Selected DNA aptamers bind potato spindle tuber viroid and modulate infection

In a finding that could reshape how scientists fight one of agriculture’s most elusive classes of pathogens, researchers in Japan have discovered that short, synthetic strands of DNA can do something remarkable: when applied alongside the potato spindle tuber viroid, some of these DNA molecules dramatically suppress infection in tomato plants, while others, unexpectedly, make the disease worse. The study, led by Takashi Naoi and Teruo Sano of Hirosaki University in collaboration with Maria S. Kaponi of the Hellenic Organization of Agricultural Insurances in Greece, was published in Plant Cell Reports and marks one of the first demonstrations that DNA aptamers can biologically modulate viroid infection in living plants, not merely bind to the pathogen in a test tube.

Viroids are among the strangest infectious agents known to science. Unlike viruses, they carry no genes at all, no proteins, no protective coat. They are nothing more than small, single-stranded, circular RNA molecules, typically ranging from about 230 to 430 nucleotides, folded into elaborate rod-like structures. Yet despite this extreme minimalism, viroids wreak havoc on crops worldwide, causing devastating diseases in potatoes, tomatoes, citrus, hops, and many other species. The potato spindle tuber viroid, or PSTVd, was the first viroid ever discovered and remains a model system for understanding how these naked RNA pathogens replicate, move through plants, and trigger symptoms. Because viroids have no protein products, conventional disease-control strategies, such as those targeting viral enzymes or coat proteins, simply do not apply. This has left a glaring gap in the plant pathologist’s toolkit.

To close that gap, the Hirosaki team turned to a technique known as SELEX, short for systematic evolution of ligands by exponential enrichment. First developed in the early 1990s, SELEX is essentially an artificial Darwinian evolution experiment conducted in a laboratory tube. Researchers start with an enormous library of random DNA sequences, in this case single-stranded DNAs of 30 nucleotides in length, and repeatedly expose them to a target molecule, here purified PSTVd RNA. Sequences that happen to stick to the target are captured, amplified, and subjected to another round of selection. Over successive rounds, the population becomes progressively enriched for molecules with the strongest binding affinity, while poorly binding sequences are washed away.

What set this study apart was the level of scrutiny applied to the evolutionary process itself. Rather than waiting until the end of the selection to examine the winners, the researchers performed next-generation sequencing on the DNA population every five rounds, at rounds 5, 10, and 15. The sequencing data revealed a textbook evolutionary trajectory: with each round, the pool of DNA sequences became both more enriched and less diverse. By the fifteenth round, the number of unique sequences in the population had collapsed to just one-eighteenth of what it had been at round five, a dramatic demonstration of convergent selection pressure at work.

From this sequencing data, the team selected sixteen sequences that ranked highly in the fifth, tenth, and fifteenth rounds, along with two sequences that were actually present at low abundance in the fifth round. These eighteen candidate aptamers were then tested for their ability to physically bind PSTVd using a pull-down assay, a technique in which the viroid RNA is immobilized and candidate DNA molecules are washed over it to see which ones stay attached. The results were telling: seven of the eighteen candidates bound either in vitro-transcribed PSTVd or native PSTVd extracted from infected tissue. Perhaps most intriguingly, the strongest binder of all came not from the highly enriched late rounds but from one of the two low-abundance sequences in round five, a reminder that sheer abundance in a SELEX pool does not always predict binding superiority. Five of the other confirmed binders were among the sequences ranked highly in the fifteenth round, validating the enrichment strategy.

But the true test of any aptamer is not whether it clings to its target in a plastic tube, but whether it can do something useful in a living organism. Here the researchers made a decision that transformed the study from a routine binding exercise into something genuinely novel. Four of the confirmed binding sequences, including one that had been previously reported in the literature, were selected for co-inoculation experiments on tomato plants. In these assays, infectious PSTVd RNA transcripts were rubbed onto tomato leaves together with the candidate aptamers, and the plants were then monitored to see whether infection took hold.

The outcome defied simple expectations. Some of the aptamers significantly inhibited PSTVd infection, reducing the frequency with which plants became diseased, essentially acting as molecular shields that interfered with the viroid’s ability to establish itself. Others did the opposite: they promoted infection, apparently making it easier for the viroid to colonize the plant. The researchers analyzed infection outcomes using survival analysis, a statistical framework borrowed from clinical research and previously applied in plant pathology to time-to-event data such as leaf abscission, treating the time until a plant became infected as the measured event.

