Histidine kinase activity of the ethylene receptor ETR1 facilitates the ethylene response in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), ethylene is perceived by a receptor family consisting of five members. Subfamily 1 members ETHYLENE RESPONSE1 (ETR1) and ETHYLENE RESPONSE SENSOR1 (ERS1) have histidine kinase activity, unlike the subfamily 2 members ETR2, ERS2, and ETHYLENE INSENSITIVE4 (EIN4), which lack amino acid residues critical for this enzymatic activity. To resolve the role of histidine kinase activity in signaling by the receptors, we transformed an etr1-9;ers1-3 double mutant with wild-type and kinase-inactive versions of the receptor ETR1. Both wild-type and kinase-inactive ETR1 rescue the constitutive ethylene-response phenotype of etr1-9;ers1-3, restoring normal growth to the mutant in air. However, the lines carrying kinase-inactive ETR1 exhibit reduced sensitivity to ethylene based on several growth response assays. Microarray and real-time polymerase chain reaction analyses of gene expression support a role for histidine kinase activity in eliciting the ethylene response. In addition, protein levels of the Raf-like kinase CONSTITUTIVE TRIPLE RESPONSE1 (CTR1), which physically associates with the ethylene receptor ETR1, are less responsive to ethylene in lines containing kinase-inactive ETR1. These data indicate that the histidine kinase activity of ETR1 is not required for but plays a modulating role in the regulation of ethylene responses. Models for how enzymatic and nonenzymatic regulation may facilitate signaling from the ethylene receptors are discussed.     

Heteromeric interactions among ethylene receptors mediate signaling in Arabidopsis

The gaseous hormone ethylene is perceived in Arabidopsis by a five member receptor family that consists of the subfamily 1 receptors ETR1 and ERS1 and the subfamily 2 receptors ETR2, ERS2, and EIN4. Previous work has demonstrated that the basic functional unit for the ethylene receptor, ETR1, is a disulfide-linked homodimer. We demonstrate here that ethylene receptors isolated from Arabidopsis also interact with each other through noncovalent interactions. Evidence that ETR1 associates with other ethylene receptors was obtained by co-purification of ETR1 with tagged versions of ERS1, ETR2, ERS2, and EIN4 from Arabidopsis membrane extracts. ETR1 preferentially associated with the subfamily 2 receptors compared with the subfamily 1 receptor ERS1, but ethylene treatment affected the interactions and relative composition of the receptor complexes. When transgenically expressed in yeast, ETR1 and ERS2 can form disulfide-linked heterodimers. In plant extracts, however, the association of ETR1 and ERS2 can be largely disrupted by treatment with SDS, supporting a higher order noncovalent interaction between the receptors. Yeast two-hybrid analysis demonstrated that the receptor GAF domains are capable of mediating heteromeric receptor interactions. Kinetic analysis of ethylene-insensitive mutants of ETR1 is consistent with their dominance being due in part to an ability to associate with other ethylene receptors. These data suggest that the ethylene receptors exist in plants as clusters in a manner potentially analogous to that found with the histidine kinase-linked chemoreceptors of bacteria and that interactions among receptors contribute to ethylene signal output.     

Possible modulation of Arabidopsis ETR1 N-terminal signaling by CTR1

The mitogen-activated protein kinase kinase kinase (MAPKKK) Constitutive Triple-Response1 (CTR1) plays a key role in mediating ethylene receptor signaling via its N-terminal interaction with the ethylene receptor C-terminal histidine kinase (HK) domain. Loss-of-function mutations of CTR1 prevent ethylene receptor signaling, and corresponding ctr1 mutants show a constitutive ethylene response phenotype. We recently reported in Plant Physiology that expression of the truncated ethylene receptor Ethylene Response1 (ETR1) isoforms etr11-349 and dominant ethylene-insensitive etr1-11-349, lacking the C-terminal HK and receiver domains, both suppressed the ctr1 mutant phenotype. Therefore, the ETR1 N terminus is capable of receptor signaling independent of CTR1. The constitutive ethylene response phenotype is stronger for ctr1-1 than ctr1-1 lines expressing the etr11-349 transgene, so N-terminal signaling by the full-length but not truncated ETR1 is inhibited by ctr1-1. We address possible modulations of ETR1 N-terminal signaling with docking of CTR1 on the ETR1 HK domain.     

