Interbacterial antagonism mediates plant growth modulation by rhizosphere synthetic communities in barley

Microbial communities in plant roots are shaped by complex interbacterial interactions, yet how these interactions translate into plant fitness remains poorly understood. In this study, 127 bacterial isolates were obtained from barley (Hordeum vulgare L.) roots of two cultivars grown in a non-fertilized field, representing 45 genera and 72 species. Screening identified isolates with growth-promoting, growth-reducing, and neutral phenotypes. Co-inoculation experiments using synthetic communities (SynComs) demonstrated that growth-promoting isolates effectively cancelled the inhibitory effects of growth-reducing isolates on barley seedling growth. Mechanistic investigation revealed that growth-promoting isolates Variovorax sp. 14F-2.1 and Pseudomonas sp. 37A kill growth-reducing isolates Flavobacterium sp. 2D-1 through direct cell-to-cell contact. Deletion of the Type VI secretion system (T6SS) gene tssA in Variovorax sp. 14F-2.1 substantially reduced this activity, implicating T6SS as a key antagonistic mechanism. Phytohormone profiling revealed that growth-promoting and neutral isolates, but not growth-reducing isolates, produce cytokinins, and only Variovorax sp. 14F-2.1 could degrade IAA, suggesting a potential hormonal basis for differential growth effects. A two-year field microbiome study showed that fertilization regimen and seasonal sampling times were dominant drivers of rhizosphere community composition, while bacterial inoculation had limited and inconsistent effects on microbial diversity and plant growth under field conditions. These results demonstrate that interbacterial antagonism is a key determinant of community-level plant growth outcomes and highlight the complexity of translating laboratory inoculant effects to field settings.