The Functional Organization and Control of Plant Respiration

The respiratory pathways of glycolysis, the tricarboxylic acid (TCA) cycle, and mitochondrial electron transport chain (miETC) are central features of carbon metabolism and bioenergetics in aerobic organisms. Respiration is essential for growth, maintenance, and carbon balance of all plant cells. Although the majority of respiratory enzymes are common to all organisms, plant respiration has evolved as a complex metabolic network endowed with a wide variety of unique characteristics. Plants have the option of employing alternative enzymes that bypass several of the conventional steps in cytosolic glycolysis, the TCA cycle, and miETC. The extent and conditions under which these bypasses operate is the subject of intensive research. The highly flexible nature of respiratory metabolism in plants has likely evolved in response to the crucial biosynthetic role played by respiration beyond its role in ATP generation; both functions must proceed if plants are to survive under varying and often stressful environmental and nutritional conditions. Additional complexity arises due to the existence of tissue- and/or developmental-specific isozymes of many plant respiratory enzymes, as well as the extensive interactions between photosynthesis and respiration, and plastidic, cytosolic, and mitochondrial metabolism in general. Recent progress in biochemistry, physiology, cell biology, genomics, transcriptomics, proteomics, metabolomics, and in vivo flux analyses have resulted in exciting new insights into many aspects of plant respiratory metabolism. Experiments on transgenic or mutant plants possessing significantly elevated or reduced levels of respiratory enzymes are augmenting our understanding of the functions, organization, and control of plant respiration. Metabolic engineering of plant respiration is of significant practical interest as it provides both an important approach to enhancing crop yields, as well as a potential mechanism for mitigating global climate change due to elevated atmospheric CO 2 levels.