Researchers have studied viruses as agents of disease in humans, domestic animals and plants, but a study of plant viruses in the wild may point to a more cooperative, benevolent role of the microbe, according to a Penn State virologist.

"Most of these wild plants have viruses," said Marilyn Roossinck, professor of plant pathology and environmental microbiology and biology, who has examined more than 7,000 individual plants for viruses. "But they don't have any of the symptoms that we usually see in crop plants with viruses."

Most of the viruses Roossinck studied are new viruses, although they are related to viruses that have been examined in crops. According to the researcher, about half of the viruses that infect wild plants tend to be continually present in the plant -- persistent.

The viruses get passed from plants to their offspring through the seeds. Researchers are still trying to uncover exactly what viruses are doing in the plants. Since the viruses are found so often, they may be playing some role in the life of the plant, according to Roossinck.

In fact, studies indicate that viruses can be beneficial to some plants, making them hardier and helping them survive extreme temperatures and drought, said Roossinck, who reported on her research at the annual meeting of the American Association for the Advancement of Science.

"When most people think of viruses, they think of serious diseases and death, such as the AIDS virus," Roossinck said.

However, on a research trip in Costa Rica, a biodiversity hot spot in Central America, she noticed that unmanaged wild plants looked healthier than managed agricultural fields.

During her research, Roossinck observed that most of the approximately 10,000 species of wild plants at the study site appeared healthy. The wild plants she studied included Fabaceae, an abundant family of plants related to beans. However, commercial crops -- melons, oranges, pineapple and aloe -- that were growing near the site were not as healthy.

"When I went to the forest, the wild plants looked healthy and gorgeous," Roossinck said. "Then, I went 10 kilometers away and the plants in the agricultural field were not looking so healthy. In the forest the plants are full of microbes: viruses, fungi and bacteria, whereas in crops farmers try to eliminate the microbes. Perhaps there is a connection."

Indeed, one plant virus that was found frequently in the forest was also found in nearby melon crops. In the melons it was causing severe disease, while in the wild plants there were no symptoms. Analyzing the viruses suggested that they were moving from the crops into the wild plants, but somehow the wild plants remained healthy.

Roossinck said she is curious about how the wild plants avoid disease, and if there is a way this can be used in agriculture.