Enhancing Salt Tolerance in Tomato by Complementing Mineral N Fertilization with Azospirillum Brasilense Inoculation: Contrasting Roles of the N-fixing Sp7 and its Nif- Mutant

Abstract This study evaluated whether Azospirillum brasilense can complement mineral N fertilization and improve tomato performance under saline conditions. Tomato plants were inoculated with either the wild-type A. brasilense Sp7 (N-fixing) or its nitrogenase-deficient mutant Nif- (non-N-fixing) and exposed to NaCl concentrations ranging from 0 to 120 mM. At moderate salinity (30–90 mM NaCl), Sp7 inoculation enhanced root development, biomass accumulation, ion homeostasis, antioxidant activity, proline content, and total N accumulation, while upregulating key genes involved in N assimilation, indicating improved N metabolism and stress adaptation. Both bacterial strains conferred benefits compared with uninoculated controls, suggesting that inoculation per se, likely through auxin-mediated effects, supports plant resilience. Under severe salinity (120 mM NaCl), however, the performance of Sp7 declined, possibly reflecting the energetic constraints and downregulation of N-metabolism pathways under extreme stress, whereas Nif- maintained more stable antioxidant responses but lacked the growth advantage of Sp7 These results reveal that inoculation with A. brasilense enhances tomato tolerance to salinity through combined phytohormonal and N-assimilation mechanisms. The contrasting performance of Sp7 and Nif- underscores the context-dependent nature of PGPR efficacy and delineates the thresholds beyond which N-fixation no longer provides added value.