Arbuscular mycorrhizal fungus mediates growth-defense trade-offs in Astragalus sinicus (Fabaceae) during pathogen infection

• AMF greatly increased Astragalus sinicus biomass but also enhanced disease severity. • AMF shifted growth–defense balance, suppressing defense-related gene expression. • Pathogen infection boosted PR genes and defense enzyme activities in non-AMF plants. • AMF promoted cytokinin pathways and carbon/nitrogen metabolism gene expression. Arbuscular mycorrhizal fungi (AMF) influence plant growth, defense, and resource allocation, yet their effects on transcriptional regulation remain unclear. We investigated the impacts of AMF Funneliformis mosseae on growth and defense in Astragalus sinicus infected with the pathogen Stemphylium henanense . Our results demonstrated that AMF inoculation significantly increased shoot and root biomass by 199.33% and 1815.15%, respectively. However, F. mosseae also led to a significant increase in the disease incidence and disease index of A. sinicus , with respective rise of 75.36% and 55.56%. These outcomes were accompanied by higher levels of malondialdehyde and elevated activity of peroxidase. Furthermore, A. sinicus inoculated with F. mosseae showed stronger defense responses, including higher chitinase activity, increased salicylic acid and jasmonic acid concentrations, and upregulation of genes encoding pathogenesis-related (PR) proteins (PR3, PR5, and PR10). In contrast, in the absence of AMF, A. sinicus displayed enhanced carbon fixation and a more robust defense response against pathogens, with upregulated expression of genes related to GAPDH, PR1, PR2, PR3, PR5, PR10, and EDS1. Gene ontology (GO) analysis revealed significant enrichment in terms of cell wall organization (GO:0071555), cell wall (GO:0005618) and cell wall biogenesis (GO:0042546) in the non-mycorrhizal inoculated groups compared to their AMF-inoculated counterparts. Genes involved in trehalose-6-phosphate synthase pathway, cytokinin synthesis, and plant growth promotion were notably expressed in A. sinicus inoculated with F. mosseae . These findings indicate that AMF mediate a complex growth-defense trade-off in A. sinicus , promoting growth while simultaneously increasing susceptibility to pathogens.