The Xanthomonas fragariae effector XopK suppresses stomatal immunity by perturbing abscisic acid signaling in strawberry

Xanthomonas fragariae (Xaf) usually causes angular leaf spot (ALS) in strawberry all over the world. Recently, we isolated a new strain of Xaf called Xaf YL19. This strain causes not only typical ALS symptoms, but also dry cavity rot in the crown tissues of strawberries. This was the first time a Xaf strain had both of these effects in strawberries. Pathogen effectors play a crucial role in plant colonization and infection. Understanding how effector proteins escape from plant surveillance is important for plant breeding and resistance deployment. In this study, thirty-three putative effectors were predicted in Xaf YL19, and a virulent secreted effector called XopK was identified. XopK has a robust capacity for suppressing plant cell death and transcript response to infection host. Transgenic strawberries expressing XopK exhibit more susceptibility to Xaf YL19, and this was associated with weakened stomatal immunity. Additionally abscisic acid (ABA) accumulation and signaling were mainly suppressed in the presence of XopK. XopK also inhibited ABA- and methyl jasmonate (MeJA)-induced stomatal closure, and ABA- but not MeJA-induced reactive oxygen species (ROS) production. Moreover, endogenous ABA is critical for Xaf-induced ROS burst and stomatal closure. These results suggested that Xaf YL19 uses XopK to suppress JA, especially ABA signaling, for disrupting stomatal closure, and allowing its colonization for disease development. Taken together, our results provide a novel virulent mechanism by Xaf and an available strategy for resistant breeding in strawberries.