Alternative oxidase: a target and regulator of stress responses.

The alternative oxidase (AOX) is found in all plants examined to date, fungi and lower invertebrates. We propose that AOX is not only part of the stress response in plants, but it also plays a central role in defining the stress response. Three lines of evidence support this proposal: (1) The absence of AOX leads to an alteration of stress defences in normal and stress conditions, (2) the expression of AOX is triggered by a variety of signals indicating that it is a common response and (3) AOX acts as a buffer that determines the threshold for the induction of programmed cell death. Therefore, AOX is not only one of many components involved in the defence response, its activity or lack of activity leads to a radical alteration of the defence equilibrium at a cellular level and thus it plays a central role in programming the stress response. This programming role of AOX can be achieved directly by its ability to suppress the induction of reactive oxygen species (ROS) and indirectly by causing changes in the energy status of cells owing to the non-phosphorylating nature of the alternative respiratory pathway. The latter is likely achieved in combination with a variety of alternative NAD(P)H dehydrogenases, that are co-regulated with AOX. Additionally, we explore the possible function of AOX as a component of the stress response beyond the plant frontier.