Why would some DNA sequences help the pathogen while others hinder it? The answer likely lies in where on the viroid’s intricate RNA structure each aptamer attaches. The researchers used computational structure prediction, drawing on tools in the lineage of AlphaFold 3, to model the interaction between each aptamer and the PSTVd RNA, and found that the differentially modulating behavior correlated with the specific binding sites predicted in silico. PSTVd’s circular RNA folds into a series of structural domains with distinct functions, including a pathogenicity domain whose stability and sequence influence symptom severity, and central regions implicated in replication and systemic movement. Previous work has shown that even a single nucleotide substitution can convert PSTVd from noninfectious to infectious in tobacco, and that mutations stabilizing the pathogenicity domain suppress replication and symptom expression. An aptamer that binds over a region essential for replication or movement could plausibly block the viroid’s life cycle, while one that binds elsewhere might stabilize a transient structural conformation that facilitates infection, for example by protecting a loop region involved in host protein interactions, such as the bulged hairpin known to interact with the host protein VirP1 during systemic spread.

The implications cut both ways, and the authors are candid about this. On one hand, inhibitory aptamers represent an entirely new class of potential anti-viroid agents. If DNA sequences can be designed or selected to bind critical functional motifs on viroid RNA and block infection, they could eventually inform the development of exogenously applied protective compounds, or perhaps be expressed in transgenic plants as part of a resistance strategy. This is particularly valuable for a pathogen class against which no conventional chemical or genetic control exists. Viroids are a growing concern for global agriculture, with pospiviroids spreading through commercial pepper and tomato production and recent surveys suggesting that seed transmission may have been overestimated but that trade in ornamental hosts continues to fuel outbreaks. Japan, where the research was conducted, has experienced significant economic losses from viroid incursions, and the country’s agricultural authorities remain on high alert for newly emerged plant viruses and viroids.

On the other hand, the discovery that some aptamers enhance infection raises both a caution and an opportunity. The caution is obvious: any future attempt to deploy aptamer-based control would need to rigorously screen candidate molecules for unintended proviral effects. The opportunity is subtler but scientifically rich. Pro-infective aptamers are, in effect, tools for probing which regions of the viroid genome are vulnerable bottlenecks in the infection process. By comparing where inhibitory and enhancing aptamers bind, researchers can map the functional anatomy of a viroid with a precision that conventional mutagenesis struggles to achieve, since many viroid mutations are lethal to the pathogen itself.

It is also worth noting the methodological contribution embedded in this work. A companion protocol paper published by the same collaboration in the International Journal of Molecular Sciences laid out a high-throughput sequencing and SELEX-based workflow for selecting aptamers against PSTVd, and the current study demonstrates that workflow end to end, from a random 30-nucleotide library through iterative selection, amplicon sequencing, binding validation, and finally biological testing in planta. The sequencing datasets are available from the corresponding authors upon reasonable request, and the paper includes extensive supplementary material detailing the sequence populations and binding analyses. The work was supported by the Japan Society for the Promotion of Science, including a JSPS Postdoctoral Fellowship, a KAKENHI grant, and the J-PEAKS program for forming Japan’s peak research universities.

The broader context makes the study timely. Viroid biology is enjoying a renaissance, driven in part by metatranscriptome mining that has revealed a stunning diversity of viroid and viroid-like circular RNA agents across the biosphere, fueling speculation about their deep evolutionary origins as relics of an RNA world. Fifty years after PSTVd’s discovery opened the field, researchers are still wrestling with fundamental questions about how these minimal pathogens achieve host adaptation, subcellular targeting, and pathogenesis without a single encoded protein. The Hirosaki study adds an unexpected chapter to that story: it shows that the viroid’s folded RNA architecture, long studied as the source of its pathogenic power, is also a surface that can be recognized, bound, and manipulated by short synthetic DNA molecules. Whether that manipulation can be harnessed reliably enough to protect crops in the field remains an open question, and the authors emphasize that the mechanisms underlying both inhibition and enhancement will require further dissection. But as a proof of concept, the result is striking, a bare piece of DNA, thirty letters long, capable of deciding whether a plant falls ill or stays healthy. For a pathogen with no genes, no proteins, and no cure, that may be the most meaningful pressure it has faced yet.