Plant–Microbiome Interactions in Agriculture: From Rhizosphere Biology to Crop Health and Productivity

Plant-associated microorganisms can influence nutrient acquisition, plant development and responses to pathogens and environmental stress, but their deliberate deployment frequently produces inconsistent outcomes outside controlled settings. This critical narrative review examines the transition from single-strain inoculants to defined synthetic microbial communities (SynComs), emphasising the evidence required to connect experimentally demonstrated mechanisms with sustainable crop production. Peer-reviewed research, field studies, meta-analyses and methodological reviews were critically integrated around community assembly, microbial interactions, product delivery, agronomic efficacy and ecological consequences. Controlled reconstruction of microbial communities has shown that particular strains and interkingdom interactions can causally affect root growth, nutrient–immunity signalling and disease suppression. Yet function is conditional on host genotype, resident microbiota, resource availability and colonisation history. Quantitative syntheses and selected field trials support the potential for benefits from single- and multispecies inoculation, while also revealing substantial site-dependent variation, incomplete strain persistence and limited independent validation of complex consortia. Increasing the number of microbial members does not itself establish complementarity, stability or superior field performance. Stronger translational evidence requires strain-resolved tracking, realistic comparators, multi-season testing, crop-specific outcome measures and assessment of non-target effects. Management-driven recruitment of indigenous microorganisms should be evaluated alongside inoculant introduction, particularly when locally adapted functional communities are already present. The review proposes a diagnosis-led approach in which intervention complexity is justified by demonstrable added value rather than presumed biological sophistication. The emerging opportunity is to combine causal microbial ecology with robust field evaluation so that beneficial interactions can be deployed where their agronomic and environmental effects are reproducible. Current evidence supports targeted use under defined conditions, not a universal microbiome solution for diverse agricultural systems.