Ligand-induced Degradation of the Ethylene Receptor ETR2 through a Proteasome-dependent Pathway in Arabidopsis

Advances in ethylene signalling: protein complexes at the endoplasmic reticulum membrane

The gaseous plant hormone ethylene plays critical roles in plant responses to environmental and endogenous signals that modulate growth and development. Over the past 25 years, great progress has been made in elucidating the ethylene signalling pathway. Genetic studies in Arabidopsis thaliana have identified key components of the pathway, and subcellular localization studies have shown that most of these components, other than transcription factors and protein turnover machinery, are associated with or lie within the endoplasmic reticulum (ER) membrane. The ethylene receptors are found in high-molecular-mass protein complexes and interact with the CTR1 serine/threonine protein kinase and the genetically downstream EIN2 Nramp-like protein. To more fully understand the ethylene signalling pathway, recent research has focused on examining the molecular connections between these components and how they are regulated. Here, we review recent advances and remaining gaps in our understanding of the early steps in the ethylene signalling pathway taking place at the ER.

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.

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.

Proteomic responses in Arabidopsis thaliana seedlings treated with ethylene

Ethylene (ET) is a volatile hormone that modulates fruit ripening, plant growth, development and stress responses. Key components of the ET-signaling pathway identified by genetic dissection in Arabidopsis thaliana include five ET receptors, the negative regulator CTR1 and the positive regulator EIN2, all of which localize to the endoplasmic reticulum. Mechanisms of signaling among these proteins are still unresolved and targets of ET responses are not fully known. So, we used mass spectrometry to identify proteins in microsomal membrane preparations from etiolated A. thaliana seedlings maintained in ambient air or treated with ET for 3 h. We compared 3814 proteins from ET-exposed seedlings and controls and identified 304 proteins with significant accumulation changes. The proteins with increased accumulation were involved in ET biosynthesis, cell morphogenesis, oxidative stress and vesicle secretion while those with decreased accumulation were ribosomal proteins and proteins positively regulated by brassinosteroid, another hormone involved in cell elongation. Several proteins, including EIN2, appeared to be differentially phosphorylated upon ET treatment, which suggests that the activity or stability of these proteins may be controlled by phosphorylation. TUA3, a component of microtubules that contributes to cellular morphological change, exhibited both increased accumulation and differential phosphorylation upon ET treatment. To verify the role of TUA3 in the ET response, tua3 mutants were evaluated. Mutant seedlings had altered ET-associated growth movements. The data indicate that ET perception leads to rapid proteomic change and that these changes are an important part of signaling and development. The data serve as a foundation for exploring ET signaling through systems biology.

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.

XAP5 CIRCADIAN TIMEKEEPER regulates ethylene responses in aerial tissues of Arabidopsis

The phytohormone ethylene differentially regulates plant architecture and growth in both a light- and nutrient-dependent fashion. The modulation of plant development by ethylene in response to both external and internal signals can also generate tissue-specific differential responses. Here, we report that XAP5 CIRCADIAN TIMEKEEPER (XCT) is involved in blue light-dependent ethylene responses in the aerial tissues of Arabidopsis (Arabidopsis thaliana) seedlings. XCT was first identified as a circadian clock mutant with a short free-running period. The xct mutation also causes sugar-specific hypocotyl growth defects, in which mutants are short in blue light when grown on a sucrose-rich medium but tall when grown on sucrose-deficient medium. Our data suggest that the hypocotyl defects in blue light are not directly caused by defects in clock or light signaling but rather by enhanced ethylene responses. In blue light, xct mutants have a more active ethylene response pathway and exhibit growth phenotypes similar to the constitutive ethylene signaling mutant constitutive triple response1 (ctr1). xct mutants also have reduced ethylene emission, analogous to plants that have lost CTR1 function. Genetic analysis suggests that XCT negatively regulates ethylene responses downstream of ETHYLENE-INSENSITIVE3 in aerial tissues. However, XCT is not required for all ethylene-mediated processes, such as the inhibition of root growth. Thus, XCT acts downstream of a major transcriptional regulator in an organ-specific manner, playing an environment-dependent role in the regulation of plant growth.

