April 14, 2009

Bacteria protect soybeans from aphids

U.S. entomologists say a careful choice of nitrogen-fixing bacteria might provide soybean farmers protection against an invasion of soybean aphids.

Pennsylvania State University researchers said soybeans are legumes -- plants that can have a symbiotic relationship with nitrogen-fixing bacteria called rhizobia and therefore do not need additional nitrogen fertilizer. Each type of legume -- peas, beans, lentils, alfalfa -- have their own rhizobia.

Soybeans are from Asia and so there were originally no nitrogen-fixing bacteria that would colonize soybeans in U.S. soils, said Associate Professor Consuelo De Moraes. The rhizobia had to be transferred here.

The soybean aphid, which is also not native to North America, began to infest U.S. soybean fields about 10 years ago, the researchers said. But they are now fully established pests requiring pesticide applications to avoid the loss of as much as 40 percent of a crop.

The researchers investigated the relationship between the type of rhizobia colonizing soybean plants and the plants' infestation with the aphids.

Our results demonstrate that plant-rhizobia interactions influence plant resistance to insect herbivores and that some rhizobia strains confer greater resistance to their mutualist partners than do others, the researchers said, noting they do not yet know what the natural nitrogen-fixing bacteria do to repel aphids.

The study is detailed online in the journal Plant and Soil.