Discovery that PACAP, a mammalian neuropeptide, activates plant immunity through chemical screening

Plants are constantly exposed to a variety of biotic stresses in their natural environment and rely on their immune systems to adapt to these challenges. Pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) constitute two complementary layers of the plant innate immune system, both of which can be activated by immune elicitors. In this study, a <i>pFRK1-GUS</i> reporter system was employed to screen multiple natural product libraries, leading to the identification of the animal-derived neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its truncated form, PACAP 6-38, as potential plant immune elicitors. Exogenous application of PACAP and PACAP 6-38 triggered multiple PTI-associated immune responses, including cytosolic calcium influx, MAPK phosphorylation, and induction of <i>FRK1</i> expression under the tested conditions, while notably failing to induce a detectable reactive oxygen species (ROS) burst. Moreover, pre-treatment with PACAP or PACAP 6-38 at the tested concentrations reduced bacterial titers of <i>Pseudomonas syringae</i> pv. <i>tomato</i> DC3000 by approximately 0.3-0.7 log units under single-application conditions. Notably, both peptides also enhanced plant resistance to <i>Ralstonia solanacearum</i>, indicating a broader role in bacterial disease resistance. Although the molecular receptors and downstream signaling components remain to be identified, this study establishes a proof-of-concept for cross-kingdom recognition of an animal neuropeptide by plants. Together, these findings highlight PACAP-induced immunity as being uncoupled from the canonical ROS burst, underscoring the conceptual novelty of animal-derived peptides as unconventional elicitors and providing a reference for potential future applications of PACAP in plants.