Pipecolic acid is induced in barley upon infection and triggers immune responses associated with elevated nitric oxide accumulation.

Abstract

Pipecolic acid is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi-)biotrophic pathogens. Here, we studied the potential role of pipecolic acid 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 pipecolic acid levels compared to those of mock-treated barley leaves. Exogenous application of pipecolic acid induced resistance in barley against the hemi-biotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity-inducing infection and exogenous application of pipecolic acid enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, pipecolic acid 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 pipecolic acid application. Furthermore, Xanthomonas translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in pipecolic acid-pretreated compared to mock-pretreated plants. Thus, the data suggest that pipecolic acid induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.