Foliar bacterial phytopathogens initially colonize the leaf surface as epiphytes, but subsequently become endophytes as the infection progresses. Because bacteria cannot directly penetrate the leaf epidermis, endophytic colonization occurs through natural openings, such as hydathodes and stomata, or through accidental wounds. Stomata are small pores located in the leaf surface that allow plants to exchange gases with the environment. Stomatal apertures are finely regulated in response to hormones and environmental factors such as light intensity, air humidity, and CO₂ concentration, which allow the plant to maximize CO₂ intake required for photosynthesis, while minimizing water loss. Several internal and external stimuli, such as the hormone abscisic acid (ABA), low humidity, or a high concentration of CO₂, can bring about stomatal closure through a reduction in turgor of the two guard cells that constitute the stomatal pore. This is achieved at least in part through the efflux of osmotically active ions from these cells (Schroeder et al., 2001; Pandey et al., 2007).