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An Open and Shut Case

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Science  22 Sep 2006:
Vol. 313, Issue 5794, pp. 1705
DOI: 10.1126/science.313.5794.1705c

Stomata are openings on the surfaces of leaves that mediate the exchange of gases, which is essential for respiration and osmotic balance. However, these doorways also provide a route by which infectious bacteria can gain access to plant internal tissues. Stomata open and close in response to changes in exposure to light, humidity, and other stimuli, but Melotto et al. show that they can also be shut as a defense against bacterial invasion. Arabidopsis closed their stomata within 2 hours of exposure to the pathogenic bacterium Pseudomonas syringae, but reopened them a few hours later. Microscopic observation showed that the bacteria were able to detect and migrate toward open stomata, perhaps sensing nutrients or other molecules released from the plant interior. Flg22, a peptide derived from the bacterial flagellin protein, or lipopolysaccharide, a component of the bacterial outer cell wall, could trigger stomatal closure, and plants are known to have immune receptors that recognize these molecules. The subsequent reopening of the stomata led the authors to test whether P. syringae produced a virulence factor that could override the host plant's protective mechanism. Indeed, they found that the bacterially produced polyketide toxin coronatine was required for reopening of the stomata. These results reveal that plants have developed an innate immune mechanism to protect themselves against bacterial invasion and that in response some bacteria have developed a virulence factor that reopens doors. — LBR

Cell 126, 969 (2006).

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