For the first time, researchers have demonstrated in an intact plant a long-contested process that allows some plants to rebound from extended drought. The team of Colorado State University, University of Colorado and U.S. Department of Agriculture scientists says understanding this special trait could improve agricultural productivity and food security.
Drought costs the United States billions in agricultural losses and increased irrigation. Lost productivity lowers food availability and raises prices for consumers.
When a plant dries out, its water transport system becomes impaired through the formation of gas bubble blockages, or embolisms, in the plant's water-transporting tissues – xylem. Depending on the extent of embolism, some plants never recover.
To recover, the gas bubbles must be removed and water flow restored through a process called "refilling." Plant scientists have been divided on whether refilling happens. Most evidence in favor of refilling has been based on destructive study methods, where a plant is cut and water is forced into tissue at a higher pressure than exists in nature.
The CSU, CU and USDA researchers said that cutting into plants creates "artifacts," or byproducts from the study method, which in this case could cause embolisms and lead to inaccurate results. Instead, they used a micro-CT scanner, a specialized X-ray machine, to observe internal plant processes as they happen in nature.
Their study, published on the cover of April's edition of Proceedings of the National Academy of Sciences , found a complete reversal of embolism and full recovery in a type of wild grass within 24 hours of watering.
"This is the first convincing evidence of the reversal, or refilling, of embolism in a vascular plant species, with the plant regaining full functional recovery afterward," said Sean Gleason, a researcher with the USDA Agricultural Research Service, CSU affiliate and co-author of the paper.
The researchers have started to look for other plants with this trait to identify the genetic mechanism behind it. Once the mechanism is identified, it could potentially be bred into crops, making them more resilient to drought.
"If a plant can recover from drought quickly by refilling, then you might be able to recoup some losses during a drought year," said co-author Troy Ocheltree, a CSU associate professor in the Warner College of Natural Resources. "If plants can refill, this may allow flexibility in the amount and timing of irrigation, although additional work is required to identify how refilling would impact crop water use."
Happenstance study subject
Embolism occurs in all vascular plants, and previous studies found that embolism can't be reversed in some plants, although they may continue to function at reduced capacity once water is restored.
For this study, lead author Jared Stewart, a postdoctoral researcher with CSU, CU and the USDA ARS, then-CSU M.S. student Brendan Allen and then-CU Ph.D. student Stephanie Polutchko examined a grass that had been growing in a parking lot to see if it would be a good candidate for their experiment. The grass was thriving in the cracks of the hot, dry asphalt lot, so they figured it might be resilient after drought.
Sure enough, despite its dead appearance and as much as 88% embolized xylem tissue after a prolonged period without water, the grass's water-transporting system bounced back literally overnight.
So far, this is the only species known to refill, but the researchers think there likely are others with this trait.
"We don't know how common this is," Ocheltree said. "But the fact that we found a plant in the parking lot that refills makes me think there's probably others out there that also refill. It shifts our mindset."
Key partnership and equipment
The study was facilitated by a partnership with CSU's College of Veterinary Medicine and Biomedical Sciences, which has a micro-CT scanner for small animals. This specialized machine emits less radiation than many CT scanners, enabling repeated scans without harming subjects.
Gleason said support from lab staff and the principal investigator, Professor Nicole Ehrhart, director of CSU's Center for Healthy Aging and Laboratory of Comparative Musculoskeletal Oncology and Traumatology, was essential for the study. Laura Chubb, a lab technician, helped the team by scanning plants initially and then trained Gleason to use the machine.
"The resilience of this humble grass – reviving its vascular system overnight – was both surprising and deeply compelling," Ehrhart said. "Collaborations like this remind us how powerful it can be when tools developed for biomedical research are applied in new ways to answer fundamental questions about life. We were thrilled to contribute to such impactful science."
"Dr. Ehrhart and Laura made this study possible with their generous offer to help us with our scans and allow us use of their micro-CT lab," Gleason said. "Without them, we wouldn't have been able to do this research."
