ABSTRACT.- Purpose: Silicon (Si) accumulation by grasses alleviates diverse biotic and abiotic stresses. Despite this important functional role, we have limited understanding of how root microbial symbionts, such as arbuscular mycorrhizal (AM) fungi, affect Si uptake and even less about how Si supply and accumulation affect AM fungal colonisation. Our objective was to determine the nature of this two-way interaction in the model grass, Brachypodium distachyon. Methods: We grew B. distachyon with five levels of Si supplementation using wild-type plants and a mutant (Bdlsi1-1) that has little capacity for Si uptake. Half of the plants were colonised by AM fungi; half were free of AM fungi. We measured Si accumulation, AM fungal colonisation, leaf carbon (C), nitrogen (N) and phosphorus (P) concentrations. Results: AM fungi did not affect Si accumulation, although small increases occurred when root mass was included as a covariate. Si supplemented soil promoted plant growth and P uptake. Si accumulation suppressed colonisation by AM fungi and C concentrations in wild type but not in Bdlsi1-1 plants. Si concentrations were negatively correlated with C and N concentrations, with correlations being stronger in wild-type plants than Bdlsi1-1 plants. Conclusions: Our results indicate that Si accumulation in the plant, rather than Si availability in the soil, underpinned reduced AMF colonisation. We propose that Si accumulation is unlikely to be impacted by AM fungi in plants with inherently high Si accumulation, but Si accumulation may suppress AM fungal colonisation in such plants. © 2022, The Author(s).