Alleviation of Cr(VI) Toxicity and Improve Phytostabilization Potential of Vigna radiata Using a Novel Cr(VI) Reducing Multi-Stress-Tolerant Plant Growth Promoting Rhizobacterial Strain Bacillus flexus M2

Chromium (Cr) is a toxic heavy metal discharged into the environment through various anthropogenic sources, which affects soil properties and fertility. Hence, an effective soil restoration strategy is the need of the hour. In this study, a potent Cr(VI)-reducing strain M2 was isolated from the rhizosphere of Zea mays L. grown in leather industrial effluent contaminated sites and identified as Bacillus flexus through 16S rDNA sequencing. Strain M2 exhibited strong tolerance to multi-stresses such as temperature (up to 45 °C), pH (up to 9.0), Sodium chloride (NaCl) (up to 7%) and PEG 6000 (up to 50%) and showed strong Cr(VI) reduction with the presence of multi-stresses. The interaction of Cr(VI) with strain M2 was elucidated through various instrumentation analyses. Fourier Transform Infra-red (FTIR) Spectroscopy analysis confirmed that Cr(VI) exposures induce significant changes in the cell-surface functional groups. Raman spectrum and Transmission Electron Microscopy–Energy Dispersive X-ray spectroscopy (TEM-EDX) analysis confirmed the bio-reduction of Cr(VI) to Cr(III) and their intracellular localization as Cr(III). Further, strain M2 produced a significant quantity of Indole acetic acid (IAA), ammonia, and exopolysaccharide (EPS) and showed positive results for various plant-growth-promoting activities with the presence of Cr(VI). In greenhouse experiments, the strain M2 inoculation progressively increased the plant growth parameters and stabilized the antioxidant system of Vigna radiata under Cr stress. However, Cr(VI) exposure decreased the growth parameters and increased the level of proline content, Hydrogen peroxide (H2O2) accumulation, and antioxidant enzymes expression in V. radiata. Interestingly, strain M2 inoculation significantly reduced the accumulation of Cr in root and shoot of V. radiata when compared to the uninoculated Cr(VI) treatment. Hence, this study confirms that rhizobacterial inoculation markedly reduced the negative impact of Cr toxicity and improved V. radiata growth even in harsh environments by stabilizing the mobility of Cr in the rhizosphere.