Community-level AM fungal interactions lead to unexpected growth and disease outcomes in tomato

Abstract Background and Aims Arbuscular mycorrhizal (AM) fungi can enhance plant growth and resistance to pathogens through Mycorrhiza-Induced Resistance (MIR). While individual AM fungal species differ in their effects on plant performance, the consequences of multi-species AM fungal communities remain poorly understood. We investigated whether AM fungal communities generate additive, synergistic or antagonistic effects on tomato growth and defence relative to their constituent single isolates. Methods Tomato plants ( Solanum lycopersicum cv. Micro-Tom) were inoculated with five single AM fungal isolates or six AM fungal community combinations and exposed to the root pathogen Rhizoctonia solani . Plant growth dynamics were monitored using high-throughput RGB phenotyping, and projected shoot area and absolute growth rate were analysed over time using linear mixed models. Results AM fungal communities produced highly variable growth and defence responses that frequently differed from expectations based on single-isolate performance. Community effects ranged from additive and synergistic to antagonistic, and several community treatments reduced pathogen-associated biomass loss, including combinations containing isolates previously classified as MIR-ineffective. Conclusions AM fungal community composition strongly influenced tomato growth and disease responses, producing complex and sometimes unpredictable outcomes. These findings demonstrate that single-isolate performance is insufficient to predict the functional effects of AM fungal communities on plant health.