Oxygen and the regulation of N2 fixation in legume nodules under P scarcity

Abstract

Oxygen (O2) is vital for nodule metabolism owing to its role in mitochondrial respiration for ATP generation, a vital component in N2 fixation. However, the concentration of O2 must be carefully regulated, because O2 can also reduce inhibit nitrogenase. Phosphorus (P) deficiency can increase the nodule's permeability to O2 and thereby exert a deleterious effect on N2 fixation. Although the mechanism by which the P deficiency increases the O2 permeability is not known, it has been attributed to a reduction in the O2 diffusion barrier within the nodule. In order to maintain N2 fixation, the nodules have several adaptations at the structural and metabolic levels to prevent and ameliorate these negative impacts. These adaptations will be evaluated in terms of structural and metabolic responses to O2 diffusion. The structural responses are based on the physical barrier of cortical cells and their intercellular spaces, while the metabolic responses include respiratory alteration and binding of O2 by leghemoglobin. We provide a critical evaluation of the current knowledge regarding the P effect on O2 permeability and propose new theories on potential mechanisms of P deficiency on the O2 diffusion barrier.