Fungus Turns Plant Sugar Into a Microbial Recruitment Tool to Boost Roots

In the crowded underground world of the rhizosphere, plants, fungi, and bacteria are locked in constant chemical negotiation. A new study published in the journal Microbiome has now decoded one of these negotiations in remarkable molecular detail, revealing how a plant-beneficial fungus acts as a kind of ecological broker, taking sugar from a plant’s roots and converting it into a currency that pays the right bacterial partners to move in. The fungus, Gongronella butleri strain w5, belongs to the Mucoromycotina, an ancient lineage of fungi that has long been associated with helping plants cope with nutrient-poor soils. What makes the new findings striking is that the fungus delivers its benefits even in soil that is already rich in phosphorus, a condition in which most conventional explanations of fungal benefit simply do not apply.

The research team, led by Li Li and Junnan Fang of Anhui University in Hefei, China, together with colleagues including corresponding authors Xiaojie Wang and Zemin Fang, set out to test whether w5 could promote the growth of tomato plants, Solanum lycopersicum, when phosphorus was not limiting. This question matters because the dominant narrative in soil ecology has centered on nutrient mobilization: fungi help plants by unlocking phosphorus, nitrogen, or iron that roots cannot access on their own. If a fungus can boost plant growth without changing nutrient availability at all, something else must be going on. The team combined greenhouse experiments, soil microcosm analyses, high-throughput DNA sequencing, functional assays, and a sophisticated genetic technique called host-induced gene silencing to chase down the answer.

The results were unambiguous. When tomato plants were inoculated with w5, they grew significantly larger, developed more extensive root systems, and showed improved photosynthetic efficiency compared with uninoculated controls. Crucially, the fungus did none of this by altering the chemical availability of nutrients in the soil. Instead, sequencing of the rhizosphere microbiome revealed that w5 inoculation reshaped the community of bacteria living around the roots, selectively enriching taxa capable of producing indole-3-acetic acid, or IAA, the most important naturally occurring auxin hormone in plants. Auxins are the molecular signals that drive root elongation, branching, and the formation of lateral roots, so a rhizosphere teeming with IAA producers is effectively a growth-promoting environment.

From the hyphosphere, the narrow zone of soil immediately surrounding fungal filaments, the researchers isolated two key bacterial players: Bacillus megaterium and Bacillus zanthoxyli. Both proved to be prolific IAA producers, and tripartite interaction assays demonstrated that the growth benefits to tomato seedlings depended on the combined activity of the fungus and these bacterial partners. The picture that emerged was not one of a fungus acting alone but of a three-way partnership in which each participant contributes something the others need: the plant supplies carbon, the fungus supplies a metabolic conversion service, and the bacteria supply the hormonal signal that stimulates root development.

The central discovery of the study lies in how the fungus pays its bacterial collaborators. Using tracer experiments and functional assays, the team showed that w5 imports sucrose, the primary sugar that plants transport through their phloem and leak into the rhizosphere, through a fungal sucrose transporter named GspSUT1. Once inside the fungal cells, the sucrose is hydrolyzed, or split, into its two component hexose sugars, glucose and fructose. The fungus then releases these hexoses back into the surrounding soil. This conversion is far from trivial. Sucrose is a relatively poor carbon source for many rhizosphere bacteria, whereas glucose and fructose are among the most readily consumed sugars in nature. By inverting the sugar, the fungus effectively transforms a plant payment that few bacteria can cash into one that many can, and in doing so it creates a metabolic niche that favors fast-growing, IAA-producing colonizers.

The researchers put this mechanism to a direct test using host-induced gene silencing, a technique that allows a plant’s RNA interference machinery to be aimed at a gene in an associated organism. When the fungal GspSUT1 transporter was silenced, the consequences cascaded through the entire partnership. Hexose secretion into the rhizosphere dropped, the enrichment of IAA-producing bacteria weakened, bacterial IAA production declined, and the synergistic promotion of plant growth collapsed. This loss-of-function experiment is the strongest evidence in the study that the sucrose-to-hexose conversion is not a side effect of fungal metabolism but the load-bearing pillar of the entire interaction. Remove the transporter, and the broker loses its ability to pay its partners.

