A research team led by Purdue University has begun translating the complex molecular language of petunias. The grammar and vocabulary of this language are buried deep in the countless proteins and other compounds that populate the flower cells.

Because plants are rooted in the soil, they cannot escape insects, pathogens, or other threats to their survival. But plant scientists have long known that they send signals to each other through aromatic chemicals called volatile organic compounds.

“They use inhalants because they can’t talk,” says Natalia Dudareva, professor emeritus of biochemistry, horticulture and landscape architecture at Purdue. “Plants notify neighboring plants of pathogen attacks. It’s almost like a vaccine. Normally you won’t see any changes in the recipient plant. But when the recipient plant is infected, it responds much more quickly. It’s ready to respond.”

Plant scientists have known about preparations similar to this grafting for a long time, but until a few years ago they did not have the opportunity to study the process. They needed a marker to show that plants were releasing volatile compounds.

New discoveries in plant signaling

Dudareva and 13 co-authors describe new details of the detection process in a recently published article in the journal Science. The team includes researchers from Purdue, the Jean Monnet University of Saint-Etienne in France, and the University of California, Davis.

Scientists know little about plant receptors for phytoncides. Mammals and insects also have them, but the way they sense volatiles is too different to help researchers study the process in plants, Dudareva said. A research team led by Purdue University scientists has documented new details about how petunias use volatile organic compounds to communicate.

The role of phytoncides in plant protection

In the magazine in 2019 Nature Chemical Biology Dudareva and colleagues published the discovery of a new physiological process called “Natural fumigation as a mechanism for the transport of volatile substances between flower organs.” The study describes how the plant’s flower tubes produce volatile compounds to sterilize the stigma, the pollen-collecting part of the pistil, to protect against pathogen attack.

“Especially in petunia, there is a lot of sugar in the stigma. This means that without these phytoncides, bacteria will grow very well,” Dudareva said. “But if the imprint cannot take in the volatiles that form in the tube, it is also smaller. This was an interorgan connection. We now had a good marker (imprint size) to study this communication process.” Measurements taken from photographs show statistical differences in stigma size with exposure to volatiles, said Shannon Stirling, Ph.D., lead author of the Science study. A student in the Department of Horticulture and Landscape Architecture at Purdue. “You see this is an ongoing trend,” she said. “Enough.” “If you look at the stamp, you will notice a slight difference in size.”

Groundbreaking discovery in understanding plant response

Combined with genetic manipulation of potential proteins of interest, the study unexpectedly revealed that a karrikin-like signaling pathway plays a key role in petunia cell signaling.

“Carrikins are not produced by plants,” Stirling said. “They form when plants burn, and our plants were never exposed to smoke or fire.”

The team also documented the importance of the karrikin-like pathway in the detection of volatile sesquiterpenes. Many plants use sesquiterpenes to communicate with other plants, among other functions.

Surprisingly, the identified karrikin receptor demonstrated the ability to selectively detect signals from a type of sesquiterpene compound and not from its mirror image, a property called “stereospecificity.” The receptor appears to be highly selective for the compound, said study co-author Matthew Bergman, a doctoral student in biochemistry at Purdue.

“The plant produces many different volatile compounds and is exposed to many other things,” Bergman said. said. “It is very surprising that this receptor is so selective and specific only for this signal sent through the tubules. This specificity ensures that no other unstable signals get through. There are no false signals.”

Methodological challenges and innovations

According to Stirling, the research required mastering the painstaking method of temporarily altering protein levels in petunia pistils to identify signal-receptor protein interactions. “Pistils and stigmas are small. They’re kind of hard to work with because of their size,” he said. “Even the number of scales you need to sample enough for anything is pretty large because they don’t weigh very much.”

This method involved injecting a specific bacterial strain into the imprint to insert target genes and then isolating the resulting proteins.

“It’s not easy to manipulate such a small organ,” Bergman said. “But Shannon was able to gently puncture the stigma with a syringe and penetrate this bacteria very precisely. He’s a real expert on this.”

Petunias are often brightly colored and fragrant, but Purdue scientists value them as well because they serve as a productive model system for their research.