Prof Marilyn Roossinck,
Professor, Plant Pathology and Environmental Microbiology, and Biology, Penn State University and Sir Walter Murdoch Adjunct Professor, Murdoch University, Perth, Western Australia
Viruses were first discovered in plants, over 100 years ago. The plants were tobacco, and they were displaying a mosaic of light and dark green areas on their leaves, that could be transmitted by rubbing the sap from an infected plant onto a healthy plant. It turned out that the infectious agent could still be found in sap that had passed through a very small filter that removes bacteria. This agent was named a virus from the Latin word for poison, and we now know this as Tobacco mosaic virus. Since then hundreds of plant viruses have been described that cause disease in many crops. They can cause symptoms like stunting, mosaics, necrosis (dead patches), bumpy leaves (called rugose), streaking, leaf curling, etc. Just as in humans and other animals, most people think of plant viruses as bad! However, this is not an accurate picture of plant viruses. In studies of viruses found in wild plants, most don’t cause any symptoms, but seem to live in harmony with their plant hosts. Are they doing anything, or just “hanging out”?
It turns out that benign viruses are not just in wild plants. Many of the foods you eat have viruses that we call “persistent” because they are passed down from one generation to the next, and seem to stay with their plants indefinitely. Peppers (bell and jalapeño) have these, as do beans, radishes, rice, and probably many more crops (they just haven’t been looked for). In at least one plant, white clover, a persistent virus benefits the plant by preventing it from making nodules when it doesn’t need them. Nodules are found in the roots of legumes and are formed by the plant and a bacteria, and they are the organs that fix nitrogen. If there is already enough nitrogen in the soil the plant doesn’t really need to make the nodules, and the virus suppresses this.
Quite a few highly prized ornamental plants derive their beauty from viruses. The most well known is Tulip breaking virus that is responsible for the vivid and beautiful striped coloring in some tulips. The striped tulips became so sought after in Holland in the 17th century that they ignited the first “economic bubble”.
In a few cases we know that plant viruses are benefitting their plant hosts. For example viruses that can cause disease under normal conditions can make plants more tolerant to drought and cold. Red beets that are generally very sensitive to freezing temperatures can tolerate a night of light frost when infected by a virus. This could extend the growing season in temperate areas.
In addition to the beneficial plant viruses, there are also viruses in fungal endophytes (fungi that colonize plants) that indirectly benefit plants. In Yellowstone National Park plants can grow in very hot soils in geothermal areas if they are colonized by a fungus that is infected with the virus. If you remove any of the players, plant, fungus or virus, they rest cannot survive the hot soils. This virus-infected fungus can be transferred to other plants too, and works well in tomato that is used as a model plant in the lab.