More than 50 years, ago researchers discovered that plants can sense carbon dioxide (CO2) concentrations. As CO2 levels change, “breathing” pores in leaves called stomata open and close, thus controlling evaporation of water, photosynthesis and plant growth. Stomata are small openings or pores found on the surface of leaves and other plant organs. They are important for a number of functions, including the exchange of gases, such as oxygen and carbon dioxide, between the plant and the atmosphere.
Stomata are typically found on the undersides of leaves and are controlled by tiny guard cells that open and close the stomata to regulate gas exchange. This process is important for photosynthesis, the process by which plants convert sunlight into energy. It is also important for transpiration, the process by which plants release water vapor into the air. Stomata play a critical role in plant physiology and are essential for the health and growth of plants.
Plants lose more than 90% of their water by evaporation through stomata. The regulation of stomatal pore openings by CO2 is crucial for determining how much water plants lose, and is critical due to increased carbon dioxide effects on climate and water resources in a warming world.
Typically, it is believed that guard cells are able to sense the levels of carbon dioxide in the air and adjust the size of the stomata accordingly. When the levels of carbon dioxide are high, the guard cells will open the stomata to allow more carbon dioxide to enter the plant for photosynthesis. When the levels of carbon dioxide are low, the guard cells will close the stomata to conserve water and prevent excess water loss through transpiration.
In this way, the guard cells act as carbon dioxide sensors within the plant, helping the plant to regulate its gas exchange and maintain optimal conditions for growth and development. But still identifying the carbon dioxide sensor and explaining how it operates within plants has remained a longstanding puzzle.
Using a mix of tools and research approaches, scientists at the University of California San Diego recently achieved a breakthrough in identifying the long-sought CO2 sensor in Arabidopsis plants and unraveled its functioning parts. UC San Diego project scientist Yohei Takahashi, School of Biological Sciences Distinguished Professor Julian Schroeder and their colleagues identified the CO2 sensor mechanism and detailed its genetic, biochemical, physiological and predicted structural properties. Their results are published December 7 in Science Advances.
Since the stomatal pores control plant water loss, the sensor is vital for water management and holds implications for climate-induced drought, wildfires, and agricultural crop management. For example, in times of drought, plants may close their stomata to conserve water and prevent excess water loss through transpiration. This can help the plants to survive in harsh conditions, but it can also reduce their ability to photosynthesize and grow. In the case of wildfires, high levels of carbon dioxide in the air can cause plants to open their stomata, which can increase the risk of the plants catching fire. In agricultural crop management, the ability of plants to sense and respond to changes in carbon dioxide levels can be used to improve crop yields and quality.
“For each carbon dioxide molecule taken in, a typical plant loses some 200 to 500 water molecules to evaporation through the stomatal pores,” said Schroeder, Novartis Chair and faculty member in the Department of Cell and Developmental Biology. “The sensor is extremely relevant because it recognizes when CO2 concentrations go up and determines how much water a plant loses as carbon dioxide is taken in.”
One critical surprise from the new research was the composition of the sensor. Rather than tracing it to a single source or protein, the researchers found that the sensor operates through two plant proteins working together. These were identified as 1) a “high leaf temperature1” protein kinase known as HT1 and 2) specific members of a mitogen-activated protein kinase family, or “MAP” kinase enzyme, known as MPK4 and MPK12.
“Our findings reveal that plants sense changes in CO2 concentration by the reversible interaction of two proteins to regulate stomatal movements,” said Takahashi, who is now based at the Institute of Transformative Bio-Molecules, Japan. “This could provide us a new plant engineering and chemical target towards efficient plant water use and CO2 uptake from the atmosphere.”
The team’s findings, which have been filed in a UC San Diego patent, could lead to innovations in efficient water use by plants as CO2 levels rise.
“This finding is relevant for crops but also for trees and their deep roots that can dry out soils if there’s no rain for long periods, which can lead to wildfires,” said Schroeder. “If we can use this new information to help trees respond better to increases in CO2 in the atmosphere, it’s possible they would more slowly dry out the soil. Similarly, the water use efficiency of crops could be improved — more crop per drop.”
To further explore their sensor discovery, the researchers collaborated with graduate student Christian Seitz and Professor Andrew McCammon in the Department of Chemistry and Biochemistry. Using cutting-edge techniques, Seitz and McCammon created a detailed model of the intricate structure of the sensor. The model implicated areas where genetic mutations have been known to restrict the ability of plants to regulate transpiration in response to carbon dioxide. The new imagery showed that the mutants cluster in an area where the two sensor proteins, HT1 and MPK, come together.
“This work is a wonderful example of curiosity-driven research that brings together several disciplines — from genetics to modeling to systems biology — and results in new knowledge with the ability to aid society, in this case by making more robust crops,” said Matthew Buechner, a program director in the U.S. National Science Foundation’s Directorate for Biological Sciences, which supported the research.
The paper’s full author list: Yohei Takahashi, Krystal Bosmans, Po-Kai Hsu, Karnelia Paul, Christian Seitz, Chung-Yueh Yeh, Yuh-Shuh Wang, Dmitry Yarmolinsky, Maija Sierla, Triin Vahisalu, J. Andrew McCammon, Jaakko Kangasjarvi, Li Zhang, Hannes Kollist, Thien Trac and Julian I. Schroeder.