VIROIDES PDF

Transmission[ edit ] The reproduction mechanism of a typical viroid. Leaf contact transmits the viroid. The viroid enters the cell via its plasmodesmata. RNA polymerase II catalyzes rolling-circle synthesis of new viroids.

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Transmission Edit The reproduction mechanism of a typical viroid. Leaf contact transmits the viroid. The viroid enters the cell via its plasmodesmata. RNA polymerase II catalyzes rolling-circle synthesis of new viroids. Viroid infections can be transmitted by aphids , by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact.

Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants.

Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid-like symptoms. If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matter abiogenesis.

Their high guanine and cytosine content, which increases stability and replication fidelity. Their circular structure, which assures complete replication without genomic tags. Existence of structural periodicity, which permits modular assembly into enlarged genomes.

Their lack of protein-coding ability, consistent with a ribosome -free habitat. Replication mediated in some by ribozymes —the fingerprint of the RNA world. History Edit In the s, symptoms of a previously unknown potato disease were noticed in New York and New Jersey fields.

Because tubers on affected plants become elongated and misshapen, they named it the potato spindle tuber disease. A fungus or bacterium could not be found consistently associated with symptom-bearing plants, however, and therefore, it was assumed the disease was caused by a virus. Despite numerous attempts over the years to isolate and purify the assumed virus, using increasingly sophisticated methods, these were unsuccessful when applied to extracts from potato spindle tuber disease-afflicted plants.

Viroids were shown to consist of short stretches a few hundred nucleobases of single-stranded RNA and, unlike viruses, did not have a protein coat. Compared with other infectious plant pathogens, viroids are extremely small in size, ranging from to nucleobases; they thus consist of fewer than 10, atoms. In comparison, the genomes of the smallest known viruses capable of causing an infection by themselves are around 2, nucleobases long. Viroids thus are true circular RNAs. The single-strandedness and circularity of viroids was confirmed by electron microscopy, [21] and Gross et al.

Over thirty plant diseases have since been identified as viroid-, not virus-caused, as had been assumed.

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