Glomeromycota

Glomeromycota (also classified as subphylum Glomeromycotina within Mucoromycota) is a monophyletic lineage of obligate biotrophic soil fungi comprising approximately 350 morphologically described species (and several thousand environmental operational taxonomic units) that form arbuscular mycorrhizas (AM) with the roots, rhizoids, or thalli of 70–80% of all extant vascular plant species, as well as non-vascular hornworts (Anthocerotophyta) and liverworts (Marchantiophyta). Every glomeromycotan species is an obligate symbiont that cannot complete its life cycle or synthesize fatty acids independently of a living photosynthetic plant host (Rhizophagus irregularis, Glomus, Funneliformis mosseae, Gigaspora, and one exceptional cyanolichen-forming species, Geosiphon pyriformis).

Inside host root cortical cells, glomeromycotan hyphae differentiate into finely branched, dichotomo-fractal haustorial trees called arbuscules (from the Latin arbuscula, "little tree"). Across the periarbuscular interface, the fungus delivers inorganic orthophosphate (H₂PO₄⁻), ammonium (NH₄⁺), nitrate (NO₃⁻), and trace micronutrients (Zn, Cu) scavenged from soil micropores in exchange for 4–20% of the plant's photosynthetically fixed carbon (supplied as hexoses and palmitic acid / C_16:0 β-monoacylglycerol). Fossil arbuscules preserved inside the prostrate axes of Aglaophyton major in the 407-million-year-old Early Devonian Rhynie Chert indicate that the glomeromycotan symbiosis enabled rootless early embryophytes to colonize nutrient-poor continental rocks.

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All Glomeromycota produce large, multinucleate asexual soil spores (glomerospores, ranging from 40 to 1,000 μm in diameter—among the largest fungal spores known) borne either singly in the soil or aggregated in sporocarps, along with non-septate (coenocytic) hyphae lacking cross-walls:

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Establishment of the arbuscular mycorrhizal symbiosis requires a bidirectional chemical conversation between the phosphorus-starved plant root and the dormant soil glomerospore:

1. Host Strigolactone Exudation (Pre-Symbiotic Branching Factor): Under inorganic phosphate (P_i) deficiency, plant roots upregulate carotenoid-derived phytohormones called strigolactones (e.g., 5-deoxystrigol, orobanchol) and export them into the rhizosphere via the ABCG transporter PDR1. Nanomolar concentrations of strigolactones bind fungal α/β-hydrolase receptors, triggering rapid mitochondrial biogenesis, oxidative respiration, and profuse hyphal branching in the germinating glomeromycotan spore so that it intercepts the root before exhausting its triacylglycerol reserves.
2. Fungal "Myc Factors" and Host Perception: In response, the fungus secretes Myc Factors—a mixture of lipochitooligosaccharides (Myc-LCOs) and short-chain chitotetraose/chitoheptaose oligosaccharides (CO4/CO8)—which are recognized at the root epidermal plasma membrane by LysM receptor-like kinases (CERK1 / LYK and SYMRK / DMI2).
3. The Common Symbiosis Signaling Pathway (CSSP) and Nuclear Calcium Spiking: Activation of SYMRK triggers generation of secondary messengers that gate nuclear-envelope ion channels (CASTOR, POLLUX / DMI1, and cyclic nucleotide-gated channels CNGC15), eliciting rhythmic perinuclear calcium oscillations (Ca²⁺ spiking). Nuclear Ca²⁺ spikes are decoded by calcium- and calmodulin-dependent protein kinase (CCaMK / DMI3), which phosphorylates the transcription factor CYCLOPS / IPD3 and activates GRAS-family regulators (RAM1, RAD1, DELLA) to orchestrate root entry. (More than 300 million years later, Fabaceae legumes co-opted this exact CSSP signaling module to evolve nitrogen-fixing root nodules with Rhizobium bacteria.)
4. The Pre-Penetration Apparatus (PPA): Upon contact of a fungal hyphopodium with the root epidermis, the plant nucleus migrates to the contact site and constructs a trans-cellular cytoplasmic bridge of endoplasmic reticulum, microtubules, and exocytic vesicles—the Pre-Penetration Apparatus (PPA)—forming an apoplastic tunnel that guides the fungal hypha across epidermal and outer cortical layers toward the inner cortex.

