Purpose: The purpose of this study was to determine if FB17 was able to induce ISR in lettuce and spinach through stomatal closure during inoculation of human pathogens, Listeria spp, Salmonella spp., and plant pathogen Pseudomonas syringae.
Methods: Plants were grown in a controlled biochamber until 2 true leaves were present. Plant leaves were inoculated with sterile H2O or bacterial pathogens only (1 x 108, S. Newport, L. innocua, or P. syringae DC3000); while, FB17 was inoculated onto the roots only, or on both leaves and roots. Two different leaves from each plant (n = 40) were collected at 1 and 3h post-inoculation (hpi) and were visually assessed using cryo-scanning electron microscopy (SEM) where stomatal aperture size was quantified using ImageJ software.
Results: Stomatal aperture size was significantly reduced compared to controls for all treatments indicating ISR was induced in lettuce and spinach (P = < 0.0001). ISR was not induced when lettuce was inoculated with FB17 on roots only at 1 and 3 hpi (P = 0.23), or with Listeria on lettuce only at 1 hpi (P = 0.21). However; FB17 triggered ISR at 3 hpi in the presence of human pathogens, thereby inducing stomatal closure in lettuce and spinach. Results were confirmed by root inoculation with heat-killed FB17, which did not induce stomatal closure. Significant differences were observed between plant species and pathogen interactions (P < 0.0001).
Significance: The results of this study indicate that plant growth promoting rhizobacteria B. subtilis FB17 may be able to prevent contamination by plant pathogens as well as internalization of human pathogens. This is the first study that assesses the effects of biocontrol agents and human pathogens on plants in terms of plant immune response through stomata.