Water immunity overrides stomatal immunity in plant resistance to Pseudomonas syringae

Stomata play crucial roles in the multilayered defense system against pathogens. Upon pathogen perception, stomata close promptly, establishing the first line of defense known as stomatal immunity. The bacterial pathogen Pseudomonas syringae (Pst) exploits open stomata for entry, however, it can also induce stomatal closure at post-invasive stages to enhance apoplastic hydration. This creates a favorable environment for Pst proliferation, evident as water-soaked lesions on leaves. During the post-invasive stages of Pst infection, plants deploy a second layer of stomatal defense by reopening their stomata, a process termed water immunity. To evaluate the relative importance of stomatal versus water immunity, we utilized a diverse set of Arabidopsis (Arabidopsis thaliana) mutants with impaired stomatal function and monitored bacterial growth, stomatal behavior, and water-soaking capacity after Pst pv. tomato DC3000 infection. Most mutants with constitutively open stomata and disrupted stomatal closure were more resistant against Pst than wild-type plants. Also, while some stomatal mutants displayed similar stomatal behavior at the initial steps of defense, their disease outcomes were the opposite, suggesting that stomatal immunity is not the determining factor in disease resistance. Instead, we discovered that the water-soaking capacity, associated with stomatal status at later stages of infection, i.e., water immunity, dictates the disease outcome. Our results show that water immunity can override the lack of stomatal immunity in plant resistance to Pst. We also address previous discrepancies in literature showing contradicting results for pathogen growth on stomatal mutants, highlighting the challenges in dissecting stomatal effects on plant resistance.