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Final Report: New Biogenic VOC Emissions Model
EPA Grant Number: R831453Title: New Biogenic VOC Emissions Model
Investigators: Monson, Russell K. , Fall, Ray
Institution: University of Colorado at Boulder
EPA Project Officer: Bloomer, Bryan
Project Period: January 1, 2004 through December 31, 2006 (Extended to December 31, 2007)
Project Amount: $644,044
RFA: Consequences of Global Change for Air Quality: Spatial Patterns in Air Pollution Emissions (2003)
Research Category: Air Quality and Air Toxics , Global Climate Change
Description:
Objective:The objectives of this project was to better understand the biochemical basis for the response of isoprene and acetaldehyde emissions to elevated atmospheric CO2 concentration and climate warming. We focused on the role of key enzymes, phosphoenolpyruvate carboxylase (PEPC) and pyruvate decarboxylase (PDC), in the control of leaf isoprene and acetaldehyde emissions, respectively, and their responses to growth CO2 concentration. We used the new knowledge we gained at the biochemical level to construct a new biochemically-based model of the emissions of these two VOCs that is more accurate in its representation of the response of forest VOC emissions to future global change.
Summary/Accomplishments (Outputs/Outcomes):- During the course of this research we produced over 30 new genetically-engineered lines of poplar trees that are designed to over-express levels of leaf PEPC enzyme. These plants were produced in collaboration with workers at the Institute of Tree Physiology in Freiburg, Germany. So far, we have analyzed 9 of these novel lines using molecular biological techniques to confirm the overexpression of the transgenic PEPC enzyme. We obtained the necessary permits to import these lines to the U.S. and received representative plants of all lines in February 2007 to the Department of Biology, Portland State University (in the charge of Dr. Todd Rosenstiel). Using funds from the EPA grant, Dr. Rosenstiel and post-doctoral associate, Dr. Tami Sivi, initiated screening the lines for expression of the transfer vector and the transferred enzyme, PEPC. We have screened our entire collection of Poplar PEPc over-expressors for PEPc mRNA content. Initial efforts to use RT-PCR to screen specifically for the Flaveria PEPc transgene were unsuccessful, as a wide array of primers were unable to distinguish endogenous and transgene PEPc mRNA. However, despite the inability to identify a transgene specific primer pair, we have screened over 30 lines for total PEPc mRNA content, and identified 8 lines which vary drastically with respect to PEPc mRNA content. As in vitro cultures do not yield sufficient tissue material for enzymatic analysis. The 8 chosen lines (including an untransformed wild-type control) have been transferred to soil culture, in a controlled-environment room. Initial morphologic measurements indicated that high PEPc mRNA content was significantly correlated to plant height, a pattern which continues to hold. As tissue becomes available for enzymatic analysis, we have now screened four of the eight selected transgenes for total leaf PEPc activity, and our initial results indicate a significant increase in PEPc activity in at least three independent lines. Further several lines, display symptoms of extreme chlorosis when grown with moderately low nutrient amounts, consistent with previous reports of PEPc over-expression in C3 systems. Dr. Rosenstiel and two undergraduate students are currently overseeing the continued propagation, maintenance, and analyses of the transgenic lines and are currently finishing the leaf enzymatic analysis of the remaining lines (a broken spectrophotometer slowed progress for a few weeks). In addition, we have also begun to measure PEPc activity in the shoots and petioles of selected transgenic lines, to determine if the increased plant height may be a consequence of enhanced PEPc activity in the internodal regions, a strong possibility with the promoter selected for this transformation (STLS-1; stem and leaf specific promoter).
- We conducted field studies of isoprene emission at three global change experiments (College Station, TX, Oak Ridge, TN and Rhinelander, WI) to determine the response of isoprene emissions to growth of natural forest stands in elevated atmospheric CO2 concentration. Coupled surface-atmosphere models are being used with increased frequency to make predictions of tropospheric chemistry on a ‘future’ earth characterized by a warmer climate and elevated atmospheric CO2 concentration. One of the key inputs to these models is the emission of isoprene from forest ecosystems. Most models in current use rely on a scheme by which global change is coupled to changes in terrestrial net primary productivity (NPP) which, in turn, is coupled to changes in the magnitude of isoprene emissions. In this study, we conducted measurements of isoprene emissions at three prominent global change experiments in the United States. Our results showed that growth in an atmosphere of elevated CO2 inhibited the emission of isoprene at levels that completely compensate for possible increases in emission due to increases in aboveground NPP. Exposure to a prolonged drought caused leaves to increase their isoprene emissions despite reductions in photosynthesis, and presumably NPP. Thus, the current generation of models intended to predict the response of isoprene emission to future global change likely contain large errors. A framework is offered as a foundation for constructing new isoprene emission models based on the responses of leaf biochemistry to future climate change and elevated atmospheric CO2 concentrations. This work was published in a paper in the Philosophical Transactions of the Royal Society in 2007 (see citations below).
- We characterized the pyruvate decarboxylase (PDC) enzyme from poplar leaves, and we compared its biochemical properties with the PDC from poplar roots. We have found that in cottonwood leaves the enzyme is highly concentrated in vascular bundles and not mesophyll cells, and PDC activity is highly induced by root flooding. We hypothesize that as with the root PDC the leaf PDC acts as a safety valve to prevent over accumulation of pyruvate in vascular bundle cells. We also cloned several poplar leaf-specific PDC genes, and are working to over express these genes in E. coli and characterize the recombinant PDC and also use the protein for antibody production. The work with E. coli will continue past the end of the EPA grant.
