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Arabidopsis Ethylene Signaling Pathway

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Science's STKE  22 Mar 2005:
Vol. 2005, Issue 276, pp. cm4
DOI: 10.1126/stke.2762005cm4

Abstract

In plants, ethylene gas functions as a potent endogenous growth regulator. In the model system Arabidopsis thaliana, the molecular mechanisms that underlie perception and transduction of the ethylene signal to the nucleus, where the transcription of hundreds of genes is altered, are being elucidated. In the current view, ethylene is sensed by a family of five receptors that show similarity to the bacterial two-component histidine kinases, and in plants function as negative regulators of the pathway. Binding of the ethylene gas turns off the receptors, resulting in the inactivation of another negative regulator of ethylene signaling, CTR1, a Raf-like protein kinase that directly interacts with the receptors. EIN2, a protein of unknown biochemical activity that functions as a positive regulator of the pathway, acts downstream of CTR. Derepression of EIN2 by ethylene upon disabling of the receptors and CTR1 leads to the activation of EIN3 and EIN3-like transcription factors. In the absence of ethylene, the levels of EIN3 protein are extremely low because of the function of two F-box-containing proteins, EBF1 and EBF2, that target EIN3 for proteosome-mediated degradation. In the presence of ethylene, the EIN3 protein accumulates in the nucleus and initiates a transcriptional cascade, resulting in the activation and repression of hundreds of genes. To date, the only empirically demonstrated direct target of EIN3 is the APETALA2 (AP2)-domain–containing transcription factor gene ERF1. The coregulation of ERF1 by another plant hormone, jasmonic acid, illustrates how a transcriptional cascade could be utilized in a combinatorial fashion to generate a large diversity of responses using a limited number of input signals. As new components and points of intersection with other pathways are identified, the Connections Map will be updated.

Description

This record contains information specific to the Arabidopsis Ethylene Signaling Pathway.

In Arabidopsis, ethylene is perceived by a family of receptors that share sequence similarity with the bacterial two-component histidine kinases (1). The five members of the Arabidopsis receptor family can be classified into two groups according to their structural characteristics (2, 3). The group I receptors, ETR1 and ERS1, possess in their N termini a canonical histidine kinase and three putative transmembrane domains that form a pocket for ethylene binding through coordination with copper (4, 5). The group II receptors, ETR2, EIN4, and ERS2, harbor an additional hydrophobic domain in their N termini, and their kinase domain lacks several of the key features present in the canonical histidine kinases (2, 3). Although ETR1 possesses a functional histidine kinase moiety, the role of this activity in the ethylene response is controversial (68). ETR1 and possibly other ethylene receptors are localized in the endoplasmic reticulum (ER) (9), where they interact with the Raf-like kinase CTR1 (10) (Fig. 1). The colocalization of CTR1 with the receptors in the endoplasmic reticulum is mediated by its interaction with the receptor proteins (11). CTR1 is a serine-threonine kinase, and this kinase activity is essential for the function of CTR1 in ethylene signaling (12).

Fig. 1.

Pathway image captured from the dynamic graphical display of the information in the Connections Maps available 3 March 2005. In this version of the pathway, the MAPK module has been removed, with the updated version of the pathway reflecting the new experimental data. For a key to the colors and symbols and to access the underlying data, please visit the pathway (About Connections Map).

Genetic and biochemical studies have suggested the presence of a mitogen-activated protein kinase (MAPK) module acting between CTR1 and EIN2 (13, 14) (Fig. 2). Recent reports (15, 16), however, seriously question the contribution of this MAPK module to ethylene signaling. Several problems have been found in the experimental design of the original report (13), such as use of toxic levels of inhibitors (15) and irreproducibility of the MAPK6 activity induction by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (16). Moreover, loss-of-function MAPK6 plants show normal response to ethylene, suggesting that the MAPK6 module may not participate in ethylene signaling (15). Thus, the MAPK module has been removed from the Pathway (Fig. 1). Although a MAPK cascade may still function downstream of CTR1, no conclusive experimental evidence is available currently.

Fig. 2.

Historic pathway image captured from the dynamic graphical display of the information in the Connections Maps available 7 February 2005. The MAPK module is present in the form of MEK and MAPK6. This version of the pathway is not supported by the newest data. For a key to the colors and symbols and to view the most current information, please visit the pathway (About Connections Map).

Both the receptors and CTR1 are negative regulators of the pathway. In the absence of ethylene, the receptors are active and turn on CTR1, the function of which is to repress the ethylene pathway. Binding of ethylene gas, on the other hand, shuts down the receptors, resulting in the deactivation of CTR1 and induction of the ethylene responses (17, 18).

The first positive regulator of the ethylene pathway is EIN2. The complete lack of ethylene sensitivity in the ein2 loss-of-function mutants suggests an essential role of this protein in mediating ethylene responses (19). EIN2 possesses an N-terminal domain that shows similarity to the NRAMP family of metal ion transporters, and a unique C-terminal domain of unknown function (19). To date, no transport activity has been demonstrated for EIN2. Overexpression studies using the C terminus of EIN2 uncovered the ability of this domain to activate the downstream components of the pathway (19).

The next known component of the ethylene pathway that works downstream of EIN2 is EIN3, a member of a plant-specific transcription factor family (20). Five additional EIN3-like (EIL) genes have been identified in the Arabidopsis genome (21). Of these, only EIL1 has been unequivocally proven to participate in the transduction of the ethylene signal (20).

Ethylene regulates the activity of EIN3, at least in part, by stabilizing the EIN3 protein. Two F-box proteins, EBF1 and EBF2, directly interact with EIN3 and mediate its degradation by a proteosome-dependent mechanism (2224). F-box proteins represent a subunit of the SKP1/Cullin/F-box (SCF) complex, which mediates ubiquitination of the protein substrates to be degraded. It remains unknown how EIN2 regulates the activity of EIN3.

In the presence of ethylene, EIN3 accumulates in the cell and binds to the promoters of primary ethylene-response genes such as ERF1 and activates their transcription. ERF1 protein, in turn, triggers transcription of the secondary ethylene-response genes, such as basic chitinase (25). Global expression analysis has uncovered a large number of ethylene-regulated genes involved in various metabolic, developmental, and structural functions (26). Conversely, beyond ERF1, no other ethylene-responsive genes have been shown to function as the primary transcriptional targets of EIN3 (25). Genes encoding the EDF family of early ethylene-inducible transcription factors, which includes EDF1, 2, 3, and 4, represent the most likely candidates. Genetic data are consistent with ERF1 and EDF1 to 4 regulating specific branches of the ethylene response pathway downstream of EIN3 (26).

For recent reviews of this signaling pathway, see (18) and (27).

Pathway Details

URL: About Connections Map

Scope: Specific

Organism: plant: Arabidopsis

Canonical Pathway: Ethylene Signaling Pathway (About Connections Map)

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