| Summary: | <p>Abstract</p> <p>Background</p> <p>The <it>rel </it>gene of <it>Rhizobium etli </it>(<it>rel</it><sub><it>Ret</it></sub>), 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.</p> <p>Results</p> <p>Growth of the <it>R. etli rel</it><sub><it>Ret </it></sub>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 <it>rel</it><sub><it>Ret </it></sub>mutant cells was obtained after H<sub>2</sub>O<sub>2</sub>, 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 <it>rel</it><sub><it>Ret </it></sub>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 <it>rel</it><sub><it>Ret </it></sub>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 <it>rel</it><sub><it>Ret </it></sub>mutant to H<sub>2</sub>O<sub>2 </sub>and a slightly increased resistance to elevated temperature (45°C) or NaCl shock, compared to wild-type bacteroids.</p> <p>Conclusion</p> <p>The <it>rel</it><sub><it>Ret </it></sub>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.</p>
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