Further analysis of XBAT32, an Arabidopsis RING E3 ligase, involved in ethylene biosynthesis

The Arabidopsis RING E3 ligase, XBAT32, was previously characterized as a regulator of lateral root initiation. However, how XBAT32 function to modulate lateral root initiation was unknown. In our recent paper, we demonstrated that XBAT32 is involved in ethylene biosynthesis and it is through this function that XBAT32 is able to regulate lateral root production. Here we discuss a few other findings, observed in the ethylene overproducing mutant, xbat32, that reflect the effect of elevated ethylene levels on plant growth and development. Ethylene signaling also regulates plant responses to adverse environmental conditions such as high salinity. Consistent with ethylene's role as a stress hormone, xbat32 exhibited increased sensitivity to salt stress during seed germination and postgerminative growth. Thus, XBAT32 may also play a role in ethylene mediated response to abiotic stresses.

Turnover of LeACS2, a wound-inducible 1-aminocyclopropane-1-carboxylic acid synthase in tomato, is regulated by phosphorylation/dephosphorylation

1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) is the rate-limiting enzyme of the ethylene biosynthesis pathway. ACS is regulated both transcriptionally and post-translationally. We previously reported that LeACS2, a wound-inducible ACS in tomato (Solanum lycopersicum), is phosphorylated in vivo, and suggested that phosphorylation regulates protein stability rather than enzymatic activity. In this report, we demonstrate that phosphorylation/dephosphorylation of LeACS2 regulates its turnover upstream of the ubiquitin-26S-proteasome degradation pathway. Pulse-chase experiments coupled with treatment with protein kinase/phosphatase inhibitors demonstrated that LeACS2 is stabilized by phosphorylation and degraded after dephosphorylation. The amount of LeACS2 affected by the protein kinase/phosphatase inhibitors significantly influenced cellular ACS activity, ACC content, and ethylene production levels in tomato fruit tissue, suggesting that post-translational regulation by phosphorylation plays an important role in the control of ethylene production. A calcium-dependent protein kinase (CDPK), LeCDPK2, was isolated as one of the protein kinases that are able to phosphorylate LeACS2 at Ser-460. LeACS2 was immediately phosphorylated after translation by CDPK and mitogen-activated protein kinase at different sites in response to wound signaling and almost all functional LeACS2 molecules are phosphorylated in the cell. Phosphorylation at both sites was required for LeACS2 stability.

The copper transporter RAN1 is essential for biogenesis of ethylene receptors in Arabidopsis

Plants utilize ethylene as a hormone to regulate multiple developmental processes and to coordinate responses to biotic and abiotic stress. In Arabidopsis thaliana, a small family of five receptor proteins typified by ETR1 mediates ethylene perception. Our previous work suggested that copper ions likely play a role in ethylene binding. An independent study indicated that the ran1 mutants, which display ethylene-like responses to the ethylene antagonist trans-cyclooctene, have mutations in the RAN1 copper-transporting P-type ATPase, once again linking copper ions to the ethylene-response pathway. The results presented herein indicate that genetically engineered Saccharomyces cerevisiae expressing ETR1 but lacking the RAN1 homolog Ccc2p (Δccc2) lacks ethylene-binding activity. Ethylene-binding activity was restored when copper ions were added to the Δccc2 mutants, showing that it is the delivery of copper that is important. Additionally, transformation of the Δccc2 mutant yeast with RAN1 rescued ethylene-binding activity. Analysis of plants carrying loss-of-function mutations in ran1 showed that they lacked ethylene-binding activity, whereas seedlings carrying weak alleles of ran1 had normal ethylene-binding activity but were hypersensitive to copper-chelating agents. Altogether, the results show an essential role for RAN1 in the biogenesis of the ethylene receptors and copper homeostasis in Arabidopsis seedlings. Furthermore, the results indicate cross-talk between the ethylene-response pathway and copper homeostasis in Arabidopsis seedling development.

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.

