Research article

Pleiotropic effects of a rel mutation on stress survival of Rhizobium etli CNPAF512

Kristien Braeken^1* , Maarten Fauvart^1* , Maarten Vercruysse¹ , Serge Beullens¹ , Ivo Lambrichts² and Jan Michiels¹

¹  Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium

²  Biomedical Research Institute, Hasselt University, B-3590 Diepenbeek, Belgium

author email corresponding author email* Contributed equally

BMC Microbiology 2008, 8:219doi:10.1186/1471-2180-8-219

Published:	10 December 2008

Abstract

Background

The rel gene of Rhizobium etli (rel_Ret), the nodulating endosymbiont of the common bean plant, determines the cellular level of the alarmone (p)ppGpp and was previously shown to affect free-living growth and symbiosis. Here, we demonstrate its role in cellular adaptation and survival in response to various stresses.

Results

Growth of the R. etli rel_Ret mutant was strongly reduced or abolished in the presence of elevated NaCl levels or at 37°C, compared to the wild type. In addition, depending on the cell density, decreased survival of exponentially growing or stationary phase rel_Ret mutant cells was obtained after H₂O₂, heat or NaCl shock compared to the wild-type strain. Survival of unstressed stationary phase cultures was differentially affected depending on the growth medium used. Colony forming units (CFU) of rel_Ret mutant cultures continuously decreased in minimal medium supplemented with succinate, whereas wild-type cultures stabilised at higher CFU levels. Microscopic examination of stationary phase cells indicated that the rel_Ret mutant was unable to reach the typical coccoid morphology of the wild type in stationary phase cultures. Assessment of stress resistance of re-isolated bacteroids showed increased sensitivity of the rel_Ret mutant to H₂O₂ and a slightly increased resistance to elevated temperature (45°C) or NaCl shock, compared to wild-type bacteroids.

Conclusion

The rel_Ret gene is an important factor in regulating rhizobial physiology, during free-living growth as well as in symbiotic conditions. Additionally, differential responses to several stresses applied to bacteroids and free-living exponential or stationary phase cells point to essential physiological differences between the different states.