1、AbstractEthylene is a gaseous plant hormone involved in several important physiological processes throughout a plants life cycle.Decades of scientific research devoted to deciphering how plants are able to sense and respond to this key molecule have culminated in the establishment of one of the best
2、 characterized signal transduction pathways in plants.The ethylene signaling pathway starts with the perception of this gaseous hormone by a family of membrane-anchored receptors followed by a Raf-like kinase CTR1 that is physically associated with the receptors and actively inhibits downstream comp
3、onents of the pathway.A major gap is represented by the mysterious plant protein EIN2 that genetically works downstream of CTR1 and upstream of the key transcription factor EIN3.Transcriptional regulation by EIN3 and EIN3-family members has emerged as a key aspect of ethylene responses.The major com
4、ponents of this transcriptional cascade have been characterized and the involvement of post-transcriptional control by ubiquitination has been determined.Nevertheless,many aspects of this pathway still remain unknown.Recent genomic studies aiming to provide a more comprehensive view of modulation of
5、 gene expression have further emphasized the ample role of ethylene in a myriad of cellular processes and particularly in its crosstalk with other important plant hormones.This review aims to serve as a guide to the main scientific discoveries that have shaped the field of ethylene biology in the re
6、cent yearsEFFECTS OF ETHYLENEFruit Ripening Abscission;leaf flower fruits(thinning,harvesting)Hook Closure Maintenance Initiates Germination in Grains Activates dormant buds(potatoes in storage)Stem elongation in deep-water rice Induces Flowering in Pineapple Promotes Female Expression in Flowers Fl
7、ower and Leaf Senescence:Ag preventative(vase life)Ethylene1.Introduction Hormones act as chemical messengers in the control of the molecular,biochemical,and physiological events underlying growth and development.Hormones also serve as essential integrators of developmental programs with the environ
8、mental signals.A basic challenge in biology is,therefore,to understand the molecular mechanisms that underlie hormone action,in other words,how those chemical signals are sensed by and communicated within the cells to trigger the relevant responses.This process can be divided into three main steps:(
9、1)signal perception,(2)signal transduction or a cascade of biochemical events that ultimately leads to the induction of the final step,(3)the response.Among the plant hormones,ethylene distinguishes itself by its simple hydrocarbon chemical structure(C2H4)and its gaseous nature.This simple molecule,
10、however,plays a major role in plant growth and development by influencing a wide range of complex physiological processes throughout the entire plant life cycle,from seed germination to flowering,fruitripening,and senescence One of the most dramatic effects of ethylene on plant morphogenesis is the
11、classical triple response exhibited by dark-grown seedlings exposed to ethylene.The triple response in Arabidopsis is characterized by(1)exaggerated curvature of the apical hook,(2)radial swelling of the hypocotyl,and(3)inhibition of hypocotyl and root growthFig.1 Phenotypes of dark-grown three-day-
12、old seedlings ofArabidopsis thaliana.The plant on the left was grown without hormonal supplementation,whereas the plant on the right was exposed to 10 mM ethylene precursor ACC and thus shows a typical triple response.A series of elegant genetic,molecular,and biochemical studies are uncovering a lar
13、gelylinear pathway that transduces the ethylene signal from the endoplasmic reticulum membrane to the nucleusFig.2 The ethylene signaling pathway and its genetically characterized components.The signaling pathway components are shown in their sequential order of action.Components drawn in white repr
14、esent active forms,whereas gray ovals represent their inactive versions.Binding of ethylene to the receptors,represented by ETR1,leads to activation of ethylene responses.Dotted oval represents EIN3 degradation by the 26S proteasome pathway due to action of EBF1 and EBF2.Arrows indicate activation s
15、teps,whereas a blocked arrow depicts repression of downstream elements by CTR1.This review aims to summarize the current state of knowledge in the ethylene field.We will start with the description of the main experimental breakthroughs that resulted in the discovery of the known ethylene signaling c
16、omponents to then describe the genomic approaches employed to characterize the molecular aspects of the ethylene response.2.Ethylene perception is mediated by a small family of receptors The development of modern molecular genetic approaches and the selection of Arabidopsis as a plantmodel system op
17、ened a new door not only for the identification of the ethylene receptors,but for the elucidation of the entire signal transduction pathway.The new quest for the molecular components of the ethylene signal transduction machinery first led to the isolation of ethylene response1,etr1,a dominant mutati
