Researchers reveal mechanisms of stomatal response to rising temperatures in plants

Researchers at the University of California (San Diego) have discovered new mechanisms by which plants respond to rising temperatures. The regulation of stomatal aperture, a key process for controlling photosynthesis and water loss in plants, has been the subject of a new experimental approach. The research revealed that, although stomata respond to rising temperatures, the molecular mechanisms behind these reactions are still poorly understood. Using a technique that maintains a constant difference in vapor pressure between the leaf and the air, scientists were able to observe rapid and reversible changes in stomatal aperture in intact plants subjected to heat.

The study analyzed mutants of Arabidopsis e Brachypodium, model plants for dicotyledons and monocotyledons, respectively. These mutants presented deficiencies in cellular signaling pathways responsible for the response to blue light, abscisic acid (ABA), carbon dioxide (CO2), as well as temperature-sensitive proteins, such as phytochrome B (phyB) and EARLY-FLOWERING-3 (ELF3). Among the results, it was clear that heat-sensitive proteins and ABA were not essential for the stomatal response to heating. However, the absence of CO2 receptors in the guard cells completely eliminated this response.

Stomata are microscopic pores present in leaves, responsible for the exchange of gases between the plant and the environment. They allow the entry of CO2 necessary for photosynthesis and regulate water loss through transpiration. The functioning of these pores is controlled by a series of environmental stimuli, such as light, CO2 and humidity. The impact of temperature changes on stomata is still not well understood.

With global warming, the increase in average global temperature is a growing concern for agriculture. The average surface temperature of the planet has already risen by 1,09°C compared to the pre-industrial era. Projections indicate that this upward trend will continue.

The team of researchers used a technique that measures changes in stomatal conductance—the ability of stomata to open or close—in response to rising temperatures. This technique differs from traditional methods because it controls the vapor pressure between the leaf and the air. The traditional method could confuse the results, since rising temperatures increase this pressure, causing the stomata to close. With the new approach, the researchers were able to isolate the effects of temperature on stomata more precisely.

The results indicate that proteins that regulate the response to blue light in leaves, such as phototropin receptors, play a role in the heat response. Mutant plants lacking these receptors showed a partial reduction in heat-induced stomatal opening. However, proteins that normally regulate stomatal closure in response to heat stress, such as phytochrome B and ELF3, did not play a relevant role in the heat response in this study.

Warming rapidly stimulates photosynthesis, leading to a reduction in internal CO2 levels in leaves. This triggers the opening of stomata, suggesting that the heat response mechanism is related to CO2 assimilation. However, under extreme heat conditions, stomata continued to open, but independently of photosynthesis, indicating the existence of another regulatory mechanism. This process has not yet been fully elucidated.

This discovery has important implications for agriculture, especially in a climate change scenario. Rising temperatures can directly affect plant productivity by altering the way they use water and assimilate CO2 for photosynthesis. Plants such as Arabidopsis thaliana, used in the study, are sensitive to temperatures above 27°C. This can lead to reduced growth.

The researchers also found that mutant plants lacking CO2 sensors, such as HT1 and MPK12/MPK4, completely lost the ability to open their stomata in response to warming. This suggests that the CO2 sensing mechanism is critical to the stomatal response to heat. These results have been replicated in monocots, such as Brachypodium. This suggests that this mechanism is conserved in different groups of plants.

The discovery that stomatal response to heat can be decoupled from photosynthesis at higher temperatures is especially relevant for the development of new agricultural management strategies. This includes the use of cultivars better adapted to hot environments, as well as irrigation management in regions that experience frequent heatwaves.

More information can be found at doi.org/10.1111/nph.20121