Role of Nitric Oxide of Bacterial Origin in the Medicago truncatula-Sinorhizobium meliloti Symbiosis

Nitric oxide (NO) is a small ubiquitous gaseous molecule that has been found in many host-pathogen interactions. NO has been shown to be part of the defense arsenal of animal cells and more recently of plant cells. To fight this molecular weapon, pathogens have evolved responses consisting of adaptation to NO or degradation of this toxic molecule. More recently, it was shown that NO could also be produced by the pathogen and contributes likewise to the success of the host cell infection. NO is also present during symbiotic interactions. Despite growing knowledge about the role of NO during friendly interactions, data on the specificity of action of NO produced by each part-ner are scarce, partly due to the multiplicity of NO production systems. In the nitrogen-fixing symbiosis between the soil bac-terium Sinorhizobium meliloti and the model legume Medicago truncatula, NO has been detected at all steps of the interaction, where it displays various roles. Both partners contribute to NO production inside the legume root nodules where nitrogen fixa-tion occurs. The study focuses on the role of bacterial NO in this interaction. We used a genetic approach to identify bacterial NO sources in the symbiotic context and to test the phenotype in planta of bacterial mutants affected in NO production. Our results show that only denitrification is a source of bacterial NO in Medicago nodules, giving insight into the role of bacteria -derived NO at different steps of the symbiotic interaction.

Automated summary

Nitric oxide (NO) plays important roles in both host-pathogen interactions and symbiotic interactions. Pathogens have evolved mechanisms to adapt to or degrade NO, while NO produced by pathogens also contributes to the success of host cell infection. In nitrogen-fixing symbiosis, NO has various roles, but the specificity of NO produced by each participant is not well understood.