Mechanistic Insights in Ethylene Perception and Signal Transduction

Abstract

The gaseous hormone ethylene profoundly affects plant growth, development, and stress responses. Ethylene perception occurs at the endoplasmic reticulum membrane, and signal transduction leads to a transcriptional cascade that initiates diverse responses, often in conjunction with other signals. Recent findings provide a more complete picture of the components and mechanisms in ethylene signaling, now rendering a more dynamic view of this conserved pathway. This includes newly identified protein-protein interactions at the endoplasmic reticulum membrane, as well as the major discoveries that the central regulator ETHYLENE INSENSITIVE2 (EIN2) is the long-sought phosphorylation substrate for the CONSTITUTIVE RESPONSE1 protein kinase, and that cleavage of EIN2 transmits the signal to the nucleus. In the nucleus, hundreds of potential gene targets of the EIN3 master transcription factor have been identified and found to be induced in transcriptional waves, and transcriptional coregulation has been shown to be a mechanism of ethylene cross talk.

Figure 1.

Genetic diagram of the core ethylene signaling pathway. The ethylene signal represses the function of the five ethylene receptor genes (ETHYLENE RESPONSE1 [ETR1], ETHYLENE RESPONSE SENSOR1 [ERS1], ETR2, ETHYLENE INSENSITIVE4 [EIN4], and ERS2), which otherwise repress ethylene responses through the negative regulator CONSTITUTIVE RESPONSE1 (CTR1). Ethylene responses are positively regulated by EIN2, EIN3, and downstream primary and secondary ethylene-responsive genes, such as ETHYLENE RESPONSE FACTOR1 (ERF1). Representative seeding phenotypes in the triple response assay (ethylene insensitivity or constitutive ethylene response) are shown for the dominant gain-of-function mutations in the ethylene receptor genes and the loss-of-function mutations in CTR1, EIN2, and EIN3/ETHYLENE INSENSITIVE3-LIKE1 (EIL1). Arrows indicate activation, and T-bars indicate repression of the pathway.

Figure 2.

Schematic model of the ethylene signaling pathway. In the absence of ethylene perception (left), the formation of functional ethylene receptors depends on a copper cofactor provided by the copper transporter RESPONSIVE TO ANTAGONIST1 (RAN1), as well as activation by REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1), which depends on cytochrome b5 (Cb5). The ethylene receptors (represented here by ETR1 and ERS1 homodimers) at the endoplasmic reticulum (ER) membrane are in a protein complex with downstream components EIN2 and CTR1. The receptors associate with and activate (by an undefined signaling mechanism) the CTR1 protein kinase domain (KD), which phosphorylates the EIN2 C-terminal domain. Phosphorylation prevents EIN2 from signaling, and EIN2 is targeted for 26S proteasomal degradation by F-box proteins ETHYLENE INSENSITIVE2 TARGETING PROTEIN1 (ETP1) and ETP2. Meanwhile, in the nucleus, the F-box proteins ETHYLENE INSENSITIVE3 BINDING F-BOX1 (EBF1) and EBF2 target the EIN3/EIL1 transcription factors for 26S proteasomal degradation (only EIN3 is shown), preventing induction of gene expression such that there is no ethylene response. Additionally, there is a postulated secondary pathway from the receptors involving autophosphorylation of the His by the receptor His kinase (HK) domain, and transfer of the phosphate to the receiver (R) domain, followed by transfer of the phosphate to ARABIDOPSIS HIS PHOSPHOTRANSFER (AHP), which is released by a conformational change in the receptors (indicated by the altered shapes of the HK and R domains between the left and right sides) when they bind ethylene (right). The binding of ethylene (right) inactivates ethylene receptor signaling (indicated by the altered shapes of the HK and R domains between the left and right sides). In addition, the levels of ERS1 and other ethylene receptor isoforms (not shown) increase (represented by the darker color on the right side relative to the left side) due to transcriptional induction, but reach an equilibrium state due to being degraded by the 26S proteasome. CTR1 levels increase in the complex as well (represented by the darker color on the right side relative to the left side) due to the increased level of ethylene receptors and protect the ETR1 receptor from proteolysis. However, the ethylene receptors no longer activate CTR1, and therefore, EIN2 is no longer phosphorylated. Instead, a cytoplasmic portion of EIN2 is proteolytically cleaved by an unidentified protease, and the liberated C-terminal portion of EIN2 (C-END) moves into the nucleus where signal transmission results in EIN2-dependent 26S proteasomal degradation of the F-box proteins EBF1/2 and, consequently, the stabilization and accumulation of master transcription factors EIN3/EIL1. EIN3/EIL1 activate a transcriptional cascade that includes the downstream ERF1 transcription factor gene. An exoribonuclease (EXORIBONUCLEASE4 [XRN4]) also plays an indirect role in the repression of EBF1/2 mRNA.

Figure 3.

Examples of the EIN3-mediated regulatory network involving ethylene and other signals. A, Ethylene-stabilized EIN3 directly binds to the promoters of various target genes (described in the text) that control a diverse array of responses. B, The EIN3 protein can physically associate with other transcriptional activators or repressors, including DELLA, JASMONATE ZIM-DOMAINs (JAZs), MYC2, and FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), which are regulated by GA, JA, and iron, respectively, to coactivate (arrows) or corepress (T-bars) transcription in various processes.