Synthetic microbial communities for plant growth promotion and soil-borne disease suppression: design principles, mechanisms, and translational challenges.

Declining soil health and soil-borne diseases threaten sustainable crop production, while single-strain inoculants often perform inconsistently in the field due to poor establishment, limited persistence, and narrow functional capacity within resident microbiomes. Synthetic microbial communities (SynComs), assembled from defined and functionally characterized microorganisms, offer a tractable strategy to improve plant growth and suppress disease through complementary and redundant functions. This review critically examines recent advances in plant-associated SynComs, emphasizing rational design over empirical strain mixing. We compare construction strategies based on core microbiome mining, trait-based screening, microbial interaction networks, host and niche adaptation, and multi-omics- or model-guided assembly. Available available evidence suggests that no single SynCom construction strategy is universally superior; rather, robust design requires an integrated, context-specific approach combining host- or core-microbiome-guided candidate selection, trait validation, interaction-informed assembly, and model-guided optimization. We synthesize direct and plant-mediated mechanisms, including nutrient mobilization, phytohormone modulation, stress alleviation, niche pre-emption, resource competition, antibiosis, rhizosphere restructuring, and immune priming. Evidence from representative crop-pathogen systems is evaluated based on mechanistic support, community stability, and validation under non-sterile greenhouse and field conditions. Major barriers to deployment include context-dependent efficacy, instability of community composition and function, formulation and delivery constraints, quality control, ecological safety, and regulatory uncertainty. We propose a design-to-deployment framework linking community assembly with ecological validation, formulation engineering, and multi-environment testing, providing a plant-centered roadmap for developing reliable SynCom-based biofertilizers as well as biocontrol products.