Editors' ChoicePlant biology

Touché! Plants and Bacteria Battle at Leaf Pores

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Science's STKE  12 Sep 2006:
Vol. 2006, Issue 352, pp. tw315
DOI: 10.1126/stke.3522006tw315

Plants have special openings on the surface of the leaf known as stomata, which allow gas exchange essential for respiration and osmotic balance. However, the stomata also provide a route by which infectious bacteria can gain access to internal tissues. The stomata are opened and closed in response to changes in exposure to light, humidity, and other stimuli, but new evidence shows that they can also be closed as part of the plants' immune defense against bacterial infection. Melotto et al. showed that Arabidopsis plants closed their stomata within 2 hours of exposure to the pathogenic bacterium P. syringae but then reopened them within a couple more hours. Microscopic observation of the bacteria showed that they were able to detect and migrate toward open stomata, perhaps sensing nutrients or other molecules released from the plant interior. The authors showed that flg22, a peptide derived from the bacterial flagellin protein, or lipopolysaccharide, a component of the bacterial outer cell wall, could also trigger stomatal closure. Plants are known to have immune receptors that recognize these molecules. The reopening of the stomata observed when leaves were exposed to whole bacteria led the authors to test whether the strain of P. syringae that they used produced a virulence factor to override the host plant's protective mechanism. Indeed, they found that the bacterially produced polyketide toxin coronatine was required to allow reopening of the stomata. The work reveals that plants have developed an innate immune mechanism to protect themselves from bacterial invasion and that, in response, some bacteria have developed a virulence factor that forces the pores open again to allow further infection.

M. Melotto, W. Underwood, J. Koczan, K. Nomura, S. Y. He, Plant stomata function in innate immunity against bacterial invasion. Cell 126, 969-980 (2006). [Online Journal]

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