Viroids – the smallest pathogens raising big questions (and issues)

What are viroids? Are they viruses? Bacteria? Or something else entirely? And why should anyone other than a biologist even care about something so small that it doesn’t have a single protein-coding gene?

The documentary from the Biotechnical Faculty of the University of Ljubljana opens the door to a world so invisible that most of us never notice it – yet it affects the food we eat, agriculture, the economy, and perhaps even our understanding of the origin of life.

Viroids are not viruses. They are not bacteria. They are even simpler. They are small, circular RNA molecules, without a protein coat and without the capacity for protein coding – and yet they replicate in plants and can act as pathogens. Scientists often describe them as the “smallest known pathogens” or even as potential living fossils from a time when life was based primarily on RNA.

But this isn’t just a story about molecular biology. It’s a story about potatoes with failing yields. About fruit trees that no longer bear fruit as they should. About vineyards and hop fields. About global food security. About millions in agricultural losses. Viroids can spread unnoticed – through infected planting material, vegetative propagation and international plant trade – and are often only discovered once the damage has already been done.

In the documentary, dr. Jernej Jakše and dr. Helena Volk, researchers from the Biotechnology study programme at the University of Ljubljana’s Biotechnical Faculty reveal how one even studies something that codes nothing. How do you analyse RNA that is so small, yet so effective? How do you distinguish a viroid from a virus? And why are viroids such an exceptional model for understanding the spread of disease and the evolution of the simplest biological systems?

The ViroiDoc project explores, among other things, the diversity of viroids, their modes of transmission, and their impact on plants that are crucial for nutrition. Using modern molecular methods, sequencing and bioinformatic analyses, scientists are discovering where viroids are hiding – often in places where no one expected them. One of the most astonishing thoughts raised by the documentary is this: could viroids be living fossils from the “RNA world” era – a time before the emergence of complex cells?

If something so simple can persist and replicate, what does that tell us about what life actually is?
The film doesn’t offer simple answers. It does, however, offer curiosity. Surprise. And a sense that beneath the surface of the everyday world, there exists a layer of reality that is quieter, less visible – yet immensely influential.

The next time you look at an apple, a potato or a vine, you might ask yourself: what else is hidden within their cells? This documentary about viroids offers the answers – or at least the right questions.

The presented achievements in the documentary were produced as part of activities funded by ARIS.
Production by Inhouse Marketing Agency d.o.o. in cooperation with XOD and ViroiDoc, October 2025.