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 receptors function as components of high-molecular-mass protein complexes in Arabidopsis

Background

The gaseous plant hormone ethylene is perceived in Arabidopsis thaliana by a five-member receptor family composed of ETR1, ERS1, ETR2, ERS2, and EIN4.

Methodology/Principal Findings

Gel-filtration analysis of ethylene receptors solubilized from Arabidopsis membranes demonstrates that the receptors exist as components of high-molecular-mass protein complexes. The ERS1 protein complex exhibits an ethylene-induced change in size consistent with ligand-mediated nucleation of protein-protein interactions. Deletion analysis supports the participation of multiple domains from ETR1 in formation of the protein complex, and also demonstrates that targeting to and retention of ETR1 at the endoplasmic reticulum only requires the first 147 amino acids of the receptor. A role for disulfide bonds in stabilizing the ETR1 protein complex was demonstrated by use of reducing agents and mutation of Cys4 and Cys6 of ETR1. Expression and analysis of ETR1 in a transgenic yeast system demonstrates the importance of Cys4 and Cys6 of ETR1 in stabilizing the receptor for ethylene binding.

Conclusions/Significance

These data support the participation of ethylene receptors in obligate as well as ligand-dependent non-obligate protein interactions. These data also suggest that different protein complexes may allow for tailoring of the ethylene signal to specific cellular environments and responses.

Ethylene signaling and response: where different regulatory modules meet

The structural simplicity of the gaseous hormone ethylene stands in contrast with the complexity of the physiological processes ethylene regulates. Initial studies suggested a simple linear arrangement of signaling molecules leading from the ethylene receptors to the EIN3 family of transcription factors. Recent discoveries have substantially changed this view. Current models suggest existence of a complex signaling pathway composed of several phosphorylation cascades, feedback-regulated transcriptional networks, and protein and mRNA turnover regulatory modules. Interactions between ethylene and other signals determine which of the ethylene-mediated responses get activated in a particular cell at a particular time. Tissue-specific regulation of auxin biosynthesis, transport, and response by ethylene is emerging as a key element in this signal integration process.

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.

Apple ethylene receptor protein concentrations are affected by ethylene, and differ in cultivars that have different storage life

Ethylene plays a crucial role in apple fruit ripening. Ethylene receptors have been identified and are known to be negative regulators of ethylene signalling. We examined ethylene receptors MdERS1 and MdERS2 in 1-MCP-treated and untreated fruit and leaves of cultivar 'Orin' and 'Fuji' apples. MdERS1 and MdERS2 transcription increased rapidly after harvest in control fruit, but in 1-MCP-treated fruit, increases were delayed for 30 days. However, MdERS1 and MdERS2 protein levels behaved differently. MdERS1 decreased gradually in both the control and 1-MCP treatments. MdERS2, however, increased gradually in control 'Fuji' and remained steady in 1-MCP-treated 'Fuji' but remained low in 'Orin'. Exogenous ethylene treatment of fruit increased MdERS1 and MdERS2 expression with slightly decreased protein levels. The ratios of proteins to mRNAs were much lower in 'Orin' fruit, and they decreased with ethylene treatment in both cultivars. However, protein to transcript ratio was higher in 'Fuji' ethylene treated fruit than in air- and ethylene-treated 'Orin' fruit. MdERS1 and MdERS2 transcript levels were increased by exogenous ethylene treatment in air pre-treated leaves, but MdERS1 and MdERS2 protein levels did not change or decrease with ethylene treatment, and the ratio of protein to mRNA was lower in ethylene-treated leaves. Differences between transcription and protein levels may be due to receptor turnover differences in the presence or absence of ethylene. Furthermore, MdERS1 and MdERS2 protein stabilities in the presence of ethylene were different in the two cvs. 'Orin' and 'Fuji'.

The ubiquitin-26S proteasome system at the nexus of plant biology

Plants, like other eukaryotes, rely on proteolysis to control the abundance of key regulatory proteins and enzymes. Strikingly, genome-wide studies have revealed that the ubiquitin-26S proteasome system (UPS) in particular is an exceedingly large and complex route for protein removal, occupying nearly 6% of the Arabidopsis thaliana proteome. But why is the UPS so pervasive in plants? Data accumulated over the past few years now show that it targets numerous intracellular regulators that have central roles in hormone signalling, the regulation of chromatin structure and transcription, tailoring morphogenesis, responses to environmental challenges, self recognition and battling pathogens.

