Celebrating 50 Years of Viroid Discovery

Dear Colleagues,

Theodor O. Diener of the US Department of Agriculture, Beltsville, Maryland, U.S, discovered the first viroid in 1971, the causal agent of potato spindle tuber disease. He showed that the agent is a free RNA of 25,000–110,000 Daltons, much smaller than a viral genome, and that no viral coat proteins were synthesized in infected plants. He concluded that the RNA is too small to contain the genetic information necessary for self-replication and it must rely on host enzymes for its replication. Also, in the U.S., Joseph S. Semancik of the University of California, Riverside, showed in 1972 that the causal agent of citrus exocortis disease is also a viroid of 125, 000 Daltons. By 1977 it was found that viroids cause economically important diseases in vegetable crops, fruit trees, ornamentals and palm species. The host range of viroids has expanded over the years to include grapevine and other cultivated and wild plant species. Viroids, the smallest known infectious agents (246-401 nt) exist as circular and linear RNAs with a high degree of base pairing and replicate autonomously in infected cells. In contrast to viruses, viroids lack capsid proteins, do not code for proteins and are completely dependent on host cellular RNA polymerases and processing enzymes for their replication. Viroids belong to the new order of subviral agents, which currently includes two families, eight genera, and 32 species of viroids. Viroids have had an impact on the science of virology, plant pathology, botany, microbiology, genetics, genomics, molecular biology, and molecular evolution as they represent the frontiers of life.

The objective of this Special Issue is to present a collection of original research, review articles, brief reports, case reports, project reports, communications and hypotheses related to viroid research advances and developments. Viroid topics of particular interest include, but are not limited to, economic impact, characteristics, diseases and viroids associated with plant species, detection and identification methods, transmission, control measures, geographical distribution and epidemiology. 

Dr. Ahmed Hadidi
Prof. Dr. Henryk Hanokh Czosnek
Prof. Dr. Kriton Kalanditis
Dr. Robert A. Owens
Guest Editors

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