Amplification and Adaptation in the Ethylene Signaling Pathway

Abstract

Plants are exquisitely sensitive to the ethylene signal and also respond to a much wider range of ethylene concentrations than would seem possible based on the simple circuitry of its primary signal transduction pathway, suggesting the existence of mechanisms for amplification and adaptation to ethylene signals. Here, such regulatory systems are considered within the context of what is known about the plant ethylene signaling pathway as well as signaling by the animal G-protein coupled receptors, and the bacterial methyl-accepting chemotaxis proteins.Magnitude amplification and sensitivity amplification mechanisms are considered as strategies for amplification of the ethylene signal. Several families of negative feedback regulators that desensitize plants to ethylene and thereby facilitate the ethylene adaptation response of plants are described. These negative feedback regulators include the ethylene receptors themselves, the reversion-to-ethylene sensitivity1 (RTE1)/greenripe (GR) family, and the auxin-regulated gene involved in organ size (ARGOS) family, all of which function at the level of the ethylene receptors to desensitize plants to ethylene. These negative regulators also include the ein3binding F box protein (EBF) family of F-box proteins, which target the ethylene insensitive3 (ein3)/ein3-lik (EIL) family of transcription factors for degradation. Ethylene signal amplification and adaptation employ both transcriptional and post-transcriptional regulation.

Ethylene signaling in plants

“…The exact mechanisms for regulation by these proteins are under investigation. More information about this is contained in a recent review (167).…”

Ethylene signaling in rice and Arabidopsis: New regulators and mechanisms

“…The structures of these ethylene receptor family members resemble those of bacterial histidine kinases and have been proposed to have been acquired through an ancient transfer of plastid genes to plants (Mount and Chang, 2002; Chang, 2016). In Arabidopsis , five ethylene receptors have been identified: ETR1, ETHYLENE RESPONSE SENSOR 1 (ERS1), ETR2, ERS2, and EIN4 (Ju and Chang, 2015; Azhar et al, 2019). All five ethylene receptors feature N‐terminal ligand‐binding/transmembrane domains, a linkage GAF domain, and a histidine kinase‐like domain with (ETR‐type) or without (ERS‐type) an attached receiver domain (Ju and Chang, 2015).…”

“…In this model, ethylene is perceived by ethylene receptors at the endoplasmic reticulum (ER) membrane and transduced through CTR1 and EIN2. The signal is further amplified by the EIN3/EIL1‐mediated transcriptional activation cascade to activate the expression of ethylene‐response genes, triggering the ethylene response (Azhar et al, 2019; Binder, 2020). In the absence of ethylene binding, ethylene receptors may somehow activate the kinase activity of CTR1 (Hua and Meyerowitz, 1998), which subsequently phosphorylates the C‐terminal domain of EIN2 and prevents it from participating in signaling (Ju et al, 2012).…”

Role of the Cytokinin-Activated Type-B Response Regulators in Hormone Crosstalk

“…Most of these encode elements that modulate activity of the signaling pathway, rather than being within the pathway itself, although the ethylene receptor ETR2 was identified as a target that has an expression which is downregulated by cytokinin. The modulating factors include ARGOS and ARL from the ARGOS gene family which are transmembrane proteins that act at the level of the receptors and desensitize the plant to ethylene [104,111,112]; TRP1 which functions as a positive regulator of ethylene signaling potentially by binding to receptors and interfering with their ability to interact with CTR1 [113]; and ETP and EBF genes which encode F-box proteins involved in the degradation of EIN2 and EIN3, respectively [114][115][116].…”