Lead-Resistant <i>Morganella morganii</i> Rhizobacteria Reduced Lead Toxicity in <i>Arabidopsis thaliana</i> by Improving Growth, Physiology, and Antioxidant Activities

Biological remediation serves as a powerful technique for addressing heavy metals toxicity in metals-contaminated soils. The present study aimed to evaluate the efficacy of lead (Pb)-resistant rhizobacterial strains on growth, photosynthetic traits, and antioxidant activities of the <i>Arabidopsis</i> plant under lead toxicity in pot conditions. Two pre-isolated and pre-characterized Pb-resistant <i>Morganella morganii</i> (ABT3) and <i>Morganella morganii</i> (ABT9) strains were used for inoculating <i>Arabidopsis</i> plants grown under varying Pb concentrations (1.5 mM and 2.5 mM) using PbNO₃ as the lead source. The treatments were set up in a completely randomized design with four replications. Data on growth parameters, physiological characteristics, lipid peroxidation, and antioxidant activities were recorded at harvesting. It was observed that Pb contamination caused a significant reduction in <i>Arabidopsis</i> growth, chlorophyll content and quantum yield at both lead concentrations. The Pb concentration of 2.5 mM, showed a substantial decrease in all parameters, including shoot fresh weight (58.72%), shoot dry weight (59.31%), root fresh weight (67.31%), root dry weight (67.28%), chlorophyll content (48.69%), quantum yield (62.36%), catalase activity (65.30%), superoxide dismutase (60.88%), and peroxidase activity (60.54%) while increasing lipid peroxidation (113.8%). However, the inoculation with Pb-resistant <i>M. morganii</i> strains (ABT3 and ABT9) improved plant growth, photosynthesis and antioxidant activities, while reduced the malondialdehyde content of Arabidopsis compared to control plants without inoculation. The <i>M. morganii</i> strain ABT9 showed a maximum increase in the shoot fresh weight (67.18%), shoot dry weight (67.96%), root fresh weight (94.04%), root dry weight (93.92%), shoot length (148.88%), root length (123.33%), chlorophyll content (52.53%), quantum yield (58.57%), catalase activity (39.46%), superoxide dismutase (21.84%), and peroxidase activity (22.34%) while decreasing lipid peroxidation (35.28%). PCA analysis further showed that all nine treatments scattered differently across the PC1 and PC2, having 81.4% and 17.0% data variance, respectively, indicating the efficiency of Pb-resistant strains. The heatmap further validated that the introduction of Pb-resistant strains positively correlated with the growth parameters, quantum yield, chlorophyll content and antioxidant activities of Arabidopsis seedlings. Both Pb-resistant strains improved <i>Arabidopsis</i> plant growth and photosynthetic efficiency under lead stress conditions. Thus, both <i>Morganella morganii</i> ABT3 and <i>Morganella morganii</i> ABT9 strains can be considered as bio-fertilizer for reducing lead toxicity thereby improving plant growth and physiology in metal-contaminated agricultural soils.