Researchers find symbiotic relationship between fungi, plants and bacteria driving plant health

In Wisconsin, Jean-Michel Ané, a professor of agronomy at the University of Wisconsin-Madison said that new evidence is mounting that fungi and bacteria are cooperating in a symbiotic relationship in order to unlock the benefits of Mycorrhizae, a beneficial fungi that help virtually all land plants absorb the essential nutrients — phosphorus and nitrogen — from the soil. Biologists now believe this ubiquitous mechanism began about 450 million years ago, when plants first moved onto land.

It’s known that disease-causing fungi build a structure to break through the plant cell wall, “but there is growing evidence that fungi and also bacteria in symbiotic associations use a mechanical stimulation to indicate their presence,” says Ané. “They are knocking on the door, but not breaking it down.”

After the fungus announces its arrival, the plant builds a tube in which the fungus can grow. “There is clearly a mutual exchange of signals between the plant and the fungus,” says Ané. “It’s only when the path is completed that the fungus starts to penetrate.”

Mechanical signaling is only part of the story — microbes and plants also communicate with chemicals, says Ané. “So this is comparable not to breaking the door or even just knocking on the door, but to knocking on the door while wearing cologne. Clearly the plant is much more active than we thought; it can process signals, prepare the path and accept the symbiont.”

Beyond fungi, some plants engage in symbiosis with bacteria called rhizobia that “fix” nitrogen from the atmosphere, making it available to the plant. Rhizobia enable legumes like soybeans and alfalfa to grow without nitrogen fertilizer. When Ané and his colleagues looked closer, they found that rhizobium symbiosis also employs mechanical stimulation. When the bacterium first contacts a root hair, the hair curls around the bacterium, trapping it. The phenomenon of curling has been known for almost 100 years. “But why would nature develop such a complicated mechanism to entrap a bacterial colony?” Ané asks. “We propose the purpose is to apply mechanical stimulation” so the plant will start building a home for the rhizobium — for mutual benefit. “We have preliminary evidence that when the entrapment is not complete, the process of colonization does not happen,” he says.

The new knowledge around symbiotic relationships may help researchers develop new tools for develop the fixation of atmospheric fixation in other plants — thereby reducing a major and costly input for plants in ammonia-based fertilizers generated from fossil fuels.