植物乙烯信号转导研究进展

过去10年,对模式植物拟南芥的分子遗传学研究建立了植物乙烯信号转导线性模型.乙烯结合到受体上,经一条MAPK级联反应和转录级联途径将信号转导而产生乙烯反应.拟南芥乙烯受体家族由5个成员构成,ETR1、ERS1、ETR2、ERS2和EIN4.乙烯受体包括三个结构域:乙烯结合结构域、组氨酸激酶结构域和反应调控结构域.乙烯受体定位于内质网,与CTR1协同负调控乙烯反应.ENI2、EIN3/EIL、ERF1依次位于CTR1下游,正调控乙烯反应.EIN3属于转录激活因子调控蛋白家族,受转录后调控.乙烯稳定EIN3结构,EBF1/EBF2促进EIN3分解.ERF1是转录调控因子家族成员之一,是EIN3/EIL的直接作用目标.     

Molecular association of the Arabidopsis ETR1 ethylene receptor and a regulator of ethylene signaling, RTE1

The plant hormone ethylene plays important roles in growth and development. Ethylene is perceived by a family of membrane-bound receptors that actively repress ethylene responses. When the receptors bind ethylene, their signaling is shut off, activating responses. REVERSION-TO-ETHYLENE SENSITIVITY (RTE1) encodes a novel membrane protein conserved in plants and metazoans. Genetic analyses in Arabidopsis thaliana suggest that RTE1 promotes the signaling state of the ethylene receptor ETR1 through the ETR1 N-terminal domain. RTE1 and ETR1 have been shown to co-localize to the endoplasmic reticulum (ER) and Golgi apparatus in Arabidopsis. Here, we demonstrate a physical association of RTE1 and ETR1 using in vivo and in vitro methods. Interaction of RTE1 and ETR1 was revealed in vivo by bimolecular fluorescence complementation (BiFC) in a tobacco cell transient assay and in stably transformed Arabidopsis. The association was also observed using a truncated version of ETR1 comprising the N terminus (amino acids 1-349). Interaction of RTE1 and ETR1 was confirmed by co-immunoprecipitation from Arabidopsis. The interaction occurs with high affinity (K(d), 117 nM) based on tryptophan fluorescence spectroscopy using purified recombinant RTE1 and a tryptophan-less version of purified recombinant ETR1. An amino acid substitution (C161Y) in RTE1 that is known to confer an ETR1 loss-of-function phenotype correspondingly gives a nearly 12-fold increase in the dissociation constant (K(d), 1.38 μM). These findings indicate that a high affinity association of RTE1 and ETR1 is important in the regulation of ETR1.     

乙烯的生物合成与信号传递

乙烯是气体植物激素, 它在植物的生长发育过程中有很多作用。所以了解乙烯的生物合成及其信号转导是非常重要的。二十年来, 通过筛选有异于正常三重反应的突变体, 人们发现了乙烯信号转导的粗略轮廓。在拟南芥中, 有5个受体蛋白感受乙烯, ETR1、ERS1、ETR2、ERS2、EIN4。它们表现出功能冗余, 是乙烯信号的负调控因子, 在植物体内以二聚体的形式存在。ETR1的N端与乙烯结合时需要 铜离子(Ⅰ)的参与。尽管已经发现ETR1有组氨酸激酶活性, 而其它受体有丝氨酸/苏氨酸激酶活性, 但受体参与乙烯信号转导的机制还不是很清楚。受体与Raf类蛋白激酶CTR1相互作用, CTR1是乙烯反应的负调控因子。CTR1蛋白失活使EIN2蛋白活化。EIN2的N端是跨膜结构域, 与Nramp家族金属离子转运蛋白的跨膜结构域类似。EIN2的C端是一个新的未知结构域, 与乙烯信号途径的下游组分相互作用。EIN3位于EIN2的下游, EIN3和EILs诱导ERF1和其它转录因子的表达, 这些转录因子依次激活乙烯反应目的基因的表达, 表现出乙烯的反应。EIN3受到蛋白酶体介导的蛋白降解途径的调节。由于乙烯是一种多功能的植物激素, 其信号途径与其它信号途径有多重的交叉。     

