The crucial role of plant mitochondria in orchestrating drought tolerance

Around the world, the frequency and intensity of droughts is increasing as a result of global climate change, with important consequences for the growth and survival of agricultural and native plant species. Understanding how plants respond to water stress is thus crucial for predicting the impacts of climate change on the crop productivity and ecosystem functioning. In contrast to the large number of studies assessing drought impacts on photosynthesis, relatively little attention has been devoted to understanding how mitochondrial respiratory metabolism is altered under water stress conditions. This review provides an overview of the impacts of water stress on mitochondrial respiration (R), combining studies at the whole-plant, individual organ, cellular and organelle levels. To establish whether there are clear patterns in the response of in vivo R to water stress, a wide range of root, leaf and whole-plant studies are reviewed. It is shown that water stress almost always inhibits R in actively growing roots and whole plants. However, in fully expanded, mature leaves the response is more variable, with water stress reducing R in near two-thirds of reported studies, with most of the remainder showing no change. Only a few studies reported increases in leaf R under severe water stress conditions. The mechanisms responsible for these variable responses are discussed. Importantly, the fact is highlighted that irrespective of whether drought increases or decreases respiration, overall the changes in R are minor compared with the large decreases in photosynthetic carbon gain in response to drought. Based on recent work highlighting the link between chloroplast and mitochondrial functions in leaves, we propose a model by which mitochondrial R enables survival and rapid recovery of productivity under water stress conditions. Finally, the effects of water stress on mitochondrial function, protein abundance and overall metabolism are reviewed.