18、on that confers ethylene insensitivity.ETR1 encodes a histidine kinase with similarity to the classical bacterial two-component histidine kinases.The novel hydrophobic amino-terminal domain of ETR1 heterologously expressed in yeast was shown to possess high-affinity binding properties to the gas eth
19、ylene.ETR1 was found to act as a dimer that localizes to a cellular membrane system.A series of elegant studies performed in the nineties revealed that ethylene receptors are encoded by a small gene family that in Arabidopsis consists of five members:ETR1,ethylene response2(etr2),ethylene insensitiv
20、e4(ein4),ethylene resistant1(ers1),and ethylene resistant2(ers2).Their sequence similarity and structural organization,the five receptors are categorized into two subfamilies.Subfamily I members(ETR1 and ERS1)harbor three hydrophobic transmembrane domains in the amino-terminus followed by a conserve
21、d histidine kinase domain.Subfamily II members(ETR2,ERS2,and EIN4)possess four predicted amino-terminal hydrophobic transmembrane regions followedby a less conserved kinase domain that lacks several of the canonical features required for histidine-kinase activity.Furthermore,three of the five recept
22、ors,ETR1,ETR2,and EIN4,also possess a carboxyl-terminal receiver domain.A major breakthrough came with the isolation of loss-offunction(LOF)alleles of the receptors.Each of the single LOF alleles was still able to respond to ethylene,indicating a high degree of functional redundancy among the recept
23、ors.Furthermore,triple and quadruple LOF mutants displayed a constitutive ethylene response in the absence of the hormone.Both receptor subfamilies appear to be able to sense ethylene,as double etr1;ers1 and triple etr2;ers2;ein4 LOF mutants are still able to respond to ethylene in the triple respon
24、se assay.However,a particular role for subfamily I receptors in light-grown plants has been recently suggested.Double etr1;ers1 LOF mutants displayed severe phenotypes,including miniature rosettes,fertility defects,and altered flower morphology.All of these effects were dependent on a functional eth
25、ylene signaling pathway,implying that the observed growth defects arose from a misregulation of ethylene responses.Overexpression of the subfamily II members was unable to rescue the observed phenotypes,whereas ectopic expression of either wild-type ETR1 or ERS1 restored normal development,further s
26、upporting the notion of a unique role for the subfamily I receptors.Ethylene was found to bind to the receptors through a transition metal cofactor,copper.Moreover,the copper cofactor was shown to be essential for ethylene binding,and thus,proper receptor function.The current structural model for th
27、e ethylene binding domain suggests that the copper(I)cofactor is located in the electron-rich hydrophobic pocket formed by the N-terminal transmembrane domains of the receptors.In particular,residues Cys65 and His69 are thought to play a fundamental role in this protein-metal-hormone interaction.In
28、planta,the relevance of this interaction was further confirmed with the identification of RESPONSIVE TO ANTAGONIST1(RAN1).RAN1 was isolated using a screening for mutants with altered specificity in hormone binding by employing the ethylene antagonist trans-cyclooctene(TCO).ran1 plants are defective
29、in a copper transporter similar to P-type ATPases.The lack of the metal cofactor impairs receptor function in ran1 mutants by causing altered ligand specificity and thus rendering the plants responsive to the antagonist TCO.Furthermore,a strong LOF ran1 allele results in a constitutive ethylene resp
30、onse phenotype in the absence of the gaseous hormone.Autophosphorylation activity has been demonstrated for ETR1 and all other members of the receptor family.However,while ETR1 autophosphorylates in the predicted conserved histidine residue,ERS1 and all of the subfamily II members display a serine-k
31、inase activity in vitro.ERS1 also possesses a histidine-kinase activity,whereas its serine autophosphorylation is thought not to be significant in vivo.Another interesting study addressed the roles of the kinase activity and of the carboxyl-terminal receiver domain of ETR1,ETR2,and EIN4 in ethylene
32、signaling by looking at the effect of expressing truncated versions of ETR1 in a triple LOF etr1;etr2;ein4 mutant background.Transformation of the triple mutant with a truncated version of ETR1 that lacks both the histidine-kinase and the receiver domain failed to restore ethylene responsiveness to
33、the triple mutant.Conversely,a second truncated ETR1 construct in which only the receiver domain was missing did rescue the partial constitutive triple response phenotype of the triple knockout line.Moreover,transgenic plants harboring this particularconstruct displayed hypersensitivity to ethylene.
