Arbuscular Mycorrhizal Fungi Dominate Over Maternally Inherited Epichloë Endophytes in Controlling Rhizosphere Processes and Pathogen Resistance.

Plant-microbe symbioses form a multi-layered system integrating vertically transmitted Epichloë endophytes, arbuscular mycorrhizal fungi (AMF), and the rhizosphere microbiome, with implications for nutrient acquisition and pathogen resistance. Epichloë endophytes are maternally inherited and may exert priority effects that influence subsequent associations with AMF and root microorganisms, ultimately shaping defensive pathways. Here, we manipulated symbiosis of perennial ryegrass (Lolium perenne) with Epichloë sp. LpTG-3 strain AR37 and the AM fungus Acaulospora delicata to examine exudate metabolites and the recruited rhizosphere microbiome in relation to host responses to the pathogen Bipolaris sorokiniana. Dual symbiosis with Epichloë and AMF increased host growth and pathogen resistance through enhanced nutrient uptake, elevated defensive enzyme activities in leaves and rhizosphere, and reduced malondialdehyde concentrations. It also recruited potentially beneficial microorganisms and enriched metabolites negatively associated with disease severity; notably, the metabolite Acetamide 1, which accumulated under dual symbiosis, strongly inhibited the pathogen in vitro. Significant correlations among metabolites, rhizosphere microbial communities, and rhizosphere soil properties revealed coordinated belowground responses under the synergistic regulation of AMF and Epichloë that reduced disease severity. Although both symbionts enhanced host performance, AMF played a stronger role than maternally inherited Epichloë in shaping the rhizosphere processes driving growth and pathogen resistance.