Ethylene insensitivity conferred by Arabidopsis ERS gene

Cooperative ethylene receptor signaling

The gaseous plant hormone ethylene is perceived by a family of five ethylene receptor members in the dicotyledonous model plant Arabidopsis. Genetic and biochemical studies suggest that the ethylene response is suppressed by ethylene receptor complexes, but the biochemical nature of the receptor signal is unknown. Without appropriate biochemical measures to trace the ethylene receptor signal and quantify the signal strength, the biological significance of the modulation of ethylene responses by multiple ethylene receptors has yet to be fully addressed. Nevertheless, the ethylene receptor signal strength can be reflected by degrees in alteration of various ethylene response phenotypes and in expression levels of ethylene-inducible genes. This mini-review highlights studies that have advanced our understanding of cooperative ethylene receptor signaling.

ethylene receptor 1 (etr1) Is Sufficient and Has the Predominant Role in Mediating Inhibition of Ethylene Responses by Silver in Arabidopsis thaliana

Ethylene influences many processes in Arabidopsis thaliana through the action of five receptor isoforms. All five isoforms use copper as a cofactor for binding ethylene. Previous research showed that silver can substitute for copper as a cofactor for ethylene binding activity in the ETR1 ethylene receptor yet also inhibit ethylene responses in plants. End-point and rapid kinetic analyses of dark-grown seedling growth revealed that the effects of silver are mostly dependent upon ETR1, and ETR1 alone is sufficient for the effects of silver. Ethylene responses in etr1-6 etr2-3 ein4-4 triple mutants were not blocked by silver. Transformation of these triple mutants with cDNA for each receptor isoform under the promoter control of ETR1 revealed that the cETR1 transgene completely rescued responses to silver while the cETR2 transgene failed to rescue these responses. The other three isoforms partially rescued responses to silver. Ethylene binding assays on the binding domains of the five receptor isoforms expressed in yeast showed that silver supports ethylene binding to ETR1 and ERS1 but not the other isoforms. Thus, silver may have an effect on ethylene signaling outside of the ethylene binding pocket of the receptors. Ethylene binding to ETR1 with silver was ∼30% of binding with copper. However, alterations in the K(d) for ethylene binding to ETR1 and the half-time of ethylene dissociation from ETR1 do not underlie this lower binding. Thus, it is likely that the lower ethylene binding activity of ETR1 with silver is due to fewer ethylene binding sites generated with silver versus copper.

Role of ethylene receptors during senescence and ripening in horticultural crops

The past two decades have been rewarding in terms of deciphering the ethylene signal transduction and functional validation of the ethylene receptor and downstream genes involved in the cascade. Our knowledge of ethylene receptors and its signal transduction pathway provides us a robust platform where we can think of manipulating and regulating ethylene sensitivity by the use of genetic engineering and making transgenic. This review focuses on ethylene perception, receptor mediated regulation of ethylene biosynthesis, role of ethylene receptors in flower senescence, fruit ripening and other effects induced by ethylene. The expression behavior of the receptor and downstream molecules in climacteric and non climacteric crops is also elaborated upon. Possible strategies and recent advances in altering the ethylene sensitivity of plants using ethylene receptor genes in an attempt to modulate the regulation and sensitivity to ethylene have also been discussed. Not only will these transgenic plants be a boon to post-harvest physiology and crop improvement but, it will also help us in discovering the mechanism of regulation of ethylene sensitivity.

Cyanide is an adequate agonist of the plant hormone ethylene for studying signalling of sensor kinase ETR1 at the molecular level

The plant hormone ethylene is involved in many developmental processes and responses to environmental stresses in plants. Although the elements of the signalling cascade and the receptors operating the ethylene pathway have been identified, a detailed understanding of the molecular processes related to signal perception and transfer is still lacking. Analysis of these processes using purified proteins in physical, structural and functional studies is complicated by the gaseous character of the plant hormone. In the present study, we show that cyanide, a π-acceptor compound and structural analogue of ethylene, is a suitable substitute for the plant hormone for in vitro studies with purified proteins. Recombinant ethylene receptor protein ETR1 (ethylene-resistant 1) showed high level and selective binding of [14C]cyanide in the presence of copper, a known cofactor in ethylene binding. Replacement of Cys65 in the ethylene-binding domain by serine dramatically reduced binding of radiolabelled cyanide. In contrast with wild-type ETR1, autokinase activity of the receptor is not reduced in the ETR1-C65S mutant upon addition of cyanide. Additionally, protein–protein interaction with the ethylene signalling protein EIN2 (ethylene-insensitive 2) is considerably sustained by cyanide in wild-type ETR1, but is not affected in the mutant. Further evidence for the structural and functional equivalence of ethylene and cyanide is given by the fact that the ethylene-responsive antagonist silver, which is known to allow ligand binding but prevent intrinsic signal transduction, also allows specific binding of cyanide, but shows no effect on autokinase activity and ETR1–EIN2 interaction.

Arabidopsis ETR1 and ERS1 differentially repress the ethylene response in combination with other ethylene receptor genes

The ethylene response is negatively regulated by a family of five ethylene receptor genes in Arabidopsis (Arabidopsis thaliana). The five members of the ethylene receptor family can physically interact and form complexes, which implies that cooperativity for signaling may exist among the receptors. The ethylene receptor gene mutations etr1-1((C65Y))(for ethylene response1-1), ers1-1((I62P)) (for ethylene response sensor1-1), and ers1(C65Y) are dominant, and each confers ethylene insensitivity. In this study, the repression of the ethylene response by these dominant mutant receptor genes was examined in receptor-defective mutants to investigate the functional significance of receptor cooperativity in ethylene signaling. We showed that etr1-1((C65Y)), but not ers1-1((I62P)), substantially repressed various ethylene responses independent of other receptor genes. In contrast, wild-type receptor genes differentially supported the repression of ethylene responses by ers1-1((I62P)); ETR1 and ETHYLENE INSENSITIVE4 (EIN4) supported ers1-1((I62P)) functions to a greater extent than did ERS2, ETR2, and ERS1. The lack of both ETR1 and EIN4 almost abolished the repression of ethylene responses by ers1(C65Y), which implied that ETR1 and EIN4 have synergistic effects on ers1(C65Y) functions. Our data indicated that a dominant ethylene-insensitive receptor differentially repressed ethylene responses when coupled with a wild-type ethylene receptor, which supported the hypothesis that the formation of a variety of receptor complexes may facilitate differential receptor signal output, by which ethylene responses can be repressed to different extents. We hypothesize that plants can respond to a broad ethylene concentration range and exhibit tissue-specific ethylene responsiveness with differential cooperation of the multiple ethylene receptors.

Genome-wide transcriptome dissection of the rice root system: implications for developmental and physiological functions

The root system is a crucial determinant of plant growth potential because of its important functions, e.g. uptake of water and nutrients, structural support and interaction with symbiotic organisms. Elucidating the molecular mechanism of root development and functions is therefore necessary for improving plant productivity, particularly for crop plants, including rice (Oryza sativa). As an initial step towards developing a comprehensive understanding of the root system, we performed a large-scale transcriptome analysis of the rice root via a combined laser microdissection and microarray approach. The crown root was divided into eight developmental stages along the longitudinal axis and three radial tissue types at two different developmental stages, namely: epidermis, exodermis and sclerenchyma; cortex; and endodermis, pericycle and stele. We analyzed a total of 38 microarray data and identified 22,297 genes corresponding to 17,010 loci that showed sufficient signal intensity as well as developmental- and tissue type-specific transcriptome signatures. Moreover, we clarified gene networks associated with root cap function and lateral root formation, and further revealed antagonistic and synergistic interactions of phytohormones such as auxin, cytokinin, brassinosteroids and ethylene, based on the expression pattern of genes related to phytohormone biosynthesis and signaling. Expression profiling of transporter genes defined not only major sites for uptake and transport of water and nutrients, but also distinct signatures of the radial transport system from the rhizosphere to the xylem vessel for each nutrient. All data can be accessed from our gene expression profile database, RiceXPro (http://ricexpro.dna.affrc.go.jp), thereby providing useful information for understanding the molecular mechanisms involved in root system development of crop plants.

