A diagram depicting stomata as entry sites for bacterial invasion. (a) A cross-sectional view of leaf epidermis and mesophyll cells showing that stomata, formed by pairs of guard cells (GC), in light-adapted Arabidopsis leaves are mostly fully open. (b) Upon exposure to bacteria, guard cells perceive PAMPs and many stomata close within 1 h. Because not all stomata are closed, those ‘non-responsive’ open stomata (e.g., stomate on the left) may provide a route for bacterial entry at a basal level. Dashed arrows indicate diffusion of PAMPs on the epidermal surface or in the intercellular space (apoplast) between mesophyll cells, which can also perceive PAMPs and activate additional host defenses. (c) In the case of the virulent plant pathogen Pst DC3000, 3 h after infection bacteria produce diffusible COR in the apoplast and/or on the plant surface to re-open closed stomata (e.g., middle stomate), thereby increasing the number of sites for bacterial invasion. Pst DC3000 bacteria also inject TTSS effectors into mesophyll cells to suppress host defenses and to release nutrients. COR and TTSS effectors have overlapping functions in the suppression of host defenses in the apoplast and in the promotion of disease symptoms (see text). COR also induces disease susceptibility systemically in leaves (; see text). This figure is adapted from Underwood et al. () with permission of the Publisher.