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

Cucumis sativus 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)—Pseudomonas paralactis, Bacillus cereus, Sinorhizobium meliloti, and Acinetobacter radioresistens—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% (A. radioresistens, 150 mM NaCl) and nitrilase activity by 50.5% (S. meliloti, 50 mM NaCl). Auxin-related pathways were strongly induced, as reflected by increases of up to 51.1% in the IAM pathway (P. paralactis) and 42.9% in the IPA pathway (A. radioresistens). 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.