Tomato TILLING technology: development of a reverse genetics tool for the efficient isolation of mutants from Micro-Tom mutant libraries

To accelerate functional genomic research in tomato, we developed a Micro-Tom TILLING (Targeting Induced Local Lesions In Genomes) platform. DNA pools were constructed from 3,052 ethyl methanesulfonate (EMS) mutant lines treated with 0.5 or 1.0% EMS. The mutation frequency was calculated by screening 10 genes. The 0.5% EMS population had a mild mutation frequency of one mutation per 1,710 kb, whereas the 1.0% EMS population had a frequency of one mutation per 737 kb, a frequency suitable for producing an allelic series of mutations in the target genes. The overall mutation frequency was one mutation per 1,237 kb, which affected an average of three alleles per kilobase screened. To assess whether a Micro-Tom TILLING platform could be used for efficient mutant isolation, six ethylene receptor genes in tomato (SlETR1-SlETR6) were screened. Two allelic mutants of SlETR1 (Sletr1-1 and Sletr1-2) that resulted in reduced ethylene responses were identified, indicating that our Micro-Tom TILLING platform provides a powerful tool for the rapid detection of mutations in an EMS mutant library. This work provides a practical and publicly accessible tool for the study of fruit biology and for obtaining novel genetic material that can be used to improve important agronomic traits in tomato.

Effects of abscisic acid on ethylene biosynthesis and perception in Hibiscus rosa-sinensis L. flower development

The effect of the complex relationship between ethylene and abscisic acid (ABA) on flower development and senescence in Hibiscus rosa-sinensis L. was investigated. Ethylene biosynthetic (HrsACS and HrsACO) and receptor (HrsETR and HrsERS) genes were isolated and their expression evaluated in three different floral tissues (petals, style-stigma plus stamens, and ovaries) of detached buds and open flowers. This was achieved through treatment with 0.1 mM 1-aminocyclopropane-1-carboxylic acid (ACC) solution, 500 nl l(-1) methylcyclopropene (1-MCP), and 0.1 mM ABA solution. Treatment with ACC and 1-MCP confirmed that flower senescence in hibiscus is ethylene dependent, and treatment with exogenous ABA suggested that ABA may play a role in this process. The 1-MCP impeded petal in-rolling and decreased ABA content in detached open flowers after 9 h. This was preceded by an earlier and sequential increase in ABA content in 1-MCP-treated petals and style-stigma plus stamens between 1 h and 6 h. ACC treatment markedly accelerated flower senescence and increased ethylene production after 6 h and 9 h, particularly in style-stigma plus stamens. Ethylene evolution was positively correlated in these floral tissues with the induction of the gene expression of ethylene biosynthetic and receptor genes. Finally, ABA negatively affected the ethylene biosynthetic pathway and tissue sensitivity in all flower tissues. Transcript abundance of HrsACS, HrsACO, HrsETR, and HrsERS was reduced by exogenous ABA treatment. This research underlines the regulatory effect of ABA on the ethylene biosynthetic and perception machinery at a physiological and molecular level when inhibitors or promoters of senescence are exogenously applied.

EIN2 regulates salt stress response and interacts with a MA3 domain-containing protein ECIP1 in Arabidopsis

Ethylene signalling regulates plant growth and development. However, its roles in salt stress response are less known. Here we studied functions of EIN2, a central membrane protein of ethylene signalling, and its interacting protein ECIP1 in salt stress responses. Mutation of EIN2 led to extreme salt sensitivity as revealed by phenotypic and physiological changes, and overexpression of C-terminus of EIN2 suppressed salt sensitivity in ein2-5, indicating that EIN2 is required for salt tolerance. Downstream components EIN3 and EIL1 are also essential for salt tolerance because ein3-1eil1-1 double mutant showed extreme salt-sensitive phenotype. A MA3 domain-containing protein ECIP1 was further identified to interact with EIN2 in yeast two-hybrid assay and GST pull-down assay. Loss-of-function of ECIP1 resulted in enhanced ethylene response but altered salt response during seed germination and plant growth. Double mutant analysis revealed that ein2-1 was epistatic to ecip1, and ecip1 mutation partially suppressed ethylene-insensitivity of etr2-1 and ein4-1. These studies strengthen that interactions between ECIP1 and EIN2 or ethylene receptors regulate ethylene response and stress response.

Expression of ACO1, ERS1 and ERF1 genes in harvested bananas in relation to heat-induced defense against Colletotrichum musae

The aim of this study was to investigate the connection between heat-induced ethylene signal changes and enhanced disease resistance. Heat enhanced ripening and elevated MaACO1 expression in naturally ripened bananas (NRB), while it delayed ripening and reduced MaACO1expression in the ethephon-treated bananas (ETB). However, in both cases, heat reduced lesion sizes infected by Colletotrichum musae. This indicates that heat-induced disease resistance in bananas was independent of ripening rate. The expression of MaERS1 gene was inhibited by heat treatment in both NRB and ETB, implying that heat as a physical signal could be sensed by banana fruits through the inhibition of ethylene receptor gene expression. The intensity of MaERF1 transcript signals was elevated in heated bananas, suggesting that the enhanced accumulation of MaERF1 transcript following heat treatment could play an important role in activation of the defense system. In ETB, inhibition of JA biosynthesis by application of IBU down-regulated the expression of MaERF and significantly weakened disease resistance, suggesting involvement of endogenous JA in induction of the gene expression, which was reconfirmed by the fact that exposure to exogenous MeJA following the combination of heat plus IBU treatment restored part of the gene expression. On the other hand, in NRB, application of IBU elevated level of MaERF1 expression at 24h and enhanced disease resistance, suggesting that, when banana was not exposed to ethephon, the expression of MaERF1 gene was not JA dependent, which was verified by the fact that MeJA application did not enhance MaERF1 gene expression. In conclusion, heat-induced disease resistance in harvested bananas could involve down-regulation of MaERS1 expression and up-regulation of MaERF1 expression and JA pathway could be involved in heat activation of the defense system in bananas exposed to ethephon.

