Cornell researchers have discovered a previously unknown way plants regulate water that is so fundamental it may change plant biology textbooks - and open the door to breeding more drought-tolerant crops.
Until now, scientists believed that pores on a leaf's surface, called stomata, which exchange both water vapor and carbon dioxide with the atmosphere, were the plant's only means for regulating water loss.
But a new study describes for the first time how water regulation also occurs under the leaf's surface, at the membranes of photosynthesizing cells. The result was made possible thanks to AquaDust, a Cornell-developed nanoscale sensor that measures water status inside leaves.
"What we've discovered occurs within the last 100 microns of the long path that water follows during transpiration from the root to the site where it evaporates inside the leaves," said Abe Stroock, the Gordon L. Dibble '50 Professor in the Smith School of Chemical and Biomolecular Engineering in Cornell Engineering.
Sabyasachi Sen, a doctoral student in Stroock's lab, and Piyush Jain, a former Stroock lab member who developed AquaDust, now co-founder and CEO of a start-up called Kubbly, are co-first authors of the study, which published Nov. 19 in the Proceedings of the National Academy of Sciences. Jain and Stroock are the paper's co-corresponding authors.
In plants, water absorbed by roots travels up vascular tissue called xylem. In the leaves, water from the xylem evaporates from photosynthesizing mesophyll cells into tiny intercellular air spaces, where it leaves as water vapor out the stomata.
This study found that while the mesophyll cells remain saturated with water, the tiny intercellular spaces around them can become very dry. This difference in water status is created as the water moves across the mesophyll cells' membranes, providing a second regulator of water flow along with stomata.
Having a window into how water moves inside the leaf made the finding possible.
"Our lab developed a tool to peer into the tiny airspaces in leaves to measure the dynamics of water stress at the cellular scale," Stroock said of AquaDust, a soft, synthetic, water-absorbent gel that occupies intercellular spaces in the mesophyll area and swells and shrinks based on water availability in the leaves.
AquaDust also contains dyes that fluoresce depending on how close dye molecules are to each other. Fiber optics or fluorescence microscopy let the researchers shine a light and receive a spectral measurement that reveals the local water stress in the leaf.
The discovery is important because plant breeders are looking for new ways to breed more water-use efficient crops, but the processes that stomata regulate have created unique challenges.
Plants take in carbon dioxide for photosynthesis to make sugars for energy and growth. But when water is scarce, stomata close to conserve moisture, which also limits carbon intake and slows growth. The trade-off between growth and saving water creates obstacles for breeders trying to improve water-use efficiency, but the study reveals a potential way around it.
"We have discovered a new trait that selectively blocks excess water loss from leaves to the atmosphere without compromising carbon assimilation, thereby improving water-use efficiency," Sen said.
The researchers are already working with Corteva Agriscience, a global agriculture company, to better understand how to breed more water-use efficient maize. "If the world wants to breed for this, the next step is to understand which genes control this trait" Sen said.
Collaborators include Robert Twohey III and Anthony Studer at the University of Illinois, Urbana-Champaign, Fulton Rockwell and N. Michele Holbrook at Harvard University, Annika Huber, Sahil Desai, I-Feng Wu, and Mehmet M. Ilman at Cornell University, and Tom De Swaef at Flanders Research Institute for Agriculture, Fisheries and Food at Belgium.
The study was conducted through the Center for Research on Programmable Plant Systems at Cornell, which is funded by the National Science Foundation.
