Abstract
Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain. While respiratory carbon oxidation pathways, electron transport, and ATP turnover are tightly coupled processes, AOX provides a means to relax this coupling, thus providing a degree of metabolic homeostasis to carbon and energy metabolism. Beside their role in primary metabolism, plant mitochondria also act as “signaling organelles”, able to influence processes such as nuclear gene expression. AOX activity can control the level of potential mitochondrial signaling molecules such as superoxide, nitric oxide and important redox couples. In this way, AOX also provides a degree of signaling homeostasis to the organelle. Evidence suggests that AOX function in metabolic and signaling homeostasis is particularly important during stress. These include abiotic stresses such as low temperature, drought, and nutrient deficiency, as well as biotic stresses such as bacterial infection. This review provides an introduction to the genetic and biochemical control of AOX respiration, as well as providing generalized examples of how AOX activity can provide metabolic and signaling homeostasis. This review also examines abiotic and biotic stresses in which AOX respiration has been critically evaluated, and considers the overall role of AOX in growth and stress tolerance.
Highlights
A common theme of many of the above studies is that Alternative oxidase (AOX) activity may optimize photosynthesis by oxidizing “excess” reducing power, when the generation of NADPH in the chloroplast exceeds its use by the Calvin cycle and other anabolic metabolism
It was shown that susceptibility to salicylic acid (SA) and nitric oxide (NO) correlated with the steady-state cellular level of reactive oxygen species (ROS) and that AOX amount contributed to this steady-state in part by influencing expression of the cellular ROS-scavenging network
This study suggested that, at least in a system with single plant cells bathed in abundant external carbohydrate, AOX was a critical component to fine-tune growth rate in response to nutrient availability [176]
Summary
Photosynthesis and respiration are the primary pathways of carbon and energy metabolism in plants. Photosynthesis uses light energy, CO2 and H2O to drive the synthesis of carbohydrates and release of O2 Respiration uses these carbohydrates to support growth and maintenance through the provision of carbon intermediates, reducing equivalents and ATP. Glycolysis, the oxidative pentose phosphate (OPP) pathway and the mitochondrial tricarboxylic acid (TCA) cycle are the central respiratory pathways using photosynthesis-derived carbohydrate to supply carbon intermediates for biosynthesis, as well as coupling carbon oxidation with the reduction of NAD(P) to NAD(P)H. These reducing equivalents are used to support biosynthetic reactions or can be oxidized by the mitochondrial electron transport chain (ETC), localized in the inner mitochondrial membrane (IMM). Components that allow for a dramatic modulation of ATP yield depending on the components of the path used for NAD(P)H oxidation and O2 reduction [1,2] (Figure 1)
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