A plastidial retrograde-signal potentiates biosynthesis of systemic stress response activators

O_LIPlants employ an array of intricate and hierarchical signaling cascades to perceive and transduce informational cues to synchronize and tailor adaptive responses. Systemic stress response (SSR) is a recognized complex signaling and response network quintessential to plants local and distal responses to environmental triggers, however, the identity of the initiating signals has remained fragmented. C_LIO_LIHere, we show that both biotic (aphids and viral pathogens) and abiotic (high-light and wounding) stresses induce accumulation of the plastidial-retrograde-signaling metabolite, methylerythritol cyclodiphosphate (MEcPP), leading to reduction of the phytohormone, auxin, and the subsequent decreased expression of the phosphatase, PP2C.D1. C_LIO_LIThis enables phosphorylation of mitogen-activated protein kinases (MAPK3/6), and the consequential induction of the downstream events ultimately resulting in biosynthesis of the two SSR priming metabolites, pipecolic- and N-hydroxy-pipecolic acid. C_LIO_LIThis work identifies plastids as the initiation site, and the plastidial retrograde-signal, MEcPP as the initiator of a multi-component signaling cascade potentiating the biosynthesis of SSR activators, in response to biotic and abiotic triggers. C_LI