Pipecolic Acid Is Induced in Barley upon Infection and Triggers Immune Responses Associated with Elevated Nitric Oxide Accumulation.

Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in 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 pv. induced immune responses in 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 pv. . Furthermore, both a systemic immunity-inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen f. sp. . In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus . Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, pv. 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.