Interplay between viroid-induced pathogenesis and RNA silencing pathways.

Of all known plant pathogens, viroids have the lowest biological complexity. Their genome consists of a naked RNA without protein-encoding capacity. However, viroids contain sufficient genetic information to establish infection in susceptible hosts. The process by which this tiny RNA subverts the plant cell machinery by coercing the host to express symptoms of viroid infection is the 'Holy Grail' that has been searched for since the first viroid-induced disease was described. Recently, a large body of evidence has led to the emergent view that RNA silencing has a crucial role in viroid pathogenesis and evolution. Here, we chronologically analyse the relevant findings supporting this idea and propose a model to explain the possible interrelation between the trans-acting small interfering RNA (ta-siRNA) biogenesis pathway and viroid replication and pathogenesis.