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# Blurred lines in the mycorrhiza world

Juliana Almario

► To cite this version:

Juliana Almario. Blurred lines in the mycorrhiza world. Nature Reviews Microbiology, 2025, 23 (10), pp.619-619. 10.1038/s41579-025-01215-5. hal-05380066

HAL Id: hal-05380066

https://hal.science/hal-05380066v1

Submitted on 24 Nov 2025

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# Journal club

Strapline

## Blurred lines in the mycorrhiza world

In middle school we learn that the soil teems with tiny animals, bacteria and fungi. These organisms decompose plant and animal debris, releasing nutrients that return to the plants and fuel their growth. However, in university-level Soil Microbiology classes, we begin to grasp the incredible complexity of the biogeochemical processes behind plant nutrient uptake from the soil. We delve into the roles of detritivores, decomposers and organic matter turnover in soil. Most importantly, we discover the mighty mycorrhizal fungi, which scavenge nutrients from soil, transport them along their long hyphae and deliver them directly to plants. A key lesson is that these nutritional exchanges are possible only because these symbiotic fungi form specialized structures within plant tissues. These structures allow the translocation of nutrients from fungal cells to plant cells, in exchange for carbon coming from the plant's photosynthesis. Mycorrhizae were first discovered in the 1880s and since then a wide diversity of associations have been described, with 4-5 broad classes recognized, including, for example, the arbuscular mycorrhizal and ectomycorrhizal symbioses. Yet, over the last 15 years, a growing number of studies have been challenging this view, notably by questioning what differentiates a "true" mycorrhizal fungus from other root-colonizing fungi that also facilitate plant nutrition.

In my case this questioning was fuelled by a ground-breaking study published in 2012 by the group of Prof. Michael Bidochka at Brock University in Canada, titled "Endophytic insect-parasitic fungi translocate Nitrogen directly from insects to plants". Metarhizium fungi from the Ascomycota phylum, have long been known as insect-pathogenic fungi and have been used as commercial biocontrol agents for over 20 years. Prof. Bidochka's group had been studying the genetics of these fungi when they observed that many were also plant root endophytes, colonizing diverse plant species across various ecosystems. This suggested that these fungi were living a "double life", both as an insect-killing fungi and as plant root colonizers. Given that insects are primarily protein-rich, containing abundant nitrogen (N), and that N is often a limiting nutrient in the rhizosphere, the authors hypothesized a mechanism by which plants could acquire N from insects via these fungi. Although this might seem like a straight-forward idea now, at the time it challenged

the long-standing paradigm that only mycorrhizal fungi could transfer nutrients to plants, and thus that fungus-to-plant nutrient transfers were the exclusive hallmark of mycorrhizal associations.

To put their idea to test, the authors imagined a simple but elegant experiment inspired from mycorrhizal research. They injected isotopically-labelled nitrogen (¹⁵N) into fungus-inoculated larvae cadavers, which they then buried in the soil near the plant roots. A mesh barrier was added to prevent direct uptake of ¹⁵N by plant roots in case of ¹⁵N leakage from the larvae. As suspected, after only 2 weeks, plants had obtained 30% of their N via fungal delivery of insect N, and this increased to as much as 50% after 4 weeks. Considering the enormous amount of N potentially available in the 200 Megatons of insect biomass buried beneath our feet, these fungi might represent a key pathway by which plants acquire N from the soil.

More striking, in a different study the authors went on to prove that in return the fungus was receiving carbon back from the plant, mirroring the classical description of plant-fungus mycorrhizal associations. Through these ground-breaking discoveries, this study exemplifies how the lines that differentiate a mycorrhizal fungus from a fungal root endophyte are getting blurred. The fact that fungus-to-plant nutrient transfers occur beyond "classical" textbook mycorrhizal associations, should invite us to reconsider more broadly the importance of fungi (and not only described mycorrhizal fungi) for nutrient flow from soil to roots.

Juliana Almario
The Microbial Ecology Laboratory, CNRS, University of Lyon, Lyon, France
e-mail: juliana.almario@univ-lyon1.fr

**Competing interests**

The author declares no competing interests.

**ORIGINAL ARTICLE** Behie, S. W. et al. Endophytic insect-parasitic fungi translocate nitrogen directly from insects to plants. *Science* https://doi.org/DOI: 10.1126/science.1222289 (2012)

**ORIGINAL ARTICLE** Behie, S. W. et al. Carbon translocation from a plant to an insect-pathogenic endophytic fungus. *Nature Communications* https://doi.org/DOI: 10.1038/ncomms14245 (2017)

**Pull quote:** this study exemplifies how the lines that differentiate a mycorrhizal fungus from a fungal root endophyte are getting blurred.