Exogenous pipecolic acid modulates plant defence responses against Podosphaera xanthii and Pseudomonas syringae pv. lachrymans in cucumber (Cucumis sativus L.).

Systemic acquired resistance (SAR) is a long-lasting and broad-based resistance that can be activated following infection with (a)virulent pathogens and treatment with exogenous elicitors. Pipecolic acid (Pip), a Lys-derived non-protein amino acid, naturally occurs in many different plant species, and its N-hydroxylated derivative, N-hydroxypipecolic acid (NHP), acts as a crucial regulator of SAR. In the present study, we conducted a systemic analysis of the defence responses of a series of D,L-Pip-pretreated Cucumis sativus L. against Podosphaera xanthii (P. xanthii) and Pseudomonas syringae pv. lachrymans (Psl). The effects of D,L-Pip on ROS metabolism, defence-related gene expression, SA accumulation and activity of defence-related enzymes were evaluated. We show that exogenously applied D,L-Pip successfully induces SAR in cucumber against P. xanthii and Psl, but not Fusarium oxysporum f. sp. cucumerinum (Foc). Exogenous application of D,L-Pip via the root system is sufficient to activate the accumulation of free and conjugated salicylic acid (SA), and earlier and stronger upregulation of SAR-associated gene transcription upon P. xanthii infection. Furthermore, D,L-Pip treatment and subsequent pathogen inoculation promote hydrogen peroxide and superoxide accumulation, as well as Rboh transcription activation in cucumber plants, suggesting that D,L-Pip-triggered ROS production might be involved in enhanced defence reactions against P. xanthii. We also demonstrate that D,L-Pip pretreatment increases the activity of defence-associated enzymes, including peroxidase, chitinase and β-1,3-glucanase. The results presented in this report provide promising features of Pip as an elicitor in cucumber and call for further studies that may uncover its potential in production areas against different phytopathogens.