乙烯信号转导的分子机制

气态植物激素乙烯在植物生长发育和应对生物及非生物胁迫过程中起着重要作用。在过去的十几年中, 对模式植物拟南芥的分子遗传研究已建立从信号感知到转录调控的乙烯信号转导线性模型。拟南芥共有5个乙烯受体ETR1、ERS1、ETR2、ERS2和EIN4, 目前已知ETR1定位在内质网上, 与类似于Raf的蛋白激酶CTR1协同负调控乙烯反应。EIN2和EIN3/EILs位于CTR1下游, 正调控乙烯反应。两个F-box蛋白EBF1和EBF2通过泛素/26S蛋白体降解途径调控EIN3的稳定性。5’→3’的外切核酸酶EIN5通过启动EBF1和EBF2 mRNA的降解, 拮抗EBF1和EBF2对EIN3的负反馈调控。目前对于乙烯信号转导途径关键组分的生化功能和乙烯下游反应途径的了解甚少, 乙烯信号转导途径与其它途径之间还存在着广泛的交叉反应, 这些问题的解决将大大增加我们对乙烯信号转导途径的了解。     

Structural Model of the Cytosolic Domain of the Plant Ethylene Receptor 1 (ETR1)

Ethylene initiates important aspects of plant growth and development through disulfide-linked receptor dimers located in the endoplasmic reticulum. The receptors feature a small transmembrane, ethylene binding domain followed by a large cytosolic domain, which serves as a scaffold for the assembly of large molecular weight complexes of different ethylene receptors and other cellular participants of the ethylene signaling pathway. Here we report the crystallographic structures of the ethylene receptor 1 (ETR1) catalytic ATP-binding and the ethylene response sensor 1 dimerization histidine phosphotransfer (DHp) domains and the solution structure of the entire cytosolic domain of ETR1, all from Arabidopsis thaliana. The isolated dimeric ethylene response sensor 1 DHp domain is asymmetric, the result of different helical bending angles close to the conserved His residue. The structures of the catalytic ATP-binding, DHp, and receiver domains of ethylene receptors and of a homologous, but dissimilar, GAF domain were refined against experimental small angle x-ray scattering data, leading to a structural model of the entire cytosolic domain of the ethylene receptor 1. The model illustrates that the cytosolic domain is shaped like a dumbbell and that the receiver domain is flexible and assumes a position different from those observed in prokaryotic histidine kinases. Furthermore the cytosolic domain of ETR1 plays a key role, interacting with all other receptors and several participants of the ethylene signaling pathway. Our model, therefore, provides the first step toward a detailed understanding of the molecular mechanics of this important signal transduction process in plants.     

Loss-of-function mutations in the ethylene receptor ETR1 cause enhanced sensitivity and exaggerated response to ethylene in Arabidopsis

Ethylene signaling in Arabidopsis begins at a family of five ethylene receptors that regulate activity of a downstream mitogen-activated protein kinase kinase kinase, CTR1. Triple and quadruple loss-of-function ethylene receptor mutants display a constitutive ethylene response phenotype, indicating they function as negative regulators in this pathway. No ethylene-related phenotype has been described for single loss-of-function receptor mutants, although it was reported that etr1 loss-of-function mutants display a growth defect limiting plant size. In actuality, this apparent growth defect results from enhanced responsiveness to ethylene; a phenotype manifested in all tissues tested. The phenotype displayed by etr1 loss-of-function mutants was rescued by treatment with an inhibitor of ethylene perception, indicating that it is ethylene dependent. Identification of an ethylene-dependent phenotype for a loss-of-function receptor mutant gave a unique opportunity for genetic and biochemical analysis of upstream events in ethylene signaling, including demonstration that the dominant ethylene-insensitive phenotype of etr2-1 is partially dependent on ETR1. This work demonstrates that mutational loss of the ethylene receptor ETR1 alters responsiveness to ethylene in Arabidopsis and that enhanced ethylene response in Arabidopsis not only results in increased sensitivity but exaggeration of response.     

