Newly discovered corn trait may help improve crop drought tolerance

UNIVERSITY PARK, Pa. — Some corn plants are genetically predisposed to develop longer, less constricted water-conducting tissues and deeper roots, which helps them deal with drought. That’s the conclusion of a team led by Penn State researchers that conducted a study of the plant’s xylem tissue that moves water upward from the roots out to the leaves.

The researchers, who focused on metaxylem vessel element length — the length of the individual tube-like cells inside the xylem — recently published their findings in Crop Science. They found that corn plants with longer metaxylem vessel elements also tend to have rapidly elongating roots, deeper root systems, stronger water transport capacity, greater water capture and improved drought adaptation. These traits form a connected syndrome the team called the “stretch phenotype.”

“Drought is a primary limitation for crop production and is projected to worsen due to climate change,” said study senior author Jonathan Lynch, distinguished professor of plant nutrition in the Penn State College of Agricultural Sciences. “Understanding and managing crop drought tolerance is an urgent priority for global agriculture. This study shows that corn plants with longer xylem vessel elements move water more efficiently, grow deeper roots, access deeper soil moisture and produce better yields during drought.”

Corn plants naturally vary in metaxylem vessel element length, the researchers found. They analyzed hundreds of corn plants from different genetic lines, from different regions, with varying characteristics and found that some had the longer cells in their xylem; others had shorter cells.

The team reported that longer xylem vessel elements were associated with lower perforation plate height — meaning that there was less constriction where those cells grew together, making longer continuous vessels and better hydraulic conductance — a measure of how easily water moves through the tissue or root. A xylem perforation plate is the porous end wall connecting two adjacent vessel elements in a plant's vascular tissue. By dissolving these end walls during cell maturation, plants create continuous, open tubes that allow water and minerals to flow efficiently from the roots to the leaves.

“Think of it like this — short pipes with many barriers result in slower flow and conversely, long, smooth pipes conduct faster flow,” Lynch explained. “The perforation plates are like tiny partitions between xylem cells. Smaller/lower barriers mean less resistance to water flow. So, longer xylem vessel elements create a more efficient water-transport system.”

Perhaps more importantly, Lynch said, the stretch phenotype consists of longer cells in various pant tissues, including the roots, meaning that roots grow faster and reach deeper in the soil, which helps them acquire water under drought conditions.

The researchers used computer simulations to confirm the stretch phenotype by modeling how xylem traits affect water flow. They then checked real root tissue in corn they grew under drought stress in the greenhouse and at two field locations — one at Penn State’s Russell E. Larson Experimental Farm in central Pennsylvania and the other at the Tuniche Research Farm near Graneros, Chile. The team employed rain-exclusion structures to simulate drought conditions at the Pennsylvania site, whereas the site in Chile has a Mediterranean climate that is naturally dry all summer.

Computer simulations, plants grown in the greenhouse and the field-grown corn all demonstrated the same pattern: Plants with the stretch phenotype had better water capture and water transport, which led to better growth and yield under drought.

Using a genome-wide association study — a research method that scans the DNA of many corn plants to find genetic variations linked to a specific trait — the researchers found DNA markers associated with longer xylem vessel elements and perforation plate height. That shows these traits are at least partly genetically controlled, Lynch noted.

In the study, cryo-scanning electron microscopy — an imaging technique that allows scientists to view delicate samples such as biological tissues by flash-freezing them first — was used for closer observation of xylem perforation plates. This work was completed at Penn State’s Huck Institutes of the Life Sciences Microscopy Core Facility.

The researchers used laser ablation tomography — an advanced imaging technique developed by the Lynch lab a decade ago that combines a pulsed UV laser and serial imaging to capture high-resolution, three-dimensional images of corn root cross sections — to view the differences between corn plants with and without longer xylem vessel elements and lower perforation plate height.

This study’s findings suggest that plant breeders could improve drought tolerance in corn by selecting for plants with the stretch phenotype, Lynch noted. The stretch syndrome the team detected was characterized by the researchers as “pleiotropic,” meaning it occurs when a single genetic location affects multiple, seemingly unrelated traits. In this case, Lynch explained, the syndrome appears to be controlled by one or two major genes, which makes it easier to isolate.

“The seed companies are always interested in traits that can be used to breed better crops, and certainly drought is the biggest risk to crop production anywhere on Earth, including in rich countries like the U.S.,” he said. “Farmers do not usually irrigate corn, so they’re depending on the climate, which is quite variable. For example, this year significant parts of the U.S. corn belt have been pretty dry. So, anything we can do to improve corn’s ability to withstand drought would be important. The findings from this research identify a potential avenue to breed more drought-tolerant crops.”

Penn State-affiliated contributors include first author and lead researcher Christopher Strock, a postdoctoral scholar in the Lynch lab when the study was done, now crop phenotyping scientist with John Deere; Cody DePew, Penn State research technologist in plant sciences; and Jagdeep Sidhu, who earned his doctorate in agricultural and environmental plant science at Penn State, currently an assistant professor of root biology at the University of Missouri. A full list of authors and their affiliations are available in the paper.

This research was supported by the Foundation for Food and Agriculture Research’s Crops of the Future and the U.S. Department of Agriculture National Institute of Food and Agriculture.