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Plant growth promotion traits and Cr (VI) reduction potentials of Cr (VI) resistant Streptomyces strains

The most toxic form of chromium [Cr (VI)] can be converted to less toxic Cr (III) by reduction with the help of microbes. A total of 6 Streptomyces strains (S. matansis BG5, Streptomyces sp. RSF17, S. vinaceus CRF2, Streptomyces sp. CRF14, S. pulcher CRF17, S. griseoincarnatus SCF18) which were previously isolated from saline farmlands of Punjab, Pakistan, were used in this work. These strains showed varying levels of resistance to Cr (VI) from 800 μg ml−1 to 1000 mg l−1. Their plant growth promoting and Cr (VI) reduction potentials were assessed. Two strains showed positive phosphate solubilization activity. All the strains had ability to produce indole acetic acid (IAA) except one strain (CRF17). The maximum production of IAA was observed by strain BG5 that was 16 mg l−1 in the presence of 50 mg l−1 of Cr (VI). All the strains were able to produce ammonia both in the absence and presence of Cr (VI). The highest Cr (VI) reduction in majority of the strains was observed at 28 °C and pH 9. The complete reduction of 150 mg l−1 of Cr (VI) occurred after 168 hrs. The chromium (VI) concentration of 200 mg l−1 could be reduced above 90% by most of these strains. The presence of other metals also enhanced Cr (VI) reduction by most of the strains. The results indicate the potential capacity of Streptomyces strains as tool for plant growth promotion and Cr (VI) bioremediation and also is the first report about plant growth promoting traits of Cr (VI) resistant Streptomyces strains.