Microbial synergies enhance tomato tolerance to abiotic stress via hormonal and physiological modulation

Fusarium solani strain K (FsK) and arbuscular mycorrhizal fungi (AMF) are soilborne symbionts that colonize plant roots and modulate stress responses. While most studies focus on individual microbial partners, understanding multipartite microbial interactions under realistic conditions is essential for designing effective inoculants. Here, we investigated the individual and combined effects of FsK, Funneliformis mosseae (F. mosseae) and Rhizophagus irregularis (R. irregularis) on tomato (Solanum lycopersicum) performance under drought and salinity stress in a greenhouse experimental set up. Under stress conditions, each endophyte showed enhanced root colonization. Co-inoculation with multiple microbes diminished this effect, however the functional outcomes were not directly dependent on the extent of microbial establishment. Under drought, FsK consistently promoted shoot growth, water retention and abscisic acid accumulation, while AMF improved nutrient status. Co-inoculation with FsK and F. mosseae led to synergistic improvements in physiological traits, but only under drought conditions. In contrast, salinity responses were less consistent and revealed functional divergence among microbial partners. These findings demonstrate that context-specific microbial combinations can enhance stress resilience in tomato.