Localization of the ethylene receptor ETR1 to the endoplasmic reticulum of Arabidopsis

The ethylene receptor ETR1 of Arabidopsis contains transmembrane domains responsible for ethylene binding and membrane localization. Sequence analysis does not provide information as to which membrane system of the plant cell ETR1 is localized. Examination by aqueous two-phase partitioning, sucrose density-gradient centrifugation, and immunoelectron microscopy indicates that ETR1 is predominantly localized to the endoplasmic reticulum. Localization of ETR1 showed no change following a cycloheximide chase. Ethylene binding by ETR1 did not affect localization to the endoplasmic reticulum, based upon analysis of plants treated with the ethylene precursor 1-aminocyclopropane- 1-carboxylic acid and by examination of a mutant receptor that does not bind ethylene. Determinants within the amino-terminal half of ETR1 are sufficient for targeting to and retention at the endoplasmic reticulum. These data support a central role of the plant endoplasmic reticulum in hormone perception and signal transduction.     

A MAPK pathway mediates ethylene signaling in plants

Ethylene signal transduction involves ETR1, a two-component histidine protein kinase receptor. ETR1 functions upstream of the negative regulator CTR1. The similarity of CTR1 to members of the Raf family of mitogen-activated protein kinase kinase kinases (MAPKKKs) suggested that ethylene signaling in plants involves a MAPK pathway, but no direct evidence for this has been provided. Here we show that distinct MAPKs are activated by the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC) in Medicago and ARABIDOPSIS: In Medicago, the ACC-activated MAPKs were SIMK and MMK3, while in Arabidopsis MPK6 and another MAPK were identified. Medicago SIMKK specifically mediated ACC-induced activation of SIMK and MMK3. Transgenic Arabidopsis plants overexpressing SIMKK have constitutive MPK6 activation and ethylene-induced target gene expression. SIMKK overexpressor lines resemble ctr1 mutants in showing a triple response phenotype in the absence of ACC. Whereas MPK6 was not activated by ACC in etr1 mutants, ein2 and ein3 mutants showed normal activation profiles. In contrast, ctr1 mutants showed constitutive activation of MPK6. These data indicate that a MAPK cascade is part of the ethylene signal transduction pathway in plants.     

Mutational analysis of the ethylene receptor ETR1. Role of the histidine kinase domain in dominant ethylene insensitivity

The ethylene receptor family of Arabidopsis consists of five members, one of these being ETR1. The N-terminal half of ETR1 contains a hydrophobic domain responsible for ethylene binding and membrane localization. The C-terminal half of the polypeptide contains domains with homology to histidine (His) kinases and response regulators, signaling motifs originally identified in bacteria. The role of the His kinase domain in ethylene signaling was examined in planta. For this purpose, site-directed mutations were introduced into the full-length wild-type ETR1 gene and into etr1-1, a mutant allele that confers dominant ethylene insensitivity on plants. The mutant forms of the receptor were expressed in Arabidopsis and the transgenic plants characterized for their ethylene responses. A mutation that eliminated His kinase activity did not affect the ability of etr1-1 to confer ethylene insensitivity. A truncated version of etr1-1 that lacks the His kinase domain also conferred ethylene insensitivity. Possible mechanisms by which a truncated version of etr1-1 could exert dominance are discussed.     

