidated, circular RNAs with sizes ranging from 250 to 400 nucleotides and infect certain plants. These RNAs are transcribed by rolling-circle mechanisms in the plant host's nuclei (Pospiviroidae) or chloroplasts (Avsunviroidae). Since viroids lack any open reading frame, their pathogenicity has for a long time been a conundrum. Recent findings, however, show that viroid infection is associated with the appearance of viroid-specific small RNA (vsRNA). These have sizes similar to endogenous small interfering RNA and microRNA and thus might alter the normal gene expression in the host plant. In this review we will summarize the current knowledge on vsRNA and discuss the current hypotheses how they connect to the induced symptoms, which vary dramatically, depending on both the plant cultivar and the viroid strain.