Editing Viroid (section)

==Transmission and replication==
[[File:Viroids- how it do.gif|thumb|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.]]
Viroids are only known to infect plants, and infectious viroids can be transmitted to new plant hosts by [[aphid]]s, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact.<ref name=nt/><ref>{{cite journal | vauthors = De Bokx JA, Piron PG | year = 1981 | title = Transmission of potato spindle tuber viroid by aphids. | journal = Netherlands Journal of Plant Pathology | volume = 87 | issue = 2 | pages = 31–34 | doi=10.1007/bf01976653| bibcode = 1981EJPP...87...31D | s2cid = 44660564 }}</ref> Upon infection, viroids replicate in the nucleus (''Pospiviroidae'') or [[chloroplasts]] (''Avsunviroidae'') of plant cells in three steps through an RNA-based mechanism. They require [[RNA polymerase II]], a host cell enzyme normally associated with synthesis of [[messenger RNA]] from DNA, which instead catalyzes "[[rolling circle]]" synthesis of new RNA using the viroid as template.<ref name="pmid22719735">{{cite journal | vauthors = Flores R, Serra P, Minoia S, Di Serio F, Navarro B | title = Viroids: from genotype to phenotype just relying on RNA sequence and structural motifs | journal = Frontiers in Microbiology | volume = 3 | pages = 217 | year = 2012 | pmid = 22719735 | pmc = 3376415 | doi = 10.3389/fmicb.2012.00217 | doi-access = free }}</ref>

Unlike plant viruses which produce [[movement proteins]], viroids are entirely passive, relying entirely on the host. This is useful in the study of RNA kinetics in plants.<ref name="pmid16078879"/>