More is not always better: phosphorus context shapes the effectiveness of single-strain and consortium plant growth-promoting bacteria in tomato.

Phosphorus availability determines whether single or consortium PGPB maximize tomato performance, demonstrating that rational inoculant design should prioritize functional complementarity over increasing microbial complexity. Phosphorus (P) availability is a major constraint for crop development and productivity, and beneficial bacteria are increasingly proposed to enhance plant P-acquisition efficiency. However, their comparative performance remains variable across studies and experimental contexts. This study investigated whether single plant growth-promoting bacteria (PGPB) strains and a defined bacterial consortium differentially modulate P nutrition and early growth of tomato under contrasting P availability. Tomato plants were grown hydroponically in a factorial design combining single-strain inoculations (Azospirillum brasilense, Pantoea agglomerans, Priestia aryabhattai), a defined consortium, and a non-inoculated control under five P regimes differing in concentration and chemical form (soluble vs. insoluble). Plant growth and root architecture traits were evaluated alongside ionomics and targeted expression analysis of SlPHT1 transporters, thus linking plant performance with nutrient status and regulation. Results indicated that P. agglomerans showed the most stable benefits under soluble P, while A. brasilense strongly promoted adaptive root and transporter responses under moderate limitation. P. aryabhattai induced subtler growth effects but reshaped nutrient associations, particularly with insoluble P. By contrast, the consortium did not consistently outperform single strains and showed clear advantages only when insoluble P was the sole source. Overall, these findings demonstrate that the effectiveness of single versus multi-strain inoculation is strongly context-dependent and emerges from the interaction between P availability, strain identity, and plant response integration. Accordingly, effective consortia should be designed based on functional complementarity and balance, rather than on increasing inoculant complexity, as overlapping PGP traits may lead to non-additive or even counterproductive effects.