VIROIDS - THE SMALLEST AND SIMPLEST AGENTS OF INFECTIOUS-DISEASE - HOW DO THEY MAKE PLANTS SICK

Viroids are autonomously replicating pathogens of higher plants that c onsist solely of unencapsidated, single-stranded, circular RNAs of 246 -375 nucleotides. Despite their extreme simplicity, viroids cause synd romes in plants that are about as varied as those caused by plant viru ses. Because viroids are not translated, their effects on plants must be a consequence of direct interaction of the viroid with host constit uents. Although the molecular mechanisms of viroid pathogenesis are st ill unknown, analysis of molecular chimeras between viroids of differe nt pathogenicity levels has revealed that the severity of symptoms is the result of complex interactions among three of the five viroid doma ins. In vitro experiments with purified mammalian protein kinase P68 h ave shown that the enzyme is strongly activated (phosphorylated) by vi roid strains that incite moderate to severe symptoms, but far less by a mild strain. Activation of a plant homolog of P68 may be the trigger ing event in viroid pathogenesis.