Autophosphorylation activity of the Arabidopsis ethylene receptor multigene family

Receptors for the gaseous phytohormone ethylene show sequence similarity to bacterial two-component histidine kinases. These receptors are encoded by a multigene family that can be divided into subfamilies 1 and 2. It has been previously shown that a subfamily 1 Arabidopsis thaliana ethylene receptor, ETR1, autophosphorylates in vitro on a conserved histidine residue (1). However, sequence comparisons between the five ethylene receptor family members suggest that subfamily 2 members do not have all the motifs necessary for histidine kinase activity. Further, a tobacco subfamily 2 receptor, NTHK1, autophosphorylates on serines and threonines in vitro (2). Here we show that all five Arabidopsis ethylene receptor proteins autophosphorylate in vitro. We analyzed the nature of the phosphorylated amino acids by acid/base stability and bi-dimensional thin layer electrophoresis and demonstrated that unlike ETR1 all other ethylene receptors autophosphorylate predominantly on serine residues. ERS1, the only other subfamily 1 receptor, is able to phosphorylate on both histidine and serine residues in the presence of Mn2+. However, histidine autophosphorylation is lost when ERS1 is assayed in the presence of both Mg2+ and Mn2+, suggesting that this activity may not occur in vivo. Furthermore, mutation of the histidine residue conserved in two-component systems does not abolish serine autophosphorylation, eliminating the possibility of a histidine to serine phosphotransfer. Our biochemical observations complement the recently published genetic data that histidine kinase activity is not necessary for ethylene receptor function in plants and suggest that ethylene signal transduction does not occur through a phosphorelay mechanism.     

Biochemical and functional analysis of CTR1, a protein kinase that negatively regulates ethylene signaling in Arabidopsis

CTR1 encodes a negative regulator of the ethylene response pathway in Arabidopsis thaliana. The C-terminal domain of CTR1 is similar to the Raf family of protein kinases, but its first two-thirds encodes a novel protein domain. We used a variety of approaches to investigate the function of these two CTR1 domains. Recombinant CTR1 protein was purified from a baculoviral expression system, and shown to possess intrinsic Ser/Thr protein kinase activity with enzymatic properties similar to Raf-1. Deletion of the N-terminal domain did not elevate the kinase activity of CTR1, indicating that, at least in vitro, this domain does not autoinhibit kinase function. Molecular analysis of loss-of-function ctr1 alleles indicated that several mutations disrupt the kinase catalytic domain, and in vitro studies confirmed that at least one of these eliminates kinase activity, which indicates that kinase activity is required for CTR1 function. One missense mutation, ctr1-8, was found to result from an amino acid substitution within a new conserved motif within the N-terminal domain. Ctr1-8 has no detectable effect on the kinase activity of CTR1 in vitro, but rather disrupts the interaction with the ethylene receptor ETR1. This mutation also disrupts the dominant negative effect that results from overexpression of the CTR1 amino-terminal domain in transgenic Arabidopsis. These results suggest that CTR1 interacts with ETR1 in vivo, and that this association is required to turn off the ethylene-signaling pathway.     

AHK5 histidine kinase regulates root elongation through an ETR1-dependent abscisic acid and ethylene signaling pathway in Arabidopsis thaliana

The Arabidopsis thaliana genome encodes a small family of histidine (His) protein kinases, some of which have redundant functions as ethylene receptors, whereas others serve as cytokinin receptors. The most poorly characterized of these is authentic histidine kinase 5 (AHK5; also known as cytokinin-independent 2, CKI2). Here we characterize three independent ahk5 mutants, and show that they have a common phenotype. Our results suggest that AHK5 His-kinase acts as a negative regulator in the signaling pathway in which ethylene and ABA inhibit the root elongation through ETR1 (an ethylene receptor).     

The ethylene signal transduction pathway in Arabidopsis

The gaseous hormone ethylene is an important regulator of plantgrowth and development. Using a simple response of etiolatedseedlings to ethylene as a genetic screen, genes involved inethylene signal transduction have been identified in Arabidopsis.Analysis of two of these genes that have been cloned revealsthat ethylene signalling involves a combination of a protein(ETR1) with similarity to bacterial histidine kinases and aprotein (CTR1) with similarity to Raf-1, a protein kinase involvedin multiple signalling cascades in eukaryotic cells. Severallines of investigation provide compelling evidence that ETR1encodes an ethylene receptor. For the first time there is aglimpse of the molecular circuitry underlying the signal transductionpathway for a plant hormone. Key words: Ethylene, plant growth, plant development, regulation, signal transduction, Arabidopsis     

