Bradyrhizobium diazoefficiens; NtrYX two component regulatory system; Soybean nodulation; Nitrogen fixation; Cytochrome cbb3 oxidase

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.