Ethylene Signaling in Arabidopsis
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Ethylene Signaling in Arabidopsis

The Hydrocarbon Ethylene (C2H4) is a Gaseous Plant hormone, which is involved in a multitude of Physiological and Developmental processes. Responses to Ethylene include Fruit Ripening, Leaf Senescence and Abscission, Promotion or Inhibition of Seed Germination, Flowering and Cell Elongation. Environmental Stresses, such as Chilling, Flooding, Wounding and Pathogen Attack increase Ethylene Synthesis and thereby control Gene Expression. A combination of genetic, biochemical, and molecular approaches is uncovering this remarkable signaling pathway in plants. Although the initial hunt for the major elements of the Ethylene pathway was performed in the model plant Arabidopsis thaliana, identification and functional analysis of the corresponding genes in other plant species uncovered a high degree of conservation of this Signaling Cascade in the Plant Kingdom. Molecular genetic studies on the plant Arabidopsis have established a largely Linear Signal Transduction Pathway for the response to Ethylene gas. The signaling components of the Ethylene pathway include five Ethylene Receptors (ETR1, ETR2, EIN4, ERS1 and ERS2), which resemble Bacterial Two-component regulators; the MAPKKK (Mitogen-Activated Protein Kinase Kinase Kinase)-like protein Ctr1 (Serine/Threonine-Protein Kinase Ctr1); EIN2 (Ethylene Insensitive Protein-2), a member of the N-Ramp family of metal-transporters; and the EIN3 and ERF families of transcription factors (Ref.1).

A family of ER (Endoplasmic Reticulum)-associated Receptors ETR (Ethylene Receptor) perceives Ethylene. There are two types of Ethylene receptors in Arabidopsis. Members of the Type-I Subfamily, which include ETR1 (Ethylene Receptor-1) and ERS1 (Ethylene Response Sensor-1), contain an Amino-terminal Ethylene-Binding Domain (also called the Sensor Domain) and a well-conserved Carboxy terminal His (Histidine) Kinase Domain. The Amino-terminal Sensor Domains of the Receptors contain a Cu (Copper) Cofactor, which is delivered by the Copper transporter RAN1 (Responsive To Antagonist-1), and is needed for Ethylene Binding and are associated with the ER membrane. The Type-II Subfamily Receptors, which include ETR2 (Ethylene Receptor-2), ERS2 (Ethylene Response Sensor-2) and EIN4 (Ethylene Insensitive-4), contain an Amino-terminal Ethylene-Binding Domain and a Degenerate His Kinase Domain that lacks one or more elements that are necessary for catalytic activity. ETR1 (Type I), ETR2 and EIN4 (Type II) also have an additional Receiver domain at their carboxyl termini whose function is unknown (Ref.2).

In the absence of Ethylene, the Receptors remain in a functionally Active state, which is able to interact with Ctr1. Ctr1 is a Serine-Threonine Kinase, which is composed of an Amino-terminal domain of unknown function, and a Carboxy-terminal kinase domain that is most related to Raf-like MAPKKKs (Mitogen-Activated Protein Kinase Kinase Kinases). All five Ethylene Receptors are able to interact with Ctr1 via their Carboxy-terminal Kinase domains. However, Type-I Receptors (i.e. ETR1 and ERS1) have a high affinity for Ctr1, whereas Type II Receptors (at least ETR2) possess a low binding affinity for Ctr1.  Association with the ER-bound receptors activates Ctr1. Activated Ctr1 then represses the downstream Ethylene responses by a mechanism that requires its Carboxyl terminal Ser/Thr Kinase domain. When Ethylene is present, it binds to the sensor domain of the receptor and presumably causes a conformational change, resulting in an inactive receptor. Ctr1 is then released from the ER and also becomes inactivated. Inactivated Ctr1 causes induction of the Ethylene responses. A MAPK module, consisting of SIMKK and MPK6 (Mitogen Activated Protein Kinase-6), is believed to act downstream of Ctr1, although the biochemical consequence of this MAPK pathway is not evident (Ref.3).

Downstream components in the Ethylene pathway include several positive regulators (EIN2, EIN5, EIN6 and the transcription factors EIN3 and EIL1). EIN2 is an integral membrane protein whose exact function is not understood. EIN5 (Ethylene Insensitive-5) and EIN6 (Ethylene Insensitive-6) have not yet been characterized at the molecular level. The nuclear protein EIN3 (Ethylene Insensitive-3) is a transcription factor that regulates the expression of its immediate target genes such as ERF1 (Ethylene Response Factor-1), EDF1 (Ethylene Response DNA-Binding Factor-1), EDF2 (Ethylene Response DNA-Binding Factor-2), EDF3 (Ethylene Response DNA-Binding Factor-3), and EDF4 (Ethylene Response DNA-Binding Factor-4).  ERF1 belongs to a large family of Apetala2- domain-containing Transcription factors that bind to a GCC-box present in the promoters of many Ethylene inducible, defense-related genes. Thus, a transcriptional cascade that is mediated by EIN3/ EIL and ERF proteins leads to the regulation of Ethylene controlled gene expression (Ref.2 and Ref.4).

Two F-box proteins, EBF1 (EIN3-Binding F Box Protein-1) and EBF2 (EIN3-Binding F Box Protein-2), whose transcription is induced by Ethylene, regulate the levels of EIN3. The level of EIN3 protein is controlled by Ethylene, via the Ubiquitin-Proteasome Complex (Ub/26S). Several EREBP (Ethylene-Responsive Element Binding Proteins) transcription factors are known to be immediate targets of EIN3/EIL1 (Ethylene-Insensitive-3-Like-1 Protein), which binds to PERE (Primary Ethylene response element) in the promoters of EREBP genes. One EREBP, called ERF1, is also involved in JA (Jasmonic Acid) mediated gene regulation. An unidentified JA regulated transcription factor also binds to the promoter of ERF1 to activate its expression. Therefore, the promoter of ERF1 functions to integrate signals from both the Ethylene and JA signaling pathways. Many EREBP proteins are known to regulate gene expression through interaction with a Cis-element called the GCC-box, which is found in several Ethylene-responsive genes including PDF1.2 (Plant Defensin Protein-1.2), ChiB (Acidic Endochitinase) and Hls1 (Hookless-1). These genes encode effector proteins that are needed to execute a wide variety of Ethylene responses, from disease resistance to differential cell growth (Ref.5). Although Ethylene is structurally the simplest of all plant hormones, it has a strong influence on many different developmental processes, from germination to senescence. In the last decade, molecular and genetic investigations have contributed enormously to the understanding of Ethylene perception and signal transduction. The rate of hormone biosynthesis, changes in the sensitivity to the hormone, or the overlap in expression of the specific subsets of hormone-responsive genes each a role in the coordination of the plants response to multiple growth regulators (Ref.6).