Pipecolic acid: A positive regulator of systemic acquired resistance and plant immunity

Pipecolic acid (Pip) is a naturally occurring non-protein amino acid, that builds up in plants in response to pathogen infection. Pip is upregulated in autophagy mutants, indicating its role as a crucial regulator of plant immunity by upregulating systemic acquired resistance (SAR). This broad-spectrum defense mechanism protects uninfected parts of the plant during subsequent pathogen attacks. Pip has been identified as a SAR chemical signal and acts before the NO-ROS-AzA-G3P. The biosynthesis of Pip begins with lysine by the activity of ALD1 and SARD4 in a sequential manner; ALD1, a lysine aminotransferase, catabolizes lysine to Δ 1-piperidine-2-carboxylic acid, which is further modified to Pip by the activity of ornithine cyclodeaminase activity of SARD4. Additionally, FMO 1, a flavin monooxygenase, catalyzes the synthesis of N-hydroxy-pipecolic acid (NHP, the final, SAR-inducing defense hormone) from Pip. Pip and its active form accumulate at the infection site in the phloem and are transported to distal parts of the plant via symplast to trigger SAR. This review focuses on the roles of Pip and NHP in regulating SAR and how they interact with other defense signals like salicylic acid (SA) to modulate plant immunity.