Photosynthesis underlies human existence. Understanding how this process copes with large environmental changes—such as shifts in temperature and ozone—leads to the improvement of engineered crops with growth-promoting properties. Thomas D. Sharkey, Distinguished Professor of Biochemistry and Molecular Biology at Michigan State University (MSU) is a global leader in the study of gas exchange between plants, the atmosphere, and microbes. His lab focuses on alternative routes of carbon dioxide (CO2) fixation in photosynthesis metabolism and the production of photosynthetic isoprene—an organic gas compound—by trees. Though it is the single largest input of hydrocarbon into the atmosphere and can contribute to toxic ozone, isoprene protects plants against environmental stress. Understanding why certain plants emit isoprene and its effect on the atmosphere will lead to both more resilient crops and the mitigation of air pollution.

Isoprene has very large effects—both positive and negative—on atmospheric chemistry. Isoprene can contribute to air pollution in cities. The negative effects occur when nitric oxide and nitrogen dioxide (NOx), produced by cars and gas or coal-burning power plants, are present. Dr. Sharkey is very interested in looking at the hard questions regarding environmental stewardship. Presidential candidate Ronald Reagan suggested that trees pollute and so regulations to keep the air clean were not justified. It is true that trees emit more hydrocarbon into the atmosphere than people, but Los Angeles air or Beijing air does not look that way because of the trees. Nevertheless, we need to understand the trees we hug.

Dr. Sharkey, with his team of undergraduate, graduate, and postdoctoral students who specialize in the fields of botany, biology, molecular biology, biophysics, and biochemistry, collaborate with interdisciplinary MSU groups and colleagues both nationally and internationally. Dr. Sharkey and his team study how climate change will affect isoprene emission. Increased temperature will increase isoprene emission while increased CO2 might reduce it. Predicting how global change will affect the atmosphere of the future requires more information about climate change effects on isoprene emission from trees. By measuring the amount of hydrocarbon plants emit and helping model global change effects, regulatory rules to control NOx emissions can be established to reduce ozone pollution, both now and in the future. 

When a plant is first exposed to isoprene, whether it is isoprene the plant made or isoprene in the air, a complex change in gene expression patterns appear. Dr. Sharkey and his team will perform an in-depth analysis to identify the original signals in plant gene cascades that are turned on by isoprene. This enables them to understand what gene expression changes provide the plant with protection against abiotic stress, the consequences of those changes, and will narrow down their search for the critical isoprene receptor. This may lead to the engineering of isoprene-producing crops that are better able to tolerate heat, drought and ozone.

Current research includes:

• The Future of Isoprene Emission from Plants - Predictions of future isoprene emission to the atmosphere depend on understanding the interacting effects of CO2 and temperature on isoprene emission. An extensive series of measurements of isoprene emission over a range of CO2 and temperature made on many representative isoprene emitting species is needed. This can be completed in six to nine months. The results would improve global models of the effects of climate change on the atmosphere so that action could be taken now to continuously improve the air we breath.
• Analysis of Isoprene Plant Receptors - Dr. Sharkey and his team recently obtained evidence that points to a receptor that detects isoprene, possibly similar to the way mammals are affected by volatile anesthetics. By modeling the structure of membranes and proteins in the membranes, they will identify where isoprene is working in the membrane and protein interface. This will enable Dr. Sharkey and his students to identify how isoprene has its effects in plants. They primarily work with Arabidopsis—a small plant that grows quickly and enables rapid progress—which doesn’t naturally produce isoprene. They have genetically engineered Arabidopsis plants to make isoprene. Adding just one gene coding for one enzyme by genetic engineering causes plants to emit isoprene. Plants that lack that enzyme cannot. They’ve also discovered that plants without naturally occurring isoprene emission respond to isoprene the same as plants that emit it; isoprene effectively protects both plant types from stressors. Dr. Sharkey predicts that in two years, the mechanism by which isoprene is detected by plants could be known.
• Interaction between Plants and Microbiomes for Growth - Due to recent novel discoveries in their lab, Dr. Sharkey and his team believe that microbes in soil make important gasses that plants can detect, which improves plant growth. They found that a plant grown in sand—which lacks organic matter and bacteria—does not grow as well as one in microbe-rich soil. By identifying the gases made by bacteria and comparing the effects on plants relative to effects of isoprene, they aim to identify how beneficial bacteria can improve plant growth and resilience. To do this, they will build a special chamber around soil, which will enable them to slowly pull the gas out of the soil. Then, Dr. Sharkey and his team will either accumulate the gas in a large vacuum container or absorb it onto activated carbon. Heating the activated carbon will cause the gas to leave the charcoal so that it can be analyzed. This will allow them to identify which microbiomes stimulate plant growth, and select the appropriate bacteria for improved crop growth. This project likely will take three to five years to make important new insights into this process.
• Improving the Isoprene Enzyme in Plants - Isoprene is a very effective building block for chemicals. Dr. Sharkey and his team are exploring ways to improve this plant enzyme, making it more efficient for other uses. When made in high concentrations, isoprene can be collected and turned into rubber or other chemical products. Most of the world’s rubber supply comes from rubber trees, but they’re very susceptible to diseases. Because there’s an increasing threat to the world’s supply, Dr. Sharkey and his team are making isoprene biologically.