Editing Viroid (section)

==RNA silencing==
There has long been uncertainty over how viroids induce [[symptom]]s in plants without encoding any [[protein]] products within their sequences.<ref name="pmid28738223">{{cite journal |vauthors=Flores R, Navarro B, Kovalskaya N, Hammond RW, Di Serio F |title=Engineering resistance against viroids |journal=Current Opinion in Virology |volume=26 |issue= |pages=1–7 |date=October 2017 |pmid=28738223 |doi=10.1016/j.coviro.2017.07.003}}</ref> Evidence suggests that [[RNA silencing]] is involved in the process. First, changes to the viroid [[genome]] can dramatically alter its [[virulence]].<ref name="pmid1546460">{{cite journal |vauthors=Hammond RW |title=Analysis of the virulence modulating region of potato spindle tuber viroid (PSTVd) by site-directed mutagenesis |journal=Virology |volume=187 |issue=2 |pages=654–662 |date=April 1992 |pmid=1546460 |doi=10.1016/0042-6822(92)90468-5 |url=https://zenodo.org/record/1258280}}</ref> This reflects the fact that any [[siRNA]]s produced would have less complementary [[base pairing]] with target [[messenger RNA]]. Secondly, [[siRNA]]s corresponding to sequences from viroid genomes have been isolated from infected plants. Finally, [[transgenic]] expression of the noninfectious [[Hairpin RNA|hpRNA]] of [[potato spindle tuber viroid]] develops all the corresponding viroid-like symptoms.<ref>{{cite journal |vauthors=Wang MB, Bian XY, Wu LM, Liu LX, Smith NA, Isenegger D, Wu RM, Masuta C, Vance VB, Watson JM, Rezaian A, Dennis ES, Waterhouse PM |display-authors=6 |title=On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=101 |issue=9 |pages=3275–3280 |date=March 2004 |pmid=14978267 |pmc=365780 |doi=10.1073/pnas.0400104101 |bibcode=2004PNAS..101.3275W |doi-access=free}}</ref> This indicates that when viroids replicate via a double stranded intermediate [[RNA]], they are targeted by a [[dicer]] enzyme and cleaved into siRNAs that are then loaded onto the [[RNA-induced silencing complex]]. The viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs, and induction of degradation or inhibition of translation causes the classic viroid symptoms.<ref>{{cite book |vauthors=Pallas V, Martinez G, Gomez G |chapter=The Interaction Between Plant Viroid-Induced Symptoms and RNA Silencing |title=Antiviral Resistance in Plants |volume=894 |pages=323–343 |year=2012 |pmid=22678590 |doi=10.1007/978-1-61779-882-5_22 |isbn=978-1-61779-881-8 |series=Methods in Molecular Biology |hdl=10261/74632 |hdl-access=free}}</ref>