Although they are composed only of a small single-stranded circular RNA molecule (300-400 bases) that does not encode any protein, viroids contain enough information to specifically infect their host and modulate gene expression of the plant in their favor, which results in the appearance of certain symptoms ultimately leading to the disease. These properties of viroids, in addition to the presence of a self-catalytic RNA motif within the Avsunviroidae family, support the idea that viroids may have a very ancient origin, which may even be a remnant of an RNA world. preceding that of DNA and proteins. This "simplistic" molecule considered as the boundary of life, seems to be an excellent model for the study of the interactions between pathogens and the cellular machinery of the host and for the study of the evolution of biological molecules.
The latent peach mosaic viroid (PLMVd) and potato spindle tuber viroid (PSTVd) are used as models in the laboratory to understand how viroids cause diseases in plants. We use in vitro techniques that allow us to determine the secondary and tertiary structure of these viroids (by mutagenesis, RNase, SHAPE, retention gel, bioinformatics, etc.) to discover new patterns within these molecules that could explain their pathogenicity and replication. We also use high throughput sequencing techniques to better understand the concept of quasispecies that characterizes the evolution of viroids. This technique also allows us to highlight the composition of interfering RNAs of the plant during infection. This study is very important because it allowed us to identify important host genes that contribute to the infection.