Microbes include fungi, bacteria and viruses. Farmers and ranchers often think of microbes as pests that are destructive to their crops or animals (as well as themselves), but many microbes are beneficial. Soil microbes (bacteria and fungi) are essential for decomposing organic matter and recycling old plant material. Some soil bacteria and fungi form relationships with plant roots that provide important nutrients like nitrogen or phosphorus. Fungi can colonize upper parts of plants and provide many benefits, including drought tolerance, heat tolerance, resistance to insects and resistance to plant diseases.
Viruses are almost always thought of as agents of disease. This is because the ones that cause disease are the ones that have been studied. We have been looking for viruses in wild plants from the Nature Conservancy's Tall Grass Prairie Preserve in northeastern Oklahoma. About half the plants have viruses, but most don't seem to be sick at all. The viruses seem to be living in the plants without doing any harm.
Recently we stressed some plants that were infected with viruses by not watering them. This was part of another experiment, but, to our surprise, all of the plants infected by viruses were much more tolerant of drought. The plants included 10 different species, and we used four different viruses. In all cases, the virus-infected plants did much better under drought stress.
Drought-stressed rice plants after six days without water. The plant on the right is infected with Brome mosaic virus; the one on the left is "healthy" (i.e., virus free).
We also found that viruses can benefit more complicated relationships. In Yellowstone National Park, soil temperatures can get pretty hot in the geothermal areas, but some plants can grow very well in these places with soil temperatures of 115°F. One plant that tolerates the heat is hot springs panic grass. A few years ago, other researchers found that the plant was colonized by a fungus. Without the fungus, the plant could not tolerate the heat. We further found that there was a virus in the fungus. When we were able to cure the fungus of its virus, it could still colonize the plants, but it no longer conferred tolerance to heat. When we reintroduced the virus, it restored heat tolerance.
These studies are making us think very differently about viruses.
There may be many viruses that benefit their hosts and most probably do not harm their hosts. We hope we will find new ways for viruses to benefit agriculture.