Poltergeist-Like 2 (PLL2)-dependent activation of the wound response distinguishes systemin from other immune signaling pathways

Systemin, the first signaling peptide identified in plants, mediates induced resistance against insect herbivores in tomato ¹ . Initially, systemin was perceived as a hormone-like, long-distance messenger that triggers systemic defense responses far from the site of insect attack. It was later found to rather act as a phytocytokine, amplifying the local wound response for the production of downstream signals that activate defense gene expression in distant tissues ² . Systemin perception and signaling rely on the systemin receptor SYR1 ³ . However, the specifics of SYR1-dependent signaling and how systemin signaling differs from other phytocytokine signaling pathways remain largely unknown. Here, we report that systemin activates the poltergeist-like phosphatase PLL2 in a SYR1-dependent manner. PLL2, in turn, regulates early systemin responses at the plasma membrane, including the rapid inhibition of proton pumps through the dephosphorylation of their regulatory C-termini. PLL2 was found to be essential for downstream defense gene induction, ultimately contributing to insect resistance.