Cloning and molecular characterization of an ethylene receptor gene, MiERS1, expressed during mango fruitlet abscission and fruit ripening

We isolated and characterized a mango (Mangifera indica L.) cDNA homolog of the ethylene receptor gene ERS1, designated MiERS1. Genomic Southern blot analysis suggested the existence of a second gene with homology to MiERS1. Spatial and temporal expression patterns of MiERS1 were first studied during fruitlet drop and compared with those of a previously identified MiETR1 gene that encodes an ETR1-type ethylene receptor. Experiments were conducted on developing fruitlet explants in which fruitlet abscission was induced by ethephon treatment. Northern analysis revealed a notable increase in MiERS1 mRNA levels in the fruitlet's activated abscission zone within 24 h of ethephon application, followed by a decreasing pattern 48 h post-treatment. A transient, albeit lesser, increase in MiERS1 mRNA levels was also observed in treated fruitlet seed and mesocarp tissues. In contrast, in the abscission zone, accumulation of MiETR1 transcript remained unchanged; a temporal increase in MiETR1 transcript level was observed in the fruitlet mesocarp, whereas in the seed, MiETR1 expression had already dropped by 24 h. Expression profiles of MiERS1 and MiETR1 were then studied during fruit ripening. In agreement with a previous study and coinciding with the climacteric rise in ethylene production, RNA blot analysis revealed that during fruit ripening, MiETR1 mRNA level increases in both mesocarp and seed tissues. Unexpectedly, however, in those same tissues, MiERS1 transcript accumulation was barely detected. Collectively, our data highlight MiERS1's possible specific function in regulating fruitlet abscission rather than fruit ripening.

Paradigms and paradox in the ethylene signaling pathway and interaction network

Phytohormone ethylene plays pivotal roles in plant response to developmental and environmental signals. During the past few years, the emerging evidence has led us to a new understanding of the signaling mechanisms and regulatory networks of the ethylene action. In this review, we focus on the major advances made in the past three years, particularly the findings leading to new paradigms and the observations under debate. With the recent demonstration of the regulation of the protein stability of numerous key signaling components including EIN3, EIL1, EIN2, ETR2, EBF1/EBF2, and ETP1/ETP2, we highlight proteasome-dependent protein degradation as an essential regulatory mechanism that is widely adopted in the ethylene signaling pathway. We also discuss the implication of the negative feedback mechanism in the ethylene signaling pathway in light of ethylene-induced ETR2 and EBF2 gene expression. Meanwhile, we summarize the controversy on the involvement of MKK9-MPK3/6 cascade in the ethylene signaling versus biosynthesis pathway, and discuss the possible role of this MAPK module in the ethylene action. Finally, we describe the complex interactions between ethylene and other signaling pathways including auxin, light, and plant innate immunity, and propose that EIN3/EIL1 act as a convergence point in the ethylene-initiated signaling network.

Ethylene and 1-methylcyclopropene differentially regulate gene expression during onion sprout suppression

Onion (Allium cepa) is regarded as a nonclimacteric vegetable. In onions, however, ethylene can suppress sprouting while the ethylene-binding inhibitor 1-methylcyclopropene (1-MCP) can also suppress sprout growth; yet, it is unknown how ethylene and 1-MCP elicit the same response. In this study, onions were treated with 10 μL L(-1) ethylene or 1 μL L(-1) 1-MCP individually or in combination for 24 h at 20°C before or after curing (6 weeks) at 20°C or 28°C and then stored at 1°C. Following curing, a subset of these same onions was stored separately under continuous air or ethylene (10 μL L(-1)) at 1°C. Onions treated with ethylene and 1-MCP in combination after curing for 24 h had reduced sprout growth as compared with the control 25 weeks after harvest. Sprout growth following storage beyond 25 weeks was only reduced through continuous ethylene treatment. This observation was supported by a higher proportion of down-regulated genes characterized as being involved in photosynthesis, measured using a newly developed onion microarray. Physiological and biochemical data suggested that ethylene was being perceived in the presence of 1-MCP, since sprout growth was reduced in onions treated with 1-MCP and ethylene applied in combination but not when applied individually. A cluster of probes representing transcripts up-regulated by 1-MCP alone but down-regulated by ethylene alone or in the presence of 1-MCP support this suggestion. Ethylene and 1-MCP both down-regulated a probe tentatively annotated as an ethylene receptor as well as ethylene-insensitive 3, suggesting that both treatments down-regulate the perception and signaling events of ethylene.

The role of the pod in seed development: strategies for manipulating yield

Pods play a key role in encapsulating the developing seeds and protecting them from pests and pathogens. In addition to this protective function, it has been shown that the photosynthetically active pod wall contributes assimilates and nutrients to fuel seed growth. Recent work has revealed that signals originating from the pod may also act to coordinate grain filling and regulate the reallocation of reserves from damaged seeds to those that have retained viability. In this review we consider the evidence that pods can regulate seed growth and maturation, particularly in members of the Brassicaceae family, and explore how the timing and duration of pod development might be manipulated to enhance either the quantity of crop yield or its nutritional properties.

Ethylene receptor ETHYLENE RECEPTOR1 domain requirements for ethylene responses in Arabidopsis seedlings

Ethylene influences many processes in Arabidopsis (Arabidopsis thaliana) through the action of five receptor isoforms. We used high-resolution, time-lapse imaging of dark-grown Arabidopsis seedlings to better understand the roles of each isoform in the regulation of growth in air, ethylene-stimulated nutations, and growth recovery after ethylene removal. We found that ETHYLENE RECEPTOR1 (ETR1) is both necessary and sufficient for nutations. Transgene constructs in which the ETR1 promoter was used to drive expression of cDNAs for each of the five receptor isoforms were transferred into etr1-6;etr2-3;ein4-4 triple loss-of-function mutants that have constitutive growth inhibition in air, fail to nutate in ethylene, and take longer to recover a normal growth rate when ethylene is removed. The patterns of rescue show that ETR1, ETR2, and ETHYLENE INSENSITIVE4 (EIN4) have the prominent roles in rapid growth recovery after removal of ethylene whereas ETR1 was the sole isoform that rescued nutations. ETR1 histidine kinase activity and phosphotransfer through the receiver domain are not required to rescue nutations. However, REVERSION TO SENSITIVITY1 modulates ethylene-stimulated nutations but does not modulate the rate of growth recovery after ethylene removal. Several chimeric receptor transgene constructs where domains of EIN4 were swapped into ETR1 were also introduced into the triple mutant. The pattern of phenotype rescue by the chimeric receptors used in this study supports a model where a receptor with a receiver domain is required for normal growth recovery and that nutations specifically require the full-length ETR1 receptor.

Cloning and characterisation of two CTR1-like genes in Cucurbita pepo: regulation of their expression during male and female flower development

Ethylene is an essential regulator of flower development in Cucurbita pepo, controlling the sexual expression, and the differentiation and maturation of floral organs. To study the action mechanism of ethylene during the male and female flower development, we have identified two CTR1 homologues from C. pepo, CpCTR1 and CpCTR2, and analysed their expressions during female and male flower development and in response to external treatments with ethylene. CpCTR1 and CpCTR2 share a high homology with plant CTR1-like kinases, but differ from other related kinases such as the Arabidopsis EDR1 and the tomato LeCTR2. The C-terminal ends of both CpCTR1 and CpCTR2 have all the conserved motifs of Ser/Thr kinase domains, including the ATP-binding signature and the protein kinase active site consensus sequence, which suggests that CpCTR1 and CpCTR2 could have the same function as CTR1 in ethylene signalling. The transcripts of both genes were detected in different organs of the plant, including roots, leaves and shoots, but were mostly accumulated in mature flowers. During the development of male and female flowers, CpCTR1 and CpCTR2 expressions were concomitant with ethylene production, which indicates that both genes could be upregulated by ethylene, at least in flowers. Moreover, external treatments with ethylene, although did not alter the expression of these two genes in seedlings and leaves, were able to upregulate their expression in flowers. In the earlier stages of flower development, when ethylene production is very low, the expression of CpCTR1 and CpCTR2 is higher in male floral organs, which agrees with the role of these genes as negative regulators of ethylene signalling, and explain the lower ethylene sensitivity of male flowers in comparison with female flowers. The function of the upregulation of these two genes in later stages of female flower development, when the production of ethylene is also increased, is discussed.

