Fig. 9.

Working model of systemic and plant-to-plant propagation of innate immunity. A Systemic acquired resistance (SAR)-inducing infection triggers pipecolic acid (Pip) and glycerol-3-phosphate (G3P) accumulation, which stimulate each other in a positive feedback loop acting upstream of monoterpene emissions. Monoterpenes subsequently enhance salicylic acid (SA)-associated immunity through the SAR signaling intermediates LEGUME LECTIN-LIKE PROTEIN1 (LLP1) and AZELAIC ACID INDUCED1 (AZI1). At the same time, monoterpenes are emitted and act as cues that are perceived by systemic leaves and also neighboring plants. At the site of monoterpene perception, SA-associated immunity is enhanced through LLP1 and AZI1. In addition, LLP1 drives a positive feedback loop with Pip and G3P to stimulate monoterpene biosynthesis and emission, potentially promoting the generation of a wave of plant-derived volatile defense cues moving between leaves in a canopy or rosette and between neighboring plants in a population