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The Response to Nitric Oxide of the Nitrogen-Fixing Symbiont Sinorhizobium meliloti

June 2010 , Volume 23 , Number  6
Pages  748 - 759

Eliane Meilhoc, Yvan Cam, Agnès Skapski, and Claude Bruand

Laboratoire des Interactions Plantes Microorganismes (LIPM), UMR441-2594 INRA-CNRS BP52627, F-31320 Castanet-Tolosan, France

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Accepted 25 January 2010.

Nitric oxide (NO) is crucial in animal-- and plant--pathogen interactions, during which it participates in host defense response and resistance. Indications for the presence of NO during the symbiotic interaction between the model legume Medicago truncatula and its symbiont Sinorhizobium meliloti have been reported but the role of NO in symbiosis is far from being elucidated. Our objective was to understand the role or roles played by NO in symbiosis. As a first step toward this goal, we analyzed the bacterial response to NO in culture, using a transcriptomic approach. We identified approximately 100 bacterial genes whose expression is upregulated in the presence of NO. Surprisingly, most of these genes are regulated by the two-component system FixLJ, known to control the majority of rhizobial genes expressed in planta in mature nodules, or the NO-dedicated regulator NnrR. Among the genes responding to NO is hmp, encoding a putative flavohemoglobin. We report that an hmp mutant displays a higher sensitivity toward NO in culture and leads to a reduced nitrogen fixation efficiency in planta. Because flavohemoglobins are known to detoxify NO in numerous bacterial species, this result is the first indication of the importance of the bacterial NO response in symbiosis.

© 2010 The American Phytopathological Society