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.

Illuminating COI1: a component of the Arabidopsis jasomonate receptor complex also interacts with ethylene signaling

A significant portion of developmental and environmental responses in plants is mediated through phytohormone signaling, often if not always integrated with outputs from other signals. We have recently shown that CORONATINE INSENSITIVE1 (COI1), a component of a jasmonate receptor complex, is involved in ethylene-induced root growth inhibition of Arabidopsis, in the light. This response is neither due to elevated levels of jasmonates in response to ethylene treatment nor dependent on the known jasmonate signal-transduction cascade, except that it requires COI1. Further, we have shown that the ethylene-induced COI1-mediated pathway functions in parallel with, and additively to, the conventional ethylene signaling pathway, and that the light requirement is primarily for long photoperiods. This unexpected interaction of COI1 with ethylene signaling has also been extended to other developmental processes including germination and fertility. This addendum summarizes the earlier findings with some new insights, and describes and speculates on the mechanisms by which these processes are regulated, in the context of the interaction between COI1 and ethylene signaling.

Analysis of the functional conservation of ethylene receptors between maize and Arabidopsis

Ethylene, a regulator of plant growth and development, is perceived by specific receptors that act as negative regulators of the ethylene response. Five ethylene receptors, i.e., ETR1, ERS1, EIN4, ETR2, and ERS2, are present in Arabidopsis and dominant negative mutants of each that confer ethylene insensitivity have been reported. In contrast, maize contains just two types of ethylene receptors: ZmERS1, encoded by ZmERS1a and ZmERS1b, and ZmETR2, encoded by ZmETR2a and ZmETR2b. In this study, we introduced a Cys to Tyr mutation in the transmembrane domain of ZmERS1b and ZmETR2b that is present in the etr1-1 dominant negative mutant and expressed each protein in Arabidopsis. Mutant Zmers1b and Zmetr2b receptors conferred ethylene insensitivity and Arabidopsis expressing Zmers1b or Zmetr2b were larger and exhibited a delay in leaf senescence characteristic of ethylene insensitive Arabidopsis mutants. Zmers1b and Zmetr2b were dominant and functioned equally well in a hemizygous or homozygous state. Expression of the Zmers1b N-terminal transmembrane domain was sufficient to exert dominance over endogenous Arabidopsis ethylene receptors whereas the Zmetr2b N-terminal domain failed to do so. Neither Zmers1b nor Zmetr2b functioned in the absence of subfamily 1 ethylene receptors, i.e., ETR1 and ERS1. These results suggest that Cys65 in maize ZmERS1b and ZmETR2b plays the same role that it does in Arabidopsis receptors. Moreover, the results demonstrate that the mutant maize ethylene receptors are functionally dependent on subfamily 1 ethylene receptors in Arabidopsis, indicating substantial functional conservation between maize and Arabidopsis ethylene receptors despite their sequence divergence.

COI1, a jasmonate receptor, is involved in ethylene-induced inhibition of Arabidopsis root growth in the light

Plant response to stress is orchestrated by hormone signalling pathways including those activated by jasmonates (JAs) and by ethylene, both of which stunt root growth. COI1 is a JA receptor and is required for the known responses to this hormone. It was observed that the coi1 mutant, which is largely unresponsive to growth inhibition by JAs, was also partially unresponsive to growth inhibition by ethylene and by its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), in the light but not in the dark. Although COI1 was required for this response to ACC, other components of the JA signal perception pathway were not. Mutants selected for insensitivity to ethylene, including etr1, ein2, and ein3, showed greater ACC-induced root growth inhibition in the light than in the dark. However, the double mutants etr1;coi1, ein2;coi1, and ein3;coi1, and coi1 seedlings treated with silver ions to block the ethylene receptors showed almost complete unresponsiveness to ACC-induced root growth inhibition in the light. The light requirement for the COI1-mediated growth inhibition by ACC was for long photoperiods, and the ACC response was not abolished by mutations in the known photoreceptors. The complementation assay indicated that SCF complex assembly was not required for COI1 function in the ACC response, in contrast to the JA response. It is concluded that COI1 is required for the light-dependent, JA-independent, root growth inhibition by ethylene.

New insight in ethylene signaling: autokinase activity of ETR1 modulates the interaction of receptors and EIN2

Ethylene insensitive 2 (EIN2), an integral membrane protein of the ER network, has been identified as the central regulator of the ethylene signaling pathway. Still, the mechanism by which the ethylene signal is transferred from the receptors to EIN2 has not been solved yet. Here, we show that protein phosphorylation is a key mechanism to control the interaction of EIN2 and the receptors. In vivo and in vitro fluorescence studies reveal that the kinase domain of the receptors is essential for the interaction. Cyanide, an ethylene agonist, which is known to reduce auto-phosphorylation of the ethylene receptor ethylene resistant 1 (ETR1) or a mutation in the kinase domain of ETR1 that prevents auto-phosphorylation (H353A), increases the affinity of the receptors for EIN2. On the other hand, mimicking permanent auto-phosphorylation of ETR1 as in the mutant H353E releases the EIN2-ETR1 interaction from the control by the plant hormone. Based on our data, we propose a novel model on the integration of EIN2 in the ethylene signaling cascade.

Relationship between Rh-RTH1 and ethylene receptor gene expression in response to ethylene in cut rose

A cDNA clone encoding a putative RTE1-like protein (Rh-RTH1) was obtained from total RNA isolated from senescing rose (Rosa hybrida cv. Tineke) petals using RT-PCR and RACE techniques. The cDNA (1,061 bp) contained an open reading frame of 684 bp corresponding to 227 amino acids. The amino acid sequence had 60.0, 49.6, 61.2, 42.5 and 39.8% identity with that of Arabidopsis RTH, RTE1, tomato GRL2, GRL1 and GR, respectively. Northern hybridization indicated that Rh-RTH1 expression is enhanced by endogenous and exogenous ethylene and inhibited by 1-MCP in petals and gynoecia. Rh-RTH1 expression partly correlated with sites of the ethylene receptor gene Rh-ETR1 and Rh-ETR3 expression, such as the petals, gynoecia, roots, and buds. The induction of Rh-RTH1 and Rh-ETR3 expression was substantially suppressed by 1-MCP treatment, while Rh-ETR1 expression was not reduced by 1-MCP treatment. Following treatment of flowers with sucrose, the level of Rh-RTH1 and Rh-ETR3 mRNA was only slightly decreased in petals and gynoecia. Upon wounding treatment, Rh-RTH1, Rh-ETR1 and Rh-ETR3 showed a quick increase in mRNA accumulation which was positively correlated with the increase in ethylene production. The expression of Rh-RTH1 showed partial correlation with that of Rh-ETR1 and Rh-ETR3.

