Bacillus subtilis and Pseudomonas fluorescens Trigger Common and Distinct Systemic Immune Responses in Arabidopsis thaliana Depending on the Pathogen Lifestyle

Plants harbor various beneficial bacteria that modulate their innate immunity, resulting in induced systemic resistance (ISR) against various pathogens. However, the immune mechanisms underlying ISR triggered by Bacillus spp. and Pseudomonas spp. against pathogens with different lifestyles are not yet clearly elucidated. Here, we show that root drenching of Arabidopsis plants with <i>Pseudomonas fluorescens</i>PTA-CT2 and <i>Bacillus subtilis</i> PTA-271 can induce ISR against the necrotrophic fungus <i>B. cinerea</i> and the hemibiotrophic bacterium <i>Pseudomonas syringae Pst</i> DC3000. In the absence of pathogen infection, both beneficial bacteria do not induce any consistent change in systemic immune responses. However, ISR relies on priming faster and robust expression of marker genes for the salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) signaling pathways upon pathogen challenge. These responses are also associated with increased levels of SA, JA, and abscisic acid (ABA) in the leaves of bacterized plants after infection. The functional study also points at priming of the JA/ET and NPR1-dependent defenses as prioritized immune pathways in ISR induced by both beneficial bacteria against <i>B. cinerea</i>. However, <i>B. subtilis</i>-triggered ISR against <i>Pst</i> DC3000 is dependent on SA, JA/ET, and NPR1 pathways, whereas <i>P. fluorescens</i>-induced ISR requires JA/ET and NPR1 signaling pathways. The use of ABA-insensitive mutants also pointed out the crucial role of ABA signaling, but not ABA concentration, along with JA/ET signaling in primed systemic immunity by beneficial bacteria against <i>Pst</i> DC3000, but not against <i>B. cinerea</i>. These results clearly indicate that ISR is linked to priming plants for enhanced common and distinct immune pathways depending on the beneficial strain and the pathogen lifestyle.