34、These observations implied that the kinase domain was necessary for signal transmission by the receptors and that the receiver domain was not essential for restoring ethylene responsiveness.Modulation of the ethylene responses by the receiver domain could be achieved,for instance,through its reporte
35、d interaction with another negative regulator of the pathway,CONSTITUTIVE TRIPLE RESPONSE1(CTR1)(see below),assuming that in the absence of the receiver domain the function of CTR1 is impaired.Interestingly,a detailed kinetic analysis of seedling growth response in the presence of exogenous ethylene
36、 and of the consecutive recovery after ethylene withdrawal also indicated the importance of both the kinase and receiver domains in specific aspects of the ethylene response.These results suggest that both the kinase and the receiver domain are important for the hypocotyl recovery from short ethylen
37、e treatment and indicate that,despite the large functional overlap,some specificity can be attributed to the different receptors.The accumulated data have provided a model for the role of the receptors in ethylene signaling.(1)In the absence of the hormone,the receptors actively repress downstream c
38、omponents of the pathway and inhibit ethylene responses.(2)Ethylene gas binds to all five receptors,causing the receptors to become inactive and releasing the pathway from their repression.Although still controversial,(3)receptor kinase activity possibly plays an important role in signaling.3.A Raf-
39、like kinase acts downstream of the receptors repressing ethylene responses A subset of those was further characterized as defective in ethylene biosynthesis,namely ethylene overproducer(eto).Conversely,the ctr1 mutant did not respond to inhibitors of ethylene biosynthesis indicative of an alteration
40、 in signal transduction.CTR1 encodes a serine/threonine kinase whose carboxyl-terminus shares sequence similarity with the Raf family of protein kinases.The constitutive response phenotype exhibited by LOF mutants indicates that CTR1 negatively regulates the ethylene signaling pathway.Interestingly,
41、CTR1 was shown to physically interact with the receptors.Subsequently,CTR1 was shown to co-localize with the receptors at the ER membranes.Moreover,co-purification of an affinity-tagged CTR1 and endogenous ETR1 in transgenic Arabidopsis lines strongly supported the in vivo interaction of these prote
42、ins.The significance of this interaction in signal transmission is corroborated by the observation that double and triple LOF receptor mutants result in a loss of ER-localization of CTR1.Taken together,these data suggested that the receptors and CTR1 function as part of an ER-localized complex that
43、actively represses ethylene responses.The serine/threonine kinase activity of CTR1 was demonstrated in vitro and shown to be essential for proper functioning of the receptors/CTR1 signaling complex.It has been established that ethylene binding to the receptors affects neither the interaction between
44、 the receptors and CTR1 nor their sub-cellular localization.The current proposed model predicts that upon ethylene binding,the receptors/CTR1 signaling complexes are turned off.4.A unique plant protein is a central component of the signaling pathway and positively regulates ethylene responses Downst
45、ream of the receptors/CTR1 complexes there acts a positive regulator of the pathway,ETHYLENEINSENSITIVE2(EIN2).EIN2 is required for all ethylene responses studied and constitutes a critical step in the signal transduction.The hydrophobic amino-terminus is predicted to form twelve transmembrane domai
46、ns and shares sequence similarity with the family of NRAMP metal ion transporters.The long EIN2 carboxyl-terminus contains a coiled-coil structure(a motif typically involved in proteinprotein interactions)but otherwise displays no similarity to known protein.Conversely,the NRAMP-like amino-terminal
47、domain of EIN2 is believed to sense the upstream signaling events.5.A transcriptional cascade mediates ethylene responses at the gene expression level ETHYLENE INSENSITIVE3,EIN3,is a nuclear-localized protein required for ethylene signaling that genetically works downstream of EIN2.EIN3 belongs to a
48、 small gene family that in Arabidopsis also includes five EIN3-LIKE(EIL)proteins.These observations suggested that EIN3 and at least some of the EILs may act as transcriptional regulators of ethylene responses.In Arabidopsis,EIN3,EIL1,and EIL2 were demonstrated to bind to a short palindromic region,
49、known as the EIN3-binding site,or EBS,in the promoter of the EREBP family member ETHYLENE RESPONSE FACTOR1(ERF1).ERF1,in turn,is a GCC box-binding transcription factor that acts downstream of EIN3 and EILs and is responsible for the modulation of a set of secondary ethylene responsive genes.Consiste
50、nt with these results,transgenic lines overexpressing EIN3 show a constitutive triple response,whereas in ERF1-overexpressing plants only some of the ethylene responses are activated.ERF1 is also required for the activation of defense-related responses by the plant hormone jasmonicacid.Moreover,both