Ethylene-induced stabilization of ETHYLENE INSENSITIVE3 and EIN3-LIKE1 is mediated by proteasomal degradation of EIN3 binding F-box 1 and 2 that requires EIN2 in Arabidopsis

Plant responses to ethylene are mediated by regulation of EBF1/2-dependent degradation of the ETHYLENE INSENSITIVE3 (EIN3) transcription factor. Here, we report that the level of EIL1 protein is upregulated by ethylene through an EBF1/2-dependent pathway. Genetic analysis revealed that EIL1 and EIN3 cooperatively but differentially regulate a wide array of ethylene responses, with EIL1 mainly inhibiting leaf expansion and stem elongation in adult plants and EIN3 largely regulating a multitude of ethylene responses in seedlings. When EBF1 and EBF2 are disrupted, EIL1 and EIN3 constitutively accumulate in the nucleus and remain unresponsive to exogenous ethylene application. Further study revealed that the levels of EBF1 and EBF2 proteins are downregulated by ethylene and upregulated by silver ion and MG132, suggesting that ethylene stabilizes EIN3/EIL1 by promoting EBF1 and EBF2 proteasomal degradation. Also, we found that EIN2 is indispensable for mediating ethylene-induced EIN3/EIL1 accumulation and EBF1/2 degradation, whereas MKK9 is not required for ethylene signal transduction, contrary to a previous report. Together, our studies demonstrate that ethylene similarly regulates EIN3 and EIL1, the two master transcription factors coordinating myriad ethylene responses, and clarify that EIN2 but not MKK9 is required for ethylene-induced EIN3/EIL1 stabilization. Our results also reveal that EBF1 and EBF2 act as essential ethylene signal transducers that by themselves are subject to proteasomal degradation.

Jasmonic acid and ethylene modulate local responses to wounding and simulated herbivory in Nicotiana attenuata leaves

Jasmonic acid (JA) and ethylene (ET) are known to play important roles in mediating plant defense against herbivores, but how they affect development in herbivore-attacked plants is unknown. We used JA-deficient (silenced in LIPOXYGENASE3 [asLOX3]) and ET-insensitive (expressing a mutated dominant negative form of ETHYLENE RESPONSE1 [mETR1]) Nicotiana attenuata plants, and their genetic cross (mETR1asLOX3), to examine growth and development of these plants under simulated herbivory conditions. At the whole plant level, both hormones suppressed leaf expansion after the plants had been wounded and the wounds had been immediately treated with Manduca sexta oral secretions (OS). In addition, ectopic cell expansion was observed around both water- and OS-treated wounds in mETR1asLOX3 leaves but not in mETR1, asLOX3, or wild-type leaves. Pretreating asLOX3 leaves with the ET receptor antagonist 1-methylcyclopropane resulted in local cell expansion that closely mimicked the mETR1asLOX3 phenotype. We found higher auxin (indole-3-acetic acid) levels in the elicited leaves of mETR1asLOX3 plants, a trait that is putatively associated with enhanced cell expansion and leaf growth in this genotype. Transcript profiling of OS-elicited mETR1asLOX3 leaves revealed a preferential accumulation of transcripts known to function in cell wall remodeling, suggesting that both JA and ET act as negative regulators of these genes. We propose that in N. attenuata, JA-ET cross talk restrains local cell expansion and growth after herbivore attack, allowing more resources to be allocated to induced defenses against herbivores.

Genetic and transformation studies reveal negative regulation of ERS1 ethylene receptor signaling in Arabidopsis

BACKGROUND

Ethylene receptor single mutants of Arabidopsis do not display a visibly prominent phenotype, but mutants defective in multiple ethylene receptors exhibit a constitutive ethylene response phenotype. It is inferred that ethylene responses in Arabidopsis are negatively regulated by five functionally redundant ethylene receptors. However, genetic redundancy limits further study of individual receptors and possible receptor interactions. Here, we examined the ethylene response phenotype in two quadruple receptor knockout mutants, (ETR1) ers1 etr2 ein4 ers2 and (ERS1) etr1 etr2 ein4 ers2, to unravel the functions of ETR1 and ERS1. Their functions were also reciprocally inferred from phenotypes of mutants lacking ETR1 or ERS1. Receptor protein levels are correlated with receptor gene expression. Expression levels of the remaining wild-type receptor genes were examined to estimate the receptor amount in each receptor mutant, and to evaluate if effects of ers1 mutations on the ethylene response phenotype were due to receptor functional compensation. As ers1 and ers2 are in the Wassilewskija (Ws) ecotype and etr1, etr2, and ein4 are in the Columbia (Col-0) ecotype, possible effects of ecotype mixture on ethylene responses were also investigated.

RESULTS

Ethylene responses were scored based on seedling hypocotyl measurement, seedling and rosette growth, and relative Chitinase B (CHIB) expression. Addition of ers1 loss-of-function mutations to any ETR1-containing receptor mutants alleviated ethylene growth inhibition. Growth recovery by ers1 mutation was reversed when the ers1 mutation was complemented by ERS1p:ERS1. The addition of the ers2-3 mutation to receptor mutants did not reverse the growth inhibition. Overexpressing ERS1 receptor protein in (ETR1 ERS1)etr2 ein4 ers2 substantially elevated growth inhibition and CHIB expression. Receptor gene expression analyses did not favor receptor functional compensation upon the loss of ERS1.

CONCLUSIONS

Our results suggest that ERS1 has dual functions in the regulation of ethylene responses. In addition to repressing ethylene responses, ERS1 also promotes ethylene responses in an ETR1-dependent manner. Several lines of evidence support the argument that ecotype mixture does not reverse ethylene responses. Loss of ERS1 did not lead to an increase in total receptor gene expression, and functional compensation was not observed. The inhibitory effects of ERS1 on the ethylene signaling pathway imply negative receptor collaboration.

Ethylene signalling and ethylene-targeted transcription factors are required to balance beneficial and nonbeneficial traits in the symbiosis between the endophytic fungus Piriformospora indica and Arabidopsis thaliana

*The endophytic fungus Piriformospora indica colonizes the roots of the model plant Arabidopsis thaliana and promotes its growth and seed production. The fungus can be cultivated in axenic culture without a host, and therefore this is an excellent system to investigate plant-fungus symbiosis. *The growth of etr1, ein2 and ein3/eil1 mutant plants was not promoted or even inhibited by the fungus; the plants produced less seeds and the roots were more colonized compared with the wild-type. This correlates with a mild activation of defence responses. The overexpression of ETHYLENE RESPONSE FACTOR1 constitutively activated defence responses, strongly reduced root colonization and abolished the benefits for the plants. *Piriformospora indica-mediated stimulation of growth and seed yield was not affected by jasmonic acid, and jasmonic acid-responsive promoter beta-glucuronidase gene constructs did not respond to the fungus in Arabidopsis roots. *We propose that ethylene signalling components and ethylene-targeted transcription factors are required to balance beneficial and nonbeneficial traits in the symbiosis. The results show that the restriction of fungal growth by ethylene signalling components is required for the beneficial interaction between the two symbionts.

EIN2, the central regulator of ethylene signalling, is localized at the ER membrane where it interacts with the ethylene receptor ETR1

Genetic studies have identified the membrane protein EIN2 (ethylene insensitive 2) as a central component of ethylene signalling in Arabidopsis. In addition, EIN2 might take part in multiple hormone signalling pathways and in response to pathogens as demonstrated by recent genetic and biochemical studies. Here we show, by an integrated approach using in vivo and in vitro fluorescence techniques, that EIN2 is localized at the ER (endoplasmic reticulum) membrane where it shows specific interaction with the ethylene receptor protein ETR1.