The ethylene signal transduction pathway in Arabidopsis

The gaseous hormone ethylene is an important regulator of plant growth and development. Using a simple response of etiolated seedlings to ethylene as a genetic screen, genes involved in ethylene signal transduction have been identified in Arabidopsis. Analysis of two of these genes that have been cloned reveals that ethylene signalling involves a combination of a protein (ETR1) with similarity to bacterial histidine kinases and a protein (CTR1) with similarity to Raf-1, a protein kinase involved in multiple signalling cascades in eukaryotic cells. Several lines of investigation provide compelling evidence that ETR1 encodes an ethylene receptor. For the first time there is a glimpse of the molecular circuitry underlying the signal transduction pathway for a plant hormone.     

Arabidopsis RTE1 is essential to ethylene receptor ETR1 amino-terminal signaling independent of CTR1

The Arabidopsis (Arabidopsis thaliana) ethylene receptor Ethylene Response1 (ETR1) can mediate the receptor signal output via its carboxyl terminus interacting with the amino (N) terminus of Constitutive Triple Response1 (CTR1) or via its N terminus (etr1^1-349 or the dominant ethylene-insensitive etr1-1^1-349) by an unknown mechanism. Given that CTR1 is essential to ethylene receptor signaling and that overexpression of Reversion To Ethylene Sensitivity1 (RTE1) promotes ETR1 N-terminal signaling, we evaluated the roles of CTR1 and RTE1 in ETR1 N-terminal signaling. The mutant phenotype of ctr1-1 and ctr1-2 was suppressed in part by the transgenes etr1^1-349 and etr1-1^1-349, with etr1-1^1-349 conferring ethylene insensitivity. Coexpression of 35S:RTE1 and etr1^1-349 conferred ethylene insensitivity in ctr1-1, whereas suppression of the ctr1-1 phenotype by etr1^1-349 was prevented by rte1-2. Thus, RTE1 was essential to ETR1 N-terminal signaling independent of the CTR1 pathway. An excess amount of the CTR1 N terminus CTR1^7-560 prevented ethylene receptor signaling, and the CTR1^7-560 overexpressor CTR1-Nox showed a constitutive ethylene response phenotype. Expression of the ETR1 N terminus suppressed the CTR1-Nox phenotype. etr1^1-349 restored the ethylene insensitivity conferred by dominant receptor mutant alleles in the ctr1-1 background. Therefore, ETR1 N-terminal signaling was not mediated by full-length ethylene receptors; rather, full-length ethylene receptors acted cooperatively with the ETR1 N terminus to mediate the receptor signal independent of CTR1. ETR1 N-terminal signaling may involve RTE1, receptor cooperation, and negative regulation by the ETR1 carboxyl terminus.The gaseous plant hormone ethylene is perceived by a small family of ethylene receptors. Arabidopsis (Arabidopsis thaliana) has five ethylene receptors that are structurally similar to prokaryotic two-component histidine kinase (HK) proteins. Mutants defective in multiple ethylene receptor genes show a constitutive ethylene response phenotype, which indicates a negative regulation of ethylene responses by the receptor genes (Hua and Meyerowitz, 1998).The receptor N terminus has three or four transmembrane domains that bind ethylene. The GAF (for cGMP-specific phosphodiesterases, adenylyl cyclases, and FhlA) domain, which follows the transmembrane helices, mediates noncovalent receptor heterodimerization and may have a role in receptor cooperation (Gamble et al., 2002; O’Malley et al., 2005; Xie et al., 2006; Gao et al., 2008). The subfamily I receptors Ethylene Response1 (ETR1) and Ethylene Response Sensor1 (ERS1) have a conserved HK domain following the GAF domain. For subfamily II members ETR2, Ethylene Insensitive4 (EIN4), and ERS2, the HK domain is less conserved, and they lack most signature motifs essential for HK activity (Chang et al., 1993; Gamble et al., 1998; Hua et al., 1998; Qu and Schaller, 2004; Xie et al., 2006). Among the five receptors, ETR1, ETR2, and EIN4 have a receiver domain following the HK domain. The ETR1 HK domain may have a role in mediating the receptor signal to downstream components, and the HK activity facilitates the ethylene signaling (Clark et al., 1998; Huang et al., 2003; Hall et al., 2012). The receiver domain can dimerize and could involve receptor cooperation (Müller-Dieckmann et al., 1999). However, differential receptor cooperation occurs between the receiver domain-lacking ERS1 and the other ethylene receptors, which does not support the hypothesis that the domains involve receptor cooperation (Liu and Wen, 2012).Acting downstream of the ethylene receptors is Constitutive Triple Response1 (CTR1), a MEK kinase (mitogen-activated protein kinase kinase kinase) with Ser/Thr kinase activity, and the kinase domain locates at the C terminus. The CTR1 N terminus does not share sequence similarity to known domains and can physically interact with the ethylene-receptor HK domain (Clark et al., 1998; Huang et al., 2003). ctr1 mutants showing attenuated CTR1 kinase activity or the ETR1-CTR1 association exhibit various degrees of the constitutive ethylene-response phenotype. For example, the ctr1-1 and ctr1^btk mutations result from the D694E and E626K substitutions, respectively, in