Viroids infecting fruit trees: from bioassays to next generation sequencing

With a genome composed of a single-stranded, circular RNA composed of only 250-400 nucleotides, viroids are hte smallest infectious agents reported so far. Although viroid RNAs apparently do not code for any protein, they replicate and accumulate in the nucleus or in the chloroplast of the infected cells and spread systemically within the host plant, often causing severe diseae. Therefore, viroids provide a valuable model system for studying the relationships between structure and function of RNAs and the role of non-coding RNAs in pathogenesis. Bioassays on indicators, the classical approach for indetification of viroids and for fulfilling Koch's postulates, can be coupled with advanced technologies (i.e. next generation sequencing, NGS) fo diagnosis, including discovery of novel viroid-like RNAs and unveiling the molecular events involved in symptom elicitation.