Effects of tobacco ethylene receptor mutations on receptor kinase activity, plant growth and stress responses

Ethylene receptor is the first component of ethylene signaling that regulates plant growth, development and stress responses. Previously, we have demonstrated that tobacco subfamily 2 ethylene receptor NTHK1 had Ser/Thr kinase activity, and overexpression of NTHK1 caused large rosette, reduced ethylene sensitivity, and increased salt sensitivity in transgenic Arabidopsis plants. Here we found that N-box mutation in the NTHK1 kinase domain abolished the kinase activity and led to disruption of NTHK1 roles in conferring reduced ethylene sensitivity and salt sensitive response in transgenic Arabidopsis plants. However, N-box mutation had partial effects on NTHK1 regulation of rosette growth and expression of salt- and ethylene-responsive genes AtNAC2, AtERF1 and AtCor6.6. Mutation of conserved residues in the H box did not affect kinase activity, seedling growth, ethylene sensitivity or salt-induced epinasty in transgenic plants but did influence NTHK1 function in control of specific salt- and ethylene-responsive gene expression. Compared with NTHK1, the tobacco subfamily 1 ethylene receptor NtETR1 had His kinase activity and played a weak role in regulation of rosette growth, triple response and salt response. Mutation of the conserved His residue in the NtETR1 H box eliminated phosphorylation and altered the effect of Ntetr1-1 on reporter gene activity. These results imply that the Ser/Thr kinase activity of NTHK1 is differentially required for various responses, and NTHK1 plays a larger role than NtETR1.

Ethylene, free radicals and the transition between stable states in plant morphology

Treatment with hydrogen peroxide has notable effects in the morphology of the root apex in Arabidopsis seedlings. The result was described as consisting in two aspects: first, a reduction in curvature values in the root profile. Second, alterations in size and shape of the cells in the root cap. Cells of the root cap were smaller and had higher circularity index (Fig. 1). The results of peroxide treatment were similar to alterations in the root apex of ethylene insensitive mutants and wild-type seedlings treated with ethylene inhibitors. This brings new evidence in favour of the association between ethylene and hydrogen peroxide signalling that was recently demonstrated in stomatal cells. Notable changes in morphology under peroxide treatment were previously reported in other biological systems (Fig. 2). In the following paragraphs we make emphasis on the need of an accurate analysis of morphology. This aspect has not received the attention required in biology, a discipline dominated by functional analysis. We suggest that the observed morphological characteristics in the root apex treated with peroxide may be the manifestation of global processes of adaptation in the organism. Alternative forms of roots grown in water or in peroxide are stable situations representing different global configurations that may have other (genomic, physiological) traits associated. Each form represents a different mode of adaptation to environmental change. The accurate description of morphology in organisms, with particular emphasis in model systems, and their variations under stress, is needed to identify and understand the basis of genomic organization and plasticity.

Emerging connections in the ethylene signaling network

The gaseous plant hormone ethylene acts as a pivotal mediator to respond to and coordinate internal and external cues in modulating plant growth dynamics and developmental programs. Genetic analysis of Arabidopsis thaliana has been used to identify key components and to build a linear ethylene-signaling pathway from the receptors through to the nuclear transcription factors. Studies applying integrative approaches have revealed new regulators, molecular connections and mechanisms in ethylene signaling and unexpected links to other plant hormones. Here, we review and discuss recent discoveries about the functional mode of ethylene receptor complexes, dual mitogen-activated protein kinase cascade signaling, stability control of the master nuclear transcription activator ETHYLENE INSENSITIVE 3 (EIN3), and the contextual relationships between ethylene and other plant hormones, such as auxin and gibberellins, in organ-specific growth regulation.

Ethylene response factor TERF1 enhances glucose sensitivity in tobacco through activating the expression of sugar-related genes

Ethylene response factor (ERF) proteins are important plant-specific transcription factors. Increasing evidence shows that ERF proteins regulate plant pathogen resistance, abiotic stress response and plant development through interaction with different stress responsive pathways. Previously, we revealed that overexpression of TERF1 in tobacco activates a cluster gene expression through interacting with GCC box and dehydration responsive element (DRE), resulting in enhanced sensitivity to abscisic acid (ABA) and tolerance to drought, and dark green leaves of mature plants, indicating that TERF1 participates in the integration of ethylene and osmotic responses. Here we further report that overexpression of TERF1 confers sugar response in tobacco. Analysis of the novel isolated tomato TERF1 promoter provides information indicating that there are many cis-acting elements, including sugar responsive elements (SURE) and W box, suggesting that TERF1 might be sugar inducible. This prediction is confirmed by results of reverse transcription-polymerase chain reaction amplification, indicating that transcripts of TERF1 are accumulated in tomato seedlings after application of glucose. Further investigation indicates that the expression of TERF1 in tobacco enhances sensitivity to glucose during seed germination, root and seedling development, showing a decrease of the fresh weight and root elongation under glucose treatment. Detailed investigations provide evidence that TERF1 interacts with the sugar responsive cis-acting element SURE and activates the expression of sugar response genes, establishing the transcriptional regulation of TERF1 in sugar response. Therefore, our results deepen our understanding of the glucose response mediated by the ERF protein TERF1 in tobacco.

Ethylene receptor ETR2 controls trichome branching by regulating microtubule assembly in Arabidopsis thaliana

The single-celled trichome of Arabidopsis thaliana is a widely used model system for studying cell development. While the pathways that control the later stages of trichome development are well characterized, the early signalling events that co-ordinate these pathways are less well understood. Hormones such as gibberellic acid, salicylic acid, cytokinins, and ethylene are known to affect trichome initiation and development. To understand the role of the plant hormone ethylene in trichome development, an Arabidopsis loss-of-function ethylene receptor mutant, etr2-3, which has completely unbranched trichomes, is analysed in this study. It was hypothesized that ETR2 might affect the assembly of the microtubule cytoskeleton based on analysis of the cytoskeleton in developing trichomes, and exposures to paclitaxol and oryzalin, which respectively act either to stabilize or depolymerize the cytoskeleton. Through epistatic and gene expression analyses it is shown that ETR2 is positioned upstream of CHROMATIN ASSEMBLY FACTOR1 and TRYPTICHON and is independent of the GLABRA2 and GLABRA3 pathways. These results help extend understanding of the early events that control trichome development and identify a signalling pathway through which ethylene affects trichome branching.

The BTB ubiquitin ligases ETO1, EOL1 and EOL2 act collectively to regulate ethylene biosynthesis in Arabidopsis by controlling type-2 ACC synthase levels

Ethylene biosynthesis is directed by a family of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS) that convert S-adenosyl-l-methionine to the immediate precursor ACC. Members of the type-2 ACS subfamily are strongly regulated by proteolysis with various signals stabilizing the proteins to increase ethylene production. In Arabidopsis, this turnover is mediated by the ubiquitin/26 S proteasome system, using a broad complex/tramtrack/bric-a-brac (BTB) E3 assembled with the ETHYLENE OVERPRODUCER 1 (ETO1) BTB protein for target recognition. Here, we show that two Arabidopsis BTB proteins closely related to ETO1, designated ETO1-like (EOL1) and EOL2, also negatively regulate ethylene synthesis via their ability to target ACSs for breakdown. Like ETO1, EOL1 interacts with type-2 ACSs (ACS4, ACS5 and ACS9), but not with type-1 or type-3 ACSs, or with type-2 ACS mutants that stabilize the corresponding proteins in planta. Whereas single and double mutants affecting EOL1 and EOL2 do not show an ethylene-related phenotype, they exaggerate the effects caused by inactivation of ETO1, and further increase ethylene production and the accumulation of ACS5 in eto1 plants. The triple eto1 eol1 eol2 mutant phenotype can be effectively rescued by the ACS inhibitor aminoethoxyvinylglycine, and by silver, which antagonizes ethylene perception. Together with hypocotyl growth assays showing that the sensitivity and response kinetics to ethylene are normal, it appears that ethylene synthesis, but not signaling, is compromised in the triple mutant. Collectively, the data indicate that the Arabidopsis BTB E3s assembled with ETO1, EOL1 and EOL2 work together to negatively regulate ethylene synthesis by directing the degradation of type-2 ACS proteins.

