Role of mycelial networks of arbuscular mycorrhizal fungi in mobilizing symbiotic N-fixing bacteria
(2025)
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RoleofmycelialnetworksofarbuscularmycorrhizalfungiinmobilizingsymbioticN-fixingbacteria.pdf
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- Legumes rely on nitrogen (N)-fixing bacteria (rhizobia) and arbuscular mycorrhizal (AM) fungi to optimize nutrient acquisition, particularly in nutrient-poor soils. Rhizobia fix atmospheric N2 by forming root nodules, while AM fungi enhance nutrients uptake (e.g. P and N) and mediate soil microbe interactions. Although rhizobia recruitment is closely linked to the production of flavonoids by plants, this is limited to a few millimeters from the roots. The mechanisms enabling long-distance rhizobial migration and ensuring host-specific recruitment remain poorly understood. Here, we investigated the role of AM fungal extraradical mycelium (ERM) and common mycorrhizal networks (CMNs) in facilitating rhizobial migration and host specific recruitment by legumes. Using Medicago truncatula and Sinorhizobium meliloti, we demonstrated that the ERM of Rhizophagus irregularis provides a physical pathway for long-distance rhizobial migration. Time-lapse microscopy revealed that the bacterium moved along the surface of the fungal hyphae supported by cytoplasmic flow within the fungal network that can improve the supply of nutrients to the bacterium and signal the presence of a suitable host. Metabolomic analyses identified eight flavonoids unique to ERM connected to M. truncatula, which enhanced rhizobial growth and expression of genes involved in nodulation. These findings demonstrate that AM fungal ERM extends rhizobial recruitment beyond the rhizosphere, allowing legumes to attract their symbiotic partners from distant soil niches. Using M. truncatula and Glycine max connected together by a common mycorrhizal network of R. irregularis, we demonstrated that S. meliloti (green fluorescent protein-tagged) and Bradyrhizobium diazoefficiens (m-cherry-tagged), migrated towards their specific legume host. Metabolomic profiling revealed distinct flavonoid signals released by each legume species, which guided their respective rhizobia through the CMN. Both in vitro and greenhouse experiments confirmed that CMNs not only facilitate rhizobial dispersal but also ensure precise host-specific recruitment, even under competitive conditions. Our thesis highlighted the dual role of AM fungi in enabling long-distance rhizobial migration and mediating host-specific recruitment through ERM and CMNs. By exploring these mechanisms, we provided novel insights into the molecular and ecological processes underpinning legume-microorganisms interactions. Specifically, our findings demonstrated that AM fungi facilitate host specificity recognition by bacteria by transporting legume-specific flavonoid signals through CMNs, which selectively attract compatible rhizobia even in the presence of competing hosts plants and microbial partners. These results pave the way for practical applications of rhizobia and AM fungi in agriculture, in particular by taking advantage of the complementary nature of these two symbionts to improve the efficiency of N2 fixation in legumes.
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He, J. (2025). Role of mycelial networks of arbuscular mycorrhizal fungi in mobilizing symbiotic N-fixing bacteria. https://hdl.handle.net/2078.5/274821