Signalling in Viroid Pathogenesis

Despite the fact that viroids are the infectious agents with the lowest complexity and best known structure, our understanding of their pathogenic interaction with the host plant is still far from complete. Viroid pathogenesis poses many intriguing questions; these have to be answered without any viroid-specified protein to which to attribute a pathogenic role. Here we present a view of viroid pathogenesis in which viroid molecules are considered as pure replicating and pathogenic signals. These act as elicitors of host responses which can also be activated by other afflicting agents. A model is outlined to explain how the viroid-induced response at the cellular level becomes a developmental disease and leads to a plant which is more resistant to subsequent infections.