Chrysanthemum chlorotic mottle viroid: a system for reverse genetics in the family Avsunviroidae (hammerhead viroids).

RNA silencing is a conserved pathway and it may result in gene expression blockage in eukaryotic organisms. RNA silencing is also part of a highly adaptable immune system response against viruses in plants and animals. It is generally thought that virus-induced RNA silencing is that double-stranded replicative intermediates of RNA viruses, and/or double-stranded RNA produced from the viral RNA by host RNA-dependent RNA polymerases are recognized by Dicer-like proteins for the production of viral siRNAs (vsiRNAs). However, recent studies show that vsiRNAs originated predominantly from highly structured single-stranded viral RNAs is a general characteristic for RNA viruses, dsDNA virus as well as for sub-viral pathogens, e.g. viroid. Increasing lines of evidence has also shown that the plant antiviral response involves hierarchical action of DCLs. To counteract antiviral silencing, many viral genomes encode suppressor proteins to combat the defense pathway. The most common strategy for viral suppressors to inhibit RNA silencing is via binding to siRNAs. Some viral suppressor can also bind to long dsRNA and maybe compete with DCLs to access viral RNA substrates, or inhibit the activity of specific DCLs in the production of vsiRNAs. This review will give an update on the current view of these researches on antiviral silencing and defense in plants.