Genetic design of soybean hosts and bradyrhizobial endosymbionts reduces NO emissions from soybean rhizosphere.

Soybeans fix atmospheric N through symbiosis with rhizobia. The relationship between rhizobia and soybeans, particularly those with high nitrous oxide (NO)-reducing (NOR) activities, can be leveraged to reduce NO emissions from agricultural soils. However, inoculating soybeans with these rhizobia under field conditions often fails because of the competition from indigenous rhizobia that possess low or no NOR activity. In this work, we utilize natural incompatibility systems between soybean and rhizobia to address this challenge. Specifically, Rj2 and GmNNL1 inhibit certain rhizobial infections in response to NopP, an effector protein. By combining a soybean line with a hybrid accumulation of the Rj2 and GmNNL1 genes and bradyrhizobia lacking the nopP gene, we develop a soybean-bradyrhizobial symbiosis system in which strains with high NOR activity predominantly infect. Our optimize symbiotic system substantially reduces NO emissions in field and laboratory tests, presenting a promising approach for sustainable agricultural practices.