Heavy metal, salinity and azo dye tolerant, Cr (VI) reducing, plant growth-promoting Pseudomonas aeruginosa R32 reverses Cr (VI) biotoxic effects in Vigna mungo

Hexavalent chromium [Cr (VI)], derived from various industries, including fly ash from coal-based Thermal Power Plants, can be a source of toxic pollution of land and water bodies. This study aimed to bioremediation of such pollutant dump sites using bacteria capable of both Cr(VI) reduction and plant growth-enhancing substance production. The bacteria were isolated from the rhizospheric fly ash of a Thermal Power Plant, Kanpur. One of the rhizospheric isolate, Pseudomonas aeruginosa R32 showed high minimum inhibitory concentration (MIC) for Cr(VI) (1250 µg/ml), heavy metal tolerance (ZnCl2, CdCl2, Pb(NO3)2) up to 100 µg/ml, Acid Red 249 (AR) tolerance and halotolerance (6% NaCl). The isolate R32 also produces plant growth-promoting (PGP) hormones in the absence or presence of Cr (VI). R32 could completely reduce Cr(VI) at a tested dose of 100 and 500 μg/ml after 24h and 72h, respectively. However, decolorization of AR was observed after 48 hours at an initial concentration of 100 µg/ml and confirmed by Fourier transform infrared spectroscopy analysis. Vigna mungo seed inoculation with isolate R32 showed increased rootling growth compared to shoot after 7 d treatment with 0, 100, 500, and 1000 μg/ml of Cr(VI) concentrations, respectively. Root length tolerance index in Cr(VI) treated V. mungo seedlings was reduced to 56%, 35%, and 29%, respectively, when treated with 100, 500, and 1000 μg/ml Cr(VI) in comparison to control. Cr(VI) sub-MIC concentrations can affect the plant growth-promoting properties of rhizospheric bacteria. Herein, we report the isolation of rhizospheric bacteria P. aeruginosa R32 showing concurrent PGP substance production and Cr(VI) bioreduction capabilities in the presence of PGP inhibitory Cr(VI) concentrations.