Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi)biotrophic pathogens. Here, we studied the potential role of Pip in bacteria-induced systemic immunity in barley. Exudates of barley leaves infected with the systemic immunity–inducing pathogen Pseudomonas syringae pv. japonica induced immune responses in A. thaliana. The same leaf exudates contained elevated Pip levels compared with those of mock-treated barley leaves. Exogenous application of Pip induced resistance in barley against the hemibiotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity–inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus Pyrenophora teres. Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, X. translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in Pip-pretreated compared with mock-pretreated plants. Thus, the data suggest that Pip induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.

Plants are exposed to numerous pathogens and fend off many of these with different phytohormone signalling pathways. Much is known about defence signalling in the dicotyledonous model plant Arabidopsisthaliana, but it is unclear to which extent knowledge from model systems can be transferred to monocotyledonous plants, including cereal crops. Here, we investigated the defence-inducing potential of Arabidopsis resistance-inducing compounds in the cereal crop barley. Salicylic acid (SA), folic acid (Fol), and azelaic acid (AzA), each inducing defence against (hemi-)biotrophic pathogens in Arabidopsis, were applied to barley leaves and the treated and systemic leaves were subsequently inoculated with Xanthomonastranslucens pv. cerealis (Xtc), Blumeria graminis f. sp. hordei (powdery mildew, Bgh), or Pyrenophora teres. Fol and SA reduced Bgh propagation locally and/or systemically, whereas Fol enhanced Xtc growth in barley. AzA reduced Bgh propagation systemically and enhanced Xtc growth locally. Neither SA, Fol, nor AzA influenced lesion sizes caused by the necrotrophic fungus P. teres, suggesting that the tested compounds exclusively affected growth of (hemi-)biotrophic pathogens in barley. In addition to SA, Fol and AzA might thus act as resistance-inducing compounds in barley against Bgh, although adverse effects on the growth of pathogenic bacteria, such as Xtc, are possible.