Identification
Rhizophagus irregularis has no mushroom. It is seen only under the microscope, after staining roots (for example with ink or trypan blue) or extracting spores from soil. Inside root cortex cells it forms highly branched arbuscules, the sites of nutrient exchange, and oval vesicles that store lipids. Outside the root, a network of fine hyphae spreads into the soil and bears spores that are round to irregular in shape, pale to yellowish, and often formed in clusters. Each spore contains hundreds to thousands of nuclei.
Spore morphology, especially the layers of the spore wall, separates genera and species, but reliable identification in roots and soil now relies on DNA sequencing. The species was described in 2008 as Glomus irregulare and moved to Rhizophagus in 2010. Many studies and products that refer to Glomus intraradices actually used R. irregularis, including the widely studied laboratory isolate DAOM 197198.
Habitat and ecology
R. irregularis is one of the most widespread arbuscular mycorrhizal fungi, found in agricultural fields, grasslands and forests on all continents except Antarctica. Arbuscular mycorrhizal fungi are very ancient partners of plants and associate with roughly 70% or more of land plant species.
The fungus is an obligate biotroph: it obtains carbon only from living roots, and receives sugars and fatty acids from its host. In return, its hyphae explore soil beyond the root's reach and deliver phosphate, nitrogen, zinc and water. It also helps build soil aggregates and can improve plant tolerance of drought and some pathogens. Intensive tillage, high phosphorus fertilisation and long fallows reduce mycorrhizal colonisation.
Uses and importance
R. irregularis is the main model organism for mycorrhizal research. Its genome was sequenced in 2013, and it has been used to study nutrient exchange, plant signalling and the unusual genetics of a fungus with many nuclei in a shared cytoplasm and no confirmed sexual cycle.
Commercially, it is the most common active ingredient in mycorrhizal inoculants sold for horticulture, agriculture, land restoration and gardening. Results in the field are variable: benefits are largest in low-phosphorus soils, for strongly mycorrhizal crops and where native mycorrhizal fungi are scarce, and smaller in fertile soils with healthy fungal communities. In the EU, mycorrhizal fungi are one of the microorganism groups permitted in CE-marked microbial plant biostimulants under the fertilising products regulation (EU) 2019/1009.
Cultivation
Because it needs a living host, R. irregularis cannot be grown alone in a flask like a yeast. Inoculum is produced in pot cultures on host plants in sterile substrates, in hydroponic or aeroponic systems, or in vitro on transformed root cultures (for example carrot roots) on agar, which yields clean spores. Product quality depends on the number of viable propagules and on freedom from contaminants.
Safety and edibility
Rhizophagus irregularis is not a human or plant pathogen and is not food. Commercial inoculants are generally regarded as low risk, but they are powders or granules that may contain dust, so follow the label instructions on handling. Introducing non-native strains into natural ecosystems is debated by ecologists, so local or certified material is often preferred for restoration projects.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Around the world
Rhizophagus irregularis is a cosmopolitan species and is found in soils worldwide. Mycorrhizal inoculants containing it are sold in many countries, with rules on registration and labelling that vary between jurisdictions.