RNA silencing as related to viroid induced symptom expression.

Evidence of post-transcriptional gene silencing (PTGS) in avocado infected by Avocado sunblotch viroid (ASBVd), the type species of family Avsunviroidae, was suggested by detection of ASBVd-specific 22-nucleotide RNAs. PTGS was observed in infected bleached and variegated symptomatic tissues as well as symptomless carrier foliar sources and fruit with typical sunblotch disease lesions. Tissues with the different symptom expressions, characterized by the presence of different predominant ASBVd variants, were found to induce PTGS at differential levels. Detection of the PTGS-associated small interfering RNAs (siRNAs) as well as relative concentration was also related to viroid titer. PTGS induced in Gynura aurantiaca infected with two closely-related variants of Citrus exocortis viroid, a member of family Pospiviroidae, was not directly related to viroid titer with initiation of symptoms.