Chemical Nature: The gas ethylene is synthesized from methionine.
Sites of Biosynthesis: In most tissues in response to stress, especially in tissues undergoing senescence or ripening. Transport: Being a gas, ethylene moves by diffusion from its site of synthesis Effects: Fruit ripening (especially in climacteric fruits, such as apples, bananas and avocados); leaf and flower senescence; leaf and fruit abscission. Modulates various developmental programs and coordinates responses to a multitude of external stress factors.
Isoenzymes of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) catalyze the rate limiting step in ethylene biosynthesis.
Ethylene is perceived by a family of ER localized receptors that share sequence similarity with the bacterial two component histidine kinases.
Ethylene gas freely diffuses through membranes and the cytoplasm, therefore it does not have a transporter system to deliver it to its receptors in the ER.
Dicot species seedlings, of ethylene insensitive mutants, germinated in the dark grow tall and spindly. In the presence of ethylene they undergo a triple response: hypocotyls and roots are shortened and thickened and the apical tip exhibits an exaggerated hook. Ethylene insensitive mutants are significantly taller than WT and their elongation is directly proportional to the loss of ethylene response.
Ethylene receptors- ETR1, ETR2, EIN4, ERS1 and ERS2. These receptors are homologous to the prokaryotic family of signal transducers-- two-component regulators. These receptors can be structurally separated into three domains. The sensor domain, the kinase domain and the receiver domain—in the absence of these hormones the receptors actively repress the ethylene response—also there is a high degree of functional redundancy between these receptors. The ethylene binding domain between these receptors is conserved but the histidine and receiver domains are not conserved between them.
The role of the receiver domain in the receptor function is unknown. Along with the kinase domain it participates in the the protein- protein interaction between the receptor and another negative regulator of the pathway. Downstream of the receptors is a Raf-like serine-threonine kinase CTR1. CTR1 interacts with ETR1 and is a negative regulator. This interaction is important not only for the activation of CTR1 by the receptors but also for its proper subcellular localization in the ER. ETR1-CTR1 complex inactivates EIN2 (not known how?), the positive component of the pathway that genetically works downstream of CTR1.
CTR1 is an MAPKKK (there may be roles in ethylene signaling for additional proteins homologous to MAPKK and MAPK??). Following the MAPK cascade is EIN2 (it has homology to Nramp metal transporters). At the end of the signaling pathway are the transcription factors EIN3 and ERF1. EIN3 binds to the promoter of ERF1. ERF1 is a positive transcription factor.
High levels of EIN2 C terminus, constitutively activates the ethylene response—the unique domain of the protein participates in the transduction of the signal to the downstream components, whereas the NRAMP-like domain may sense the upstream signals.
The ethylene signal arrives at the nucleus through EIN3 a plant specific transcription factor that belongs to a small gene family.
SCF E3 a ubiquitin ligase complex and the proteosome is involved in the regulation of EIN3 protein levels. The ethylene- mediated modification of EIN3, EBFs, or both that prevents EIN3 from being targeted for degradation remains unknown.
EIN3 protein levels rapidly increase in response to ethylene and this response requires several ethylene- signaling pathway. In the absence of ethylene, EIN3 is quickly degraded through a ubiquitin/proteosome pathway mediated by two F box proteins EBF1 and EBF2. The ubiquitin/proteosome pathway negatively regulates ethylene responses by targeting EIN3 for degradation. The stabilization of EIN3 by ethylene results in the transcriptional activation of hundreds of genes QUES 1: The roles of both the histidine and serine kinase activities need to be tested in sensitive assays using new receptor mutants harboring mutations in the kinase domain. QUES 2: Mechanisms mediating developmental switch in which ripening is initiated after a cumulative exposure to ripening is initiated.
