Viruses sometimes save their hosts, rather than killing them
HISTORY casts a long shadow. Many of the first
bacteria to be discovered were agents of disease, and that is how most people
perceive bacteria to this day, even though less than 1% of them are pathogens.
Something similar is turning out to be true of viruses, as Marilyn Roossinck of
Pennsylvania State University told the AAAS meeting in Boston. Dr Roossinck
works on plant viruses and she has assembled evidence suggesting a lot of such
viruses are harmless to their hosts, and in some cases may actually be beneficial.
That has implications for biology. It also has implications for agriculture.
Plant viruses come in two varieties: acute and
persistent. Acute viruses pass from plant to plant, and often cause
recognisable symptoms of disease. Persistent viruses are passed by a plant to
its offspring in its seeds, rather than from one grown plant to another. They
persist at low levels in a plant and rarely cause recognisable adverse
Scientific research concentrates on acute
viruses, for obvious reasons. But this has created an impression that most
plant viruses are acute. Dr Roossinck suspected that was not true and decided
to find out. Using the tools of modern genetics, she searched thousands of
plant species in two locations (one in Oklahoma and one in Costa Rica) for
Like its cousin, DNA, RNA molecules are long
strands. Unlike DNA, which is usually double-stranded, RNA is usually
single-stranded. Except in viruses, where it also has two strands. That makes
viral RNA easy to isolate and identify, which Dr Roossinck did.
She found thousands of new viruses in her trawl,
and their mixture was strikingly different from that of known plant viruses.
The viral world is divided into families, and, as far as is known, all members
of a given family are either acute or persistent. Though most of the viruses in
Dr Roossinck’s net were new to science, their RNA gave away which family they
belonged to. Around half, it turned out, were persistent viruses. Previous data
had suggested that figure was less than 1%.
What benefits these viruses might confer is hard
to determine, because in most cases all members of a plant species are
infected, so no virus-free individuals exist to make the comparison. But Dr
Roossinck has come across some examples of viruses that do help their hosts.
One discovery was the result of an experiment
that attempted to use a virus to smuggle a gene into a plant, called Nicotiana
benthamiana, that is widely used in botanical experiments. She and her
colleagues found by chance that the virus conferred resistance to drought on
this plant, and further experiments with a related virus showed that was true
of 15 other plant species, too.
These viruses, admittedly, are acute. But she
also has an example of a beneficial persistent virus. Her examination of a
grass species growing in the hot and hostile environment of a geyser field in
Yellowstone park showed that its heat tolerance was conferred by a virus that
lives in a fungus which is, itself, symbiotic with the grass: no virus, no heat
Dr Roossinck is now doing experiments in Costa
Rica to look at virally induced heat tolerance in a range of plants and hopes
to extend these in order to disentangle the advantages to their hosts of other
sorts of persistent viruses. That would help to illuminate a view which is held
by a rapidly increasing number of biologists, that many creatures (humans
included) rely on symbiosis, rather than being self-sufficient. In the human
case the symbionts are gut bacteria that help to process food, and also to regulate
Dr Roossinck’s work may have applications, too.
Plant breeders and genetic engineers have been trying for years to confer
drought-resistance on crops. Her studies suggest they are using the wrong
approach. Instead of trying to improve the crops’ own genes they should be
looking at the crops’ viruses and, paradoxically from the traditional point of
view about viruses, actually infecting plants with new viral strains rather
than doing everything in their power to keep crops virus-free.
23 February, 2013