A cross-domain synthetic community in which the ectomycorrhizal fungus Suillus clintonianus, which cannot effectively obtain or synthesize the thiamine (vitamin B1) it needs for growth, recruits the hyphae-associated bacterium Bacillus altitudinis B4 to its mycelial surface. The fungus first secretes ureidosuccinic acid and pregnenolone to recruit the bacterium; the secreted ureidosuccinic acid then stimulates B. altitudinis B4 to increase biomass and upregulate thiamine biosynthesis genes, enhancing thiamine production. The fungus absorbs the bacterially produced thiamine, which promotes fungal growth and increases the ectomycorrhizal colonization rate of the host pine Pinus massoniana. The mechanism, resolved by nontargeted metabolomics and whole-genome sequencing, illustrates a vitamin-mediated tripartite symbiosis in which a prototrophic bacterial partner fulfills a fungal thiamine auxotrophy to support host-fungal mutualism.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Suillus clintonianus | NCBITaxon:1904413 · TaxonMech · TraitMech |
CROSS_FEEDER
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DOMINANT |
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| Bacillus altitudinis B4 — ontology label: Bacillus altitudinis | NCBITaxon:293387 · TaxonMech · TraitMech |
CROSS_FEEDER
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DOMINANT |
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| Pinus massoniana | NCBITaxon:88730 · TaxonMech · TraitMech | DOMINANT | |
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Ecological Interactions
The graph shows only explicitly linked interaction participants. Community members without an asserted participant link are omitted; membership alone does not imply an interaction.
Fungal recruitment of the hyphae-associated bacterium
COLONIZATION_FACILITATIONSuillus clintonianus exudates attract Bacillus altitudinis B4 and support attachment to fungal hyphae. Ureidosuccinic acid promoted chemotaxis, whereas pregnenolone primarily enhanced swimming and swarming. Their separate effects support recruitment without showing that both compounds are equivalent chemoattractants or that no other exudate contributes.
Source Taxon: Suillus clintonianus
Target Taxon: Bacillus altitudinis B4
Metabolites: ureidosuccinic acid (CHEBI:15859 · MediaIngredientMech · CMMMech), pregnenolone (CHEBI:16581 · MediaIngredientMech · CMMMech)
HYPOTHESIZED/PARTIAL: recruitment establishes the close fungal-bacterial association in which fungal exudates can stimulate bacterial thiamine production. The authors propose this sequence, but recruitment was not isolated as a necessary mediator of the cue response; ureidosuccinic acid also acted in the separate bacterial assay.
Evidence
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PMID:41454778 - SUPPORT (IN_VITRO)"S. clintonianus first secreted ureidosuccinic acid and pregnenolone, recruiting the hyphae-associated bacterium B. altitudinis B4 to the mycelial surface"
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PMID:41454778 - SUPPORT (IN_VITRO)"However, pregnenolone could only improve the bacterial motility, including swimming and swarming of B. altitudinis B4"
Ureidosuccinic acid stimulation of bacterial thiamine production
Ureidosuccinic acid associated with S. clintonianus exudates increased B. altitudinis B4 growth, extracellular thiamine and expression of selected thiamine-metabolism genes in the tested assays. This is a measured metabolite response, not demonstrated uptake of ureidosuccinate as a carbon or nitrogen nutrient. The proposed precursor explanation and the benefit received by the bacterial partner remain unresolved.
Source Taxon: Suillus clintonianus
Target Taxon: Bacillus altitudinis B4
Metabolites: ureidosuccinic acid (CHEBI:15859 · MediaIngredientMech · CMMMech)
Biological Processes:
- thiamine biosynthetic process (GO:0009228 · PathwayMech)
Ureidosuccinic acid increased extracellular thiamine in the bacterial assay, supporting a greater vitamin supply available to the fungal recipient. Separate coculture and tracer observations support fungal acquisition of bacterial thiamine; they do not quantify what fraction of endogenous transfer is caused by this one exudate.
Evidence
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PMID:41454778 - SUPPORT (IN_VITRO)"the ureidosuccinic acid secreted by S. clintonianus further stimulated B. altitudinis B4 to enhance thiamine production"
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PMID:41454778 - SUPPORT (IN_VITRO)"However, it remains unclear what benefits the bacteria gain in this consortium."
