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

Oxygen (O) is vital for nodule metabolism owing to its role in mitochondrial respiration for ATP generation, a vital component in N fixation. However, the concentration of O must be carefully regulated, because O can also reduce inhibit nitrogenase. Phosphorus (P) deficiency can increase the nodule's permeability to O and thereby exert a deleterious effect on N fixation. Although the mechanism by which the P deficiency increases the O permeability is not known, it has been attributed to a reduction in the O diffusion barrier within the nodule. In order to maintain N 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 O 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 O by leghemoglobin. We provide a critical evaluation of the current knowledge regarding the P effect on O permeability and propose new theories on potential mechanisms of P deficiency on the O diffusion barrier.