Bioaugmentation with Plant Growth-Promoting Rhizobacteria Alleviates Chromium and Salt Stress in Rice Through the Improvement of Physiology, Ion Homeostasis, and Antioxidant Defense

Salinity and heavy metal stress significantly reduce agricultural productivity in arable lands, particularly affecting crops like rice (<i>Oryza sativa</i> L.). This study aimed to evaluate the efficacy of heavy metal-tolerant plant growth-promoting rhizobacteria (HMT-PGPR) in mitigating the harmful effects of salt (NaCl), chromium (Cr), and combined NaCl + Cr stress on rice plants. Two pre-isolated and well-characterized heavy metal-tolerant epiphytic (<i>Ochrobactrum pseudogrignonense</i> strain P14) and endophytic (<i>Arthrobacter woluwensis</i> strain M1R2) PGPR were tested. The LSD test (<i>p</i> ≤ 0.05) was used to assess the statistical significance between treatment means. Stresses caused by NaCl, Cr, and their combination were found to impair plant growth and biomass accumulation through mechanisms, including osmotic stress, oxidative damage, ionic imbalance, reduced photosynthetic pigment, lowered relative water content, and compromised antioxidant defense systems. Conversely, inoculation with HMT-PGPR alleviated these adverse effects by reducing oxidative stress indicators, including malondialdehyde (MDA), hydrogen peroxide (H₂O₂) content and electrolyte leakage (EL) and enhancing plant growth, osmolyte synthesis, and enzymatic antioxidant activity under single- and dual-stress conditions. The application of HMT-PGPR notably restricted Na⁺ and Cr⁶⁺ uptake, with an endophytic <i>A. woluwensis</i> M1R2 demonstrating superior performance in reducing Cr⁶⁺ translocation (38%) and bioaccumulation (42%) in rice under dual stress. The findings suggest that <i>A. woluwensis</i> effectively mitigates combined salinity and chromium stress by maintaining ion homeostasis and improving the plant's antioxidant defenses.