Ethylene: multi-tasker in plant-attacker interactions

In the past decades, the role of ethylene in the regulation of plant responses to biotic stress has been intensively studied. Analyses of plant genotypes that are impaired in ethylene biosynthesis, perception or signalling revealed an important role for ethylene in the primary response to pathogen attack. In addition, ethylene has been demonstrated to fulfil a key function in the control of systemic immune responses that are induced by beneficial micro‐organisms. Although the importance of ethylene in the regulation of plant immune responses is evident, its role in stimulating disease resistance or susceptibility appears to depend greatly on the plant–attacker combination. Whereas in many studies ethylene was demonstrated to facilitate disease resistance or tolerance, in other studies ethylene was shown to support pathogen infection. Recent advances in defence‐signalling research have revealed that ethylene plays an important role in modulating interactions between defence‐signalling pathways that are regulated by either salicylic acid (SA) or jasmonic acid (JA). By functioning as a modulator of these important defence‐regulatory pathways, ethylene may play a decisive positive or negative role in the final outcome of the immune response of a plant.