The ethylene receptor ETR2 delays floral transition and affects starch accumulation in rice

Ethylene regulates multiple aspects of plant growth and development in dicotyledonous plants; however, its roles in monocotyledonous plants are poorly known. Here, we characterized a subfamily II ethylene receptor, ETHYLENE RESPONSE2 (ETR2), in rice (Oryza sativa). The ETR2 receptor with a diverged His kinase domain is a Ser/Thr kinase, but not a His kinase, and can phosphorylate its receiver domain. Mutation of the N box of the kinase domain abolished the kinase activity of ETR2. Overexpression of ETR2 in transgenic rice plants reduced ethylene sensitivity and delayed floral transition. Conversely, RNA interference (RNAi) plants exhibited early flowering and the ETR2 T-DNA insertion mutant etr2 showed enhanced ethylene sensitivity and early flowering. The effective panicles and seed-setting rate were reduced in the ETR2-overexpressing plants, while thousand-seed weight was substantially enhanced in both the ETR2-RNAi plants and the etr2 mutant compared with controls. Starch granules accumulated in the internodes of the ETR2-overexpressing plants, but not in the etr2 mutant. The GIGANTEA and TERMINAL FLOWER1/CENTRORADIALIS homolog (RCN1) that cause delayed flowering were upregulated in ETR2-overexpressing plants but downregulated in the etr2 mutant. Conversely, the alpha-amylase gene RAmy3D was suppressed in ETR2-overexpressing plants but enhanced in the etr2 mutant. Thus, ETR2 may delay flowering and cause starch accumulation in stems by regulating downstream genes.

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.

F-box proteins regulate ethylene signaling and more

Ethylene regulates several aspects of plant development, such as fruit ripening. In this issue of Genes & Development, Qiao and colleagues (pp. 512-521) report that the stability of the ethylene signaling protein EIN2 is modulated by two F-box proteins ETP1/2, reminiscent of the finding that another regulator of ethylene response, EIN3, is also targeted by the F-box proteins EBF1/2. ETP1/2 and EBF1/2 show distinct phylogenetic patterns, suggesting that they have different evolutionary constraints.

AtTRP1 encodes a novel TPR protein that interacts with the ethylene receptor ERS1 and modulates development in Arabidopsis

Arabidopsis AtTRP1 is an orthologue of SlTPR1, a tomato tetratricopeptide repeat protein that interacts with the tomato ethylene receptors LeETR1 and NR in yeast 2-hybrid assays and in vitro, and modulates plant development. AtTRP1 is encoded by a single copy gene in the Arabidopsis genome, and is related to TCC1, a human protein that competes with Raf-1 for Ras binding, and distantly related to the immunophilin-like FK-binding proteins TWD1 and PAS1. The former is involved in auxin transport and the latter is translocated to the nucleus in response to auxin. AtTRP1 interacted preferentially with the Arabidopsis ethylene receptor ERS1 in yeast two-hybrid assays. This association was confirmed by in vivo co-immunoprecipitation. AtTRP1 promoter-GUS was highly expressed in vascular tissue, mature anthers, the abscission zone, and was induced by ACC. Overexpression of AtTRP1 in wild-type Arabidopsis resulted in dwarf plants with reduced fertility, altered leaf/silique morphology, and enhanced expression of the ethylene responsive gene AtChitB. Exogenous GA did not reverse the dwarf habit. Etiolated transgenic seedlings overexpressing AtTRP1 displayed enhanced sensitivity to low ACC and this was correlated with the transgene expression. Seedlings overexpressing AtTRP1 at high levels exhibited shortened and swollen hypocotyls, inhibited root growth, and an altered apical hook. Plants overexpressing AtTRP1 also showed a reduced response to exogenous IAA and altered expression of a subset of auxin early responsive genes. These results indicated that overexpression of AtTRP1 affects cross-talk between ethylene and auxin signalling and enhances some ethylene responses and alters some auxin responses. A model for AtTRP1 action is proposed.