Modulation of the Rcs-mediated signal transfer by conformational flexibility

The Rcs (regulator of capsule synthesis) signalling complex comprises the membrane-associated hybrid sensor kinases RcsC and RcsD, the transcriptional regulator RcsB and the two co-inducers RcsA and RcsF. Acting as a global regulatory network, the Rcs phosphorelay controls multiple cellular pathways including capsule synthesis, cell division, motility, biofilm formation and virulence mechanisms. Signal-dependent communication of the individual Rcs domains showing histidine kinase, phosphoreceiver, phosphoryl transfer and DNA-binding activities is characteristic and essential for the modulation of signal transfer. We have analysed the structures of core elements of the Rcs network including the RcsC-PR (phosphoreceiver domain of RcsC) and the RcsD-HPt (histidine phosphotransfer domain of RcsD), and we have started to characterize the dynamics and recognition mechanisms of the proteins. RcsC-PR represents a typical CheY-like α/β/α sandwich fold and it shows a large conformational flexibility near the active-site residue Asp875. NMR analysis revealed that RcsC-PR is able to adopt preferred conformations upon Mg2+ co-ordination, BeF3− activation, phosphate binding and RcsD-HPt recognition. In contrast, the α-helical structure of RcsD-HPt is conformationally stable and contains a recognition area in close vicinity to the active-site His842 residue. Our studies indicate the importance of protein dynamics and conformational exchange for the differential response to the variety of signals perceived by complex regulatory networks.

Involvement of RTE1 in conformational changes promoting ETR1 ethylene receptor signaling in Arabidopsis

Ethylene is an important regulator of plant growth, development and responses to environmental stresses. Arabidopsis perceives ethylene through five homologous receptors that negatively regulate ethylene responses. RTE1, a novel gene conserved in plants, animals and some protists, was recently identified as a positive regulator of the ETR1 ethylene receptor. Here, we genetically analyze the dependence of ETR1 on RTE1 in order to obtain further insight into RTE1 function. The function of RTE1 was found to be independent and distinct from that of RAN1, which encodes a copper transporter required for ethylene receptor function. We tested the ability of an rte1 loss-of-function mutation to suppress 11 etr1 ethylene-binding domain mis-sense mutations, all of which result in dominant ethylene insensitivity due to constitutive signaling. This suppression test uncovered two classes of etr1 mutations -RTE1-dependent and RTE1-independent. The nature of these mutations suggests that the ethylene-binding domain is a possible target of RTE1 action. Based on these findings, we propose that RTE1 promotes ETR1 signaling through a conformational effect on the ethylene-binding domain.

MAPK signaling in plant hormone ethylene signal transduction

The signal transduction pathway of the plant stress and defense hormone, ethylene, has been extensively elucidated using the plant genetic model Arabidopsis over the last two decades. Among others, a MAPKKK CTR1 was identified as a negative regulator that has led to the speculation of MAPK involvement in ethylene signaling. However, it remained unclear how the MAPK modules acting downstream of the receptors to mediate ethylene signaling. We have recently presented new evidence that the MKK9-MPK3/6 modules identified by combined functional genomic and genetic screens mediate ethylene signaling, which is negatively regulated by the genetically identified CTR1-dependent cascades. Our genetic studies show consistently that the MKK9-MPK3/MPK6 modules act downstream of the ethylene receptors. Biochemical and transgenic analyses further demonstrated that the positive-acting and negative-acting MAPK activities are integrated and act simultaneously to control the key transcription factor EIN3 through dual phosphorylations to regulate the EIN3 protein stability and downstream transcription cascades. This study has revealed a novel molecular mechanism that defines the specificity of complex MAPK signaling. Comprehensive elucidation of MAPK cascades and the underlying molecular mechanisms would provide more precise explanations for how plant cells utilize MAPK cascades to control specific downstream outputs in response to distinct stimuli.

Rh-PIP2;1, a rose aquaporin gene, is involved in ethylene-regulated petal expansion

Aquaporins are water channel proteins that facilitate the passage of water through biological membranes and play a crucial role in plant growth. We showed that ethylene treatment significantly reduced petal size, inhibited expansion of petal abaxial subepidermal cells, and decreased petal water content in rose (Rosa hybrida 'Samantha'). Here, we report the isolation of a plasma membrane aquaporin (PIP) gene, Rh-PIP2;1, and characterized its potential role in ethylene-inhibited petal expansion. Rh-PIP2;1 is mainly localized on the plasma membrane and belongs to the class 2 subfamily of PIP proteins. We show that Rh-PIP2;1 is an active water channel. The transcripts of Rh-PIP2;1 are highly abundant in petal epidermal cells, especially in the abaxial subepidermal cells. The expression of Rh-PIP2;1 is highly correlated with petal expansion and tightly down-regulated by ethylene. Furthermore, we demonstrate that in Rh-PIP2;1-silenced flowers, petal expansion was greatly inhibited and anatomical features of the petals were similar to those of ethylene-treated flowers. We argue that Rh-PIP2;1 plays an important role in petal cell expansion and that ethylene inhibits petal expansion of roses at least partially by suppressing Rh-PIP2;1 expression.

Two-component signaling elements and histidyl-aspartyl phosphorelays

Two-component systems are an evolutionarily ancient means for signal transduction. These systems are comprised of a number of distinct elements, namely histidine kinases, response regulators, and in the case of multi-step phosphorelays, histidine-containing phosphotransfer proteins (HPts). Arabidopsis makes use of a two-component signaling system to mediate the response to the plant hormone cytokinin. Two-component signaling elements have also been implicated in plant responses to ethylene, abiotic stresses, and red light, and in regulating various aspects of plant growth and development. Here we present an overview of the two-component signaling elements found in Arabidopsis, including functional and phylogenetic information on both bona-fide and divergent elements.

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.

Chemical genetic dissection of brassinosteroid-ethylene interaction

We undertook a chemical genetics screen to identify chemical inhibitors of brassinosteroid (BR) action. From a chemical library of 10,000 small molecules, one compound was found to inhibit hypocotyl length and activate the expression of a BR-repressed reporter gene (CPD::GUS) in Arabidopsis, and it was named brassinopride (BRP). These effects of BRP could be reversed by co-treatment with brassinolide, suggesting that BRP either directly or indirectly inhibits BR biosynthesis. Interestingly, the compound causes exaggerated apical hooks, similar to that caused by ethylene treatment. The BRP-induced apical hook phenotype can be blocked by a chemical inhibitor of ethylene perception or an ethylene-insensitive mutant, suggesting that, in addition to inhibiting BR, BRP activates ethylene response. Analysis of BRP analogs provided clues about structural features important for its effects on two separate targets in the BR and ethylene pathways. Analyses of the responses of various BR and ethylene mutants to BRP, ethylene, and BR treatments revealed modes of cross-talk between ethylene and BR in dark-grown seedlings. Our results suggest that active downstream BR signaling, but not BR synthesis or a BR gradient, is required for ethylene-induced apical hook formation. The BRP-related compounds can be useful tools for manipulating plant growth and studying hormone interactions.

