Genomic and biochemical investigation of soybean antioxidant metabolism in response to growth at elevated carbon dioxide and elevated ozone

Metabolism in an environment containing of 21% oxygen (O2) has a high risk of oxidative damage due to the formation of reactive oxygen species (ROS). Therefore, plants have evolved an antioxidant system consisting of metabolites and enzymes that either directly scavenge ROS or recycle the antioxidant metabolites. Ozone (O3) is a temporally dynamic molecule that is both naturally occurring as well as an environmental pollutant that is predicted to increase in concentration in the future as anthropogenic precursor emissions rise. It has been hypothesized that any elevation in O3 concentration ([O3]) will cause increased oxidative stress in plants and therefore enhanced subsequent antioxidant metabolism, but evidence for this response is variable. Along with increasing atmospheric O3 concentrations ([O3]), atmospheric carbon dioxide concentration ([CO2]) is also rising and is predicted to continue rising in the future. The effect of elevated [CO2] on antioxidant metabolism varies among different studies in the literature. Therefore, the question of how antioxidant metabolism will be affected in the most realistic future atmosphere, with increased [CO2] and increased [O3], has yet to be answered, and is the subject of my thesis research. First, in order to capture as much of the variability in the antioxidant system as possible, I developed a suite of high-throughput quantitative assays for a variety of antioxidant metabolites and enzymes. I optimized these assays for Glycine max (soybean), one of the most important food crops in the world. These assays provide accurate, rapid and high-throughput measures of both the general and specific antioxidant action of plant tissue extracts. Second, I investigated how growth at either elevated [CO2] or chronic elevated [O3] altered antioxidant metabolism, and the ability of soybean to respond to an acute oxidative stress in a controlled environment study. I found that growth at chronic elevated [O3] increased the antioxidant capacity of leaves, but was unchanged or only slightly increased following an iii acute oxidative stress, suggesting that growth at chronic elevated [O3] primed the antioxidant system. Growth at high [CO 2] decreased the antioxidant capacity of leaves, increased the response of the existing antioxidant enzymes to an acute oxidative stress, but dampened and delayed the transcriptional response, suggesting an entirely different regulation of the antioxidant system. Third, I tested the findings from the controlled environment study in a field setting by investigating the response of the soybean antioxidant system to growth at elevated [CO2], chronic elevated [O3] and the combination of elevated [CO 2] and elevated [O3]. In this study, I confirmed that growth at elevated [CO2] decreased specific components of antioxidant metabolism in the field. I also verified that increasing [O3] is highly correlated with increases in the metabolic and genomic components of antioxidant metabolism, regardless of [CO2] environment, but that the response to increasing [O3] was dampened at elevated [CO 2]. In addition, I found evidence suggesting an up regulation of respiratory metabolism at higher [O3], which would supply energy and carbon for detoxification and repair of cellular damage. These results consistently support the conclusion that growth at elevated [CO2] decreases antioxidant metabolism while growth at elevated [O3] increases antioxidant metabolism.