Figure 1. The ethylene signaling pathway.

An N-terminal transmembrane domain of ethylene receptors targets the receptors to the endoplasmic reticulum (ER) membrane, where they form homodimers. CTR1 associates with the His kinase and receiver domains of a receptor, and, in the absence of the hormone, ethylene receptors maintain CTR1 in an active state to phosphorylate EIN2, which inhibits the ability of the latter to induce ethylene responses. EIN2 also interacts directly with ethylene receptors. When present, ethylene binds to the copper bound in the N-terminal transmembrane domain, resulting in the inactivation of the ethylene receptor-CTR complex, perhaps through conformational changes. Loss of EIN2 phosphorylation results in the proteolytic release of its C-terminal domain, which is targeted to the nucleus. The presence of the EIN2 C-terminal domain in the nucleus prevents further degradation of the ethylene response transcription factors EIN3/EIL1 by two F-box proteins (EBF1/2) and induces EBF1/2 degradation. Dimerization of EIN3/EIL1 and their binding to the promoter region of ethylene response factor (ERF) transcription factor genes activate the gene expression of the latter, resulting in the activation of downstream ethylene response genes. CTR, constitutive triple response; EIL, ethylene insensitive 3-like; EIN, ethylene insensitive.