The Bradyrhizobium diazoefficiens two-component system NtrYX has a key role in symbiotic nitrogen fixation of soybean plants and cbb.sub.3 oxidase expression in bacteroids

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 cbb.sub.3 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 cbb.sub.3 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 cbb.sub.3 oxidase expression in bacteroids.