Spray-on antibacterial coating offers new protection for plants against disease and drought

(Press-News.org) Engineers at the University of California San Diego have developed a spray-on polymer coating that could help plants resist harmful bacterial infections and survive drought. The advance, published in ACS Materials Letters, could help strengthen global food security as increased environmental stresses continue to intensify plant disease pressures.

Bacterial infections are a growing threat to agriculture as they contribute to major crop losses worldwide. These infections, which are driven by both Gram-negative and Gram-positive bacteria, cause destructive plant diseases such as wilt, blight, speck and canker. Rising temperatures are also allowing pathogens to expand into new regions. As a result, crops are increasingly exposed to a variety of infections.

To tackle this challenge, researchers from the labs of Jon Pokorski and Nicole Steinmetz, both professors in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering at the UC San Diego Jacobs School of Engineering, and members of the UC San Diego Materials Research Science and Engineering Center (MRSEC), joined forces to develop an antibacterial coating that can be sprayed directly onto plant leaves.

The coating is made from a specially designed synthetic polymer containing positively charged chemical groups in its structure that have antibacterial properties. These positively charged groups disrupt bacterial cell membranes, which makes them effective against a wide range of harmful microbes.

Creating the spray coating — and making it plant-friendly — was made possible by modifying a common polymer synthesis method to work in gentle, water-based conditions.

“Typically, polymers are synthesized using organic solvents that are toxic to plants,” said study co-first author Luis Palomino, a chemical and nano engineering Ph.D. candidate in Pokorski’s lab. “What we did differently here is we made the polymer in buffer conditions in water. That allowed us to make a spray formulation that’s more biocompatible with plants. We can easily dissolve the polymer to the right concentration in water and just spray it on.”

Using this modified method, the researchers produced a type of polymer material, called polynorbornene, that is permeable to gases. This feature is important to allow the plant leaves to continue breathing and growing normally.

The coating was sprayed onto the leaves of Nicotiana benthamiana, a common plant model used in lab tests and plant molecular farming, which is the production of high-value molecules in plants. It protected live plants against Agrobacterium infection. When tested on individual leaves in petri dishes, the coating inhibited the growth of both Gram-negative and Gram-positive bacteria, such as Escherichia coli and Staphylococcus aureus.

A surprising discovery was that “full leaf coverage was not necessary to achieve protection,” said study co-first author Patrick Opdensteinen, a postdoctoral researcher in Steinmetz’s lab. “We can spray just a small part of the leaf, and that translates to bacterial immunity for the whole plant. That was a really cool outcome.”

The researchers believe this whole-plant protection may stem from a systemic stress response. Treated leaves briefly showed a mild increase in hydrogen peroxide, which serves as a natural signaling molecule that plants produce when responding to stress. This response declined over time, and the plants remained healthy throughout the study. The researchers hypothesize that this brief stress signal may be activating a broader defensive response throughout the plant.

Another result was that coated plants also showed improved drought tolerance. When water was withheld for four days, sprayed plants remained healthier and wilted less than untreated plants. The researchers hypothesize that the polymer coating may be acting as a physical barrier to reduce water loss, as well as inducing molecular-level stress response mechanisms that increase drought resistance.

Ongoing work will further explore the mechanisms behind both the whole-plant bacterial and drought resistance. The team will also focus on improving biodegradability and evaluating toxicity for future implementation in the field.

“Our hope is to use this in the field to benefit agriculture, and this is the first step,” Opdensteinen said. “There’s a lot of potential for plant protection.”

Full study: “Polynorbornene Spray Coating to Enhance Plant Health”

This research was primarily supported by the National Science Foundation through the UC San Diego Materials Research Science and Engineering Center (UCSD MRSEC) DMR-2011924.

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