Halotolerant Brucella intermedia PM63 enhances growth, ion homeostasis, antioxidant defense, and salt stress tolerance in rice.

Soil salinity limits rice growth through osmotic stress, ionic imbalance, and oxidative damage. In this study, halotolerant plant growth-promoting bacteria were isolated from Peganum harmala collected at the Khewra Salt Mine, Pakistan, and one of the best isolates, Brucella intermedia PM63, was evaluated for its ability to improve salt tolerance in rice. Ten bacterial isolates were screened for salt tolerance and plant growth-promoting (PGP) traits, and three selected strains were characterized by phenotypic tests, FTIR, SEM, 16S rRNA sequencing, and PCR detection of functional genes. PM63 tolerated up to 3 M NaCl and, under non-saline conditions, produced indole-3-acetic acid (167.27 µg mL⁻¹) and siderophores (110.72 PSU) and solubilized phosphate (157.72 µg mL⁻¹). PCR detected nifH, acdS, ituC, sfp, and opd in PM63. FTIR and SEM analyses showed changes in cell-surface functional groups and cell morphology under salt stress. In a greenhouse experiment with three rice varieties exposed to 0-300 mM NaCl, PM63 inoculation improved root and shoot growth, biomass, photosynthetic pigment content, and osmolyte accumulation. It also modulated antioxidant responses, increased K⁺ retention, reduced Na⁺ accumulation, and lowered oxidative damage (H₂O₂ and malondialdehyde) and electrolyte leakage. The magnitude of the response differed among varieties, with KSK-434 responding most strongly across several traits. These findings identify PM63 as a halotolerant plant growth-promoting bacterial inoculant with potential for improving rice performance under saline conditions.