From early defence priming to lasting memory: developmental and seasonal dynamics in trees.

Defence priming enhances plant responsiveness to future stress following prior exposure, and has been extensively characterised in annual model species as a reversible state associated with faster or stronger defence activation. However, studies in long-lived plants indicate that priming responses can be elicitor-specific, developmentally variable and closely linked to environmental history. In trees, priming is often strongest in early life stages, while in mature individuals defence responses are more frequently associated with direct activation and stabilised regulatory states. Evidence across systems shows that priming can operate over multiple timescales, from short-term reversible responses to longer-lasting effects that influence response thresholds and activation kinetics. Epigenetic mechanisms, including DNA methylation and chromatin modification, contribute to these processes and may persist across cell divisions or developmental transitions. Together, these findings highlight the importance of developmental stage and temporal context in shaping plant defence responses.