Figure 2.

Model of the Ethylene Signal Transduction Pathway.

There are five ethylene receptors in Arabidopsis, ETR1, ETR2, EIN4, ERS1, and ERS2. ETR1 and ERS1 contain three transmembrane domains and a conserved histidine kinase domain, and have been shown to function as homodimers. ETR2, EIN4, and ERS2 have four membrane-spanning regions and a degenerate histidine kinase domain. Only ETR1, ETR2, and EIN4 have receiver domains at their C termini. Ethylene binding occurs at the N-terminal transmembrane domain of the receptors, and a copper co-factor is required for the binding. RAN1, a copper transporter, is involved in delivery of copper to the ethylene receptor. In the absence of an ethylene signal, ethylene receptors activate a Raf-like kinase, CTR1, and CTR1 in turn negatively regulates the downstream ethylene response pathway, possibly through a MAP-kinase cascade. Binding of ethylene inactivates the receptors, resulting in deactivation of CTR1, which allows EIN2 to function as a positive regulator of the ethylene pathway. EIN2 contains the N-terminal hydrophobic domain similar to the Nramp metal transporter proteins and the novel hydrophilic C terminus. EIN2 positively signals downstream to the EIN3 family of transcription factors located in the nucleus. EIN3 binds to the promoter of ERF1 gene and activates its transcription in an ethylene-dependent manner. Transcription factors ERF1 and other EREBPs can interact with the GCC box in the promoter of target genes and activate downstream ethylene responses.