Regulation of oxygen diffusion in legume nodules

A review is presented of advances recently made in studies on the structure and mechanisms of the variable O2 diffusion barrier of legume nodules. These suggest that the barrier is a complex of structures involving intercellular space occlusions in the mid-cortex and osmocontractile responses in the inner cortex. There are also several metabolic or structural changes which could occur within the infected region. An attempt is made to integrate these various layers within a temporal framework. It is also hypothesised that this framework is integrated through the use of enhanced production of activated oxygen species, particularly H2O2, which act as sensors and signals for the diffusion barrier. Finally, the potential for exploiting the regulation of O2 diffusion is considered in terms of the requirements for low O2 conditions for gene expression, the control of O2 supply under stress conditions and the use of the O2 barrier by the host plant to regulate N2 fixation activity.