Interecations of elevted CO₂ and ozone on the respiratory control of growth in soybean and corn

SoyFace Experiment

An important agronomical question regarding plant respiration has long been whether reduced respiration increases plant productivity. The energetically wasteful alternative pathway has long been recognized as a potential target for improving agricultural productivity. We will study whether genotype selection by low respiration and low alternative pathway activity is feasible in elevated CO₂ and ozone scenarios under field conditions. It has been proposed that the alternative pathway can reduce the endogenous generation of reactive oxygen species (ROS) by avoiding over-reduction of the UQ pool. As a hypothesis, we propose that the alternative pathway will function to minimize endogenous ROS production under high levels of atmospheric ozone, which presumably will result in a greater exogenous generation of ROS. Therefore, genotypes (or transgenic plants) with low or lacking alternative pathway will not be able to avoid mitochondrial ROS production, damaging cells. Moreover, as higher rates of respiration are required for repair processes (as from ozone damage or disease), the UQ pool will be over reduced if the alternative pathway is not present or present in low amounts. To test this hypothesis, we will use antisense Arabidopsis lines lacking the alternative pathway (Gonzalez-Meler, Siedow and Umbach, unpublished) grown in greenhouses at elevated CO₂ and ozone levels. We will also test soybean genotypes with high and low expression of the alternative pathway and respiration (as tested by immunoblots and oxygen isotope fractionation) as how they respond to different levels of ozone. We propose a similar sampling protocol as for field gas exchange measurements for all the genotypes.
Elevated CO₂ is shown to inhibit enzymes of mitochondrial respiration, including cytochrome oxidase. Inhibition of cytochrome oxidase by elevated CO₂ will result in an over reduction of the UQ pool if the alternative pathway is not present, generating ROS. Also if the cytochrome pathway is partially inhibited, the electrons can be redirected to the alternative pathway with no net effect on the overall rate of respiration. If upon partial inhibition of one pathway (e.g. cytochrome pathway by high CO₂); the other pathway accommodates the electron flow, isotope fractionation should shift towards the value of the uninhibited pathway. We will measure oxygen isotope fractionation during respiration in plants grown at the two CO₂ and ozone concentrations and will estimate yields of ATP production that will be related to plants growth. Because for isotope fractionation measurements need to be done in the lab, we will place plants on pots in FACE rings. Pots will be taken to the lab for measurements. Our laboratory is also developing a portable sampling system that will be used in filed conditions. Samples collected with such device will be analyzed in the laboratory. Once this system is fully tested, we will be able to measure in situ isotope fractionation during photosynthesis and respiration.
Also, because the identified long- and short-term respiratory targets for high CO₂ are enzymes of the mitochondria, we propose to measure changes in activity and amounts of cytochrome c oxidase, succinate dehydrogenase, alternative oxidase and mitochondrial markers such as ATP synthase in response to the CO₂ and ozone treatments. We propose a similar sampling protocol as for gas exchange measurements using old and newly developed leaves to evaluate chronic CO₂xozone effects on respiration and respiratory enzymes. The results of these studies will provide a throughout picture of the effects of elevated CO₂ and ozone on plant respiration, of the extent to which genetic and biochemical mechanisms each participate in the observed responses, and how they influence the overall plant growth. Our collaborators are Steve Long and Evan DeLucia (University of Illinois).