Potential of indole-3-acetic acid-producing rhizobacteria to resist Pb toxicity in polluted soil

Heavy metal toxicity in industrial polluted area is imparting serious consequences on crops. Two Pb-tolerant bacteria were isolated from maize growing in a dumping site of Attock Oil Refinery, Rawalpindi. The oil-polluted field had a higher geo-accumulation index (I_geo) and pollution load index (PLI) value for Pb than standard. The soil accumulated higher Cd, Zn, and Mn contents too. Maize leaves and roots accumulated higher Pb and Zn and exhibited biological concentration factor (BCF), biological accumulating factor (BAC), and translocation factor (TF) Zn. Two bacterial strains (Exiguobacterium aurantiacum and Bacillus firmus) were isolated from maize rhizosphere growing in an oil-polluted field and applied as bio-inoculants on maize in a greenhouse experiment for 80 days. Both bio-inoculants were tolerant to Pb at 500 ppm and had the potential to produce indole-3-acetic acid (IAA) in the presence or absence of Pb. Results revealed that single inoculation of bio-inoculants decreased Pb contents in the soil, leaves, and roots of maize by 30% over the control. Growth and physiological attributes of maize were also improved by 25% in a single application of bio-inoculants. Application of Pb with bio-inoculants decreased the efficiency of PGPR, and there were only 10–15% increases in growth and physiological attributes over single inoculation. Bio-inoculants exhibited the best results in the presence of IAA and Pb application by intensive root growth (60% better than control), reducing Pb toxicity (38%) and increasing growth and physiological attributes by 10–15% over single inoculation of bio-inoculants. Application of bio-inoculants with IAA may decrease the deleterious effects of Pb toxicity in oil-polluted agriculture fields.