More than 50 Years of Viroid Research

Visit the ViroiDoc YouTube channel to watch a featured video lecture by Professor Kriton Kalantidis from the University of Crete.

Viroids are small, circular, non-coding RNA molecules that lack a protein coat and can infect a wide range of plant species. Viroid RNAs move intracellularly, extracellularly, and systemically within infected plants.

They were first identified in the late 1960s by Prof. T. Diener as the causal agents of Potato Spindle Tuber Disease, marking the discovery of infectious RNA molecules. The term viroid was formally introduced by Diener in a seminal 1971 paper.

The Potato Spindle Tuber Viroid (PSTVd), a quarantine plant pathogen, belongs to the family Pospiviroidae. PSTVd was initially identified in potato plants (Solanum tuberosum, Solanaceae) but is now known to infect a wide range of hosts, including other members of the Solanaceae family and various vegetable crops.

Viroids depend entirely on host cellular proteins to complete their biological cycle. Early research focused primarily on understanding their replication mechanisms, epidemiology, and genome sequencing. This work led to several landmark discoveries, including the complete sequencing of the PSTVd genome, the demonstration that viroid RNA is replicated by a DNA-dependent RNA polymerase, and the confirmation that viroid genomes are non-coding.

This last finding suggested that all viroid-related phenomena originate from the intrinsic sequence and structure of the viroid RNA, independent of translation. In subsequent years, further significant advances were made by researchers in the field of viroid biology. Nevertheless, many fundamental questions regarding viroid infection, replication, and host interaction remain unanswered.

Curious to learn more?

Visit the ViroiDoc YouTube channel to watch a featured video lecture by Prof. Kriton Kalantidis, recorded in September 2025 during the ViroiDoc Days of the EU-funded ViroiDoc project's training activities.

Kriton Kalantidis is Professor of Plant Developmental Biology at the Department of Biology at the University of Crete (UoC), Greece. His research interests focus on viroid/virus-plant interactions and RNA silencing pathways in plants.

Kriton Kalantidis also serves as the supervisor for Doctoral Candidate (DC9) who will work on the individual research project “How disrupting host factors suppresses viroid infectivity" at the University of Crete.

Explore more content on viroid research by visiting ViroiDoc YouTube channel.