Viroids

Dear Colleagues,

Viroids are small covalently closed single-stranded RNAs that infect plants. Since they were discovered in 1971-1972 as disease-causing agents of economically important crops, they were, and still are, considered as virus-like plant pathogens. However, in addition to their small size (in the range of 250-400 nt), viroids do not code for proteins and are fully associated with their hosts.

Early work was essentially based on the use of biological approaches such as the use of indexing methods using indicator plants. However, the development of a wide range of molecular approaches has been critical/essential for the discovery of new viroids, viroid strains, as well as the identification of symptomless hosts. Essentially, two types of viroids have been well identified and characterized with those belonging to the Avsunviroidae family having essential features needed for viroid replication. In addition, the fact that viroids follow a quasi-species model provides a situation that enhances their survival as well as the adaptation to new hosts.

In spite of the limited number of viroids that have been identified and characterized, they cover a wide range of plant hosts: i) Vegetatively-propagated hosts enhancing their survival in the progeny; ii) Seed-propagated hosts that may or may not transfer viroids to their progeny; iii) Hosts affected by viroids containing viroid-like satellite RNAs; iv) Wild plant species with very limited information regarding their relationship with viroids and viroid-like satellite RNAs.

Further research should also focus on viroids as biological beings (not only as plant pathogens) focusing on their survival, prevalence, and role on the living world. This would provide information in terms of their present role and also as biological ancestors of the present world.

Dr. Nuria Duran-Vila
Guest Editor

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