Induced systemic resistance against Pseudomonas syringae pv. maculicola by a long chain bacterial volatile emitted from Paenibacillus polymyxa in Arabidopsis thaliana

Abstract: Some strains of plant growth-promoting rhizobacteria (PGPR) elicit induced systemic resistance (ISR). Previously, volatile organic compounds (VOCs), including acetoin and 2,3-butanediol, emitted from PGPR were identified as bacterial determinants of ISR. We screened for ISR using a microtiter plate and I-plate bioassay in which seedlings were challenged with the pathogen Pseudomonas syringae pv. maculicola ES4326 in the presence of bacterial volatiles. To investigate the induction of ISR signaling by the VOCs emitted from the reference strain, GB03, and test strain, Paenibacillus polymyxa E681 which showed strong capacity on ISR and plant growth promotion under in vitro and field conditions. To identify plant signaling pathways involved, we screened Arabidopsis plants transformed with PR1::GUS and PDF1.2::GUS, indicators of salicylic acid and ethylene signaling, respectively. GB03 elicited ISR via ethylene-dependent signaling, as indicated by elevated PDF1.2 expression in exposed seedlings, while E681 increased SA-dependent signaling, as indicated by elevated PR1 expression suggestive that E681 and GB03 strains activate different signaling transduction pathways. The efficacy of induction was also strain-specific, with stronger protection against P. syringae in plants exposed to VOCs released from E681 versus plants exposed to GB03. Among more than thirty low molecular weight identified volatile compounds, including methanethiol, isoprene, and an acetic acid-butyl ester, hexadecane, a C16 hydrocarbon was found to be released exclusively from only strain E681 and can prime transcriptional levels of PR1 defense gene. These results provide the first evidence for the existence of a novel E681 signal molecule that can serve as a bacterial determinant in ISR against P. syringae.