Isolation and characterization of heavy metal-tolerant, plant growth-promoting bacteria and evaluation of the impact of lead [Pb(II)] on their functional traits.

In this study, six multi-metal-tolerant bacterial strains having PGP traits were isolated and examined for the effect of lead (Pb) on them. For isolate A (Agrobacterium pusense) and B2 (Arthrobacter sp.), the highest minimum inhibitory concentration (MIC) values were, Pb(II) (13,000 ppm), mercury (160–200 ppm), and arsenic (950-1,200 ppm). B3 (Bacillus paramycoides) and RS2 (Enterobacter cloacae) exhibited superior tolerance to chromium, zinc, nickel, and copper with MICs of 850, 1,000, 900, and 900 ppm, respectively. RS1 (Kosakonia oryzae) showed the highest tolerance to antimony (550 ppm) and cadmium (120 ppm), whereas B1 (Cronobacter muytjensii) showed the highest tin tolerance (500 ppm). The isolates showed multiple PGP traits; indole acetic acid (IAA) production ranged from 6 to 225.5 ± 11 µg mL− 1, while gibberellic acid (GA) ranged from 25.6 ± 0.7 to 65 ± 0.5 µg mL− 1. 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity varied from 4.3 ± 0.4 to 20.6 ± 2 µM α-ketobutyrate mg− 1 protein h− 1, while ammonia production ranged from 17.1 ± 0.56 to 39.1 ± 0.63 µM mL− 1. Furthermore, all the strains showed high Pb(II) tolerance, and no significant inhibitory effect of Pb(II) was observed on growth and PGP traits up to 200 ppm. Though some traits decreased after Pb(II) exposure, such as ACCD, while others increased (IAA) at high Pb(II) concentrations, making them interesting candidates for phytoremediation of Pb-contaminated soils and for promoting plant development in such conditions.