Figure 1. Ethylene responses in Arabidopsis. (a) A 4-day old dark-grown seedling germinated in the presence of exogenously applied ethylene exhibits the characteristic triple response phenotype (right). The defining features of the triple response include inhibition of cell elongation in the hypocotyl and root, radial swelling of the hypocotyl, exaggerated curvature of the apical hook and a proliferation of root hairs. The 4-day old dark-grown seedling (left) has not been treated with ethylene and displays the same phenotype as an ethylene-insensitive mutant. (b) The ctr1 null mutant (right) exhibits inhibition of cell expansion in leaves compared to a wild-type plant (left) of the same age. (c) A wild-type plant treated with 100 ppm ethylene gas for 3 days exhibits leaf senescence (right) in comparison to an untreated wild-type plant (left).

Figure 2. Model of the ethylene-signalling pathway in Arabidopsis. Ethylene is perceived by a family of five ethylene receptors, ETR1, ERS1, EIN4, ETR2 and ERS2, one of which (ETR1) has been localized to the endoplasmic reticulum. These receptors form homodimers and require a copper cofactor to bind ethylene. The delivery of copper is dependent on the transporter RAN1. At least one of the receptors (ETR1) is positively regulated through an unknown mechanism by RTE1. The receptors repress downstream ethylene responses through CTR1, a Raf-like MAPKKK that also acts to negatively regulate the pathway (possibly through a MAPK cascade). Upon ethylene binding, the receptor is inactivated, and no longer signals to CTR1. With CTR1 no longer repressing downstream signalling, EIN2 becomes activated. EIN2 is an integral membrane protein with some similarity to the Nramp family of metal-ion transporters. The signalling mechanism of EIN2 is unknown, but is believed to occur through the novel carboxyl-terminal domain. EIN2 is a positive regulator of ethylene responses that activates EIN3, which is a member of the EIN3/EIL1 family of transcription factors. EIN3 is regulated by two F-box proteins (EBF1/EBF2), which in the absence of ethylene, target EIN3 for degradation through the 26S proteasome pathway. EBF1 and EBF2 are negatively regulated at the mRNA level indirectly through a 5¢-3¢ exoribonuclease XRN4 (EIN5). Upon ethylene binding, EIN3 is stabilized, enabling it to activate a transcriptional cascade resulting in the expression of ethylene-response genes.

Figure 3. The ethylene biosynthesis pathway. In higher plants, ethylene is produced from the amino acid l-methionine, which is converted into S-adenosylmethionine (AdoMet) by AdoMet synthetase. Next, AdoMet is converted into 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase, which is the rate-limiting step in ethylene biosynthesis. Finally, ACC is converted into ethylene through the action of the enzyme ACC oxidase.

Figure 4. The ethylene receptor family in Arabidopsis. The five ethylene receptors in Arabidopsis, which are similar in sequence and structure to two-component histidine protein kinase receptors, fall into two subfamilies. Subfamily I receptors contain all motifs necessary for functional histidine kinase activity (indicated by horizontal black lines), whereas subfamily II receptors lack most or all of these motifs. All receptors (shown here as monomers) contain a highly conserved transmembrane ethylene-binding domain, with three membrane spanning regions in subfamily I receptors and four in subfamily II, and all receptors contain a GAF-like domain (of unknown function) and a histidine protein kinase domain. Three of the five receptors (ETR1, EIN4 and ETR2) also contain a domain that is conserved with two-component receiver domains. The GAF, histidine kinase and receiver domains comprise the region believed to be important for signal transmission. See also