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 plant 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 past 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 groundbreaking study published in 2012 by the group of 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 more than 20 years. 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 insect-killing fungi and as plant root colonizers. Given that insects are primarily protein-rich, containing abundant nitrogen, and that nitrogen is often a limiting nutrient in the rhizosphere, the authors hypothesized a mechanism by which plants could acquire nitrogen from insects through these fungi. Although this might seem like a straightforward 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.
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