Viroids | The smallest Pathogens | Diseases, symptoms, transmission and prevention

Viroids, the smallest known pathogens, are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants. Potato spindle tuber viroid, discovered in 1971, is the prototype; 29 other viroids have since been discovered ranging in length from 120 to 475 nucleotides. Viroids only infect plants; some cause economically important diseases of crop plants, while others appear to be benign. Two examples of economically important viroids are coconut cadang-cadang viroid.
The 30 known viroids have been classified in two families.
There is no evidence that viroids encode proteins or mRNA. Unlike viruses, which are parasites of host translation machinery, viroids are parasites of cellular transcription proteins: they depend on cellular RNA polymerase for replication. Such polymerases normally recognize DNA templates, but can copy viroid RNAs.
After replication, viroid progeny exit the nucleus or chloroplast and move to adjacent cells through plasmodesmata, and can travel systemically via the phloem to infect other cells. Viroids enter the pollen and ovule, from where they are transmitted to the seed. When the seed germinates, the new plant becomes infected. Viroids can also be transmitted among plants by contaminated farm machinery and insects.
Symptoms of viroid infection in plants include stunting of growth, deformation of leaves and fruit, stem necrosis, and death. Because viroids do not produce mRNAs, it was first proposed that disease must be a consequence of viroid RNA binding to host proteins or nucleic acids. The discovery of RNA silencing in plants lead to the hypothesis that small interfering RNAs derived from viroid RNAs guide silencing of host genes, leading to induction of disease. In support of this hypothesis, peach latent mosaic viroid small RNAs have been identified that silence chloroplast heat shock protein 90, which correlates with disease symptoms. The different disease patterns caused by viroids in their hosts might all have in common an origin in RNA silencing.
#ViroidInfections
#HowViroidsCauseInfection

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