Plants rely on complex innate immune systems to make up for their lack of adaptive systems. They use complex signaling cascades that can detect and respond to bacterial and fungal pathogens both inside and outside their cells. One type of response is for the plant to close their stoma--the small openings on its leaves--which denies the pathogen entry points into the plant. The RIN4 protein has recently been shown to be instrumental in this process. Under normal circumstances, proteins embedded in the cell membrane send signals to RIN4 to keep stoma opened. When chemical signals from pathogenic microbes are detected, these signals stop and the stoma close.
"Bacterial pathogens gain entry inside plant leaves through wounds or natural openings like stomata...A pair of guard cells surrounds stomatal pores. Guard cells respond to diverse stimuli in order to regulate stomatal apertures...stomatal closure is induced...in an attempt to restrict bacterial invasion." (Liu et al. 2009:2)
"RIN4 protein acts in concert with PM H+-ATPases to regulate stomatal apertures...Importantly, the RIN4 knockout line does not re-open its stomata in response to virulent Pst." (Liu et al. 2009:10)