Ancient Soil Fungus Forms a Working Partnership With a Plant Thought Unable to Have One

For decades, plant biologists have believed that one large family of food crops could not form partnerships with underground fungi. A new study from the University of Sheffield says that assumption was wrong. The work, published in Current Biology and led by professor Katie Field, shows for the first time that Arabidopsis thaliana can form a fully functional, resource-sharing relationship with an ancient soil fungus.

Arabidopsis is a small flowering mustard plant and the main genetic model in plant biology. It belongs to the mustard family, Brassicaceae, which also includes major food crops. Members of the family were long thought to be shut out of fungal symbioses. Millions of years ago they lost genes that make up the common symbiosis signaling pathway, the genetic lock-and-key system most land plants use to pair with beneficial soil fungi. Earlier attempts to force fungal partnerships mostly failed, and scientists concluded the natural network was permanently closed to the whole family.

The Sheffield team found a way around it. The fungus they used is a Mucoromycotina fine root endophyte, or MFRE, which belongs to a different lineage that diverged about 620 million years ago. Because of that ancient split, MFREs bypass the conventional signaling route, giving them a different way to communicate with plants.

Using advanced cell imaging and multi-isotope tracing, the researchers showed that a wild strain, isolate Lyc-1, extensively colonized Arabidopsis roots without hurting plant growth or causing disease. The trade went both ways. The fungal network took up nitrogen and phosphorus from the surrounding soil and delivered them directly into the plant's shoots. In return it received more than 260% more plant-fixed carbon.

"This discovery is incredibly exciting because it challenges one of the long-standing assumptions in plant biology," Field said. "We've found that a fungus can form a functional, mutually beneficial partnership with a plant thought to be incapable of such relationships, and the evidence suggests it does so in a fundamentally different way from the fungi we know best." She said it raises the possibility that scientists have been overlooking an important source of natural support for plants, including some of the most important crops.

The finding also gives researchers a powerful tool. Scientists already know a great deal about Arabidopsis genetics, development and interactions with pathogens, which makes the plant paired with MFRE an ideal model for finding the molecular mechanisms behind this alternative symbiosis. One co-author called it a chance to "rapidly identify" those mechanisms.

The practical goal is farming. Professor Jurriaan Ton said that taking the symbiosis "from the lab to the field could offer a nature-based way to improve crop nutrition and resilience, supporting soil health and regenerative farming while reducing reliance on expensive chemical fertilizers." The paper, by James Prout and colleagues, is titled "Mucoromycotina fine root endophyte engages in a nutritionally mutualistic symbiosis with Arabidopsis thaliana." No field trials have been reported yet.