Mitigation of lead-induced soil toxicity and plant growth promotion by phosphate-solubilizing bacterium Enterobacter hormaechei KR2215 isolated from mangrove rhizosphere.

Lead (Pb(II)) contamination in soil, primarily resulting from mining and agricultural practices, poses environmental and agricultural challenges due to its toxicity and persistence. This study investigated the potential of a phosphate solubilizing bacterium (PSB), Enterobacter hormaechei KR2215, isolated from mangrove rhizosphere, for Pb(II) remediation and enhancement of Cicer arietinum (chickpea) growth under Pb(II) stress. The strain tolerated Pb(II) upto 1900 ppm and achieved 93.86 % removal in biofilm mode. Biochemical analysis of extracellular polymeric substances (EPS) revealed reduction in carbohydrate content, while protein and lipid levels increased, under Pb(II) stress. Electron microscopy coupled with elemental mapping confirmed Pb(II) accumulation on the biofilm-EPS matrix, while structural analysis revealed alterations in EPS functional groups upon Pb(II) exposure. EPS protein conformation shifted from α-helix (20.2 %) to random coil (51.1 %) at 1500 ppm Pb(II). Phosphate solubilization (682.83 ± 14.26 U m/L) and acid phosphatase activity (46.67 ± 3.39 U m/L) peaked on 3rd day, but declined at higher Pb(II) levels. Gene expression analysis indicated upregulation of phoR, pqqE, pgaC, and wzc at 500 ppm, while zntA expression peaked at 1000 ppm (17.82 fold, P < 0.0001). Colonization of E. hormaechei KR2215 in the chickpea endorhizosphere was confirmed by scanning electron microscopy (SEM) and enterobacterial repetitive intergenic sequence-based polymerase chain reaction (ERIC-PCR). Inoculated plants exhibited improved growth and reduced Pb(II) accumulation in roots (12.18 %), shoots (26.81 %), leaves (33.30 %), and seeds (42.83 %). Additionally, it enhanced antioxidant activity and reduced lipid peroxidation, suggesting its potential to improve crop productivity in Pb(II)-contaminated soils.