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.

SUMMARYNucleic acid extracts from citrons (Citrus medica cv. Etrog) displaying mild and moderate symptoms associated with the exocortis disease were analysed by sequential and denaturing PAGE which revealed the presence of several viroids. A comparison was made of electrophoretic patterns displaying one or more distinct citrus viroids from field isolates of citrus with exocortis. Citrus viroids were characterized by the physical parameters of electrophoretic mobility, chromatography on CF-11 cellulose and hybridization to cDNA probes of the well characterized citrus viroids, citrus exocortis viroid, CV-Ib from the 'citron variable viroid' isolate, and citrus cachexia viroid. These characteristic properties combined with biological distinctions in the host range and symptom expression suggested a scheme for the organization of the citrus viroids into five major groups. The association of the symptoms induced by these citrus viroids in citron cv. Etrog, their organization into individual viroid groups and their presumed relationship to the exocortis disease of citrus are discussed.

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SUMMARYAnalysis by polyacrylamide gel electrophoresis of nucleic acid preparations, obtained from several varieties of grapevine by a procedure designed to isolate and purify viroids, revealed the presence of RNA species with some of the characteristic physical properties of viroids. Under non-denaturing conditions, a band with a mobility faster than that of citrus exocortis viroid (CEV) was detected, and under fully denaturing conditions two bands were observed, one co-migrating with the circular forms of CEV and a second migrating faster than the linear forms of this viroid. This RNA species did not hybridize with a cDNA probe to CEV. Some of the grapevine preparations were infective for Gynura aurantiaca, inducing symptoms similar to those caused by CEV, and the appearance of an RNA which had the same mobility as CEV in denaturing and non-denaturing electrophoretic systems and hybridized with cDNA to CEV. These results suggest that viroid-like and viroid RNAs can be recovered from grapevine, the former (with no detectable sequence homology to CEV) at a concentration sufficient to be observed as a physical entity in gels, and the latter (with close sequence homology to CEV) whose presence could only be revealed by bioassay. The possible involvement of these RNAs in some grapevine diseases of unknown aetiology is discussed.

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