Denitrification in Rhizobia-Legume Symbiosis

The genera Allorhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium, collectively referred to as rhizobia, are members, among others, of the bacterial order Rhizobiales of the α-proteobacteria [1]. Rhizobia are soil, Gram-negative bacteria with the unique ability to establish a N2-fixing symbiosis on legume roots, and on the stems of some aquatic legumes. Symbiotic nitrogen fixation (SNF) by the legume-rhizobia couple is a process of both ecological and agricultural importance. In the nodule, maintenance of nitrogenase activity is subject to a delicate equilibrium. A high rate of O2-respiration is necessary to supply the energy demands of the nitrogen reduction process, but O2 also irreversibly inactivates the nitrogenase complex. These conflicting demands are met by control of O2 flux through a diffusion barrier in the cortex of nodules which greatly limits permeability to O2 [2]. Then, O2 is delivered to the bacteroids by the plant O2-carrier, leghaemoglobin, which is present exclusively in the nodule [3]. To cope with the low ambient O2-concentration within the nodule, nitrogen-fixing bacteroids use a high-affinity cytochrome cbb3-type oxidase encoded by the fixNOQP operon to produce ATP [4]. In recent years it has emerged that many rhizobia species have genes for enzymes for some or all of the four reductases reactions (see chapter 2) for denitrification. In fact, denitrification can be readily observed in many rhizobia species, in their free-living form, in legume root nodules, or as isolated bacteroids [5]. In this chapter, we update progress on denitrification by free-living and symbiotic rhizobia.