PlantDiagram 1as in relatively stress-free environments invest most of their resources in growth and reproduction (left). If plants live in hostile environments and are attacked by harmful microbes or insects, they activate inducible defence mechanisms (right). Expression of this “induced defense” is often costly due to allocation of limited resources to defensive compounds, or toxicity of the defense to the plant’s own metabolism. Plants are also capable of sensing environemtal cues that indicate upcoming stress. These signals can “sensitize” or “prime” the plant’s immune system (middle), which results in a faster and/or stronger defence reaction when the plant is attacked at a later stage. Although the level of disease protection by priming tends to be less than that by induced defence, priming is effective against a broad range of (a)biotic stress and comes with considerably fewer costs than induced defence (Proc Natl Acad Sci USA,103: 5602-5607).