Viroids: the smallest and simplest agents of infectious disease. How do they make plants sick?

Viroids are autonomously replicating pathogens of higher plants that consist solely of unencapsidated, single-stranded, circular RNAs of 246-375 nucleotides. Despite their extreme simplicity, viroids cause syndromes in plants that are about as varied as those caused by plant viruses. Because viroids are not translated, their effects on plants must be a consequence of direct interaction of the viroid with host constituents. Although the molecular mechanisms of viroid pathogenesis are still unknown, analysis of molecular chimeras between viroids of different pathogenicity levels has revealed that the severity of symptoms is the result of complex interactions among three of the five viroid domains. In vitro experiments with purified mammalian protein kinase P68 have shown that the enzyme is strongly activated (phosphorylated) by viroid strains that incite moderate to severe symptoms, but far less by a mild strain. Activation of a plant homolog of P68 may be the triggering event in viroid pathogenesis.