Alternative Oxidase of Plant Mitochondria

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Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain that relaxes the coupling between respiratory carbon oxidation, electron transport, and ATP turnover. Despite key advances in our understanding of the genetic and biochemical control of AOX, the role of this auxiliary respiratory pathway in plant metabolism and mitochondrial physiology, as well as its overall impact on plant growth and performance remains poorly understood. As often highlighted in influential reviews, this represents a major gap in our understanding of plant primary metabolism. AOX gene expression is highly responsive to environmental cues and some evidence indicates that the pathway may be of particular importance under variable and stress conditions. Theoretically, AOX should negatively impact growth since it reduces ATP yield. However, it may have key roles in metabolism and mitochondrial physiology that outweigh this energy cost. My research proposal has two broad scientific goals. The first is to establish the role of AOX in supporting leaf metabolism. An emphasis will be to elucidate the mechanisms by which AOX supports photosynthesis. Our preliminary work shows that during drought stress, photosynthesis is strongly compromised in transgenic plants lacking AOX. Gas exchange and chlorophyll fluorescence analyses indicate that this is the result of a severe biochemical (rather than stomatal) limitation in plants lacking AOX. Detailed physiological and biochemical analyses will elucidate the nature of this limitation. We will also investigate the interaction of AOX and photosynthesis in plants grown at ambient versus high CO2. High CO2 has the potential to increase photosynthetic rate but can result instead in a down-regulation of photosynthetic capacity, such that the potential for higher source activity is not met. The propensity for such down-regulation depends strongly upon the response of sinks to high CO2. While respiration is a critical sink activity, little is known regarding specifically AOX at high CO2. This is a major gap in our understanding of how photosynthesis and growth will be impacted by future high CO2 conditions. A second broad scientific goal of the research proposal is to establish the role of AOX in supporting mitochondrial physiology. An emphasis will be elucidating the role of AOX in supporting the signaling function of mitochondria. Mitochondria act as signaling organelles involved in stress acclimation, but the primary “signals” from mitochondria that are involved in such responses are unknown. Our proposal builds upon a major finding from our previous work, where biochemical and confocal imaging analyses established that AOX controls the ETC generation of superoxide and nitric oxide, reactive species that can act as damaging molecules or signaling molecules within the cell. These reactive species, along with downstream products (hydrogen peroxide, peroxynitrite) are lead candidates as signal molecules for stress acclimation. Our ability to manipulate the level of these molecules using plants with altered AOX amount provides a powerful strategy to test hypotheses regarding the signaling capability of specific reactive oxygen and nitrogen species emanating from a specific cell compartment, the mitochondrion. We will investigate this signaling capability in support of stress acclimation responses at both the molecular level (ie. changes in nuclear gene expression) and cell level (ie. guard cell movements). Another important overall goal of the research proposal is the mentorship and training of three postdoctoral fellows, eight graduate students and numerous undergraduates. This will include a fostering of independence and multidisciplinary work.