Enhancing salinity tolerance and growth of Spinacia oleracea L. using halophilic plant growth-promoting bacteria

Salinity stress is one of the major abiotic stresses limiting agricultural productivity and reducing crop yields worldwide. Halophilic bacteria play a crucial role in mitigating the adverse effects of saline conditions by producing plant growth-promoting (PGP) substances, thereby enhancing plant stress tolerance. This study aimed to isolate, identify, and purify halophilic PGP bacteria from the Marakkanam salt pan. The isolated colonies were selected based on distinct colony morphology. Molecular characterization based on partial 16S rRNA gene sequencing confirmed diverse halophilic bacterial strains. The isolates were evaluated in vitro at different NaCl concentrations. All strains tolerated NaCl up to 15%, whereas MKM11 grew even at 20%. Multiple plant growth-promoting (PGP) traits were detected, including indole-3-acetic acid (IAA) production, ammonia production, siderophore synthesis, and phosphate and potassium solubilization. A greenhouse experiment on Spinach crop (Spinacia oleracea) under saline stress conditions showed that inoculation with halophilic PGP bacteria remarkably enhanced crop yield, physiological and biochemical performance compared with the control. Treatment with Halobacillus marinus MKM3 increased shoot height by (≈ 85-90%), fresh biomass by (≈ 120-130%), dry biomass by (≈ 220-240%), and total chlorophyll content by (≈ 55-60%), while Halobacillus halophilus MKM11 improved these parameters by (≈ 65-70%), (≈ 100-110%), (≈ 180-200%), and (≈ 40-45%), respectively. Both strains also boosted antioxidant enzyme activities (SOD, POD, and CAT) and reduced lipid peroxidation, reflecting improved stress tolerance. These findings highlight their potential as effective bioinoculants for improving plant performance and enhancing stress resilience in saline soils.