History of Research at the
U.S. Department of Agriculture and Agricultural Research Service
ARS plant pathologist Theodor O. Diener discovered a cell-invading plant
pathogen 80 times smaller than a virus-the viroid.
accepted scientific dogma, the discovery of the viroid was not supposed to
Finding out what causes potato spindle tuber
disease brought about a small revolution in the study, diagnosis, and treatment
of viral plant diseases. It also helped change approaches and attitudes in the
study of livestock and human diseases.
But as crop diseases go, it wasn't very
important. It didn't cost potato farmers millions of dollars in losses or
control measures. Of course, if it got into a potato crop, it led to a
second-year harvest of spindly, twisted tubers, but that didn't happen
Still, the disease made potato breeders
nervous. For all they knew, the disease could sweep through their stock,
damaging all the breeding potatoes in one year. And they wouldn't know why or
be able to do much about it.
And it nagged at plant pathologists. They
couldn't figure out what agent caused the disease. After eliminating all the
other possibilities, they concluded that it was some kind of virus, even though
it didn't behave the way a virus should.
But it wasn't a virus. It was something
Potato spindle tuber and at least 15 other
crop diseases are caused by viroids, an entity that nobody had ever heard of
before 1971, its official date of discovery. Theodor O. Diener, the
Agricultural Research Service plant pathologist who discovered the pathogen,
named it the viroid, because it is like a virus.
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Like a virus,
the viroid invades a cell and hijacks its reproductive mechanisms. It forces
the cell to duplicate the viroid's RNA instead of its own. The viroid has no
DNA. RNA and DNA are nucleic acids, the molecules of heredity; with the
exception of viroids and some viruses, all genes are made of DNA.
The difference between viroids and RNA
viruses is that viroids have no protective protein coat. The scientific dogma
in 1971 was that an organism with no protein wasn't supposed to be able to
replicate itself, even with a host cell's help. And an entity as small as the
PSTV (potato spindle tuber viroid)130,000 daltonswasn't supposed to
be able to infect anything, even a potato.
Until that time, scientists believed that
the minimum weight necessary for infectivity was about 1 million daltons. (A
dalton, also called an atomic mass unit, equals one-twelfth the mass of a
Diener wasn't much impressed by scientific
dogma. He'd seen it turned upside down too many times. But he was very careful
to prove that the viroid really existed. In all, it took him 6 painstaking
Genius, said Thomas Edison,
is one percent inspiration and ninety-nine percent perspiration.
The perspiration spent in pursuit of the viroid came in years of preliminary
work by William B. Raymer, Diener, and many associates.
Raymer, also a plant pathologist, was at the
ARS Potato Diseases Investigation Laboratory in Beltsville, Maryland, during
the early 1960's. It was Raymer who began the project that eventually1ed to the
discovery of the viroid.
most important lesson to be gleaned from the discovery of the viroid is the
importance of freedom for research scientists to follow leads when they become
evident, rather than be tied down by too narrow a position description and
Russell L. Steere, ARS botanist (retired),
It was Raymer
and fellow ARS plant pathologist Muriel O'Brien who came up with a breakthrough
in convenience in finding the cause of potato spindle tuber diseasea
simple bioassay for the infectious agent.
Since spindle tuber takes a couple of years
to show up in potatoes, results of many tests were excruciatingly slow to come.
But Raymer and O'Brien found that the unknown pathogen was easily transmitted
in tomatoes. Within 2 weeks, a growing tomato plant became dramatically
Now they could get lots of diseased leaves
quickly. High-speed centrifugation, a standard method to purify viruses, would
surely turn up the virus, thought Raymer and O'Brien. Bottling the
virus, capturing it, seemed imminent.
Not exactly. The standard method produced
such low amounts of infection that it was clear the cause of potato spindle
tuber disease was not a typical virus.
Baffled, Raymer went to Diener, who had
recently joined the new Plant Virology Pioneering Laboratory, one of 16
pioneering labs set up by ARS to define the laws and principles of basic
problems in agriculture.
Russell L. Steere, botanist and chief of the
lab, would later say that perhaps the most important lesson to be gleaned
from the discovery of the viroid is the importance of freedom for research
scientists to follow leads when they become evident, rather than be tied down
by too narrow a position description and predetermined goals.
For a year after they teamed up in 1965,
Raymer and Diener gradually put to rest the notion that potato spindle tuber
disease was caused by a virus. They tried a different form of centrifugation,
developed by Myron K. Brakke, an ARS chemist.
This lower leaf of Etrog citron, an indicator host, shows typical symptoms of
viroid infection-necrotic cells on the midvein and petiole, where the leaf
attaches to the stem.
gradient centrifugation technique showed that the pathogen was small and light.
So it was unlikely, says Diener, that the agent was a conventional viral
nucleoprotein. It appeared more likely that this material was a free nucleic
Procedures in enzyme chemistry were next.
Diener and Raymer treated extracts of diseased tomato leaves with an enzyme
that chews up RNA. With RNA removed from the extracts, the scientists
discovered that the treated extract failed to reinfect healthy tomato plants as
it had before the enzyme treatment.
RNA in the agent was clearly important. But
treatment with enzymes that remove DNA or protein made no difference; neither
changed the pathogen's ability to infect tomato plants.
The results told Diener and Raymer that the
essential ingredient of the spindle tuber agent was RNA and that it contained
At this point, in 1966, Raymer left for a
job in private industry. Diener spent the next 5 years isolating and
characterizing the viroid, verifying his experiments, filling in the holes,
preparing to meet the skepticism that generally greets proposals of new,
His concept did meet some resistance,
chiefly from animal virologists and medical researchers unfamiliar with his
earlier work, but his carefully prepared evidence was overwhelming. And as is
often the case, another scientist working on another disease came op with a
similar proposal at roughly the same time.
In 1975, Diener was co-recipient of the
Alexander von Humboldt Award, which is presented each year for the most
significant contribution to agriculture or the agricultural sciences for the
past 3 to 5 years.
"Tracking the Elusive
Viroid" was published in the May 1989 issue of
Agricultural Research magazine.
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