Plant defence depends on speed: the sooner a plant recognises its attacker, the more effective its defence response will be. Over the course of evolution, plants have acquired the ability to prime their immune system after perception of specific environmental signals. This heightened state of defence enables a faster and/or stronger activation of inducible defence mechanisms after pathogen attack, providing resistance against a wide range of diseases. Based on a phenotypically similar response in human immune cells, this plant resistance response is called “defence priming”.The primed defence state is not accompanied with an increased expression of costly defence mechanisms. Consequently, broad-spectrum disease resistance by defence priming is not associated with major reductions in plant growth and reproduction. Furthermore, defence priming can be maintained over a relatively long period of time, which is why it is often regarded as a form of immunological plant memory. Despite these attractive properties for application in sustainable agriculture, there are still many unanswered questions about the mechanistic basis of defence priming. The main objective of this ERC proposal is to elucidate the mechanisms underpinning two poorly understood aspects of the phenomenon:1. the onset of primed defence.2. the long-term maintenance of primed defence, including the intriguing phenomenon that the primed defence state can be transmitted from pathogen-exposed plants to their progeny.Using state-of-the-art techniques in the field of molecular biology, biochemistry, and (epi)genetics, this project will address the overarching question of how plants adapt their immune system to the conditions in their environment. This project will allow me to generate new insights in the regulation of plant defence and provide tools that can be used to develop novel strategies for sustainable crop protection.