Stomata are the plant's version of a window—an opening in the leaf epidermis where photosynthetic gas exchange occurs. This pore is a perfect way in for bacteria, which cannot themselves penetrate the epidermis. But the new findings show that, upon sensing bacterial proteins, the two guard cells that form the pore close the window.
Closure depends on signaling pathways activated by plant hormones, including salicylic acid and abscisic acid. A bacterial strain that successfully invades Arabidopsis overrides the closure and reopens stomata. This bypass requires a bacterial compound called coronatine, which antagonizes salicylic acid and abscisic acid, possibly by mimicking another hormone, called jasmonic acid.
Other successful bacteria lack coronatine. They might use other tricks to reopen stomata or wait around on the surface for heavy rain, which opens the pores for them.
Defensive stomatal shutting has been overlooked because most plant pathogen researchers deliver bacteria directly below the epidermis—a “quick and easy” laboratory technique, according to He. But he believes that surface inoculations, a more natural state, will become more common now that this plant line of defense has been identified.