Sci. STKE, 12 September 2006 PLANT BIOLOGY Touché! Plants and
Bacteria Battle at Leaf Pores 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