Infection Alert

How plants unwittingly give a green light to bacterial pathogens.

Pathogenic bacteria cause fewer annual crop losses than fungi or viruses, but those losses are significant — just ask any farmer whose tomato plants have, at what should have been the apex of their fruit-bearing prime, collapsed into twisted piles of rotting yellow vines.

Combatting similar bacteria-borne disasters typically means adopting a costly, time-consuming regimen of crop rotation, weed management, sterilization and spraying with antibacterial agents. A scientist at MU believes helping vulnerable plants help themselves would be a vast improvement, and has recently completed an important first step toward doing just that.

In a study published in the April 21 issue of the Proceedings of the National Academy of Sciences, MU’s Scott Peck, an associate professor of biochemistry, teamed up with a far-flung group of researchers to gain insight into how a tomato-wilting bacteria — Pseudomonas syringaeĀ pv. tomato DC3000 — successfully invades its host. It’s a breakthrough, Peck says, that could one day help scientists strike a blow against future infections.

The discovery involves a twist in the way plants chemically present themselves to potential bacterial intruders, one that ultimately results in their unwittingly “inviting” attacks. Bacteria, Peck explains, are looking for sugar, which the plants produce during photosynthesis, and for organic acids that the researchers identified as part of their study. When all are present together, he says, the bacteria know it’s time to start infecting. If they move quickly enough, a plant’s immune defenses can be disabled before it has time to counterattack.

“When potential pathogens enter host plants, a race ensues to deploy their respective disease and defense mechanisms,” he says. “Scientists have paid a lot of attention to how plants and other organisms recognize and respond to invading microbes, but little attention has been paid to how the signals transmitted by the organisms that are being attacked play a role in the process. Our work focuses on suppressing the signals from the plant that cue the bacteria to attack.”

Key to that work was an Arabidopsis mutant resistant to P. syringae. With the help of scientists at the Pacific Northwest National Laboratory, Peck’s MU team compared metabolites in the resistant plant to those of a normal specimen. This eventually led to the identification of five acids that were significantly decreased in the mutant; these acids, the scientists surmised, were most likely to be involved in the crucial signaling process. To be certain, they added them to the resistant plant. Sure enough, its immunity vanished.

Now identified, Peck hopes the finding will lead to new ways of thinking about thwarting infection. “Our results show that the plant can disguise itself from pathogen recognition by removing the signals needed by the pathogen to become fully virulent,” he says.

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