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 pathogen contributing to global yield losses. With rising food demand and increasing disease pressure, sustainable crop protection strategies are urgently needed to support UN Sustainable Development Goal 2: Zero Hunger. Arbuscular mycorrhizal (AM) fungi, including Rhizophagus irregularis, form symbioses with barley roots and can modulate host immunity through mycorrhiza-induced resistance (MIR). Here, we tested whether R. irregularis colonisation alters barley growth and defence responses during P. hordei infection. AM fungal colonisation did not significantly reduce disease severity or mitigate pathogen-associated biomass loss at a single post-infection time point. However, co-infected plants showed enhanced expression of defence genes (PR1, PR2, PR3 and WRKY28), which remained low in plants colonised by AM fungi alone, consistent with immune priming. RNA sequencing revealed AM fungal-associated reprogramming of the leaf transcriptome, including enrichment of defence, metabolism and ubiquitination-related processes. These results indicate that R. irregularis reshapes barley immune regulatory networks at transcriptional and post-translational levels. Although these molecular changes did not translate into measurable phenotypic protection within the short experimental timeframe, they highlight the complexity and context dependence of MIR in cereal-rust interactions.