Prevention of Oxidative Damage and Phytoremediation of Cr(VI) by Chromium(VI) Reducing Bacillus subtilus PAW3 in Cowpea Plants

Experiments were conducted to observe the role of plant growth promoting (PGP) strain PAW3 in reduction of Cr(VI) and cowpea growth. PAW3 was identified as Bacillus subtilus by 16S rRNA sequence analysis. Strain PAW3 produced substantial amounts of PGP substances such as indole acetic acid (IAA), ACC deaminase, exopolysaccharide (EPS), siderophore and solublized phosphate even at 500 mu g/mL Cr(VI). PAW3 completely reduced Cr(VI) at pH 5-7, 100-200 mu g Cr/mL and 20-35 degrees C. PAW3 reduced Cr(VI) into Cr(III) (30 +/- 1 mu g/mL in supernatant and 70 +/- 2.7 mu g/mL in debris). PCR amplification revealed the presence of Cr(VI) reductase gene (ChR) in PAW3 with a fragment size of 300 bp whereas other strains (PAW1, PAW2 and PAW5) did not express. Both malondialdehyde and antioxidant levels increased with increase in Cr(VI). Inoculation of cowpea with PAW3 resulted in the best growth and photosynthesis in pot soils amended with Cr(VI). PAW3 completely reduced Cr(VI) to Cr(III) after 30 days of growth. The capacity to secrete plant growth regulators, antioxidants, and Cr(VI) reduction could be responsible for growth promotion of cowpea.