- We conducted an experiment to verify that changes in the activity of PEPC enzyme causes reductions in leaf isoprene emission in poplar trees. We conducted a series of experiments in which we treated the roots of poplar trees with elevated concentrations of bicarbonate (HCO3-), stimulating the trees to shift their nitrate reduction activities from the leaves to the roots, and concomitantly causing a dramatic reduction in both leaf PEPC activity and transcript levels. Consistent with our working model, this dramatic reduction in foliar PEPC activity, promoted a significant increase in leaf isoprene emission rate. Surprisingly, this increase in isoprene emission occurred despite a significant reduction in both the content and activity of the isoprene synthase enzyme. Results from this study provide strong support for our hypothesis that the activity of PEPC is a principal control regulating isoprene emission rate in vivo.
- We devoted a major effort to synthesize our knowledge and produce a new biochemically-based model for isoprene emission, which was the primary aim of the proposed work. The model is based on our hypothesized connections between the activity of PEP carboxylase (PEPc) in the cytosol of isoprene-emitting leaves and the requirement for pyruvate in the chloroplasts in the face of elevated CO2. The relationship between isoprene emission rate and the leaf intercellular CO2 concentration (Ci), for the short-term effect, resembled an inverse sigmoidal response surface. Sigmoidal relationships between the velocity of a catalyzed reaction and the concentration of a substrate or allosteric effector often reflect second-order interactions between the enzyme and alternative (competing) substrates or cooperative substrates. In such systems, reaction velocity is typically affected by exponential amplification of underlying substrate interactions. Building on this past knowledge, and starting from existing models (e.g., the Hill equation), we designed an empirically-based relationship that fits the isoprene-Ci response pattern reasonably well:
During the course of this research, we were not able to screen the transgenic lines for isoprene emission because of the long delays we encountered when going from callus to plantlet in the production of the clones. We are going to continue the research on these lines beyond the scope of the EPA funding and will send eventual publication acknowledging the EPA support as the results are disseminated.
(1)
where Cci is the overall Ci scaling factor, intended to scale Is to the progressive inhibitory effects of increasing Ci, Ismax is the estimated asymptote at which further decreases in Ci have a negligible effect on the isoprene emission rate (Is), C* and h are Hill-type scaling coefficients used to calibrate the sigmoidal slope of the relationship between Is and Ci. The terms C* and h are derived empirically, with C* carrying units of ppmv (analogous to the Km of the Michaelis-Menten model of enzyme kinetics) and h is unitless.
The model presented in Equation (1) is intended to complement the scaling coefficients CL and CT, which are typically used to adjust a normalized (basal) emission rate (Isb) (sometimes called the ‘emission factor’) to incident light intensity (CL) and prevailing leaf temperature (CT). In combination with these previously-defined coefficients, the new scaling coefficient, CCi, can be used to scale Isb (typically determined at a leaf temperature of 30 °C, an incident PPFD of 1000 μmol m-2 s-1 and, in this case, the Ci that occurs at an ambient atmospheric [CO2] of 400 ppmv) to instantaneous combinations of these factors that differ from the ‘basal’ state:
Is = Isb (CT * CL * CCi) (2)
Through our research we clarified the biochemical mechanisms that control isoprene and acetaldehyde emissions from plants, and especially those components that are most susceptible to change due to increases in the atmospheric CO2 concentration. The studies revealed that in the case of isoprene emission, effects of CO2 on the cytosolic enzyme PEP carboxylase exert a principal control over the long- and short-term influences of increased atmospheric CO2 concentration. In the case of acetaldehyde emissions, the studies revealed that pyruvate decarboxylase, a principal leaf enzyme exerts major controls over acetaldehyde emission. We developed genetically-engineered lines of poplar trees with different amounts of PEP carboxylase, which are available for future studies on the biochemical controls over isoprene emission in an atmosphere with elevated CO2. Finally, we used our knowledge to develop a mathematical model that accurately describes the long- and short-term responses of isoprene emissions to elevated atmospheric CO2 concentration. The model describes a sigmoidal response that is characteristic of control over isoprene emission rate by two alternative substrates. This model is currently being inserted in global models of isoprene emission to explore how future increases in atmospheric CO2 will affect atmospheric chemistry.
Journal Articles on this Report: 1 Displayed | Download in RIS Format
| Other project views: | All 9 publications | 1 publications in selected types | All 1 journal articles |
| Type | Citation | ||
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Monson RK, Trahan N, Rosenstiel TN, Veres P, Moore D, Wilkinson M, Norby RJ, Volder A, Tjoelker MG, Briske DD, Karnosky DF, Fall R. Isoprene emission from terrestrial ecosystems in response to global change: minding the gap between models and observations. Philosophical Transactions of the Royal Society A: Mathematical, Physical & Engineering Sciences 2007;365(1856):1677-1695. |
R831453 (2006) R831453 (Final) |
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ozone, troposphere, pollution,
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Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Air, Scientific Discipline, RFA, Air Quality, Air Pollutants, climate change, Air Pollution Effects, Atmosphere, air toxics, Chemicals, Chemistry, Atmospheric Sciences, particulate matter, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, aerosols, meteorology, climate model, Global Climate Change, Volatile Organic Compounds (VOCs), atmospheric models, airborne aerosols, BVOCs, ozone, atmospheric dispersion models, greenhouse gas, climatic influence, air quality models, climate models, aerosol formation, monitoring of organic particulate matter, atmospheric chemistry, climate variability, environmental measurement, environmental stress, global change, atmospheric particulate matter, emissions monitoring, modeling, ambient air pollution, anthropogenic stress, atmospheric aerosol particles, ecological models, biogenic VOC emissions model, ambient aerosol, atmospheric transport, ecosystem models, greenhouse gases
Progress and Final Reports:
2004 Progress Report
2005 Progress Report
2006 Progress Report
Original Abstract