Plant growth-promoting microorganisms enhance biochemical traits, essential oil composition, and antioxidant defense...

An ideal sustainable agricultural system is one that promotes human health, benefits both producers and consumers, protects the environment, and provides sufficient food for the growing population. Salinity is one of the most significant abiotic stresses that disrupts plant growth by causing nutrient uptake imbalances. Plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) have been increasingly used to enhance plant tolerance to abiotic stresses, including salinity. A factorial greenhouse experiment with three replications in a completely randomized design was conducted to evaluate the effects of AMF and PGPR inoculation (non-inoculation, inoculation with Stenotrophomonas rhizophila OT29-3 (SR), Penibacillus simplex Q52-4 (PS) and Diversispora versiformis (AMF)) on biochemical traits, antioxidant enzyme activities, and essential oil (EO) composition and content of basil (Ocimum basilicum L.) under salinity stress (0, 30, 60, and 90 mM NaCl). The results showed that AMF and PGPR inoculation significantly (P < 0.05) increased EO content, proline (PC), total phenolic (TP), total flavonoid (TF) and antioxidant enzyme activities. Inoculation with AMF and PGPR reduced electrolyte leakage index (ELI), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents by 25.4%, 25.2%, and 24.1%, respectively. This improvement was associated with enhanced activities of antioxidant enzymes, including catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX) and superoxide dismutase (SOD). Inoculation with PS, AMF, and SR showed the highest efficiency, increasing EO content by 17.9% and 16.7%, respectively. Linalool, 1,8-cineole, and naphthalene were identified as the main EO components, with higher concentrations observed in PS, AMF and SR treated plants under all salinity levels. Overall, our findings demonstrate that salinity tolerance and EO production in basil can be improved through enhanced nutrient and water uptake and activation of antioxidant and non-antioxidant defense systems. AMF and PGPR inoculation, including SR, represent effective and low-cost strategies for optimizing basil cultivation in saline regions.