Systemic acquired resistance: an emerging role for jasmonates in local signal biogenesis, translocation and distal signal decoding.

Plant systemic acquired resistance (SAR) requires generation and movement of mobile signals from local leaves in which plant disease resistance has been activated (effector-triggered immunity, ETI), to distal, uninfected regions, where they prime host defences. Although salicylic acid (SA) and N-hydroxypipecolic acid (NHP) are widely recognised as key regulators of SAR, the requirement for de novo synthesis implies the existence of upstream or parallel inducing signals. Recent studies using whole plant and confocal reporter imaging, electrical signalling, and single-cell transcriptomics have revealed how specific cell types and the temporal-spatial organisation of phytohormone networks contribute to immune signalling. We summarise these findings alongside previous knowledge to highlight the collective importance of jasmonates, calcium, reactive oxygen species, and electrical signals as early initiators, coordinators, and most likely propagators of long-distance signalling during ETI-induced SAR. We draw parallels with induced systemic resistance and highlight the coordinated roles of jasmonates, volatile compounds, and the microbiome in plant-to-plant communication. Furthermore, we also review environmental modulation of defence responses, a research area deriving further attention. Evidence points towards the coordinated activation of multiple signals, including jasmonates, driving systemic immunity across biological scales from the infected cell to entire plant communities.