Oxygen and the regulation of nitrogen fixation in legume nodules

Abstract

In N2‐fixing legume nodules, O2 is required in large amounts for aerobic respiration, yet nitrogenase, the bacterial enzyme that fixes N2, is O2 labile. A high rate of O2 consumptition and a cortical barrier to gas diffusion work together to maintain a low, non‐inhibitory O2 concentration in the central, infected zone of the nodule. At this low O2 concentration, cytosolic leghemoglobin is required to facilitate the diffusion of O2 through the infected cell to the bacteria. The resistance of the cortical diffusion barrier is variable and is used by legume nodules to regulate the O2 concentration in the infected cells such that it limits aerobic respiration and N2 fixation at all times. The resistance of the diffusion barrier and therefore the degree of O2 limitation seems to be regulated in response to changes in the O2 concentration of the central infected zone, the supply of phloem sap to the nodule, and the rate of N assimilation into the end products of fixation.