Interaction of nitric oxide with the components of the plant mitochondrial electron transport chain.

Mitochondria are not only major sites for energy production but also participate in several alternative functions, among these generation of nitric oxide (NO), and its different impacts on this organelle, is receiving increasing attention. The inner mitochondrial membrane contains the chain of protein complexes, and electron transfer via oxidation of various organic acids and reducing equivalents leads to generation of a proton gradient that results in energy production. Recent evidence suggests that these complexes are sources and targets for NO. Complex I and rotenone-insensitive NAD(P)H dehydrogenases regulate hypoxic NO production, while complex I also participates in the formation of a supercomplex with complex III under hypoxia. Complex II is a target for NO which, by inhibiting Fe-S centres, regulates reactive oxygen species (ROS) generation. Complex III is one of the major sites for NO production, and the produced NO participates in the phytoglobin-NO cycle that leads to the maintenance of the redox level and limited energy production under hypoxia. Expression of the alternative oxidase (AOX) is induced by NO under various stress conditions, and evidence exists that AOX can regulate mitochondrial NO production. Complex IV is another major site for NO production, which can also be linked to ATP generation via the phytoglobin-NO cycle. Inhibition of complex IV by NO can prevent oxygen depletion at the frontier of anoxia. The NO production and action on various complexes play a major role in NO signalling and energy metabolism.