Recent advances in ethylene research

Ethylene regulates many aspects of the plant life cycle, including seed germination, root initiation, flower development, fruit ripening, senescence, and responses to biotic and abiotic stresses. It thus plays a key role in responses to the environment that have a direct bearing on a plant's fitness for adaptation and reproduction. In recent years, there have been major advances in our understanding of the molecular mechanisms regulating ethylene synthesis and action. Screening for mutants of the triple response phenotype of etiolated Arabidopsis seedlings, together with map-based cloning and candidate gene characterization of natural mutants from other plant species, has led to the identification of many new genes for ethylene biosynthesis, signal transduction, and response pathways. The simple chemical nature of ethylene contrasts with its regulatory complexity. This is illustrated by the multiplicity of genes encoding the key ethylene biosynthesis enzymes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase, multiple ethylene receptors and signal transduction components, and the complexity of regulatory steps involving signalling relays and control of mRNA and protein synthesis and turnover. In addition, there are extensive interactions with other hormones. This review integrates knowledge from the model plant Arabidopsis and other plant species and focuses on key aspects of recent research on regulatory networks controlling ethylene synthesis and its role in flower development and fruit ripening.

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.

Ethylene signaling: new levels of complexity and regulation

The gaseous plant hormone ethylene plays important roles in plant growth and development. Recent discoveries have expanded our linear view of ethylene signaling by revealing an elaborate signaling network with multiple regulatory circuits. At the membrane, the ethylene receptors form heteromeric and higher order complexes providing enhanced sensitivity and fine-tuning of signaling. Ethylene sensitivity is further enhanced by the rapid degradation of ethylene receptors upon ethylene binding and by dependence on a novel protein REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1)/GREEN-RIPE (GR). In the nucleus, EIN3-BINDING F-BOX1 and 2 (EBF1/2) coordinately control 26S proteasome degradation of the critical transcription factors EIN3 and EIL1. EBF1/2 expression is repressed by ETHYLENE-INSENSITIVE5 (EIN5), which encodes the exoribonuclease XRN4. Additionally, EIN3 possesses two mitogen-activated protein kinase (MAPK) phosphorylation sites that have opposing effects on EIN3 stability.

Ethylene signaling in Arabidopsis involves feedback regulation via the elaborate control of EBF2 expression by EIN3

EIN3 is a key transcription factor in the signaling pathway of the plant hormone ethylene in Arabidopsis. Ethylene signaling suppresses the 26S proteasome-mediated proteolysis of EIN3, the accumulation of which induces ethylene-responsive gene expression. The proteolysis of EIN3 has been suggested to be mediated by the E3 ubiquitin ligase SCF(EBF1/2) that contains either of the two closely related F-box proteins, EBF1 or EBF2. Here, we demonstrate feedback regulation of ethylene signaling that results from the direct upregulation of the EBF2 gene by EIN3. Although EBF1 and EBF2 show comparable activities as repressors of EIN3-dependent transcription, our analysis of transgenic Arabidopsis plants has revealed that the EBF2 promoter, but not the EBF1 promoter, is activated by ethylene. Furthermore, the results of protoplast transient assays in vivo and electrophoretic mobility shift assays in vitro have revealed that EIN3 can bind and activate the EBF2 promoter, indicating that EIN3 modulates EBF2 gene expression in planta. An ebf2 mutant transformed with the EBF2 gene under the control of a mutant promoter in which the EIN3-binding site was disrupted still showed an ethylene hyper-responsive phenotype, indicating the physiological relevance of EIN3-mediated activation of the EBF2 gene in the downregulation of ethylene signaling. We show an additional finding that a sequence downstream of the EBF2 coding region is also involved in modulations of both the EBF2 expression level and sensitivity to ethylene. These results thus indicate that the fine-tuning of ethylene signaling involves the intricate regulation of EBF2 expression in Arabidopsis.

