Different signaling pathways of induced resistance by rhizobacteria in Arabidopsis thaliana against two pathovars of Pseudomonas syringae

• The mechanisms by which plant growth-promoting rhizobacteria (PGPR) mediate induced systemic resistance are currently being intensively investigated from the viewpoint of signal transduction pathways within plants. • Here, we determined whether our well-characterized PGPR strains, which have demonstrated induced resistance on various plants, also elicit induced resistance in Arabidopsis thaliana . Nine different PGPR strains were evaluated for their capacity to cause induced resistance on Arabidopsis against two pathovars of Pseudomonas syringae. Six strains significantly reduced severity of P. syringae pv. tomato, whereas seven strains reduced severity of P. syringae pv. maculicola. • From the initial screenings, four strains (90-166, SE34, 89B61 and T4) were selected because of their consistent induced resistance capacity. Elicitation of induced resistance with these strains depended on how disease severity was measured. Three strains (90-166, 89B61 and T4) induced resistance in NahG plants (SA-deficient), indicating a salicylic acid-independent pathway, which agrees with the previously reported pathway for induced resistance by PGPR. However, differences from the reported pathway were noted with strain 89B61, which did not require jasmonic acid or ethylene signaling pathways for induced resistance, and with strain T4, which induced resistance in npr1 plants. • These results indicate that strains 89B61 and T4 induce resistance via a new pathway or possibly a variation of the previously reported pathway. This information will broaden our understanding of ways in which microorganisms can signal physiological changes in plants.