The hormone jasmonic acid plays a major role in the plant immune system and in regulating growth. Scientists have already learned much about how jasmonic acid works, but one important link was missing: what makes the plant’s jasmonic acid level go down once the attack by a fungus or insect has been warded off? Plant biologists at Utrecht University and colleagues from the University of Amsterdam, have now discovered how the plant metabolises jasmonic acid, issuing the signal ‘safe’. Controlling this mechanism may present new opportunities to increase resistance of crops to fungi and insects. The results of their research were published in the scientific journal PNAS on Tuesday 30 May.