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. Plants from coastal environments often develop adaptive strategies, including beneficial interactions with microbial partners, to tolerate high salinity. We investigated this potential by studying inland and coastal populations of Brassica fruticulosa with contrasting salinity tolerance. We characterized the microbiomes of salt-tolerant (PST) and salt-sensitive (PSS) populations from the bulk soil, rhizosphere, roots, and leaves, revealing distinct microbial communities associated with salinity tolerance. Reciprocal transplant experiments using rhizospheric microbiomes from salt-tolerant (MST) and salt-sensitive (MSS) populations demonstrated that MST inoculation improved growth, nutritional status, and reduced oxidative stress in PSS plants under high salinity. More than 100 bacterial strains were isolated from MST, 26 of which thrived at 150 mM NaCl and exhibited traits such as phosphate solubilization and siderophore production. These strains were further screened for nitrogen fixation, auxin (IAA) production, and ACC deaminase activity under increasing salinity, revealing strain-dependent variation in plant growth-promoting activity. Three strains, T7 and R52 (both Pantoea sp.) and R60 (Pseudomonas sp.), maintained robust growth-promoting traits under salt stress and were further assessed for motility, biofilm formation, and root adhesion. Inoculation of Sinapis alba plants with these strains enhanced biomass and altered root morphology, although ionomic analyses revealed no major changes in nutrient accumulation. A fitness assay comparing T7, R60, and their consortium demonstrated that T7 conferred the greatest improvement in plant performance, highlighting its potential as a bioinoculant for enhancing crop resilience in saline environments.