Arbuscular mycorrhizal symbiosis: A natural manganese detoxifier in plants

Manganese (Mn) is an essential plant micronutrient that becomes toxic at high concentrations. This toxicity arises in acidic, anaerobic (reducing), or organic matter-abundant soils, as well as in environments impacted by the overuse of Mn-containing agrochemicals. Arbuscular mycorrhizal fungi (AMF), symbionts of most plants, serve as effective detoxifiers against Mn phytotoxicity. This review integrates current knowledge of the mechanisms behind this detoxification of AMF. This review provides a holistic and mechanistic framework that integrates these diverse, cross-scale detoxification pathways, from biogeochemical reactions in the rhizosphere to molecular regulation within the plant. Extraradically, AMF modulate the rhizosphere pH and oxidize Mn into insoluble biogenic oxides. They also adsorb Mn onto their fungal structures (hyphae and spores), sequester it using glomalin, and shift the rhizosphere microbiota favoring Mn immobilization. Intraradically and within the host plant, AMF enhance Mn tolerance in host plants by regulating Mn uptake via root/fungal transporters and compartmentalizing the metal within fungal structures (vacuoles and cell walls). In addition, AMF activate the plant’s antioxidant systems, protect photosynthesis, and improve nutrient homeostasis (particularly P, Ca, Mg, and Fe). These processes rely on sophisticated molecular dialogue between the plant and the fungus, which fine-tunes cellular Mn homeostasis. Elucidating these AMF-mediated mechanisms holds promise for applying AMF-based strategies to remediate Mn-contaminated soils and support sustainable agriculture production in Mn-toxic soils.