Ethylene signaling: simple ligand, complex regulation

•The receptors, CTR1 and EIN2 reside in the ER membrane forming clusters.•In the absence of ethylene, CTR1 inhibits EIN2 by direct phosphorylation.•In the presence of ethylene, the C-end of EIN2 is cleaved off and moves to the nucleus.•In the nucleus, EIN3/EIL1 activate a transcriptional cascade in a four-wave manner.•Sophisticated molecular loops enable fine-tuning or resetting of the pathway.

The hormone ethylene plays numerous roles in plant development. In the last few years the model of ethylene signaling has evolved from an initially largely linear route to a much more complex pathway with multiple feedback loops. Identification of key transcriptional and post-transcriptional regulatory modules controlling expression and/or stability of the core pathway components revealed that ethylene perception and signaling are tightly regulated at multiple levels. This review describes the most current outlook on ethylene signal transduction and emphasizes the latest discoveries in the ethylene field that shed light on the mechanistic mode of action of the central pathway components CTR1 and EIN2, as well as on the post-transcriptional regulatory steps that modulate the signaling flow through the pathway.