As Temperatures Rise, Researchers Identify Mechanisms Behind Plant Response to Warming

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Microscopic pores on the surface of leaves called stomata help plants "breathe" by controlling how much water they lose to evaporation. These stomatal pores also enable and control carbon dioxide intake for photosynthesis and growth.

As far back as the 19^th century, scientists have known that plants increase their stomatal pore openings to transpire, or "sweat," by sending water vapor through stomata to cool off. Today, with global temperatures and heat waves on the rise, widening stomatal pores are considered a key mechanism that can minimize heat damage to plants.

But for more than a century, plant biologists have lacked a full accounting of the genetic and molecular mechanisms behind increased stomatal "breathing" and transpiration processes in response to elevated temperatures.

University of California San Diego School of Biological Sciences PhD student Nattiwong Pankasem and Professor Julian Schroeder have constructed a detailed picture of these mechanisms. Their findings, published in the journal New Phytologist, identify two paths that plants use to handle rising temperatures.

"With increasing global temperatures, there's obviously a threat to agriculture with the impact of heat waves," said Schroeder. "This research describes the discovery that rising temperatures cause stomatal opening by one genetic pathway (mechanism), but if the heat steps up even further, then there's another mechanism that kicks in to increase stomatal opening."

For decades, scientists struggled to find a clear method to decipher the mechanisms underlying rising temperature-mediated stomatal openings due to the intricate measurement processes required. The difficulty is rooted in the complex mechanics involved in setting air humidity (also known as the vapor pressure difference, or VPD) to constant values while the temperature increases, and the trickiness of picking apart temperature and humidity responses.

Pankasem helped solve this problem by developing a novel approach for clamping the VPD of leaves to fixed values under increasing temperatures. He then teased out the genetic mechanisms of a range of stomatal temperature responses, including factors such as blue-light sensors, drought hormones, carbon dioxide sensors and temperature-sensitive proteins.

Important for this research was a new generation gas exchange analyzer that allows improved control of the VPD (clamping the VPD to fixed values). Researchers can now conduct experiments that elucidate the temperature effects on stomatal opening without the need to remove leaves from whole living plants.

The results revealed that the stomatal warming response is dictated by a mechanism found across plant lineages. In this study, Pankasem investigated the genetic mechanisms of two plant species, Arabidopsis thaliana, a well-studied weed species and Brachypodium distachyon, a flowering plant that is related to major grain crops such as wheat, maize and rice, representing an opportune model for these crops.

The researchers found that carbon dioxide sensors are a central player in the stomatal warming-cooling responses. Carbon dioxide sensors detect when leaves undergo rapid warming. This starts an increase in photosynthesis in the warming leaves, which results in a reduction in carbon dioxide. This then initiates the stomatal pores to open, allowing plants to benefit from the increase in carbon dioxide intake.

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