Plant Respiration

Abstract Plant respiration is a metabolic gas‐exchange process involving oxygen (O 2 ) consumption and carbon dioxide (CO 2 ) release that reflects the controlled oxidation of primarily carbohydrate substrates through glycolysis, the tricarboxylic acid (TCA) cycle and the mitochondrial electron transport chain (mETC). Plant respiration serves three major functions. First, it provides ATP for cellular energy needs. Second, it supplies low‐molecular‐weight ‘building‐block’ metabolites that serve as precursors for biosynthesis and nitrogen assimilation. Third, the capacity of the plant mETC to uncouple electron flux to O 2 from ATP production plays a crucial role in regulating cellular redox balance (e.g. NAD(P)H/NAD(P) + ratios) and the stress‐induced production of harmful reactive oxygen species. Although most respiratory enzymes are highly conserved across organisms, plant respiration exhibits numerous distinctive features. These include parallel glycolytic pathways in the cytosol and plastid, glycolytic enzymes that use pyrophosphate instead of ATP, alternative routes of carbon entry into and metabolism by the TCA cycle and nonenergy‐conserving electron transport pathways of the mETC such as alternative oxidase (AOX). This metabolic flexibility represents an essential adaptation that enables sessile plants to acclimate to the wide range of environmental stresses they encounter in nature. Key Concepts Respiration is typically represented by the combined reactions of glycolysis, the TCA cycle and the mETC, which produce ATP as well as biosynthetic precursors needed for growth, N‐assimilation and stress acclimation. Plant respiration is a complex network containing alternative enzymatic reactions that circumvent classical reactions dependent upon ATP, ADP or phosphate as a co‐substrate. Carbohydrates are the dominant respiratory substrate in plants, whereas fatty acids are rarely respired. Parallel glycolytic pathways occur in the cytosol and plastid of all plant cells, with the respective reactions catalysed by distinct nuclear‐encoded isozymes. PEPC is a tightly regulated enzyme situated at a crucial branch point of central plant metabolism that controls anaplerotic replenishment of TCA cycle intermediates withdrawn for various purposes, including N‐assimilation and macromolecule biosynthesis. The operation of the TCA ‘cycle’ is flexible and changes flux modes to suit the needs of the cell. The plant mETC dynamically alters the efficiency of mitochondrial ATP production via the use of alternative dehydrogenases and oxidases, and potentially UCP. This minimises the generation of harmful ROS by the mETC while providing additional metabolic flexibility in an ever‐changing and stressful environment. There are fundamental differences in the control of plant and animal respiration, with plant glycolysis being regulated from the bottom up, and plant mETC altering the efficiency of ATP synthesis. Many enzymes within the respiratory network are subject to post‐translational mechanisms that rapidly control their activity to ensure that the rate of respiration is well balanced with the cell's need for ATP and/or biosynthetic precursors. At the ecosystem level, plant respiration has a profound impact on the CO 2 concentration in the atmosphere and is therefore a key component influencing the global C‐cycle and climate change.