Microbial networking shapes ecosystems by regulating biogeochemical fluxes and the health of higher organisms. In the rhizosphere, bacterial colonisers of plant roots drive nutrient turnover, promote plant growth, and suppress pathogens. These interactions occur under intense viral pressure: bacteriophages outnumber bacteria in most environments and can reprogram bacterial lifestyles, spatial organisation, and evolution. It remains unclear how phages shape bacterial colonisation at the plant-root interface and how bacterial resistance emerges under plant-associated conditions. Building on an established, collaborative team studying tripartite plant-bacteria-phage interactions, this project will address these questions by investigating the interplay among the Gram-positive commensal Bacillus subtilis, the plant pathogen Xanthomonas campestris pv. campestris (Xcc), and the model plant Arabidopsis thaliana in the context of phage infection. Our approach builds on preliminary data demonstrating robust B. subtilis cell-state switching between motile and sessile states during colonisation of Arabidopsis roots, and already established infection conditions in bacterial cultures and within the root environment, for a panel of B. subtilis and Xcc phages. To determine how viral pressure alters B. subtilis colonisation, we will employ RootChip-based microfluidic devices combined with ultra-sensitive live fluorescence imaging to visualise phage infection, cell-state switching, bacterial lysis and persistence on roots at single-cell resolution. Our preliminary results reveal that specific sugars, notably glucose, strongly inhibit phage φ29 infection of B. subtilis PS216, consistent with a model in which root exudate-driven modification of cell-wall teichoic acids reduces phage adsorption. We will investigate how root exudate alters bacterial susceptibility to phage infection using targeted supplementation, B. subtilis knock-out mutants, and Arabidopsis mutants with altered sugar exudation. These studies will be extended to a soil-inspired porous microfluidic chip to emulate realistic rhizosphere conditions. We will investigate if phage- induced bacterial lysis elicits plant immune responses and thereby affects pathogen invasion. Overall, this project will uncover mechanistic principles of phage-driven microbial networking at plant roots and reveal how plants, phages, and bacteria jointly shape colonisation, resistance evolution, and community resilience.