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 symptoms.

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 viral RNA.

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 tolerance.

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 physiology.

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.