This molecular mechanism allows plants to "control their mouths"

Produced by: Science Popularization China

Author: Li Yin (School of Life Sciences, Sun Yat-sen University)

Producer: China Science Expo

We have learned from books since childhood that plants can absorb carbon dioxide (CO2), release oxygen and water, and accumulate organic matter, so plants are called "producers". As the concentration of carbon dioxide in the atmosphere continues to rise, the global greenhouse effect continues to intensify. So, in this case, is there a way to make plants work harder, "eat" CO2 in large mouthfuls, and accumulate organic matter more efficiently? The answer is - yes!

Recently, scientists from the United States, Estonia and Finland discovered a molecular mechanism that allows plants to "control their mouths"!

Image source: Veer Gallery

The assimilation process of CO2 cannot be separated from this "mouth"

Seeing this, some friends may inevitably feel confused - do plants have mouths?

We all know that the assimilation of CO2 is indispensable in the Earth's life system. More than 90% of the organic matter in the dry weight of plants is converted through carbon assimilation. Plants absorb CO2 from the atmosphere, fix it through chemical processes in their cells, and then convert it into organic compounds such as sugars, amino acids and organic acids, which requires this "mouth".

What exactly is this "mouth"? In fact, it is a special structure - stomata mainly distributed on the surface of leaves.

Plants are able to regulate gas exchange between the plant and the atmosphere by opening and closing their stomata when sensing CO2 concentrations in the air spaces between leaf cells and changes in light intensity during the day and night. Increased CO2 concentrations in leaves cause the stomata to close rapidly, thereby inhibiting transpiration, which leads to water loss. Conversely, responses to low CO2 concentrations cause the stomata to open.

Stomata of plants

Image source: provided by the author

The continuous rise in CO2 concentration in the atmosphere will cause the stomata of plants to close. Once these "mouths" are closed for a long time, they will seriously affect the plant's water transpiration, photosynthesis and plant growth process.

However, the working mechanism of how plants sense changes in CO2 concentration in the environment and regulate the opening and closing of stomata through sensor-like control has not been fully understood before.

Just recently, after joint research by scientists from the United States, Estonia and Finland, they finally found a CO2 sensor in plant cells. This is a molecular pathway that plants use to regulate the inflow and outflow of CO2 by "controlling their mouths".

New discovery! Can plants also “control their mouths”?

The CO2 sensor discovered in this study involves several protein kinases, which are enzymes that catalyze protein phosphorylation and can change the conformation and activity of proteins and enzymes.

One of the protein kinases is named HT1 (high leaf temperature 1) . It is a gene that was discovered a long time ago from a mutant isolated from Arabidopsis thaliana by using infrared thermal imaging. Arabidopsis thaliana with HT1 gene mutation is insensitive to CO2 and shows higher leaf temperature than normal plants under low CO2 concentration, indicating that its transpiration, which loses water and lowers leaf temperature, is inhibited for some reason.

Scientists analyzed these mutant Arabidopsis and found that the HT1 gene mutation caused the plant's stomata to remain closed regardless of CO2 concentration. The discovery of the HT1 protein kinase indicates that protein phosphorylation is extremely important in CO2-induced stomatal movement.

Effect of atmospheric CO2 concentration on stomata

Image source: Li Y, et al. JXB, 2014, 65(13):3657–3667.

Based on previous studies, scientists already knew that two other proteins from the CBC (CONVERGENCE OF BLUE LIGHT AND CO2) protein kinase family, CBC1 and CBC2, also play a crucial role in stomatal CO2 response.

Under blue light and low CO2 concentration, CBC1 or CBC2 can stimulate stomatal opening, and CBC1 or CBC2 can interact with HT1 and be phosphorylated by it. Therefore, it is believed that HT1 and CBC kinase are both negative regulators of the stomatal closure process induced by high CO2 concentration.

In contrast, two other protein kinases, MPK4 and MPK12, from the MPK (mitogen-activated protein kinase) family, promote stomatal opening. Double mutations in the MPK4 and MPK12 genes keep the plant's stomata open and insensitive to high CO2 concentrations, indicating that MPK4 and MPK12 are positive regulators with overlapping functions in early CO2 signal transduction in stomatal guard cells.

Green Leaf

Image source: Veer Gallery

Before this, although scientists had some understanding of the role of this series of protein kinases, the exact CO2 sensor was not known, and the related signal network mechanism was also unclear.

If plants want to "control their mouths", they have to rely on this "sensor"

This latest achievement proposes a stomatal regulation signal transduction model related to CO2.

Under low CO2 concentrations, HT1 protein kinase activates the downstream negative regulatory protein kinase CBC1. The activation of CBC1 inhibits the mechanism that causes stomatal closure, forcing the guard cells on both sides of the stoma to swell, allowing the stomata to remain open for as long as possible, allowing them to "open their mouths" to absorb CO2 and meet the needs of photosynthesis.

When plants sense an increase in CO2 levels, the MPK4/MPK12 protein complex (MPK4/12) becomes involved in this regulatory process. When guard cells are exposed to high CO2 concentrations, MPK4/12 can be triggered to bind to HT1. The interaction after these proteins bind can inhibit HT1 kinase activity, leading to a decrease in CBC1 kinase activity, and further promote the induction of stomatal closure, allowing plants to quickly close their "mouths."

Based on the experimental phenomena, the research team predicted that there is an unknown protein phosphatase (PPase) that inhibits the action of CBC1 and activates the mechanism of stomatal closure under high CO2. One or more members of the CBC family homologous proteins, together with CBC1 and CBC2, may have overlapping functions that can partially replace each other in regulating stomatal opening in response to low CO2 conditions.

Plant stomatal CO2 sensor and its signal transduction mode

Image source: Takahashi Y, et al. Sci Adv, 2022, 8:eabq6161.

The CO2 response mechanism of stomata is crucial to plant growth. This process also regulates the water use efficiency of plants. This research result provides new evidence for explaining the complex regulatory process of plant respiration.

Stomata on green leaves

Image source: Veer Gallery

Conclusion

The revelation of the core signaling mechanism of CO2 regulating the degree of stomatal opening makes it possible to use the complex respiratory mechanism of plants to purposefully cultivate strong and high-yield crops.

In the future, scientists may be able to breed crop varieties that efficiently accumulate organic matter in response to the continued increase in atmospheric CO2 concentrations, allowing plants to "open their mouths" to eat more CO2 at the right time and "close their mouths" to reduce transpiration, thereby achieving the genetic improvement goal of increasing plant carbon intake and water use efficiency. Let us wait and see!

Editor: Ying Yike

References:

[1] Hashimoto M, et al. Nat Cell Biol, 2006, 8:391–397.

[2] Hiyama A, et al. Nat Commun, 2017, 8(1):1284.

[3] Li Y, et al. JXB, 2014, 65(13):3657–3667.

[4] Takahashi Y, et al. Sci Adv, 2022, 8:eabq6161.

[5] Tõldsepp K, et al. Plant J, 2018, 96(5):1018-1035.

[6] Zhang J, et al. Curr Biol, 2018, 28(23):R1356-R1363.