AM Mushroom Research

Researchers know that a type of fungus called arbuscular mycorrhizal fungi (AM) establishes symbiotic relationships with the roots of 70% of all land plants. In this relationship, plants exchange fatty acids for nitrogen and phosphorus from fungi. However, AM fungi lack the enzymes necessary to release nitrogen and phosphorus from complex organic molecules.

A trio of BTI scientists led by Maria Harrison, the William H. Crocker Professor at BTI, wondered if other microbes in the soil could help fungi access those nutrients. In a first step to examine that possibility, the team investigated whether AM fungi associate with a specific community of bacteria. His research appeared on March 1 in The ISME Journal.

Bacterial communities

The team examined bacteria that live on the surfaces of long filament-like structures called hyphae, which fungi spread into the ground away from their host plant. In hyphae of two species of fungi, the team discovered very similar bacterial communities whose composition was different from that of the surrounding soil.

“This tells us that, like the human gut or plant roots, AM fungal hyphae have their own unique microbiomes,” said Harrison, who is also an adjunct professor in the College of Integrative Plant Sciences. Cornell Agriculture and Life Sciences. . “We are already testing some interesting predictions about what these bacteria might do, such as help with phosphate acquisition.

Bacteria that can increase crop yield

"If we are correct, enriching the soil for some of these bacteria could increase crop yields and ultimately reduce the need for conventional fertilizers along with their associated costs and environmental impacts," he added.

His co-investigators on the study were former BTI scientists Bryan Emmett and Véronique Lévesque-Tremblay.

In the study, the team used two species of AM fungi, Glomus versiforme and Rhizophagus irregularis, and grew them in three different types of soil in symbiosis with Brachypodium distachyon, a species of grass related to wheat. After letting the fungus grow with the grass for up to 65 days, the researchers used gene sequencing to identify bacteria that adhere to the surfaces of the hyphae.

Two species of mushrooms

The team found remarkable consistency in the composition of the bacterial communities of the two fungal species. Those communities were similar in all three types of soil, but very different from those found in the soil away from the filaments.

The function of these bacteria is not yet clear, but their composition has already raised some interesting possibilities, Harrison said.

"We predict that some of these bacteria release phosphorus ions in the immediate vicinity of the filaments, giving the fungus the best chance of capturing those ions," Harrison said. "Learning which bacteria have this function could be key to improving the phosphate acquisition process of fungi for the benefit of plants."

Chain performance

Harrison's group is investigating the factors that control which bacteria assemble on the filaments. Harrison believes that AM fungi can secrete molecules that attract these bacteria, and in turn, bacterial communities can influence the molecules that the fungus secretes.

Among the hyphal microbiomes were members of Myxococcales and other taxa that include “bacterial predators” that kill and eat other bacteria by bursting them and releasing their contents.

These predators move by gliding along surfaces so that "the fungal filaments can serve as linear feeding lanes," said Emmett, who is currently a research microbiologist with the US Department of Agriculture's Agricultural Research Service. Soil appear to travel along fungal hyphae in the soil, and these predators can make the journey more dangerous."

Predators in filaments

While not all members of those taxa in the filaments may be predators, Harrison's group plans to investigate how and why those potential predators congregate there. "It is possible that the actions of predatory bacteria make mineral nutrients available to everyone in the surrounding soil, both predators and fungi," he said.

The research work was supported by the United States Department of Energy.