Plants have a sophisticated internal communication system to help them optimize energy production. Now, a new study by an international team of scientists led by Penn State researchers reveals for the first time the molecular messengers that control how and when plants "breathe" and "eat," which could have implications for agriculture.
"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 Sarah Assmann, Waller Professor of Plant Biology at Penn State and corresponding author on the study published today (Aug 25) in the journal Nature Plants. "Our findings open doors for future research into improving plant resilience and crop yields."
For decades, plant scientists have tried to understand how the internal cells of a leaf communicate with guard cells, specialized cells in the outermost cell layer of the leaf. Pairs of guard cells encircle and control the width of stomata, the microscopic pores in the outer layer of the leaf that impact vital processes like energy production and water loss.
Stomata serve as microscopic "mouths" on leaves, opening and closing to control the intake of carbon dioxide (CO2), essential for making the carbohydrates that provide energy to the plant. Stomata also control the release of water vapor back into the atmosphere, Assmann explained. While it was known that guard cells open stomata in response to light, which drives photosynthesis, and there has long been evidence of a chemical "messenger" from inside the leaf that guides this process, the identity of the messenger had remained elusive.
"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," Assmann said. "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."
Their research, conducted on mouse-ear cress - scientific name Arabidopsis thaliana - and fava beans, or Vicia faba, revealed that sugars, along with maleic acid, a chemical involved in energy production, act as these crucial messengers.
The scientists identified and characterized the molecular feedback loop between photosynthetic activity and stomatal regulation through a long series of painstaking experiments.
First, they carefully extracted apoplastic fluid, the liquid found between plant cells, from leaves exposed to either red light, which stimulates high photosynthesis, or darkness. By isolating and characterizing chemical compounds or "metabolites" in the fluid, they hypothesized they would be able to find the messenger traveling through the fluid, much like spotting a mail carrier on a busy city intersection.
By analyzing the chemical composition of the apoplastic fluid, the researchers identified a total of 448 unique chemical compounds - many more than were previously known - that are essential for basic plant functions like growth and development.
"We identified hundreds of metabolites in apoplastic fluid, which no one had analyzed to this extent before," Assmann said. "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."
Through extensive analysis of this fluid, the researchers identified sugars - including sucrose, fructose and glucose - and maleic acid as significant components that increased under red light, which activates photosynthesis. The researchers hypothesized that those particular metabolites would be able to enhance stomatal opening under red light.
To test their hypothesis, the researchers peeled off the thin outer layers of the leaf and exposed them to light in the presence or absence of sugars. They observed that the sugars indeed directly promoted stomatal opening in the isolated epidermis under red light. Next, they conducted a series of experiments on full leaves, using sugar feeding coupled with measurements of CO2 uptake and water loss to confirm that sugars signaled the stomata to open more widely.
Finally, they performed tests on single cells that revealed how sugars stimulate the molecular mechanisms that underlie guard cell control of stomatal opening. Overall, the work provides the first complete picture of this internal communication process within plants that can determine their survival in a range of climates, Assmann said.
"We're focused on understanding how plants sense and respond to environmental conditions," she said. "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 - 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."
Other Penn State authors are doctoral student Yunqing Zhou and Associate Professor of Biology Timothy Jegla, and postdoctoral scholars Mengmeng Zhu and Yotam Zait, who is now an assistant professor at The Hebrew University of Jerusalem and led the research. Other authors are Adi Yaaran and Sunheng Yon of The Hebrew University of Jerusalem; Eigo Ando, Yuki Hayashi and Toshinori Kinoshita of Nagoya University in Japan; Mami Okamoto and Masami Y. Hirai of the RIKEN Center for Sustainable Resource Science; and Sixue Chen of the University of Mississippi.
The U.S. National Science Foundation funded the Penn State aspects of this work.
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