The optimal defence hypothesis (ODH) predicts that tissues that contribute most to a plant’s fitness and have the highest probability of being attacked will be the parts best defended. In general, young tissues show stronger induced defence responses and contain higher basal levels of specialized defensive metabolites than other older plant parts. In this review, Meldau et al. summarize defence patterns in above- and below-ground plant tissues and their regulation by jasmonic acid (JA). They hypothesize that the interactions of growth hormones (cytokinins, auxin, gibberellins, brassinosteroids) with JA-mediated defence pathways might shed light on the molecular basis of the ODH.