Plant growth-promoting rhizobacteria (PGPR) occupy an unusual position in agricultural science. The mechanistic literature is extensive and increasingly precise, yet the translation of that knowledge into products that perform dependably across ordinary farming conditions remains partial. This review examines why the gap persists, treating mechanism, ecology, formulation and commercial architecture as one connected problem rather than as separate literatures. Peer-reviewed evidence retrieved through Europe PMC and Crossref Metadata Search, supplemented by targeted verification of foundational works, was appraised for design adequacy, scale of inference and consistency, with searches closing on 5 June 2026. Four findings emerge. Mechanistic demonstration has advanced faster than agronomic demonstration, and the inferential step from a characterised trait to a yield response is frequently asserted rather than tested. Meta-analytic estimates of yield benefit are positive but modest and strongly conditioned by environment, with the largest responses reported under water limitation and in phosphorus-deficient or otherwise constrained soils; this conditionality is a property of the technology, not a defect of individual trials. Formulation research has diversified into encapsulation, hydrogel and seed-coating platforms that demonstrably extend viability, yet very few studies follow a formulation through storage, sowing, colonisation and yield in the same experimental system, so the chain of evidence linking shelf-life to field benefit is largely inferred. Commercial and regulatory constraints, including inconsistent product quality, fragmented registration pathways and thin economic evidence for adoption, are frequently listed but rarely investigated with the rigour applied to mechanism. Priorities for the field are therefore reframed around multi-environment trials with genotype-by-environment analysis, mandatory strain-level verification in marketed products, standardised viability and colonisation reporting, and the design of consortia on ecological rather than additive principles. The evidence supports cautious optimism about conditional, well-targeted deployment and does not support claims of general substitutability for mineral fertilisation.
Keywords: Bioinoculant formulation; rhizosphere colonisation; microbial biostimulants; synthetic microbial communities; inoculant quality control; agricultural biologicals regulation; context-dependent efficacy.
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