Regulation of respiration when the oxygen availability changes

Oxygen is a vital substrate for plant energy metabolism. Since plants do not have a sophisticated mechanism to deliver oxygen to those sites where it is actually needed, a plant cell has to continuously cope with changes of the oxygen tension within the tissue. The actual internal oxygen concentration will depend on the resistance for oxygen diffusion through the tissue, as well as on the actual respiratory activity. This paper discusses the current state of knowledge on the regulation of respiration by the oxygen availability. Contradicting opinions from the literature on plant respiration are reviewed and commented upon. Also, knowledge about the regulation of respiration in animal mitochondria is included. Apart from changes in glycolytic flux, the role of both the cytochrome-c oxidase (COX) and the alternative oxidase (AOX) in the adaptive response of respiration to changes in the oxygen availability are discussed. One hypothesis is formulated which describes an alternative or additional role for AOX. It is suggested that AOX could play a role in maintaining oxygen homeostasis within the mitochondrion. Because of the relative low affinity for oxygen of AOX as compared to COX, the alternative oxidase will not interfere with COX activity, but AOX activity will reduce the free oxygen concentration, thereby decreasing the production of reactive oxygen species (ROS) inside the mitochondrion.