Fungi slay insects and feed host plants

For some plants, a nitrogen-rich diet from dead matter in soil just isn’t enough. Researchers from the University of Guelph in Ontario are discovering that these plants also rob nitrogen from the flesh of tiny soil-dwelling insects. But instead of doing the job themselves, the plants rely on fungal partners in crime.

Fungi of the species Laccaria bicolor attach to the roots of the eastern white pine, where they lure tiny insects commonly known as springtails, says study leader John N. Klironomos, a soil ecologist. The fungus then kills the insects, perhaps with a toxin, and sucks up nitrogen from the insects’ bodies to nourish itself. The host plant takes any leftover nitrogen. In return, the plant supplies the fungi with energy-rich carbohydrates.

About 95 percent of plants get nutrients such as nitrogen and phosphorus with help from various types of fungi, says Klironomos. But researchers always believed these fungi were innocuous to other soil organisms, he says. Fungi typically attach to plant roots and then send out fibrous branches, which can infiltrate dead matter. When that happens, the fungi break down the organic materials using enzymes and absorb nutrients.

Klironomos and his coworker Miranda M. Hart discovered L. bicolor‘s insecticidal tendencies while studying the feeding habits of springtails, which normally eat fungi. The researchers were wondering if the insects were having a negative effect on the environment by eating certain species of fungi and indirectly starving plants that depend on such fungi to help them get their nutrients. Klironomos and Hart tested various species of fungi to see which ones the springtails preferred. When they fed the insects L. bicolor, however, almost all of the springtails died.

“Instead of the fungus being preyed on [by the insects], it was the [insects] that were being preyed on by the fungus,” says Klironomos. It was as shocking as putting a pizza in front of a person and having the pizza eat the person instead of vice versa, he says. The researchers report their findings in the April 5 Nature.

L. bicolor first paralyzes the springtails, quite likely with a toxin, the researchers say. Then it extends nutrient-seeking filaments into the insects.

To confirm their chance findings, Klironomos and Hart treated roughly 500 springtails to a helping of L. bicolor. After 2 weeks, only 5 percent of the springtails remained alive. In contrast, all insects that were fed different species of fungi or whose diet was devoid of fungi survived.

To see how much of the plant’s nitrogen came from the insects, the researchers radioactively labeled nitrogen atoms in the insects’ bodies and followed their trails in eastern white pine seedlings. They found that after 2 months, 25 percent of the plant’s nitrogen had come from the insects. “That’s very significant,” says Gopi K. Podila, a molecular biologist at Michigan Technological University in Houghton. “It’s almost like [the plant is] actively seeking nitrogen from the insects and getting it through the fungus.”

The researchers plan on testing other fungus-plant-insect relationships to determine whether this newly found nitrogen-harvesting dynamic is widespread in the plant world.