Denitrification in Legume-associated Endosymbiotic Bacteria
Cristina Sánchez, Eulogio J. Bedmar and María J. Delgado
from: Nitrogen Cycling in Bacteria: Molecular Analysis (Edited by: James W. B. Moir). Caister Academic Press, U.K. (2011)
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. During this interaction bacteroids, as rhizobia are called in the symbiotic state, are contained in intracellular compartments within a specialized organ, the nodule, where they fix N2. When faced with a shortage of oxygen some rhizobia species are able to switch from O2-respiration to using nitrates to support respiration in a process known as denitrification. The complete denitrification pathway comprises the sequential reduction of nitrate or nitrite to dinitrogen, via the gaseous intermediates nitric oxide and nitrous oxide. The enzymes involved in denitrification are nitrate-, nitrite-, nitric oxide- and nitrous oxide reductase, encoded by nar/nap, nir, nor and nos genes, respectively. In recent years it has emerged that many rhizobia species have genes for enzymes of some or all of the four reductase reactions for denitrification. In fact, denitrification can be readily observed in many rhizobia species, in their free-living form, in legume root nodules, or in isolated bacteroids. This chapter will focus on update progress on denitrification by rhizobia under free-living and symbiotic conditions read more ...