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Applied and Environmental Microbiology, December 2005, p. 8537-8547, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8537-8547.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Two Novel Bacterial Biosensors for Detection of Nitrate Availability in the Rhizosphere

Kristen M. DeAngelis,{dagger} Pingsheng Ji,{dagger},{ddagger} Mary K. Firestone, and Steven E. Lindow*

Department of Plant and Microbial Biology and Department of Environmental Science Policy and Management, University of California, Berkeley, California 94720

Received 29 September 2004/ Accepted 12 September 2005

The nitrate-regulated promoter of narG in Escherichia coli was fused to promoterless ice nucleation (inaZ) and green fluorescent protein (GFP) reporter genes to yield the nitrate-responsive gene fusions in plasmids pNice and pNgfp, respectively. While the promoter of narG is normally nitrate responsive only under anaerobic conditions, the L28H-fnr gene was provided in trans to enable nitrate-dependent expression of these reporter gene fusions even under aerobic conditions in both E. coli DH5{alpha} and Enterobacter cloacae EcCT501R. E. cloacae and E. coli cells containing the fusion plasmid pNice exhibited more than 100-fold-higher ice nucleation activity in cultures amended with 10 mM sodium nitrate than in nitrate-free media. The GFP fluorescence of E. cloacae cells harboring pNgfp was uniform at a given concentration of nitrate and increased about 1,000-fold when nitrate increased from 0 to 1 mM. Measurable induction of ice nucleation in E. cloacae EcCT501R harboring pNice occurred at nitrate concentrations of as low as 0.1 µM, while GFP fluorescence was detected in cells harboring pNgfp at about 10 µM. In the rhizosphere of wild oat (Avena fatua), the whole-cell bioreporter E.cloacae(pNgfp) or E. cloacae(pNice) expressed significantly higher GFP fluorescence or ice nucleation activity when the plants were grown in natural soils amended with nitrate than in unamended natural soils. Significantly lower nitrate abundance was detected by the E. cloacae(pNgfp) reporter in the A. fatua rhizosphere compared to in bulk soil, indicating plant competition for nitrate. Ice- and GFP-based bacterial sensors thus are useful for estimating nitrate availability in relevant microbial niches in natural environments.

* Corresponding author. Present address: Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720-3102. Phone: (510) 642-4174. Fax: (510) 642-4995. E-mail: icelab{at}socrates.berkeley.edu.

{dagger} Kristen DeAngelis and Pingsheng Ji contributed equally to this work.

{ddagger} Present address: North Florida Research and Education Center, University of Florida, Quincy, FL 32351.

Applied and Environmental Microbiology, December 2005, p. 8537-8547, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8537-8547.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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