Fire blight, caused by Erwinia amylovora, is a destructive disease of pome fruits. Several analytical techniques, namely gas chromatography-mass spectrometry (GC-MS), proton transfer reaction- time of flight mass spectrometry (PTR-TOF-MS), laser photoacoustic detection (LPD) were used for characterizing the volatile emission by apple plants (Malus domestica cv. Golden) infected by E. amylovora. These techniques demonstrated that different volatiles are emitted by healthy and infected plants. In addition, the possible biological role of these volatiles was investigated. For this purpose, the headspace of infected plants was used to treat healthy plants which were successively inoculated with E. amylovora. Healthy plants, exposed to VOCS from infected ones, showed reduced CO2 fixation capacity, and increased total leaf area. Bacterial population and migration inside plant tissues were significantly lower than in control plants. A qPCR assay was developed to monitor the expression of key genes involved in the activation of plant defences, namely: (1) the jasmonic acid-precursor enzyme, LOX (two isoforms); (2) one of the salicylic acid precursor enzyme, PAL; (3) three SA-induced genes, PR1, PR5 and PR8; (4) two products involved in SA reception and signal transduction, SABP2 and NPR1. All the SA-related genes, except PAL, were found to be induced in healthy plants exposed to VOCs from infected ones, whereas the subsequent infection of the pre-exposed plants greatly reduced their expression. On the other hand, the LOX genes were stimulated by infection in pre-exposed plants. Those results suggest a biological, hormone-like activity of VOCs emitted by E. amylovora infected plants, on modulating the plant defence system in neighbouring plants.
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