Plants appear still, yet inside they are alive with communication. They must continuously balance how to capture energy without losing too much water. For decades, scientists suspected there were internal messengers guiding this process, but the exact molecules remained unidentified.
A new study led by Penn State researchers finally reveals the nature of these signals. This work not only solves a long-standing mystery but also suggests new directions for agriculture and plant resilience research.
How plants balance food and water
“This discovery significantly advances our understanding of how plants coordinate their internal metabolism – the chemical reactions they use to make energy – with their external environment, a fundamental process for plant growth and survival,” said Professor Sarah Assmann.
“Our findings open doors for future research into improving plant resilience and crop yields.”
Guard cells, located on the leaf surface, control stomata, the tiny pores that regulate the intake of carbon dioxide and the release of water vapor.
These pores act like microscopic mouths. When they open, plants can “eat” by absorbing CO2, but they risk losing vital water at the same time.
Mysterious plant messengers
“There is always a tradeoff for terrestrial plants between maximizing CO2 intake, which is needed for photosynthesis, and letting out water vapor, which can dry out the plant and ultimately kill it if it loses too much water,” explained Professor Assmann.
“The stomata are the pores where that tradeoff takes place. When they open, they let in CO2 that allows the plant to feed, but they also let out water vapor, which dehydrates the plant. We knew there had to be some kind of messenger telling the guard cells how to regulate that life-or-death decision.”
The new findings show that sugars, including sucrose, glucose, and fructose, as well as maleic acid, act as these critical messengers. These metabolites form a feedback loop, linking the plant’s energy production to its stomatal control.
Tracking signals in plant leaves
The researchers worked with the model plant Arabidopsis thaliana, or thale cress, and fava beans to uncover the energy production system in plants.
By extracting apoplastic fluid from leaves exposed to either red light or darkness, they were able to isolate chemical compounds. Red light stimulates photosynthesis, making it easier to detect active signals.
Through this process, the team identified 448 chemical compounds in the fluid. “We identified hundreds of metabolites in apoplastic fluid, which no one had analyzed to this extent before,” noted Professor Assmann.
“That, on its own, is an important contribution to the field, independent of the research question that we specifically were addressing, because it gives a lot of leads on other potential signaling molecules for processes throughout the plant.”
Sugars control plants water use
Further experiments revealed sugars directly promoted stomatal opening under red light. When tested in intact leaves, these compounds increased carbon dioxide uptake and also altered water release, confirming their messenger role.
Cell-level experiments explained the mechanism: sugars stimulate molecular machinery inside guard cells, activating them to open the stomata.
This research provides the first full picture of the internal dialogue between photosynthesis and water regulation.
Studying what makes plants resilient
The Nature Plants study also emphasizes that stomatal control does not rely solely on hormones, as once thought. Instead, metabolic products themselves can serve as powerful signaling agents.
This finding adds a new dimension to how scientists view plant-environment interactions. According to Professor Assmann, the team is focused on understanding how plants sense and respond to environmental conditions.
“Plants can’t uproot themselves and find somewhere else to live; they have to deal with whatever the environment throws at them – increasingly drought and heat stress,” she noted.
“So we study what makes plants resilient, from the very specific molecular level all the way up to whole plant physiology and field experiments, with the goal of improving crop productivity.”
The project brought together researchers from Penn State, The Hebrew University of Jerusalem, Nagoya University, RIKEN Center for Sustainable Resource Science, and the University of Mississippi.
The research was funded, in part, by the National Science Foundation.
The study is published in the journal Nature Plants.
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