The Mitochondrion: An Integration Point of Cellular Metabolism and Signalling

In addition to efficient synthesis of ATP by oxidative phosphorylation, acquisition of the mitochondrial endosymbiont brought a whole range of new metabolic capabilities to the ancestral eukaryotic cell lineage such that the mitochondrion retains an important role in numerous anabolic and catabolic processes. While respiration dominates metabolism of the mitochondrion, this organelle is also important in the catabolism of amino acids and the provision of carbon skeletons for biosynthesis of a wide range of compounds including amino acids, vitamins, lipids, and tetrapyrroles. However, mitochondrial metabolism is best understood in the context of cellular metabolism as a whole; this is particularly true in auxotrophic organisms such as plants. For this reason understanding of the integration of mitochondrial metabolism with associated metabolic pathways in distinct cellular locations is of great importance. The examples of photorespiration, proline, cysteine, branched chain amino acid, ascorbate and folate metabolism all indicate that mitochondrial steps in these pathways are critical to their function and regulation. Moreover, the central metabolic position of the mitochondrion and its key roles in bioenergetics and redox regulation, additionally mean that it is ideally placed to act as a sensor of the biochemical status of the cell. When taken together these observations suggest that the myriad nonrespiratory functions of the mitochondria are of vast importance in the coordination of plant cellular metabolism and function.