Salicylic acid, the active molecule in aspirin and some acne medications, is a hormone in plants that is essential for immunity, but it's a double-edged sword: Too much can cause autoimmunity and stunt growth. In a new study published April 20 in Nature Communications, University of California, Davis, researchers discovered that plants use a surprising multi-layered system to regulate salicylic acid levels and keep their immune system in check.
Rising salicylic acid levels trigger the production of enzymes that break down the hormone, but in carrying out their job the enzymes are themselves flagged for elimination, which limits how much salicylic acid they can destroy. The findings could be used in agriculture to make crops more resilient to changes in their environment.
"Through this layered control, plants are able to balance immunity with growth, responding quickly to threats while avoiding the cost of prolonged defense," said senior author Nitzan Shabek, an associate professor of plant biology at UC Davis whose lab combines biochemistry and structural biology to investigate plant signaling. "Our discovery could open the door for innovation in agriculture by enabling new ways to fine-tune crop immunity without compromising growth."
Uncovering a new layer of control
The first clue came a few years ago, when Shabek's research group was mapping proteins that interact with the ubiquitin system, the machinery that cells use to recycle proteins. In that study, published in New Phytologist, the team was surprised to spot two salicylic acid-destroying enzymes (DMR6 and DLO1) as potential ubiquitin targets.
"I was intrigued by the possibility that the same enzymes responsible for deactivating salicylic acid are themselves being destroyed," said first author Natalie Hamada, who led the project as a Ph.D. candidate in Shabek's lab.
To investigate this possibility, the team used a range of scientific methods to probe DMR6 and DLO1, including structural biology, biochemistry, genetic engineering and plant infection experiments.
By examining DMR6 and DLO1's three-dimensional atomic structures, the researchers were able to pinpoint how the enzymes' shapes change when they interact with salicylic acid. When DMR6 binds to salicylic acid, its shape changes in a way that flags it for destruction by the ubiquitin system. In contrast, when DLO1 binds to salicylic acid, it becomes less likely to be targeted by the ubiquitin system. The difference could be that DMR6 modulates early immune responses, whereas DLO1 is thought to contribute later on, the researchers said.
Next, by tagging and mapping how DMR6 and DLO1 interact with other proteins, the researchers discovered a regulatory protein that marks both enzymes for destruction by the ubiquitin system. This protein, which they named DAF1, binds more strongly to DMR6 in the presence of salicylic acid, meaning that salicylic acid is triggering its own inactivation.
Finally, the team examined how the regulatory system works during real bacterial infections. When they genetically engineered tobacco plants to lack DAF1, the plants were more susceptible to bacterial infection. In contrast, when they engineered plants to produce more DAF1 than usual, the plants developed signs of autoimmunity, suggesting that their immune response was overzealous.
"It's like a seesaw - when plants don't have DAF1, their immune response is compromised, because DMR6 removes salicylic acid too efficiently, but when they produce too much DAF1, they degrade DMR6 too efficiently, which means they end up with excess salicylic acid," said co-author Jacob Moe-Lange, who worked on the project as a postdoctoral fellow in Shabek's lab. "Regulating the regulators of salicylic acid is critical for plants to successfully grow and balance priorities when they face stress."
Enhancing pathogen resistance without compromising growth
By revealing this new layer to salicylic acid regulation, this study could help make crops more resilient by allowing them to more efficiently switch between growth and mounting an immune response. Previous studies have shown that genetically engineering plants to reduce DMR6 can boost their immunity, but this strategy compromises plant growth and introduces regulatory hurdles associated with genetic modification. Targeting DAF1 could offer a more subtle approach.
"Our findings could potentially be used to fine-tune plant disease resilience without using genetic engineering," said Hamada. "For example, it might be possible to design molecules that enhance or inhibit interactions between DMR6 and DAF1, which could be strategically applied to non-GMO crops."
Additional authors on the study are: Malathy Palayam, Gabrielle Wyatt, Sun Hyun Chang, Annie Hu, Savithramma Dinesh-Kumar and Philipp Zerbe, UC Davis; and Justin Walley and Christian Montes, Iowa State University.
The work was supported by: the National Science Foundation; the U.S. Department of Energy, Office of Science, Biological and Environmental Research, Genomic Science Program; the National Institutes of Health; the United States Department of Agriculture and UC Davis's Simon Chan Memorial Fellowship.
This project utilized the UC Davis Controlled Environment Facility and the UC Berkeley Molecular Graphics and Computation Facility.
