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ISHS Acta Horticulturae 901: XII International Symposium on Virus Diseases of Ornamental Plants

VIROIDS IN ORNAMENTALS

Author:   R. Flores
Keywords:   : small pathogenic RNAs, viroid diseases, viroid detection, viroid pathogenesis, viroid cross-protection, viroid and RNA silencing
Abstract:
With a genome composed of a small (246-401 nt), single-stranded, circular RNA, viroids are a special class of replicons infecting plants, including some ornamentals, in which they may induce specific diseases. In striking contrast to plant viruses, which encode proteins mediating their own replication and movement, viroids depend on host factors for these purposes and can be regarded as parasites of their host transcription machinery. The about 30 known viroids are classified into the families Pospiviroidae, type species Potato spindle tuber viroid (PSTVd), and Avsunviroidae, type species Avocado sunblotch viroid (ASBVd). PSTVd and ASBVd replicate (and accumulate) in the nucleus and the chloroplast, respectively, and the other members of both families most likely behave as their type species. Viroids replicate through an RNA-based rolling-circle mechanism that entails reiterative transcription of the incoming circular template to generate complementary longer-than-unit strands that, by themselves or after cleavage and ligation, prime the second half of the cycle finally leading to the mature circular forms. Remarkably, processing of the multimeric replicative intermediates is catalyzed by hammerhead ribozymes in the family Avsunviroidae. To complete their infectious cycle, viroids must recruit host factors for intracellular, cell-to-cell and long-distance movement within the plant. Viroid-derived RNAs with the typical properties of the small interfering RNAs, the hallmarks of RNA silencing, have been detected in plants infected by members of both families. This finding indicates that viroids are inducers and targets (and perhaps even suppressors) of RNA silencing, a regulatory layer that may mediate many aspects of viroid biology including pathogenesis.
Chrysanthemum stunt was one of the first diseases in which the inciting agent was identified as a viroid, Chrysanthemum stunt viroid, a member of the family Pospiviroidae that also infects other ornamentals. Chrysanthemum was additionally suspected to host a second viroid, although identification and characterization of the corresponding RNA, Chrysanthemum chlorotic mottle viroid (CChMVd), a member of the family Avsunviroidae, had to wait more than 20 years due to its low accumulation level. There are CChMVd non-symptomatic strains that cross-protect against challenge inoculation with severe strains, with the pathogenicity determinant mapping at a tetraloop of the CChMVd branched conformation. Other viroids infecting ornamentals include Columnea latent viroid, Iresine viroid and Coleus blumei viroid 1, 2 and 3, all belonging to the family Pospiviroidae. Additionally, a retroviroid-like element has been found in carnation. Some ornamental viroids are seed-transmissible. Viroids are usually detected by nucleic acid-based techniques (PAGE, molecular hybridization and RT-PCR). Control measures include the use of viroid-free propagating material together with the regular decontamination of pruning tools. Some viroids can be removed from infected material by in vitro micrografting.
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