AMF primes immune genes against Puccinia hordei (Brown rust) in Hordeum vulgare but does not reduce pathogen burden

Barley ( Hordeum vulgare ) is susceptible to Puccinia hordei (leaf rust), a biotrophic foliar fungal pathogen contributing to yield losses globally. With projected increases in food demand and ongoing threats from plant diseases, sustainable approaches to crop protection are essential to achieve the UN Sustainable Development Goal 2: Zero Hunger. Arbuscular mycorrhizal fungi (AMF), including Rhizophagus irregularis , establish symbioses with barley roots and have been implicated in the modulation of host immunity via mycorrhiza-induced resistance (MIR). In this study, we investigated whether R. irregularis AMF colonisation primes barley immune responses to P. hordei . Although AMF did not alter disease severity or growth in infected plants, co-infected tissues exhibited enhanced expression of defence-associated genes, including PR1 , PR2 , PR3 and WRKY28 . AMF alone did not induce these genes, consistent with a priming mechanism. Transcriptome profiling revealed AMF-induced reprogramming of the leaf transcriptome, with enrichment of genes related to defence, protein modification, and metabolism. AMF also modulated host protein ubiquitination during infection. These findings demonstrate that R. irregularis primes barley immune responses through transcriptional and post-translational regulation, with potential applications in the development of sustainable disease management strategies.