Rhizophagus irregularis modulates neighbour-primed defences in Medicago truncatula against foliar pathogens via common mycelial network-mediated inter-plant signals.

Arbuscular mycorrhizal fungi (AMF) are ubiquitous root symbionts that form common mycelial networks (CMN), linking multiple plants underground. CMN are thought to contribute to information exchange between plants and to neighbour-primed defences, but the transcriptomic and metabolomic responses of receiver plants connected through CMN remain poorly understood. In addition, the confounding effects of hyphal damage caused by CMN disconnection have not been clearly resolved. To assess the contribution of CMN integrity to neighbour-primed plant defences, we used the model AMF Rhizophagus irregularis to connect two Medicago truncatula plants and examined the effects of sender wounding and flg22 elicitation on receiver leaf responses and pathogen tolerance. We found that changes in receiver biotic stress and defence signalling pathways depend on CMN-mediated inter-plant signals rather than on damage to the mycelial network. These responses were associated with distinct changes in leaf isoprenoid production, including volatile monoterpene and triterpene compounds. CMN-mediated signals from stressed senders enhanced receiver resistance to Fusarium sporotrichoides while increasing susceptibility to Botrytis cinerea. These findings highlight the role of CMN in inter-plant signalling and pathogen-specific resistance and susceptibility, with implications for understanding plant community defence in natural and agricultural systems.