Rhizosphere microbiome and plant growth-promoting bacteria isolates enhance salinity tolerance in Brassicaceae species

Soil salinity is a major abiotic stress limiting agricultural productivity worldwide, yet plants from saline coastal habitats have evolved tolerance mechanisms, including associations with beneficial microbiomes, that offer potential for crop improvement. We investigated this by studying inland and coastal populations of Brassica fruticulosa with contrasting salinity tolerance. We characterised the microbiomes of salt-tolerant (PST) and salt-sensitive (PSS) plant populations across plant-associated niches, revealing distinct microbial community compositions. Reciprocal transplant experiments using rhizospheric microbiomes from salt-tolerant (MST) and salt-sensitive (MSS) populations demonstrated that MST inoculation improved growth, maintained phosphorus and iron accumulation, and reduced oxidative stress in PSS plants under salinity. Over 100 bacterial strains were isolated from MST, 26 of which exhibited both phosphate solubilization and siderophore production and were screened for nitrogen fixation and auxin production under increasing salinity, revealing strain-dependent variation in growth-promoting traits. Three strains, T7 and R52 (both Pantoea sp.) and R60 (Pseudomonas sp.), maintained robust growth-promoting capacities under salt stress. Inoculation of Sinapis alba improved plant performance and altered root architecture under salinity, with increased proline accumulation and reduced osmotic potential. T7 exerted the strongest effect on plant fitness under control conditions. Consortium inoculation showed no added benefit over T7 alone, highlighting the strain-specific nature of plant growth promotion. These results highlight the potential of salt-adapted microbiomes from wild halotolerant plants as a source of plant growth-promoting bacteria with functional resilience under salinity and emphasize that efficacy under saline conditions depends strongly on strain identity and environmental context.