Asking For Trouble: How Some Plants Invite Bacteria To Attack

By Shweta Iyer on April 27, 2014 1:11 PM EDT

tomatoes
While most plants may benefit from staying alive, some of them take a more harmful route, inviting bacteria to attack them by releasing chemical signals. (Photo: Shutterstock)

You would never invite a thief to your house and give him the keys to your safe, would you? Scientists have discovered, however, that such a scenario occurs in a certain species of plants - they actually invite pathogens to them, sending signals indicating that they're there, vulnerable to attack.

The plant, Arabidopsis, is a small flowering plant often used in biological studies. For their study, researchers from the University of Missouri, the Biological Sciences Division at Pacific Northwest National Laboratory (PNNL), and EMSL, DOE's Environmental Molecular Sciences Laboratory, analyzed the chemical composition of the signal, which in turn attracts bacteria to the plant. Upon making contact, the bacteria injects the plant with harmful proteins, thus causing infection.

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"This signaling system triggers a structure in bacteria that actually looks a lot like a syringe, which is used to inject virulence proteins into its target. It's exciting to learn that metabolites excreted by the host can play a role in triggering this system in bacteria," co-author Thomas Metz said in a statement.

The research was conducted on bacteria known as Pseudomonas syringae pv. tomato DC3000, which can ruin tomatoes as well as Arabidopsis. The bacteria infect their host via a molecular system known as the type 3 secretion system (T3SS) - infection in tomatoes causes them to develop brown spots.

To identify the compounds that triggered T3SS in the bacteria, the researchers conducted experiments on a mutant form of Arabidopsis known as Arabidopsis mkp1, which has shown resistance to Pseudomonas syringae. The researchers compared levels of metabolites in both mutant and regular forms of the plant, and found that a combination of acids in Arabidopsis triggered the bacteria's T3SS. These five acids were identified as pyroglutamic, citric, shikimic, 4-hydroxybenzoic, and aspartic acids.

These acids are present at a much lower rate in the mutant plant than in normal plants, which is why it's not susceptible to infections from Pseudomonas syringae. The researchers showed this by introducing higher levels of the acids into the mutants, which then became susceptible. "We know that microbes can disguise themselves by altering the proteins or molecules that the plant uses to recognize the bacteria, as a strategy for evading detection," lead researcher Scott Peck said in the statement. "Our results now show that the plant can also disguise itself from pathogen recognition by removing the signals needed by the pathogen to become fully virulent."

This study can help scientists develop microbial resistance in plants. The bacteria are also known to convert materials like switchgrass and wood chips into useable fuel, and a better understanding of their molecular machinery can help conserve energy. But perhaps most importantly, the study can help us understand infections better, as pathogens responsible for human infection also use a mechanism similar to T3SS.

"There isn't a single solution for disease resistance in the field, which is part of the reason these findings are important," Peck said. "The concept of another layer of interaction between host and microbe provides an additional conceptual strategy for how resistance might be manipulated. Rather than trying to kill the bacteria, eliminating the recognition signals in the plant makes the bacteria fairly innocuous, giving the natural immune system more time to defend itself."

Photo courtesy of Shutterstock.

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