Redox-sensitive fluorescent biosensors detect Sinorhizobium meliloti intracellular redox changes under free-living and symbiotic lifestyles

Abstract

Reactive oxygen species such as hydrogen peroxide (H2O2) are key signaling molecules that control the setup and functioning of Rhizobium-legume symbiosis. This interaction results in the formation of a new organ, the root nodule, in which bacteria enter the host cells and differentiate into nitrogen (N2)-fixing bacteroids. The interaction between Sinorhizobium meliloti and Medicago truncatula is a genetic model to study N2-fixing symbiosis. In previous work, S. meliloti mutants impaired in the antioxidant defense, showed altered symbiotic properties, emphasizing the importance of redox-based regulation in the bacterial partner. However, direct measurements of S. meliloti intracellular redox state have never been performed. Here, we measured dynamic changes of intracellular H2O2 and glutathione redox potential by expressing roGFP2-Orp1 and Grx1-roGFP2 biosensors in S. meliloti. Kinetic analyses of redox changes under free-living conditions showed that these biosensors are suitable to monitor the bacterial redox state in real-time, after H2O2 challenge and in different genetic backgrounds. In planta, flow cytometry and confocal imaging experiments allowed the determination of sensor oxidation state in nodule bacteria. These cellular studies establish the existence of an oxidative shift in the redox status of S. meliloti during bacteroid differentiation. Our findings open up new possibilities for in vivo studies of redox dynamics during N2-fixing symbiosis.