Ethylene signaling in plants

Abstract

Ethylene is a gaseous phytohormone and the first of this hormone class to be discovered. It is the simplest olefin gas and is biosynthesized by plants to regulate plant development, growth, and stress responses via a well-studied signaling pathway. One of the earliest reported responses to ethylene is the triple response. This response is common in eudicot seedlings grown in the dark and is characterized by reduced growth of the root and hypocotyl, an exaggerated apical hook, and a thickening of the hypocotyl. This proved a useful assay for genetic screens and enabled the identification of many components of the ethylene-signaling pathway. These components include a family of ethylene receptors in the membrane of the endoplasmic reticulum (ER); a protein kinase, called constitutive triple response 1 (CTR1); an ER-localized transmembrane protein of unknown biochemical activity, called ethylene-insensitive 2 (EIN2); and transcription factors such as EIN3, EIN3-like (EIL), and ethylene response factors (ERFs). These studies led to a linear model, according to which in the absence of ethylene, its cognate receptors signal to CTR1, which inhibits EIN2 and prevents downstream signaling. Ethylene acts as an inverse agonist by inhibiting its receptors, resulting in lower CTR1 activity, which releases EIN2 inhibition. EIN2 alters transcription and translation, leading to most ethylene responses. Although this canonical pathway is the predominant signaling cascade, alternative pathways also affect ethylene responses. This review summarizes our current understanding of ethylene signaling, including these alternative pathways, and discusses how ethylene signaling has been manipulated for agricultural and horticultural applications.

Keywords: Arabidopsis thaliana; bioengineering; constitutive triple response 1 (CTR1); ethylene; ethylene-insensitive 2 (EIN2); hormone receptor; phytohormone; plant hormone; signal transduction; signaling.

Figure 1.

Simple genetic model of ethylene signaling. In black is shown a model for ethylene signaling based on molecular genetic experiments in Arabidopsis. These experiments showed that ethylene signaling involves ethylene receptors (ETR1, ERS1, ETR2, EIN4, and ERS2), the protein kinase CTR1, and EIN2 that signals to the transcription factors EIN3, EIL1, and EIL2. These, in turn, signal to other transcription factors, such as the ERFs, leading to ethylene responses. This has long been considered the canonical signaling pathway. In this model, CTR1 is a negative regulator of signaling. Ethylene functions as an inverse agonist, where it inhibits the receptors, which leads to lower activity of CTR1 releasing downstream components from inhibition by CTR1. More recent evidence has shown the existence of an alternative, “noncanonical” pathway (in gray), where ETR1 signals to histidine-containing AHPs and then to ARRs to modulate responses to ethylene.

Figure 2.

Diagram of domains of receptor isoforms. The receptors are dimers located in the ER membrane. Each dimer is stabilized by two disulfide bonds near the N terminus. All of the receptors contain transmembrane helices that comprise the ethylene-binding domain followed by a GAF and kinase domain. ETR1 is a histidine kinase, and the other four isoforms are serine/threonine kinases. Three of the five contain a receiver domain at the C terminus of the protein. The models for the receptors are based on published structural and computational studies on ETR1 (43, 69), where each monomer coordinates a copper ion required for ethylene binding. In ETR1, the DHp domain of the kinase dimerizes, and a flexible region allows each kinase catalytic domain to associate with the DHp domain. It is unknown whether the kinase domains of the other isoforms also dimerize. The receiver domains are predicted to be orientated away from the central axis of the receptor dimer.

Figure 3.

Model for ethylene signaling. RAN1 is a copper transporter that delivers copper to the lumen of the ER, where it is required for the biogenesis of the receptors and is used as a cofactor by the receptors to bind ethylene. A, in the absence of ethylene, the receptors signal to CTR1, which phosphorylates EIN2. This results in the ubiquitination of EIN2 by an SCF E3 containing the ETP1/2 F-box proteins, leading to EIN2 degradation by the proteasome. Because EIN2 levels are low, an SCF-E3 containing the EBF1/2 F-box proteins ubiquitinates EIN3 and EIL1, leading to their degradation by the proteasome and preventing them from affecting transcription in the nucleus. B, in the presence ethylene, the receptors bind ethylene via a copper cofactor. The binding of ethylene is modeled to cause a conformational change that either reduces CTR1 kinase activity or, as shown, results in CTR1 being sequestered by the receptors so that CTR1 can no longer phosphorylate EIN2. The reduction in EIN2 phosphorylation results in less EIN2 ubiquitination and an increase in EIN2 levels. An unknown protease cleaves EIN2, releasing the C-terminal end (EIN2-C) from the N-terminal end (EIN2-N). One fate of EIN2-C is to bind the RNAs for EBF1 and EBF2 and become sequestered in processing bodies (P-bodies). The reduction of EBF1/2 results in less ubiquitination of EIN3 and EIL1, causing higher EIN3/EIL1 levels. The other fate of EIN2-C is to translocate to the nucleus, where it increases the transcriptional activity of EIN3/EIL1 via ENAP1. This leads to numerous transcriptional changes. In parallel with this pathway, phosphoryl transfer from a conserved histidine in the ETR1 DHp domain to an aspartate in the receiver domain occurs. This is followed by phosphoryl transfer from this residue to AHPs and finally ARRs resulting in transcriptional changes.

Figure 4.

Chemicals that affect ethylene responses in plants. Many strained alkenes, such as 2,5-norbornadiene, trans-cyclooctene, and 1-methylcyclopropene, have been demonstrated to be effective antagonists of ethylene responses that function on the ethylene receptors. Other compounds, such as triplin, are agonists of ethylene responses. Triplin is believed to function by altering the delivery of copper ions to the receptors.