Connections Map Overview

Ethylene Signaling Pathway

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Science's STKE  22 Mar 2005:
Vol. 2005, Issue 276, pp. cm3
DOI: 10.1126/stke.2762005cm3

Abstract

The structural simplicity of the plant hormone ethylene contrasts with its dramatic effects in various developmental processes, as well as in the cellular processes that ethylene initiates in response to a diversity of environmental signals. A single well-conserved signaling cascade mediates this broad spectrum of responses. Ethylene is perceived by a family of two-component histidine kinase receptors that become inactivated upon ethylene binding. In the absence of the hormone, the receptors activate CTR1, a negative regulator of ethylene responses. Sequence similarity between CTR1 and the Raf protein kinases implies involvement of a mitogen-activated protein kinase cascade in this signaling pathway. The protein EIN2 acts downstream of CTR1 and the possible kinase cascade. Although the biochemical function of EIN2 is not understood, its critical role is manifested by the complete ethylene insensitivity of EIN2 loss-of-function mutants. Downstream of EIN2, a family of plant-specific EIN3-like transcription factors mediate ethylene responses. The regulation of EIN3 stability by ethylene is accomplished by F-box–containing proteins that participate in the formation of a SKP1/cullin/F-box complex that targets proteins for degradation by the proteasome. A large number of ethylene-regulated genes have been identified, including the APETALA2 domain–containing transcription factor genes ERF1 and EDF1 to 4, which suggests the participation of a transcriptional cascade in the ethylene response. The differential regulation of some components of this complex nuclear cascade by other signaling pathways provides a possible mechanism for interaction and signal integration. As new points of intersection with other pathways and additional participants in the pathway are identified, the Connections Map will be updated to include this new information.

Description

This record contains general information about the ethylene signaling pathway collected across species.

Ethylene is a plant hormone involved in the regulation of a large number of processes, ranging from seed germination to senescence and organ abscission. For recent reviews of this signaling pathway, see (1, 2). Research publications are cited in the descriptions of the signaling components accessible through the pathway in the Connections Maps (Fig. 1).

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).

Although most of the components of this pathway have been identified and extensively characterized in Arabidopsis (Table 1), their participation in ethylene signaling in other plant species appears to be universal. Although the basic genetic architecture of this pathway is conserved, the number of family members in each signaling step varies from species to species.

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, please visit the pathway (About Connections Map).

Table 1.

Specific examples of the ethylene signaling pathway in the Connections Maps database. The specific pathway is based on the canonical ethylene signaling pathway.

Ethylene is sensed by a family of endoplasmic reticulum (ER)–localized membrane-bound receptors (3) that share sequence similarity with bacterial two-component histidine kinases [reviewed in Bleecker and Kende (4)]. Ethylene receptors can be classified into two subfamilies, I or II, depending on the presence or lack of a canonical histidine kinase domain, respectively (5, 6). Although kinase activity has been demonstrated for both receptor subfamilies, its role in signal transduction is not clear (710). The receptors work as negative regulators upstream of the signaling component CTR1 (11, 12). Sequence analysis implies that CTR1 is a Raf-like protein kinase (13). It serves as another negative regulator of the pathway that colocalizes and directly interacts with the receptors (14, 15).

Downstream of CTR1, a cascade of mitogen-activated protein kinases (MAPKs) is thought to operate (Fig. 2). Biochemical studies in both Medicago and Arabidopsis suggest a role for salt stress–inducible MAPK kinase (SIMKK) and MAPK6 in the ethylene response (16). Recent reports (17, 18), 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 (16), such as use of toxic levels of inhibitors (17) and irreproducibility of the MAPK6 activity induction by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (18). Moreover, loss-of-function MAPK6 plants show normal response to ethylene, suggesting that the MAPK6 module may not participate in ethylene signaling (17). Thus, the MAPK module has been removed from the Pathway (Fig. 1). Although an MAPK cascade may still function downstream of CTR1, there is no conclusive experimental evidence available currently.

Downstream of CTR1 is EIN2, a novel plant-specific protein of unknown function. EIN2 has two well-defined domains: (i) an N terminus with similarity to the NRAMP family of metal ion transporters and (ii) a unique hydrophilic C terminus. Overexpression of the C-terminal domain constitutively activates ethylene responses, suggesting that this domain is responsible for the transduction of the signal to yet unknown downstream components (19). Further downstream in the pathway is a family of EIN3-like proteins (20). These plant-specific transcription factors are structurally and functionally conserved among several plant species (2123). Their activation by ethylene is, at least in part, mediated by the regulation of their protein abundance through a ubiquitin-mediated proteasomal pathway. Two F-box proteins that form part of an SKP1/cullin/F-box (SCF) complex have been implicated in the ethylene-mediated regulation of EIN3 levels in Arabidopsis (24, 25). Functional studies of EIN3 have shown that this protein can bind to the promoter sequences of an ethylene-inducible transcription factor gene ERF1, a member of the ethylene response element binding protein (EREBP) family of genes (26, 27). Overexpression experiments suggest that ERF1 participates in the regulation of a number of ethylene-responsive genes, as well as of jasmonic acid (JA)-responsive genes (26, 28). Ethylene controls expression levels of a large number of target genes through what seems to be a transcriptional cascade (29).

Pathway Details

URL: About Connections Map

Scope: Canonical

Related Resources

Reviews

J. M. Alonso, J. R. Ecker, The ethylene pathway: A paradigm for plant hormone signaling and interaction. Sci. STKE 2001, re1 (2001). [Gloss] [Abstract] [Full Text]

T. Urao, K. Yamaguchi-Shinozaki, K. Shinozaki, Plant histidine kinases: An emerging picture of two-component signal transduction in hormone and environmental responses Sci. STKE 2001, re18 (2001). [Gloss] [Abstract] [Full Text]

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