The authors describe w5 as acting like an ecological invertase, a reference to the enzyme that cleaves sucrose into glucose and fructose. The metaphor captures the essence of the mechanism: the fungus does not create new carbon, nor does it fix nutrients from the air or rock. It reallocates carbon that the plant has already exported, changing its chemical form to reshape which microbes thrive. This carbon-hormone coupling, in which fungal sugar processing enriches hormone-producing bacteria, is what ultimately improves root architecture, photosynthetic capacity, and nutrient uptake in the host plant. Better roots feed the plant more efficiently, which in turn sustains the carbon flow that keeps the partnership running, a positive feedback loop established by a single fungal transporter.

Why does this matter for agriculture? Most biofertilizer research has focused on fungi and bacteria that mobilize nutrients in deficient soils, and on arbuscular mycorrhizal fungi that trade phosphorus for carbon in classic symbioses. The new study demonstrates that Mucoromycotina fungi possess a functional plasticity that operates independently of soil nutrient status, meaning their value may extend to the intensively fertilized soils that dominate modern agriculture. In phosphorus-rich fields, where adding more nutrient-mobilizing microbes would accomplish little, a fungus that engineers the microbial community through carbon reallocation could still deliver measurable gains in root development and plant vigor. The work also suggests that the composition of the rhizosphere microbiome can be steered not only by adding microbes but by managing the carbon chemistry that determines which microbes can compete.

The findings also carry evolutionary weight. Mucoromycotina fungi are among the earliest diverging lineages of terrestrial fungi, and their associations with plants are thought to predate the arbuscular mycorrhizal symbioses that dominate modern land plants. Evidence that a member of this ancient lineage can actively farm its bacterial surroundings, converting plant sugar into a selective subsidy for beneficial partners, hints that carbon-mediated microbiome engineering may be a deeply rooted fungal strategy rather than a recent innovation. The study reframes the rhizosphere not as a passive meeting ground but as a marketplace in which a fungus can control the terms of trade.

Practical applications are likely to follow. The researchers suggest that fungi like w5 could serve as components of synthetic biofertilizers, formulations designed to assemble beneficial microbial communities around crop roots rather than simply delivering nutrients. Because the mechanism depends on a defined transporter gene, GspSUT1, it also offers a molecular marker for screening other fungal strains for similar capabilities, and potentially a target for breeding or engineering crops and their fungal partners for stronger partnerships. The work was supported by the National Natural Science Foundation of China and the Natural Science Research Project of Higher Education of Anhui Province. As agriculture searches for ways to sustain yields while reducing chemical inputs, the image of a fungus quietly converting root sugar into a bacterial recruitment bonus may prove to be one of the more consequential discoveries in the effort to manage the hidden half of crop production.

Subject of Research: How the fungus Gongronella butleri w5 reallocates plant-derived sucrose to enrich IAA-producing rhizosphere bacteria and promote root development in phosphorus-rich soil

Article Title: The plant-beneficial fungus Gongronella butleri w5 reallocates plant-derived sucrose to enrich IAA-producing bacteria to promote plant root development in phosphorus-rich soil

Article References: The plant-beneficial fungus Gongronella butleri w5 reallocates plant-derived sucrose to enrich IAA-producing bacteria to promote plant root development in phosphorus-rich soil. (n.d.). https://doi.org/10.1186/s40168-026-02543-1

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02543-1

Keywords: Gongronella butleri, rhizosphere microbiome, indole-3-acetic acid, sucrose transporter, Mucoromycotina, Bacillus megaterium, plant growth promotion, host-induced gene silencing, carbon metabolism, biofertilizer, tomato, root development