Extremophilic and conventional PGPR consortia enhance wheat tolerance to microplastic-osmotic co-stress

Microplastic contamination and water limitation increasingly co-occur in agricultural systems, creating complex constraints on plant growth that may be alleviated through microbial biostimulation. This study evaluated the ability of a conventional plant growth-promoting rhizobacterium, B. subtilisubtilis RC72, and an extremophilic P. poaeoae strain, applied individually or in combination, to improve wheat (Triticum aestivum L.) performance under polypropylene microplastic and polyethylene glycol-induced osmotic stress. A controlled glasshouse experiment comprising 16 treatment combinations was used to assess plant growth, biomass allocation, root architecture, water relations, photosynthetic pigments, oxidative damage, osmolyte accumulation and antioxidant responses. Microplastic and osmotic stress reduced plant growth, biomass and physiological performance while altering root traits and increasing oxidative stress. Bacterial inoculation improved several stress-responsive traits, including biomass, pigment accumulation and root development, although oxidative responses were strongly treatment dependent; notably, the Mp + Peg + Pp + Ba treatment exhibited the highest MDA accumulation. Multivariate analyses further showed that microbial treatments associated with improved growth and pigment traits were separated from treatments characterized by stronger oxidative responses. Overall, B. subtilis and P. poae modulated wheat responses in a trait- and treatment-dependent manner, and the data did not demonstrate consistent superiority of the extremophilic strain across the measured parameters. The effectiveness of stress-adapted PGPR therefore appears to depend on bacterial identity and the specific environmental stress combination rather than on extremophilic origin alone. These findings indicate that PGPR responses under combined microplastic and osmotic stress are strongly dependent on bacterial identity and stress context, and that selection of microbial inoculants should therefore be based on treatment-specific performance rather than an assumption of universally beneficial consortium or extremophile effects.