Thiamine provisioning fulfilling the fungal requirement
CROSS_FEEDINGB. altitudinis B4 supplies thiamine that S. clintonianus acquires under thiamine-deficient conditions. Bacterial supernatant rescued fungal growth, and labeled thiamine derived from bacterial metabolites was detected in fungal hyphae, supporting actual bacterial-to-fungal vitamin transfer. Genomic evidence and vitamin-omission phenotypes support fungal thiamine auxotrophy; the biosynthetic annotation refers to the bacterial donor.
Source Taxon: Bacillus altitudinis B4
Target Taxon: Suillus clintonianus
Metabolites: thiamine (CHEBI:18385 · MediaIngredientMech · CMMMech)
Biological Processes:
- thiamine biosynthetic process (GO:0009228 · PathwayMech)
HYPOTHESIZED/PARTIAL: bacterial thiamine supports fungal growth and is proposed to contribute to the higher ectomycorrhizal colonization observed with B4 in the pot experiment. The vitamin-mediated route is supported across the assays but is not isolated as the exclusive mediator of the in-plant phenotype.
Evidence
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PMID:41454778 - SUPPORT (IN_VITRO)"S. clintonianus absorbed the thiamine secreted by the B. altitudinis B4, promoting fungal growth"
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PMID:41454778 - SUPPORT (IN_VITRO)"13C12-thiamine was detected within the S. clintonianus hyphae following their cultivation with metabolites from B. altitudinis B4"
Enhanced ectomycorrhizal colonization of the pine host
MUTUALISMThe combined S. clintonianus-B4 treatment increased fungal relative biomass and ectomycorrhizal colonization compared with the fungus-only treatment. Pine dry weight, root activity and phosphorus content also increased, supporting a beneficial ectomycorrhizal association rather than colonization alone. B4 alone did not significantly improve seedling performance. The authors interpret this as support for fungal-host mutualism; the pot comparison does not isolate thiamine as the sole mediator or measure bidirectional host-fungal nutrient flux.
Source Taxon: Suillus clintonianus
Target Taxon: Pinus massoniana
Biological Processes:
- biological process involved in interspecies interaction between organisms (GO:0044419 · PathwayMech)
Evidence
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PMID:41454778 - SUPPORT (IN_VIVO)"promoting fungal growth and increasing the colonization rate in association with Pinus massoniana"
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PMID:41454778 - SUPPORT (IN_VIVO)"the dry weight, root activity, and phosphorus content of P. massoniana were significantly greater in the combined treatment compared to the S. clintonianus-only treatment"
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which parts of recruitment and pine improvement are specifically mediated by the identified fungal cues and bacterial thiamine?
Direct thiamine transfer and positive fungal/plant outcomes are retained. The source leaves bacterial benefit and cue mechanism unresolved, and the pot treatment does not isolate thiamine as the only driver. Retain the proposed sequence with these limits, without turning cue exposure into demonstrated nutrient consumption or universal reciprocity.
Attaches to: ecological_interactions#Fungal recruitment of the hyphae-associated bacterium, ecological_interactions#Ureidosuccinic acid stimulation of bacterial thiamine production, ecological_interactions#Thiamine provisioning fulfilling the fungal requirement, ecological_interactions#Enhanced ectomycorrhizal colonization of the pine host
2 evidence item(s)
However, it remains unclear what benefits the bacteria gain in this consortium.
The source explicitly leaves bacterial benefit unresolved; it does not establish reciprocal nutrient exchange.
13C12-thiamine was detected within the S. clintonianus hyphae following their cultivation with metabolites from B. altitudinis B4
The reported isotope observation directly supports transfer, beyond bulk supernatant depletion.
External Resources
| Name | Repository | Resource ID |
|---|---|---|
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Primary publication - thiamine cross-feeding in an ectomycorrhizal SynCom
Primary study establishing that Suillus clintonianus recruits Bacillus altitudinis B4 to metabolize thiamine and promote Pinus massoniana symbiosis, using nontargeted metabolomics and whole-genome sequencing. |
OTHER | PMID:41454778 |
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Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Thiamine limitation of the fungal partner | fungus cannot effectively obtain or synthesize thiamine | N/A |
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Growth Media
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