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Once inside an inner root cortical cell, the fungal hypha penetrates the cell wall without breaching the host plasma membrane. Instead, the plant plasma membrane invaginates and proliferates extensively around each dichotomous branch of the fungal arbuscule, creating a specialized symbiotic membrane—the periarbuscular membrane (PAM)—separated from the fungal cell wall by an acidic (~pH 5.0–5.5) apoplastic compartment, the periarbuscular space (PAS):

• Phosphate and Nitrogen Delivery: Extraradical hyphae extending several centimeters beyond the root's 1–2 mm phosphate-depletion zone absorb soil H₂PO₄⁻ via high-affinity fungal H⁺/P_i symporters (GintPT) and polymerize it into vacuolar polyphosphate (poly-P) chains. Translocated rapidly via tubular vacuoles to the arbuscule, poly-P is hydrolyzed back to orthophosphate, exported into the periarbuscular space, and actively imported into the plant cell by PAM-localized, mycorrhiza-specific H⁺/P_i symporters (PT4 / PT11) energized by the host H⁺-ATPase HA1. Ammonium is simultaneously transferred via PAM-localized AMT2 transporters.
• Fatty Acid Auxotrophy (FAS-I Loss) and Lipid Transfer: Whole-genome sequencing of Rhizophagus irregularis revealed that Glomeromycota completely lack the cytosolic Type I fatty acid synthase (FAS-I) gene, rendering them obligate fatty acid auxotrophs. Within arbuscule-containing cells, the host transcription factor RAM1 induces plastidial fatty acid synthesis (FatM acyl-ACP thioesterase and RAM2 glycerol-3-phosphate acyltransferase) to synthesize 16:0 β-monoacylglycerol (β-MAG), which is exported across the periarbuscular membrane by the half-size ABCG transporter heterodimer STR1 / STR2 alongside monosaccharides exported by SWEET transporters.
• Transient Arbuscule Turnover: Each individual arbuscule has a functional lifespan of only 2 to 7 days, after which it senesces, collapses, and is digested by the host cell without harming the cortical cell itself, which can then be re-colonized by a new arbuscule.

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1. Multinucleate Coenocytic Organization and Homokaryon–Dikaryon Biology: A single Rhizophagus irregularis spore contains hundreds to thousands of nuclei migrating freely through continuous, non-septate hyphae connected by frequent hyphal fusions (anastomoses). Although long assumed to be strictly ancient asexuals, genomic resolution demonstrates that R. irregularis strains exist either as homokaryons (carrying one nuclear genotype) or dikaryons (carrying two distinct nuclear genotypes coexisting in the same cytoplasm and encoding divergent MAT-locus homeodomain transcription factors), capable of parasexual or cryptic meiotic recombination and adjusting the ratio of their two nuclear genotypes in response to shifting host plant species.
2. Soil Aggregation and Glomalin-Related Soil Proteins: Dense extraradical mycelial networks (reaching 10 to 100 meters of hyphae per gram of prairie or forest soil) physically enmesh mineral grains and deposit recalcitrant hydrophobic glycoproteins (glomalin-related soil proteins, GRSPs) and necromass that stabilize soil macroaggregates against wind and water erosion while sequestering stable soil organic carbon.

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1. Schüßler, A., Schwarzott, D., & Walker, C. (2001). A new fungal phylum, the Glomeromycota: phylogeny and evolution. Mycological Research, 105(12), 1413–1421. Google Scholar
2. Tisserant, E., et al. (2013). Genome of an arbuscular mycorrhizal fungus provides insight into the oldest plant symbiosis. Proceedings of the National Academy of Sciences, 106(50), 20117–20122. Google Scholar
3. Luginbuehl, L. H., et al. (2017). Fatty acids in arbuscular mycorrhizal fungi are synthesized by the host plant. Science, 356(6343), 1175–1178. Google Scholar
4. Akiyama, K., Matsuzaki, K. I., & Hayashi, H. (2005). Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature, 435(7043), 824–827. Google Scholar

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• Fungi
• Chytridiomycota
• Vascular Plants
• Bryophyta
• Fabaceae