Alternative oxidase and plant stress tolerance

Abstract

Alternative oxidase (AOX) is one of the terminal oxidases of the plant mitochondrial electron transport chain. AOX acts as a means to relax the highly coupled and tensed electron transport process in mitochondria thus providing and maintaining the much needed metabolic homeostasis by directly reducing oxygen to water. In the process AOX also act as facilitator for signaling molecules conveying the metabolic status of mitochondria to the nucleus and thus able to influence nuclear gene expression. Since AOX indirectly, is able to control the synthesis of important signaling molecules like hydrogen peroxide, superoxide, nitric oxide, thus it is also helping in stress signaling. AOX mediated signaling and metabolic activities are very much important for plant stress response. This include both biotic (fungal, bacterial, viral, etc.) and abiotic (drought, salinity, cold, heavy metal, etc.) stresses. The review provides a gist of regulation and functioning of AOX.

Keywords: Abiotic stress; Alternative oxidase (AOX); biotic stress; retrograde signaling.

Figure 1.

Schematic representation for AOX induction under stress and response of the plant Electron Transport Chain (ETC). Under unstressed condition NADH oxidation by complex I is coupled to proton transport from matrix to inter-membrane space (IMS), whereas oxidation of FADH₂ and NAD(P)H doesn't lead to such fate. Similar is the electron flow from ubiquinol to complex III and then to complex IV which is additionally associated with reduction of O₂ to H₂O. Proton transport across the membrane generates a proton motive force which is dissipated by complex V to produce ATP. But under stress due ETC complex dysfunction, citrate accumulation there is electron leakage from complexes as ROS and RNS (shown in dotted lines) which induces AOX. AOX puts a branch in ETC after ubiquinol pool and directly reduces O₂ to H₂O with production of heat. I, II, III, IV, V: Complex 1–5; IMS: Inter Mitochondrial Space; IMM: Inner Mitochondrial Membrane.