Ethylene: Multi‐Tasker in Plant–Attacker Interactions

In the past decades, the role of ethylene in the regulation of plant re- sponses to biotic stress has been intensively studied. Analyses of plant genotypes that are impaired in ethylene biosynthesis, perception or signalling revealed an im- portant 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 demon- strated to facilitate disease resistance or tolerance, in other studies ethylene was shown to support pathogen infection. Recent advances in defence-signalling re- search have revealed that ethylene plays an important role in modulating inter- actions 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.