the CTR1 kinase domain, and ctr1-1 shows a stronger ethylene-response phenotype than ctr1^btk, with ctr1-1 having much weaker kinase activity than ctr1^btk (Kieber et al., 1993; Huang et al., 2003; Ikeda et al., 2009). The ctr1-8 mutation results in the G354E substitution that prevents the ETR1-CTR1 association, and the mutant exhibits a constitutive ethylene-response phenotype. Overexpression of the CTR1 N terminus CTR1^7-560, which is responsible for interaction with ethylene receptors, leads to constitutive ethylene responses, possibly by titrating out available ethylene receptors (Kieber et al., 1993; Huang et al., 2003). These studies suggest that CTR1 kinase activity and the interaction of CTR1 with the receptor HK domain may be important to the ethylene receptor signal output in suppressing constitutive ethylene responses.Although the ETR1-CTR1 interaction via the HK domain is essential to the ethylene receptor signal output, evidence suggests that the ETR1 receptor signal output can also be independent of the HK activity or domain. The etr1 ers1 loss-of-function mutant displays extreme growth defects. The etr1[HGG] mutation inactivates ETR1 HK activity, and expression of the getr1[HGG] transgene rescues the etr1 ers1 growth defects, which indicates a lack of association of ETR1 receptor signaling and its kinase activity (Wang et al., 2003). The dominant etr1-1 mutation results in the C65Y substitution and confers ethylene insensitivity (Chang et al., 1993), and the expression of the HK domain-lacking etr1^1-349 and ethylene-insensitive etr1-1^1-349 isoforms partially suppresses the growth defects of etr1 ers1-2. Loss-of-function mutations of subfamily II members do not affect etr1-1^1-349 functions. Therefore, etr1-1^1-349 predominantly cooperates with subfamily I receptors to mediate the ethylene receptor signal output (Xie et al., 2006). Biochemical and transformation studies showing that ethylene receptors can form heterodimers and that each receptor is a component of high-molecular-mass complexes explain how ethylene receptors may act cooperatively (Gao et al., 2008; Gao and Schaller, 2009; Chen et al., 2010).Reversion To Ethylene Sensitivity1 (RTE1), a Golgi/endoplasmic reticulum protein, was isolated from a suppressor screen of the dominant ethylene-insensitive etr1-2 mutation. The cross-species complementation of the rte1-2 loss-of-function mutation by the rice (Oryza sativa) RTE Homolog1 (OsRTH1) suggests a conserved mechanism that modulates the ethylene receptor signaling across higher plant species (Zhang et al., 2012). RTE1 and OsRTH1 overexpression led to ethylene insensitivity in wild-type Arabidopsis but not the etr1-7 loss-of-function mutant, and expression of etr1^1-349 restored ethylene insensitivity with RTE1 overexpression in etr1-7 (Resnick et al., 2006; Zhou et al., 2007; Zhang et al., 2010). Coimmunoprecipitation of epitope-tagged ETR1 and RTE1 and Trp fluorescence spectroscopy revealed the physical interaction of RTE1 and ETR1 (Zhou et al., 2007; Dong et al., 2008, 2010). Therefore, RTE1 may directly promote ETR1 receptor signal output through the ETR1 N terminus, but whether RTE1 has an essential role in ETR1 N-terminal signaling remains to be addressed.Currently, the biochemical nature of the ethylene receptor signal is unknown, and the underlying mechanisms of mediation of the ethylene receptor signal output remain uninvestigated. Genetic and biochemical studies suggest that activation of CTR1 by ethylene receptors may suppress constitutive ethylene responses; upon ethylene binding, the receptors are converted to an inactive state and fail to activate CTR1, and the suppression of ethylene responses by CTR1 is alleviated (Hua and Meyerowitz, 1998; Klee, 2004; Wang et al., 2006; Hall et al., 2007). However, this model does not address how the ETR1 N terminus, which does not have the CTR1-interacting site, mediates the receptor signal to suppress constitutive ethylene responses. The receptor signal of the truncated etr1 isoforms may be mediated by other full-length ethylene receptors and then activate CTR1; alternatively, the ETR1 N-terminal signal may be mediated by a pathway independent of CTR1 (Gamble et al., 2002; Qu and Schaller, 2004; Xie et al., 2006). Results showing that mutants defective in multiple ethylene receptor genes exhibit a more severe ethylene-response phenotype than ctr1 and that ctr1 mutants are responsive to ethylene support the presence of a CTR1-independent pathway (Hua and Meyerowitz, 1998; Cancel and Larsen, 2002; Huang et al., 2003; Liu et al., 2010).In this study, we investigated whether mediation of ETR1 N-terminal signaling is independent of CTR1 and whether RTE1 is essential to the CTR1-independent ETR1 N-terminal signaling. The ETR1 N-terminal signaling was not mediated via other full-length ethylene receptors, but the signal of full-length ethylene receptors could be mediated by the ETR1 N terminus independent of CTR1. The ETR1 C terminus may inhibit ETR1 N-terminal signaling, whereby deletion of the C terminus facilitates N-terminal signaling. We propose a model for the possible modulation of ETR1 receptor signaling.     

