Augmenting Cr(VI) phytoremediation potential of Ricinus communis through rhizospheric crosstalk with multi stress tolerant plant growth promoting Bacillus altitudinis M1.

Plant growth promoting rhizobacteria are cost-effective and eco-friendly alternative for bioremediation of Cr(VI). This study investigated the effects of rhizobacterial strain Bacillus altitudinis M1 on Cr(VI) reduction, plant growth promotion and Cr(VI) stress mitigation in Ricinus communis. Biosorption and bioreduction of Cr(VI) up to 300 mg/l by the strain M1 was confirmed by FTIR, Raman Spectrum and TEM-EDX analysis. Moreover, the strain M1 exhibited high tolerance to temperature (up to 40 °C), pH (up to 8.0), NaCl (up to 6%) and various heavy metals (Pb, Cd, Ni, Cu, Mn and Zn). The strain M1 produced significant IAA, ammonia and EPS under higher concentration of Cr(VI). The strain improved the growth and development of test crop R. communis under higher Cr(VI) concentration. Inoculation of the strain M1 alleviated Cr(VI)-induced oxidative stress in roots and leaves of R. communis by decreasing proline (up to 24 and 33%), HO (up to 56 and 43%), and MDA (up to 42 and 40%) by regulating the activity of antioxidant enzymes. These findings suggest that the strain M1 promotes plant growth under Cr(VI) stress through multiple mechanisms, including phytohormone production, nutrient mobilization, stress metabolite modulation, and antioxidant defense system regulation. Thus the application of the strain M1 potentially reduces Cr(VI) bioavailability, making it a promising candidate for Cr(VI) bioremediation.