In the early 1920s, farmers in New Jersey noticed their potatoes were shriveling, their leaves becoming deformed. The plants were sick with an illness that came to be known as potato spindle tuber disease. But it took almost five decades for someone to find the cause.
In 1971, Theodor O. Diener, a plant pathologist at the Department of Agriculture, discovered that the culprit is an inconceivably tiny pathogen — one-80th the size of a virus. Dr. Diener called it a viroid.
Since Dr. Diener’s initial discovery, scientists have identified nearly three dozen species of viroids that attack crops from tomatoes to coconuts, as well as flowers such as dahlias and chrysanthemums. In many cases, the only way to stop an outbreak is to destroy all the infected plants. These days many countries require that plants be certified viroid-free before being imported.
But viroids may be much more than agricultural pests. New research suggests that they existed at the earliest stages of life on Earth, enduring in their primitive state for billions of years. These are the pterodactyls of the microbial world — except that they are still very much with us. We just didn’t realize it.
Today, most living things are composed of three basic ingredients. They contain proteins, which give a body structure and carry out chemical reactions. They also contain double-stranded DNA, which encodes genes.
And they contain RNA, a single-stranded molecule similar to DNA. Among many other jobs, RNA carries the information for building proteins from a cell’s genes to its protein factories.
Many scientists have argued that before this kind of life emerged, life was based solely on RNA. RNA can store genetic information, but scientists have discovered that some RNA molecules also carry out chemical reactions. In other words, this single molecule might have been able to handle all the basic tasks required for life. Only later did DNA and proteins evolve.
At first, the proponents of the so-called RNA-world theory assumed that RNA-based life had become extinct long ago, driven to extinction with the arrival of superior DNA-based life. Researchers have relied only on indirect hints to infer what RNA-based life was like.
But in the current issue of Annual Reviews of Microbiology, a team of Spanish scientists argues that these primitive life forms share the planet with us today. “Viroids are probably relics of the RNA world,” said Santiago F. Elena, an evolutionary biologist at Spain’s National Research Council in València.
Dr. Elena and his colleagues base their argument on the bizarre biology of viroids, which are nothing more than naked loops of RNA. Viruses, by contrast, package their genetic material in a protein shell.
A viroid contains astonishingly little genetic material. DNA and RNA are made from building blocks called nucleotides. All the genetic material in a viroid may total less than 400 nucleotides. A flu virus is gigantic by comparison, with 14,000 nucleotides; the human genome contains 3.2 billion pairs of nucleotides.
As scant as the viroid’s genome may be, it’s enough for reproduction. The first step is to slip into a plant, usually through a wound (pruning shears can spread viroids from flower to flower, for example). Once inside a cell, the viroid tricks the host into making new copies of its genes.
As the new strand of RNA grows, it cuts itself, creating a newly liberated strand of RNA. The strand then loops into a circle — a new viroid.
To Dr. Elena and his colleagues, that suggests that viroids behave exactly as the organisms in an RNA world might have — their RNA carries genes and also performs a chemical task. That viroids might be holdovers from an ancient, almost mythical time seems more logical to Dr. Elena and his colleagues than to think these odd organisms recently evolved.
It’s unlikely, for example, that viroids are just viruses that lost their shells. “The evolutionary origins of viruses and viroids are totally different,” said Ricardo Flores, a biologist also at the National Research Council and co-author on the new paper.
Originally, Dr. Flores and Dr. Elena argue, the ancestors of today’s viroids were free-living organisms. But when DNA-based life emerged and conquered the world, viroids evolved into parasites. They started taking advantage of the cellular machinery of their DNA-based hosts, using it to churn out new viroids.
But it’s a long road from the primal ooze to today’s potatoes and dahlias. If viroids really did first evolve billions of years ago, then they ought to infect more species than domesticated plants.
Dr. Flores says he thinks this gap has more to do with science than with nature. Scientists notice viroids only when they harm the plants we raise. If researchers looked beyond farms, they might find new species.
Wild plants would be a good place to start, Dr. Flores said. It’s possible that wild plants serve as a reservoir where viroids can lurk, spilling over from time to time onto farms.
But viroids may be lurking elsewhere, too. The ancestors of plants gained the ability to capture sunlight by swallowing up photosynthetic microbes. Dr. Flores speculated that these microbes, called cyanobacteria, may have been ancient hosts for viroids, eventually passing them on to plants.
“I wouldn’t be surprised if viroids are found in cyanobacteria,” Dr. Flores said. It would be a remarkable discovery, a link in a chain connecting the food on our tables to the dawn of life.