ORIGINAL ARTICLE Growth stimulatory effect of Ochrobactrum intermedium and Bacillus cereus on Vigna radiata plants M. Faisal and S. Hasnain Department of Botany, University of the Punjab, Quaid-e-Azam Campus, Lahore, Pakistan Introduction Chromium is one of the most common polluting metals arising from the discharged effluents of leather tanning, electroplating and alloy preparation (Stoychev et al. 2002). Interest has increased from national and interna- tional organizations because of its widespread use and adverse environmental impact on both fauna and flora (Zayed and Terry 2003). Chromium mostly occurs as Cr(III) and Cr(VI). Cr(VI) is more mobile and permeable to cells and is taken up by plants so enters into the food chain (Bluskov et al. 2005). Hence, there must be efforts to limit chromium mobility and permeability. Several bacterial genera are now known which detoxify chro- mium by reducing Cr(VI) in to Cr(III). (Mabbett et al. 2002; Chen and Cutright 2003). Microbes in the rhizo- sphere interact with plants thereby affecting plant growth, enhancing mineral and water uptake, providing antibiot- ics to inhibit soil pathogens and producing the plant growth regulators (Kapulnik 1996). The present work was undertaken to determine whether chromium resistant bacteria might also contribute to improvements in plant growth by chelating or reducing the toxic Cr(VI) to less permeable Cr(III) form. Here, the effect of two chromium resistant bacterial inoculations on the germination and the growth of mungbean under Cr(III) and Cr(VI) stres- ses was investigated. Material and methods Bacterial strains and culture conditions Strain CrT-1 was isolated from wastewater of Prime tan- nery, Muridkey, Lahore while strain S-6 from chromium contaminated the soil sample from Sheikhupura, Pakistan. Initially, strains were isolated on nutrient agar plates sup- plemented with 1 mg ml )1 of K 2 CrO 4 . Colonies obtained were picked, purified and were taken to higher level of K 2 CrO 4 . Strains were normally stored at 4°C on nutrient agar plates amended with 40 mg ml )1 of K 2 CrO 4 . Keywords Bacillus cereus, Ochrobactrum intermedium, Vigna radiata, Chromium, Cr(VI) reduction, heavy metals. Correspondence S. Hasnain, Department of Botany, University of the Punjab, Quaid-e-Azam Campus, Lahore 54590, Pakistan. E-mail: genetic@brain.net.pk 2005/0149: received 12 February 2005, revised 25 March 2006 and accepted 12 April 2006 doi:10.1111/j.1472-765X.2006.01977.x Abstract Aims: This study assessed the plant growth-promoting ability of the bacterial strains Ochrobactrum intermedium (isolate CrT-1) and Bacillus cereus (isolate S-6). Methods and Results: Two chromium resistant bacterial strains isolated from chromium-contaminated wastewater and soils were identified as O. intermedi- um CrT-1 and B. cereus S-6. These strains were inoculated on seeds of mung- bean Vigna radiata var NM-92, which were germinated and grown under chromate salts (300 lg ml )1 of CrCl 3 or K 2 CrO 4 ). The data show that Cr(VI) was more toxic because of its better availability to plants roots when compared with Cr(III). The major part of Cr(VI) supplied to the seedlings was reduced to Cr(III) in the rhizosphere by the bacterial strains, thus lowering the toxicity of chromium to seedlings. Conclusions: Strains have significant Cr(VI) resistance and reduction potential and have ability to enhance mungbean plant growth under chromium stress. Significance and Impact of the study: These strains could be utilized for the growth of economically important cash crops as well as for the bioremediation of chromium-polluted soils. Letters in Applied Microbiology ISSN 0266-8254 ª 2006 The Authors Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 461–466 461