1. Host Recognition and Root Colonization

Rhizophagus intraradices, a species of arbuscular mycorrhizal fungus (AMF) in the phylum Glomeromycota, initiates symbiosis through a sophisticated chemical signaling exchange with host plants. Root exudates, particularly strigolactones, trigger spore germination and hyphal branching. In response, R. intraradices produces Myc-LCOs (Mycorrhizal lipochitooligosaccharides), which activate host plant receptors and initiate symbiotic signaling pathways via the common symbiosis signaling pathway (CSSP).

Once recognition is achieved, the fungus penetrates the root epidermis and cortex via appressoria, establishing intraradical colonization. Within cortical cells, it forms arbuscules, finely branched hyphal structures that serve as the interface for bi-directional nutrient exchange. In some host species, vesicles are also formed, acting as lipid-rich storage and reproductive structures.

2. Nutrient Foraging and Transfer

The most direct agronomic benefit of R. intraradices lies in its capacity to enhance nutrient acquisition:

• The fungus develops an extensive extraradical hyphal network that significantly increases the absorptive surface area of the root system, accessing nutrients beyond the rhizosphere depletion zone.

  
  

• Key nutrients mobilized include phosphorus (Pi), zinc (Zn), copper (Cu), and other micronutrients, often bound in forms that are otherwise unavailable to plants.

  
  

• High-affinity phosphate transporters (e.g., GintPT) in fungal hyphae facilitate Pi uptake, which is then translocated via the fungal cytoskeleton to the arbuscules.

  
  

• Inside the arbuscule interface, nutrient exchange occurs via a periarbuscular membrane, where plant Pi and metal transporters (e.g., PT4) retrieve the nutrients.

  
  

In return, the plant supplies the fungus with photosynthetically derived carbon, mainly in the form of hexoses, transported through plant sugar transporters , supporting fungal metabolism and reproduction.

3. Abiotic Stress Alleviation

R. intraradices significantly modulates plant physiological responses under abiotic stress conditions:

• Enhances water acquisition through extended hyphal reach and improved root hydraulic conductivity.

• Increases osmoprotectant synthesis, including proline, glycine betaine, and soluble sugars, aiding in osmotic adjustment under drought and salinity stress.

• Activates antioxidant enzyme systems, including superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), reducing oxidative damage from ROS generated during stress.

• Influences the synthesis and signaling of phytohormones such as abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), and auxins, which regulate stress adaptation, stomatal closure, and root architecture.

  
  

4. Soil Aggregation and Health

The extraradical hyphae of R. intraradices play a critical role in soil structure and fertility:

• Secrete glomalin-related soil proteins (GRSPs) that stabilize soil aggregates by binding mineral particles and organic matter.

• Improve soil porosity, water infiltration, and bulk density, contributing to enhanced root penetration and aeration.

• Support carbon sequestration by promoting stable soil organic carbon pools.

• Increase microbial biomass and enzymatic activity, such as phosphatases, ureases, and dehydrogenases, which further enhance nutrient cycling and microbial community function.

  
  
  
  

5. Biotic Stress Resistance and Pathogen Suppression

R. intraradices contributes to plant immunity and disease resistance through several pathways:

• Competes with soil pathogens for space and resources in the rhizosphere and root cortex.

• Activates induced systemic resistance (ISR) via jasmonate and ethylene signaling pathways, enhancing the plant’s defense readiness.

• Alters rhizosphere microbiome composition, often increasing populations of beneficial microorganisms (e.g., Pseudomonas, Trichoderma) that further antagonize pathogens.

• Reduces the translocation of heavy metals and xenobiotics to aerial parts, providing a protective buffer in contaminated soils.

  
  

6. Ecological and Agronomic Integration

In sustainable agriculture, R. intraradices is increasingly applied as a bioinoculant, either alone or in combination with other beneficial microbes.

Its efficacy depends on:

• Soil conditions (pH, organic matter, nutrient availability)

• Host plant genotype and mycorrhizal compatibility

• Co-inoculation strategies (e.g., with nitrogen-fixing bacteria like Azospirillum brasilense)

• Reduction in synthetic fertilizer inputs, which can suppress AMF colonization when in excess