Slippery, drippy goop produced by Ralstonia bacteria has been identified as a key factor in the rapid wilting of crops such as tomatoes and potatoes, according to groundbreaking research. This study, published on January 22 in the Proceedings of the National Academy of Sciences, is the result of a unique collaboration between plant pathologists and engineers at the University of California, Davis.
Ralstonia solanacearum can persist in damp soils for years before infecting plants, spreading swiftly through the xylem, the plant’s water-carrying vessels. Once infected, plants can wilt and die within days. “My analogy is that they cause a heart attack for plants, because they clog up the vessels and cause plants to wilt and die,” explained Tiffany Lowe-Power, an associate professor of plant pathology at UC Davis.
The Role of Bacterial Goop
Like many bacteria, Ralstonia colonies secrete a film around themselves. This film, however, is particularly sloppy and challenging to work with, according to Lowe-Power. “Ralstonia are charismatically disgusting, there’s this like, real grossness to them,” she noted.
The secreted film is composed of a sugar-like molecule known as exo polysaccharide 1 (EPS-1). While it has been known that EPS-1 is linked to the bacteria’s lethal effect on plants, the exact mechanism remained elusive. “With the ways that microbiologists and geneticists go about answering questions, we are able to get somewhat close, but not really to the mechanism,” Lowe-Power stated. “We need a physicist.”
A Cross-Disciplinary Approach
Enter Hari Manikantan, an associate professor in the UC Davis Department of Chemical Engineering, who specializes in the mechanics of complex fluids. “I love goop of all forms — saliva, foams, lung surfactants, tears,” Manikantan said, highlighting his enthusiasm for the project.
Goopy fluids possess both viscous and elastic properties. Elasticity refers to a material’s ability to return to its original shape after being stretched, while viscosity measures its flow rate. “Silly putty, for example, is elastic over a short time scale. You bounce it, it’s a perfectly solid object. If you keep it on a table, it slowly flows out over minutes to hours,” Manikantan explained. “The question is what’s the relevant time scale.”
Discovering the Dynamics
Manikantan and Lowe-Power’s collaboration began during a new faculty training session before the pandemic, where they discovered their shared interest in goop. Utilizing equipment in Manikantan’s lab, they made precise measurements of the viscoelastic properties of secretions from Ralstonia colonies, collected by graduate student Matthew Cope-Arguello.
Their findings revealed that the pathogenic goop flows easily under shear forces similar to those in plant xylem vessels, enabling the bacteria to spread rapidly through infected plants. To determine the prevalence of this trait, Cope-Arguello developed a simple test: if bacteria grown on a plate with a biofilm drip when the plate is tilted, they possess this characteristic.
“We were really able to show, both from the data that our collaborators collected as well as data that we mined through publicly available genomes, that this polysaccharide is unique to the plant pathogens,” Cope-Arguello said.
Implications for Science and Agriculture
For biologists, this research elucidates why EPS-1 makes these bacteria particularly pathogenic. For engineers and physicists, it offers a new experimental system to explore. “Now we have this actual relevant change that’s guided by genetics that my community can begin to mathematically model. So I’m very excited about how this feeds back into that soft matter physics world,” Manikantan remarked.
The study’s coauthors include researchers from UC Davis, The Ohio State University, University of South Alabama, University of Massachusetts, Amherst, University of Dayton, University of Wisconsin-Madison, University of Michigan Medical School, and University of Vermont. The work received support from the UC Davis Academic Senate, the U.S. Department of Agriculture, and the National Science Foundation.
This development represents a significant step forward in understanding the mechanisms by which Ralstonia devastates crops, potentially paving the way for new strategies to combat plant diseases and improve agricultural resilience.