LeCTR2, a CTR1-like protein kinase from tomato, plays a role in ethylene signalling, development and defence

Arabidopsis AtCTR1 is a Raf-like protein kinase that interacts with ETR1 and ERS and negatively regulates ethylene responses. In tomato, several CTR1-like proteins could perform this role. We have characterized LeCTR2, which has similarity to AtCTR1 and also to EDR1, a CTR1-like Arabidopsis protein involved in defence and stress responses. Protein–protein interactions between LeCTR2 and six tomato ethylene receptors indicated that LeCTR2 interacts preferentially with the subfamily I ETR1-type ethylene receptors LeETR1 and LeETR2, but not the NR receptor or the subfamily II receptors LeETR4, LeETR5 and LeETR6. The C-terminus of LeCTR2 possesses serine/threonine kinase activity and is capable of auto-phosphorylation and phosphorylation of myelin basic protein in vitro . Overexpression of the LeCTR2 N-terminus in tomato resulted in altered growth habit, including reduced stature, loss of apical dominance, highly branched inflorescences and fruit trusses, indeterminate shoots in place of determinate flowers, and prolific adventitious shoot development from the rachis or rachillae of the leaves. Expression of the ethylene-responsive genes E4 and chitinase B was upregulated in transgenic plants, but ethylene production and the level of mRNA for the ethylene biosynthetic gene ACO1 was unaffected. The leaves and fruit of transgenic plants also displayed enhanced susceptibility to infection by the fungal pathogen Botrytis cinerea , which was associated with much stronger induction of pathogenesis-related genes such as PR1b1 and chitinase B compared with the wild-type. The results suggest that LeCTR2 plays a role in ethylene signalling, development and defence, probably through its interactions with the ETR1-type ethylene receptors of subfamily I.