Viroids: Non-coding Circular RNAs Are Tiny Pathogens Provoking a Broad Response in Host Plants

More than 50 years ago, viroids were firstly described as the smallest RNA molecules capable to infect certain plants and to autonomously self-replicate in host plants. Viroids are covalently closed circular single-stranded RNAs that are non-coding and depend for most of their infection cycle on host proteins. Today, viroids are subdivided into the two families Avsunviroidae and Pospiviroidae. Members of Avsunviroidae replicate in the chloroplast and have a highly bifurcated structure including hammerhead ribozymes, which cleave oligomeric replication intermediates into monomers and ligate them to mature circles. Members of Pospiviroidae accumulate in the nucleus, have a rod-like structure and depend on host proteins for cleavage and ligation. We will describe our present knowledge on sequence and structural elements of viroids in connection to their replication and trafficking.