Mitochondrial uncoupling protein-dependent signaling in plant bioenergetics and stress response

The biological function of plant mitochondrial uncoupling proteins (pUCPs) has been a matter of considerable controversy. For example, the pUCP capacity to uncouple respiration from ATP synthesis in vivo has never been fully acknowledged, in contrast to the mammalian UCP1 (mUCP1) role in uncoupling respiration-mediated thermogenesis. Interestingly, both pUCPs and mUCPs have been associated with stress response and metabolic perturbations. Some central questions that remain are how pUCPs and mUCPs compare in biochemical properties, molecular structure and cell biology under physiological and metabolically perturbed conditions. This review takes advantage of the large amount of data available for mUCPs to review the biochemical properties, 3D structure models and potential physiological roles of pUCPs during plant development and response to stress. The biochemical properties and structure of pUCPs are revisited in light of the recent findings that pUCPs catalyse the transport of metabolites across the mitochondrial inner membrane and the resolved mUCP2 protein structure. Additionally, transcriptional regulation and co-expression networks of UCP orthologues across species are analysed, taking advantage of publicly available curated experimental datasets. Taking these together, the biological roles of pUCPs are analysed in the context of their potential roles in thermogenesis, ROS production, cell signalling and the regulation of plant cellular bioenergetics. Finally, pUCPs biological function is discussed in the context of their potential role in protecting against environmental stresses.