Rhizobacteria Bacillus subtilis restricts foliar pathogen entry through stomata.

Plants exist in a complex multitrophic environment, where they interact with and compete for resources with other plants, microbes and animals. Plants have a complex array of defense mechanisms, such as the cell wall being covered with a waxy cuticle serving as a potent physical barrier. Although some pathogenic fungi infect plants by penetrating through the cell wall, many bacterial pathogens invade plants primarily through stomata on the leaf surface. Entry of the foliar pathogen, Pseudomonas syringae pathovar tomato DC3000 (hereafter PstDC3000), into the plant corpus occurs through stomatal openings, and consequently a key plant innate immune response is the transient closure of stomata, which delays disease progression. Here, we present evidence that the root colonization of the rhizobacteria Bacillus subtilis FB17 (hereafter FB17) restricts the stomata-mediated pathogen entry of PstDC3000 in Arabidopsis thaliana. Root binding of FB17 invokes abscisic acid (ABA) and salicylic acid (SA) signaling pathways to close light-adapted stomata. These results emphasize the importance of rhizospheric processes and environmental conditions as an integral part of the plant innate immune system against foliar bacterial infections.