Plant Growth-Promoting Rhizobacteria as a Strategy to Enhance Enzymatic and Metabolic Tolerance of Cucumis sativus L. Under Salinity Stress

<i>Cucumis sativus</i> L., a salt-sensitive horticultural crop, is severely affected by soil salinity, which disrupts photosynthetic efficiency and metabolic homeostasis. This study quantified the effects of Plant Growth-Promoting Rhizobacteria (PGPR)-<i>Pseudomonas paralactis</i>, <i>Bacillus cereus</i>, <i>Sinorhizobium meliloti</i>, and <i>Acinetobacter radioresistens</i>-on key enzymatic indicators of cucumber seedlings exposed to 0, 50, 100, and 150 mM NaCl. PGPR inoculation significantly enhanced bacterial stress-mitigation and hormonal pathways, with ACC-deaminase activity increasing by up to 78.8% (<i>A. radioresistens</i>, 150 mM NaCl) and nitrilase activity by 50.5% (<i>S. meliloti</i>, 50 mM NaCl). Auxin-related pathways were strongly induced, as reflected by increases of up to 51.1% in the IAM pathway (<i>P. paralactis</i>) and 42.9% in the IPA pathway (<i>A. radioresistens</i>). In plant tissues, key metabolic enzymes exhibited high stability under salinity, with ProDH and NDPK activities increasing by up to 4.5% and 2.35%, respectively, while RuBisCO activity remained unaffected across treatments. These results demonstrate that PGPR function as effective bioestimulants by coordinating hormonal regulation and metabolic resilience, providing a sustainable biotechnological strategy to enhance cucumber tolerance to salinity stress.