Unlocking the Mystery of Plant Breathing – Scientists Discover Mechanism Plants Use To Control “Mouths”

A Magnified View of a Leaf Stoma Which Somewhat Resembles a Human Mouth

A magnified view of a stoma on the leaf of a Tradescantia albiflora albovittata plant, commonly known as a giant white inch plant. Credit: Douglas Clark

A significant discovery about the mechanisms by which plants open and close their stomata could lead to new methods of protecting crops from the effects of climate change, particularly the rising levels of carbon dioxide in the atmosphere.

While breathing is often taken for granted as an involuntary process, it is actually a complex mechanism. Biologists are now gaining a deeper understanding of the intricacies of breathing in plants, with important implications for meeting the future food needs of the world.

A team of researchers from the University of California San Diego, in collaboration with scientists from Estonia and Finland and funded by the U.S. National Science Foundation, have discovered a previously unknown molecular pathway that plants use to control their intake of carbon dioxide. The researchers believe that by utilizing this mechanism, it could be possible to improve the water-use efficiency and carbon intake of plants, which is crucial as the levels of carbon dioxide in the atmosphere continue to rise. In light of this, the team has filed a patent and is exploring ways to apply their findings to the development of tools for crop breeders and farmers.

The research was recently published in the journal Science Advances.

A Magnified View of Many Plant Stomata on a Begonia Leaf

A magnified view of many plant stomata on the leaf of a Begonia rex cultorum plant. The width of each stoma is about 80 microns. Credit: Douglas Clark

Stomata, so what-a?

Plants take in carbon dioxide and water and then use light to turn these into the nutrients they need to grow. This process also emits oxygen, which humans and other animals then breathe. That’s the basic summary of photosynthesis. But how exactly does it work?

The process becomes a bit clearer on the microscopic level. On the underside of leaves and elsewhere, depending on the plant, are tiny openings called stomata — thousands of them per leaf with variations by plant species. Like little castle gates, pairs of cells on the sides of the stomatal pore — known as guard cells — open their central pore to take in the carbon dioxide. However, when stomata are open, the inside of the plant is exposed to the elements and water from the plant is lost into the surrounding air, which can dry out the plant. Plants, therefore, must balance the intake of carbon dioxide with water vapor loss by controlling how long the stomata remain open.

A highly magnified video of a single stoma opening and closing on a leaf of a Tradescantia spathacea plant, commonly known as a boat lily. Credit: Douglas Clark

“The response to changes is critical for plant growth and regulates how efficient the plant can be in using water, which is important as we see increased drought and rising temperatures,” said Julian Schroeder, Torrey Mesa Research Institute chair in plant science at UC San Diego, who led the new research.

As the climate changes, both atmospheric carbon dioxide concentration and temperature increase, affecting the balance between carbon dioxide entry and water vapor loss through the stomata. If plants, especially crops like wheat, rice, and corn, can’t strike a new balance, they risk drying out, farmers risk losing valuable output, and more people across the world risk going hungry. Even with advances in agriculture, an NSF-funded study published in 2021 found that global agricultural productivity over the past 60 years is still 21% lower than it could have been without climate change.

Scientists have long understood stomata and the balance between carbon dioxide intake and water loss. What they haven’t known, until now, is how plants sense carbon dioxide to signal stomata to open and close in response to changing carbon dioxide levels. Knowing this will now enable researchers to edit those signals — so plants can strike the right balance between taking in carbon dioxide versus losing water — and allow scientists and plant breeders to produce crops robust enough for the environment of the future.

Calling the guards

The researchers identified a series of proteins that work like a chain of soldiers sensing the carbon dioxide level and calling out “CLOSE THE GATES!” to get the guard cells to relax and shut the stomata.

“Finding that the CO2 sensor in plants is made up of two proteins was enlightening and may be a reason the mechanism hadn’t been identified until now,” Schroeder said. “NSF support over the last two decades was critical to locating this elusive pathway.”

“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 NSF’s Directorate for Biological Sciences.

In a low-carbon dioxide environment where the plant needs to keep the stomata open longer to get the amount it needs for photosynthesis, a protein known as HT1 activates an enzyme that forces the guard cells to swell up, keeping the stoma open.

When the plant senses increased levels of carbon dioxide, a second protein blocks the first one from keeping the stomata open, and the stomata shut. If the stomata close before the plant can get enough resources for photosynthesis, agricultural yield can be lower or non-existent.

“Determining how plants control their stomata under changing CO2 levels creates a different kind of opening — one to new avenues of research and possibilities for addressing societal challenges,” said Richard Cyr, an NSF program director who studied plant cell biology prior to joining the agency.

Reference: “Stomatal CO2/bicarbonate sensor consists of two interacting protein kinases, Raf-like HT1 and non-kinase-activity activity requiring MPK12/MPK4″ by Yohei Takahashi, Krystal C. Bosmans, Po-Kai Hsu, Karnelia Paul, Christian Seitz, Chung-Yueh Yeh, Yuh-Shuh Wang, Dmitry Yarmolinsky, Maija Sierla, Triin Vahisalu, J. Andrew McCammon, Jaakko Kangasjärvi, Li Zhang, Hannes Kollist, Thien Trac and Julian I. Schroeder, 7 December 2022, Science Advances.
DOI: 10.1126/sciadv.abq6161

The study was funded by the National Science Foundation

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