Genetic basis of ethylene perception and signal transduction in Arabidopsis

Ethylene is a simple gaseous hormone in plants. It plays important roles in plant development and stress tolerance. In the presence of ethylene treatment, all ethylene receptors are in an activated form, which can physically interact with CTR1 and consequently recruit CTR1 protein to endoplasmic reticulum membraneto activate it. Activated CTR1 suppresses the downstream signal transduction by an unknown mechanism. Upon binding to its receptors, ethylene will inactivate the receptor/CTR1 module and in turn alleviate their inhibitory effect on two positive regulators acting downstream of CTR1: EIN2 and EIN3. Genetic study reveals that EIN2 is an essential component in the ethylene signaling pathway but its biochemical function remains a mystery. EIN3 is a plant-specific transcription factor and its protein abundance in the nucleus is rapidly induced upon ethylene treatment. In the absence of ethylene signal, EIN3 protein is degraded by an SCF complex containing one of the two F-box proteins EBF1/EBF2 in a 26S proteasome-dependent manner. EIN3 can bind to the promoter sequences of a number of downstream components, such as ERFs, which in turn bind to a GCC box, a cis-element found in many ethylene-regulated defense genes. Ethylene has been shown to also regulate many other hormones' signaling pathways including auxin, abscisic acid and jasmonic acid, implying the existence of complicated signaling networks in the growth, development and defense responses of various plants.

Ethylene receptor antagonists: strained alkenes are necessary but not sufficient

Plants use ethylene as a hormone to control many physiological processes. Ethylene perception involves its binding to an unusual copper-containing, membrane-bound receptor. Inhibitors of ethylene action are valuable to study signaling and may have practical use in horticulture. Past investigation of alkene ligands for this receptor has identified strain as the key factor in antagonism of ethylene binding and action, consistent with known trends in metal-alkene complex stability. However, in this work, this principle could not be extended to other alkenes, prompting development of the proposal that a ring-opening reaction accounts for the unusual potency of cyclopropene ethylene antagonists. Another factor augmenting the affinity of alkenes for the copper binding site is pyramidalization, as in trans-cycloalkenes. The enantiomeric selectivity in the binding of one such alkene to the ethylene receptor demonstrates its protein-composed asymmetric environment.

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.

Subcellular localization and in vivo interactions of the Arabidopsis thaliana ethylene receptor family members

The gaseous phytohormone ethylene regulates many developmental processes and responses to environmental conditions in higher plants. In Arabidopsis thaliana, ethylene perception and initiation of signaling are mediated by a family of five receptors which are related to prokaryotic two-component sensor histidine kinases. The transient expression of fluorescence-tagged receptors in tobacco (Nicotiana benthamiana) epidermal leaf cells demonstrated that all ethylene receptors are targeted to the ER endomembrane network and do not localize to the plasmalemma. In support of in planta overlay studies, the ethylene receptors form homomeric and heteromeric protein complexes at the ER in living plant cells, as shown by membrane recruitment assays. A comparable in vivo interaction pattern was found in the yeast mating-based split-ubiquitin system. The overlapping but distinct expression pattern of the ethylene receptor genes suggests a differential composition of the ethylene receptor complexes in different plant tissues. Our findings may have crucial functional implications on the ethylene receptor-mediated efficiency of hormone perception, induction of signaling, signal attenuation and output.

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 role of protein turnover in ethylene biosynthesis and response

Plant growth and development is controlled by a set of hormones whose responses are tightly regulated in order to direct appropriate responses. In several hormone signaling pathways, protein turnover has emerged as a common regulatory element. Ethylene is a phytohormone that controls a variety of processes, including fruit ripening, senescence, and stress response. This review focuses on the regulation of the ethylene response pathway through protein degradation. Protein turnover has been found to regulate both ethylene biosynthesis and ethylene response. Ethylene production is regulated through the turnover of the biosynthetic enzyme ACS. Recently it was found that ethylene receptors are controlled by protein turnover as well. A third process in the control of ethylene signaling is the targeting of the ethylene response transcription factor ETHYLENE INSENSITIVE3 (EIN3) for degradation by the proteins EIN3-BINDING F-BOX 1 and 2 (EBF1 and EBF2).