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Potato spindle tuber disease wasn't a life-or-death situation, but it led to the discovery of viroids: infectious, replicating bits of RNA

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♪ INTRO ♪ In 1971, a USDA plant scientist named Theodor Otto Diener discovered what could be the smallest pathogen on the planet.

It was unlike anything biologists had ever seen: it could make copies of itself, like a virus, but was much, much simpler. It didn’t have any kind of outer protein coating, and just a tiny bit of RNA.

At the time, scientists didn't think something like this could even exist. But these pathogens, called viroids, were very real. They infected potatoes, turning them into gnarly twists that looked more like ginger than something you’d top with sour cream.

And researchers eventually realized that they might be a clue to how life got started on Earth. As far as plant diseases go, potato spindle tuber disease wasn’t a life-or-death situation like the Irish potato famine in the mid-1800s. But it was still a problem for farmers, and no one knew what caused it.

So the USDA was put on the case. Theodor Diener and his colleagues figured the potato disease was caused by a virus. But when they extracted samples of it, they discovered it was much smaller than any known virus.

It was subviral. And that was baffling. Tests revealed that, unlike viruses, this thing didn’t have a protein coat.

It was just a chunk of RNA, or ribonucleic acid, the molecule we usually think of as the template used to make proteins. At the time, scientists had also assumed that even small pathogens needed a certain amount of DNA or RNA to replicate themselves… at least a couple thousand nucleotides. That’s tiny compared to most living things, but enough instructions for a virus to take over a host cell and make copies.

But this potato menace, whatever it was, seemed to have just a tenth of that. Because Diener’s mystery agent was similar to — but also clearly different from — true viruses, he called it a viroid. We now know that viroids are tiny, single-stranded circles of RNA.

Unlike RNA viruses, the RNA isn’t used to make proteins. Instead, viroids make their way into a plant cell’s nucleus or its chloroplasts, where photosynthesis happens. There, the viroid cranks out copies of itself using the host’s RNA polymerase, the enzyme that strings together pieces of RNA based on a DNA sequence.

Some viroids also have the ability to act as ribozymes, and cut RNA, which helps their replication process. Today, we know about dozens of viroids, which infect lots of crops besides potatoes. There’s the avocado sunblotch viroid, the apple scar skin viroid, and the hop stunt viroid.

Some viroids cause minor, cosmetic damage, but others — like a viroid that infects coconuts — will kill the entire plant. They’re also sneaky little devils. Different viroids can get into plants in a lot of different ways.

Farmers need to be on the lookout for contaminated farm tools, seeds, pollen, and even insects. Aphids, for instance, can suck up millions of copies of viroids when feeding on an infected plant, which they can then pass on to an unsuspecting victim with their next snack. Fortunately, viroids don’t infect humans.

The closest thing is hepatitis D, which looks an awful lot like a viroid. But it technically isn’t, since its RNA codes for a protein, and it doesn’t self-replicate. But even if we don’t need to worry about viroids giving us sunblotch or scarred skin like a poor infected apple, they’re still important to study.

They could even give us insight into where we, and all life, came from. That’s because some scientists see viroids as living fossils, precursors to all of the DNA and protein molecules in organisms today. RNA is a unique molecule: it can be passed on between generations to preserve information, and it can sometimes do things, like catalyze chemical reactions.

Those are two things that cells really need! Many biologists support an idea known as the RNA World hypothesis, which suggests life on Earth began with RNA. And, later, evolution drove the creation of the DNA molecule, which is a more reliable way to store genetic information, and the protein, which is a better catalytic workhorse.

For some scientists, the viroid is an especially good candidate for the RNA that started it all. Since viroids are very small and circular, replication would have been easier and more accurate. And some viroids are ribozymes, one of the few RNA molecules that can do things to other molecules.

So, it’s possible you, me, and everything living on Earth has a viroid to thank. And we here at SciShow, have you to thank for watching this episode—which was brought to you by our patrons on Patreon. If you want to help support this show, you can go to

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