Role of mycelial networks of arbuscular mycorrhizal fungi in mobilizing symbiotic N-fixing bacteria, by Jiadong He

Biological nitrogen fixation, driven by legume-rhizobia symbiosis, is key to sustainable agriculture. While legumes attract rhizobia over short distances via root-secreted flavonoids, long-distance dispersal and specific host recognition in complex soil environments remain unclear. 

This research reveals that arbuscular mycorrhizal (AM) fungal hyphae and common mycorrhizal networks (CMNs) act as critical conduits and signaling pathways.

AM fungal extraradical hyphae (ERM) serve as “fungal highways”, enabling rapid, directed migration of rhizobia, such as Sinorhizobium meliloti, to Medicago truncatula roots. Time-lapse microscopy demonstrates bacteria move along hyphal surfaces, possibly aided by fungal cytoplasmic flow, enhancing nutrients and signal delivery.

Additionally, AM fungal networks transport plant-derived flavonoids, identified via metabolomic analysis of hyphal exudates. These flavonoids stimulate rhizobial growth and activate nod gene expression remotely, signaling compatible hosts. In silico studies confirm strong binding between flavonoids and rhizobial NodD proteins, key nodulation gene regulators.

The research further shows that CMNs connecting different legumes (M. truncatula and Glycine max) enable selective recruitment of host-specific rhizobia (S. meliloti and Bradyrhizobium diazoefficiens). Host-specific flavonoid profiles in hyphal exudates act as “identity signals”, guiding rhizobia with precision through soil.

This work establishes AM fungal networks as a sophisticated belowground system, combining physical transport via fungal highways with host-specific chemical signaling to enhance rhizobial access to legume roots and nodulation efficiency. These tripartite interactions offer potential for improving nitrogen fixation in sustainable agriculture.

Conducted using in vitro systems, greenhouse microcosms, time-lapse microscopy, metabolomics, and molecular docking, this research highlights the ecological and agricultural significance of AM fungi.

Jury members :

Prof. Stéphane Declerck  (UCLouvain) (Supervisor)
Dr. Maryline Calonne-Salmon (UCLouvain) (Supervisor)
Prof. Marnik Vanclooster  (UCLouvain) (Chairperson)
Prof. Claude Bragard  (UCLouvain) (Secretary)
Prof. Philippe Jacques (ULiège, Belgique)
Prof. Sofie Goormachtig (UGent, Belgique)

The public defense of Jiadong He will also take place in the form of a videoconference.