Tomato ethylene receptor-CTR interactions: visualization of NEVER-RIPE interactions with multiple CTRs at the endoplasmic reticulum

In the model plant Arabidopsis, members of a family of two-component system His kinase-like ethylene receptors have direct protein-protein interactions with a single downstream Ser/Thr kinase CTR1. These components of the ethylene signalling network found in Arabidopsis are conserved in the climacteric fruit tomato, but both the ethylene receptors and CTR1-like proteins (LeCTRs) in tomato are encoded by multigene families. Here, using a yeast two-hybrid interaction assay, it is shown that the tomato receptors LeETR1, LeETR2, and NEVER-RIPE (NR) can interact with multiple LeCTRs. In vivo protein localization studies with fluorescent tagged proteins revealed that the ethylene receptor NR was targeted to the endoplasmic reticulum (ER) when transiently expressed in onion epidermal cells, whereas the four LeCTR proteins were found in the cytoplasm and nucleus. When co-expressed with NR, three LeCTRs (1, 3, and 4), but not LeCTR2, also adopted the same ER localization pattern in an NR receptor-dependent manner but not in the absence of NR. The receptor-CTR interactions were confirmed by biomolecular fluorescence complementation (BiFC) showing that NR could form a protein complex with LeCTR1, 3, and 4. This suggested that ethylene receptors recruit these LeCTR proteins to the ER membrane through direct protein-protein interaction. The receptor-CTR interactions and localization observed in the study reinforce the idea that ethylene receptors transmit the signal to the downstream CTRs and show that a single receptor can interact with multiple CTR proteins. It remains unclear whether the different LeCTRs are functionally redundant or have unique roles in ethylene signalling.

Subcellular co-localization of Arabidopsis RTE1 and ETR1 supports a regulatory role for RTE1 in ETR1 ethylene signaling

Ethylene is an important plant growth regulator perceived by membrane-bound ethylene receptors. The ETR1 ethylene receptor is positively regulated by a predicted membrane protein, RTE1, based on genetic studies in Arabidopsis. RTE1 homologs exist in plants, animals and protists, but the molecular function of RTE1 is unknown. Here, we examine RTE1 expression and subcellular protein localization in order to gain a better understanding of RTE1 and its function in relation to ETR1. Arabidopsis plants transformed with the RTE1 promoter fused to the beta-glucuronidase (GUS) reporter gene revealed that RTE1 expression partly correlates with previously described sites of ETR1 expression or sites of ethylene response, such as the seedling root, root hairs and apical hook. RTE1 transcript levels are also enhanced by ethylene treatment, and reduced by the inhibition of ethylene signaling. For subcellular localization of RTE1, a functional RTE1 fusion to red fluorescent protein (RFP) was expressed under the control of the native RTE1 promoter. Using fluorescence microscopy, RTE1 was observed primarily at the Golgi apparatus and partially at the endoplasmic reticulum (ER) in stably transformed Arabidopsis protoplasts, roots and root hairs. Next, a functional ETR1 fusion to a 5xMyc epitope tag was expressed under the control of the native ETR1 promoter. Immunohistochemistry of root hairs not only showed ETR1 residing at the ER as previously reported, but revealed substantial localization of ETR1 at the Golgi apparatus. Lastly, we demonstrated the subcellular co-localization of RTE1 and ETR1. These findings support and enhance the genetic model that RTE1 plays a role in regulating ETR1.

The plant stress hormone ethylene controls floral transition via DELLA-dependent regulation of floral meristem-identity genes

The length of the Arabidopsis thaliana life cycle depends on the timing of the floral transition. Here, we define the relationship between the plant stress hormone ethylene and the timing of floral initiation. Ethylene signaling is activated by diverse environmental stresses, but it was not previously clear how ethylene regulates flowering. First, we show that ethylene delays flowering in Arabidopsis, and that this delay is partly rescued by loss-of-function mutations in genes encoding the DELLAs, a family of nuclear gibberellin (GA)-regulated growth-repressing proteins. This finding suggests that ethylene may act in part by modulating DELLA activity. We also show that activated ethylene signaling reduces bioactive GA levels, thus enhancing the accumulation of DELLAs. Next, we show that ethylene acts on DELLAs via the CTR1-dependent ethylene response pathway, most likely downstream of the transcriptional regulator EIN3. Ethylene-enhanced DELLA accumulation in turn delays flowering via repression of the floral meristem-identity genes LEAFY (LFY) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1). Our findings establish a link between the CTR1/EIN3-dependent ethylene and GA-DELLA signaling pathways that enables adaptively significant regulation of plant life cycle progression in response to environmental adversity.

Molecular characterization of an ethylene receptor gene (CcETR1) in coffee trees, its relationship with fruit development and caffeine content

To understand the importance of ethylene receptor genes in the quality of coffee berries three full-length cDNAs corresponding to a putative ethylene receptor gene (ETR1) were isolated from Coffea canephora cDNA libraries. They differed by their 3'UTR and contained a main ORF and a 5'UTR short ORF putatively encoding a small polypeptide. The CcETR1 gene, present as a single copy in the C. canephora genome, contained five introns in the coding region and one in its 5'UTR. Alternative splicing can occur in C. canephora and C. pseudozanguebariae, leading to a truncated polypeptide. C. pseudozanguebariae ETR1 transcripts showed various forms of splicing alterations. This gene was equally expressed at all stages of fruit development. A segregation study on an inter-specific progeny showed that ETR1 is related to the fructification time, the caffeine content of the green beans, and seed weight. Arabidopsis transformed etiolated seedlings, which over-expressed CcETR1, displayed highly reduced gravitropism, but the triple response was observed in an ethylene enriched environment. These plants behaved like a low-concentration ethylene-insensitive mutant thus confirming the receptor function of the encoded protein. This gene showed no induction during the climacteric crisis but some linkage with traits related to quality.

Modulation of ethylene responses affects plant salt-stress responses

Ethylene signaling plays important roles in multiple aspects of plant growth and development. Its functions in abiotic stress responses remain largely unknown. Here, we report that alteration of ethylene signaling affected plant salt-stress responses. A type II ethylene receptor homolog gene NTHK1 (Nicotiana tabacum histidine kinase 1) from tobacco (N. tabacum) conferred salt sensitivity in NTHK1-transgenic Arabidopsis (Arabidopsis thaliana) plants as judged from the phenotypic change, the relative electrolyte leakage, and the relative root growth under salt stress. Ethylene precursor 1-aminocyclopropane-1-carboxylic acid suppressed the salt-sensitive phenotype. Analysis of Arabidopsis ethylene receptor gain-of-function mutants further suggests that receptor function may lead to salt-sensitive responses. Mutation of EIN2, a central component in ethylene signaling, also results in salt sensitivity, suggesting that EIN2-mediated signaling is beneficial for plant salt tolerance. Overexpression of the NTHK1 gene or the receptor gain-of-function activated expression of salt-responsive genes AtERF4 and Cor6.6. In addition, the transgene NTHK1 mRNA was accumulated under salt stress, suggesting a posttranscriptional regulatory mechanism. These findings imply that ethylene signaling may be required for plant salt tolerance.