Viroids and RNA silencing: mechanism, role in viroid pathogenicity and development of viroid-resistant plants.

Viroids are autonomously replicating, small single-stranded circular RNA pathogens that do not code for proteins and may cause diseases in infected, susceptible plants. They have the ability to induce both RNA-mediated transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS), or RNA silencing, in infected plants. Their induced RNA silencing has also been demonstrated in a wheat germ extract system. A possible role of RNA silencing in viroid pathogenicity and evolution has been discussed. It is suggested that RNA silencing can be employed to engineer plants for viroid resistance and attempts to produce these plants have been also discussed.