The way to slip inside a leaf is through tiny pores called stomata. Typically, these pores release moisture and take in gases needed for the plant to thrive, while closing down during droughts to conserve water. Now researchers have discovered that pores also batten down their hatches when assaulted by bacteria. But plant pathogens, it turns out, can trick the pores into opening again.
Unlike fungi, which can infiltrate plant surfaces with ease, bacteria must finagle a passage through existing openings to access the cells inside. Stomata are the obvious choice. But Sheng Yang He, a molecular plant pathologist at Michigan State University in East Lansing, suspected stomata might play an active role in fighting infection.
He and his colleagues tested this in the lab by looking at how pores responded to various pathogens. When they exposed Arabidopsis leaves to bacteria, pores began shutting down within an hour. The stomata were still closed 8 hours later if the pathogen was an innocuous bacterium. But to their surprise the pores opened back up within 3 hours of exposure to a true plant pathogen, the researchers report in the 8 September Cell.
They discovered why: the pathogen, Pseudomonas syringae, contains a plant hormone mimic called coronatine, which causes the pores to open. Pseudomonas mutants that lack coronatine won't open stomata but can still cause disease when injected into the plant, suggesting that coronatine's sole purpose is to pry the pores open. According to co-author Maeli Melotto, many plant pathogens may use coronatine to neutralize plant defenses.
The key insight of the paper is that stomata "are a fundamental defense against invasion," says John Mansfield, a plant pathologist at Imperial College in London. The study "gets to grips with the microenvironment within which bacterial pathogens have to operate" by imitating field conditions, he says. In most such studies, researchers inject the bacteria into the plant, thus bypassing the natural route for infections.
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