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N-acyl-homoserine lactones (AHLs), a well-described group of quorum sensing (QS) molecules, may modulate plant defense responses and plant growth. However, there is limited knowledge regarding defense responses of non-model crops to AHLs and the mechanism of action responsible for the modulation of defense responses against microbial pathogens. In the present study, long-chained oxo-C14-HSL (N-3-oxo-tetradecanoyl-L-homoserine lactone) was shown to a distinct potential to prime cucumber for enhanced defense responses against biotrophic oomycete pathogen Pseudoperonospora cubensis (P. cubensis) and hemibiotroph bacterium P. syringae pv. lachrymans (Psl). We provide evidence that AHL-mediated enhanced defense against downy mildew disease is based on cell-wall reinforcement by lignin and callose depositions, activation of defense-related enzymes (peroxidase, β-1,3-glucanase, phenylalanine ammonia-lyase), accumulation of ROS (hydrogen peroxide, superoxide) and phenolic compounds. Quantitative analysis of salicylic acid (SA) and jasmonic acid (JA) and transcriptional analysis of several of genes associated with these phytohormones revealed that defense priming with oxo-C14-HSL is commonly regulated by SA signaling pathway. Here we also show that treatment with short- (N-hexanoyl-L-homoserine lactone) and medium-chained (N-3-oxo-decanoyl-L-homoserine lactone) AHLs promoted primary root length and modified root architecture, respectively, which is resulted in enhanced plant growth.

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