Rhizosphere-competent actinobacterial consortium alleviates salinity stress in tomato through plant growth-promoting traits and ACC deaminase activity

Actinobacterial strains obtained from the rhizosphere of tomato (Solanum lycopersicum) plants were evaluated for their production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase (ACCD) and plant growth regulators (PGRs). Here, we aimed to determine the effects of rhizosphere-competent (RC) and non-rhizosphere-competent (NRC) consortia on tomato growth under salt stress (200 mM NaCl) by assessing morphological, physiological, and biochemical mechanisms. Our greenhouse experiments revealed that inoculation with actinobacterial consortia significantly increased the length and biomass of shoots and roots compared to control plants; however, RC consortium demonstrated overall greater plant growth promotion efficacy, including higher biomass accumulation and improved root-shoot development than plants inoculated with NRC consortia under both normal and salt stress conditions. This suggests that the high rhizosphere competence is crucial for plant growth promotion and salt tolerance. Individually, isolate #36 (the PGR-producer) significantly improved plant growth and biochemical photosynthetic components (net photosynthesis and chlorophyll content index), while its combination with RC isolate #53 (the ACCD-producer) further enhanced salt stress resilience. In particular, inoculation with RC consortium of actinobacteria positively influenced photosynthetic parameters, priming plants to respond more robustly to salt stress, likely by efficiently activating antioxidant metabolism, detoxifying Na⁺, and enhancing polyamine, auxin, gibberellin, and cytokinin levels. Notably, tomato plants treated with isolates #36 (Streptomyces violaceus UAE1) and #53 (Streptomyces levis UAE1) exhibited a threefold reduction in endogenous ACC levels compared to salt-stressed controls, indicating effective ethylene regulation. This study is the first to demonstrate the synergistic effect of RC actinobacterial consortium in mitigating salt stress in tomato plants, highlighting their potential as bioinoculants for sustainable agriculture.