Chapter 6 - Denitrification in Rhizobia-Legume Symbiosis

Abstract

This chapter describes the denitrification in rhizobia legume symbiosis and provides an update on the progress on denitrification by free-living and symbiotic rhizobia. A high rate of O2-respiration is necessary to supply the energy demands of the N reduction process, but O2 also irreversibly inactivates the nitrogenase complex. These conflicting demands are met by a control of O2 flux through a diffusion barrier in the cortex of nodules, which greatly limits permeability to O2. Denitrification genes encoding either periplasmic nitrate reductase , Cu-containing nitrite reductase , nitric oxide reductase , or nitrous oxide reductase have been identified in several rhizobia species, such as R. sullae, R. etli, S. meliloti, and B. japonicum. When a rhizobial species is used to inoculate the corresponding legume host, bacteroids within the nodules are also able to express the denitrification pathway. The measurements of denitrification activity in intact, the root nodules of alfalfa upon NO3– supply demonstrated that NO3– reached the infected zone and was metabolized by the bacteroids to produce NOx. The chapter also presents the symbiotic phenotype of B. japonicum strains carrying a mutation in the nirK, norC, or nosZ structural genes in comparison with that of the wild-type strain. The role of denitrification in nodules covers rhizobial denitrification as an energy-producing mechanism and rhizobial denitrification as a nitrite and nitric-oxide-detoxifying mechanism.