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Aims: Bradyrhizobium diazoefficiens is a soil bacterium capable of establishing nitrogen-fixing symbiosis with soybean plants, an important crop for food production worldwide. This interaction is under the control of several regulatory factors that affect the bacterial efficiency to nodulate legume plants and fix nitrogen within root nodules. To date, the function of the NtrYX two-component system in B. diazoefficiens remains unclear. In this work, we investigated the role of NtrY in the symbiotic interaction of B. diazoefficiens with soybean plants. Methods: We constructed a non-polar mutant of the sensor protein NtrY of the system, and then analysed the symbiosis of this mutant with soybean plants. Results: We found that the ntrY mutant was defective in plant dry weight, nitrogen content and nitrogenase activity. Haem c-staining of cbb3 oxidase components showed a clear reduction in the expression of this terminal oxidase in ntrY bacteroids with respect to wild type bacteroids. Such defect in cbb3 expression correlated with a decreased respiratory capacity of the bacteroids produced by the ntrY mutant. Conclusions: These results suggest that the role of B. diazoefficiens NtrYX in symbiotic nitrogen fixation might be a consequence of its involvement in cbb3 oxidase expression in bacteroids., This work was supported by the Agencia Nacional de Promoción de la Investigación Científica y Tecnológica (ANPCyT; project PICT 2013–2864), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Secyt-UNLP, Argentina. MJD received financial support from the European Regional Development Fund (ERDF), cofinanced grants AGL2013–45087-R and AGL2017–85676-R from the Ministerio de Economía y Competitividad (Spain) and PE2012-AGR1968 from the Junta de Andalucía. MFL was supported by fellowships from CONICETand by a travel grant from Secyt-UNLP. VAH was supported by a fellowship from CONICET. SLG is researcher at CONICET.

14 Pags.- 4 Figs. © Los autores, 2011., [ES] Las especies reactivas de oxígeno (ROS), como el radical superóxido y el peróxido de hidrógeno, se generan durante el metabolismo aeróbico celular. Las ROS pueden ser perjudiciales debido a su capacidad para oxidar lípidos, proteínas y ADN, originando un “estrés oxidativo”. Asimismo, las especies reactivas de nitrógeno (RNS), como el óxido nítrico (NO) y el peroxinitrito, pueden ser citotóxicas y causar un “estrés nitrosativo”. Las plantas contienen una gran variedad de antioxidantes y otras moléculas que evitan la producción incontrolada de ROS y RNS, modulando su concentración y permitiendo que participen como moléculas seflalizadoras en procesos como el crecimiento, la respuesta al estrés y la formación de los nódulos. Los antioxidantes son abundantes en los nódulos e incluyen enzimas (superóxido dismutasas, catalasas, peroxidasas) y metabolitos (ascorbato, glutatión). El NO puede ser producido en la fracción vegetal de los nódulos y en los bacteroides, presumiblemente por las nitrato y nitrito reductasas, por actividades tipo NO sintasa y por mecanismos no enzimáticos. Recientemente, se ha propuesto que las hemoglobinas no simbióticas desempeñan un papel esencial en la modulación de la concentración de NO. Además, el NO puede unirse a las hemoglobinas simbióticas (leghemoglobinas) para formar complejos nitrosilo, detectables mediante resonancia paramagnética electrónica. Esto sugiere que ambos tipos de hemoglobinas podrían estar implicados en evitar la respuesta defensiva de la planta frente a potenciales simbiontes., [EN] Reactive oxygen species (ROS), such as the superoxide radical and H202 are generated during aerobic metabolism. ROS can be detrimental due to their capacity to oxidize lipids, proteins, and DNA, originating oxidative stress. Likewise, reactive nitrogen species (RNS), such as nitric oxide (NO) and peroxynitrite, can be cytotoxic and cause nitrosative stress. Plants contain a vast array of antioxidants and other molecules that avoid the uncontrolled production of ROS and RNS, modulating their steady-state concentration and allowing them to participate as signal molecules in processes such a growth, stress response, and nodule formation. Antioxidants are very abundant in nodules and include enzymes (superoxide dismutases, catalases, peroxidases) and metabolites (ascorbate, glutathione). NO can be produced in the plant fraction of nodules and in the bacteroids, presumably by nitrate and nitrite reductases, by a NO synthase-like activity, and by nonenzymatic mechanisms. Recently, it has been proposed that non-symbiotic hemoglobins play an essential role in modulating NO concentration. Moreover, NO can bind symbiotic hemoglobins (leghemoglobins) forming nitrosyl complexes, which are detectable by electron paramagnetic resonance. This suggests that both types of hemoglobins can be involved in avoiding the plant’s defense response against potential symbionts.