Localization of the Raf-like kinase CTR1 to the endoplasmic reticulum of Arabidopsis through participation in ethylene receptor signaling complexes

Ethylene signaling: simple ligand, complex regulation

The hormone ethylene plays numerous roles in plant development. In the last few years the model of ethylene signaling has evolved from an initially largely linear route to a much more complex pathway with multiple feedback loops. Identification of key transcriptional and post-transcriptional regulatory modules controlling expression and/or stability of the core pathway components revealed that ethylene perception and signaling are tightly regulated at multiple levels. This review describes the most current outlook on ethylene signal transduction and emphasizes the latest discoveries in the ethylene field that shed light on the mechanistic mode of action of the central pathway components CTR1 and EIN2, as well as on the post-transcriptional regulatory steps that modulate the signaling flow through the pathway.

The emerging role of photorespiration and non-photorespiratory peroxisomal metabolism in pathogen defence

Photorespiration represents one of the major highways of primary plant metabolism and is the most prominent example of metabolic cell organelle integration, since the pathway requires the concerted action of plastidial, peroxisomal, mitochondrial and cytosolic enzymes and organellar transport proteins. Oxygenation of ribulose-1,5-bisphosphate by Rubisco leads to the formation of large amounts of 2-phosphoglycolate, which are recycled to 3-phosphoglycerate by the photorespiratory C2 cycle, concomitant with stoichiometric production rates of H2 O2 in peroxisomes. Apart from its significance for agricultural productivity, a secondary function of photorespiration in pathogen defence has emerged only recently. Here, we summarise literature data supporting the crosstalk between photorespiration and pathogen defence and perform a meta-expression analysis of photorespiratory genes during pathogen attack. Moreover, we screened Arabidopsis proteins newly predicted using machine learning methods to be targeted to peroxisomes, the central H2 O2 -producing organelle of photorespiration, for homologues of known pathogen defence proteins and analysed their expression during pathogen infection. The analyses further support the idea that photorespiration and non-photorespiratory peroxisomal metabolism play multi-faceted roles in pathogen defence beyond metabolism of reactive oxygen species.

The nitrate transporter NRT2.1 functions in the ethylene response to nitrate deficiency in Arabidopsis

The ethylene signalling pathway is closely associated with complex environmental stresses. Previous studies have reported impact of high nitrate (HN) availability on ethylene biosynthesis and regulation of ethylene on nitrate transporter 2.1 (NRT2.1) expression. However, molecular interaction between NRT2.1 transcript levels and the ethylene signalling pathway under nitrate deficiency is still elusive. Here, we report a low nitrate (LN) treatment-induced rapid burst of ethylene production and regulated expression of ethylene signalling components CTR1, EIN3 and EIL1 in wild-type Arabidopsis thaliana (Col-0) seedlings, and enhanced ethylene response reporter EBS:GUS activity in both Col-0 and the ethylene mutants ein3-1eil1-1 and ctr1-1. LN treatment also caused up-regulation of NRT2.1 expression, which was responsible for an enhanced high-affinity nitrate uptake. Comparison of ethylene production and EBS:GUS activity between nrt1.1, nrt2.1 mutants and Col-0 indicated that this up-regulation of NRT2.1 expression caused a positive effect on ethylene biosynthesis and signalling under LN treatment. On the other hand, ethylene down-regulated NRT2.1 expression and reduced the high-affinity nitrate uptake. Together, these findings uncover a negative feedback loop between NRT2.1 expression and ethylene biosynthesis and signalling under nitrate deficiency, which may contribute to finely tuning of plant nitrate acquisition during exploring dynamic soil conditions.

Ethylene signaling and regulation in plant growth and stress responses

Gaseous phytohormone ethylene affects many aspects of plant growth and development. The ethylene signaling pathway starts when ethylene binds to its receptors. Since the cloning of the first ethylene receptor ETR1 from Arabidopsis, a large number of studies have steadily improved our understanding of the receptors and downstream components in ethylene signal transduction pathway. This article reviews the regulation of ethylene receptors, signal transduction, and the posttranscriptional modulation of downstream components. Functional roles and importance of the ethylene signaling components in plant growth and stress responses are also discussed. Cross-reactions of ethylene with auxin and other phytohormones in plant organ growth will be analyzed. The studies of ethylene signaling in plant growth, development, and stress responses in the past decade greatly advanced our knowledge of how plants respond to endogenous signals and environmental factors.

Temporal transcriptional response to ethylene gas drives growth hormone cross-regulation in Arabidopsis

The gaseous plant hormone ethylene regulates a multitude of growth and developmental processes. How the numerous growth control pathways are coordinated by the ethylene transcriptional response remains elusive. We characterized the dynamic ethylene transcriptional response by identifying targets of the master regulator of the ethylene signaling pathway, ETHYLENE INSENSITIVE3 (EIN3), using chromatin immunoprecipitation sequencing and transcript sequencing during a timecourse of ethylene treatment. Ethylene-induced transcription occurs in temporal waves regulated by EIN3, suggesting distinct layers of transcriptional control. EIN3 binding was found to modulate a multitude of downstream transcriptional cascades, including a major feedback regulatory circuitry of the ethylene signaling pathway, as well as integrating numerous connections between most of the hormone mediated growth response pathways. These findings provide direct evidence linking each of the major plant growth and development networks in novel ways. DOI:http://dx.doi.org/10.7554/eLife.00675.001.

Temporal transcriptional response to ethylene gas drives growth hormone cross-regulation in Arabidopsis

The gaseous plant hormone ethylene regulates a multitude of growth and developmental processes. How the numerous growth control pathways are coordinated by the ethylene transcriptional response remains elusive. We characterized the dynamic ethylene transcriptional response by identifying targets of the master regulator of the ethylene signaling pathway, ETHYLENE INSENSITIVE3 (EIN3), using chromatin immunoprecipitation sequencing and transcript sequencing during a timecourse of ethylene treatment. Ethylene-induced transcription occurs in temporal waves regulated by EIN3, suggesting distinct layers of transcriptional control. EIN3 binding was found to modulate a multitude of downstream transcriptional cascades, including a major feedback regulatory circuitry of the ethylene signaling pathway, as well as integrating numerous connections between most of the hormone mediated growth response pathways. These findings provide direct evidence linking each of the major plant growth and development networks in novel ways. DOI:http://dx.doi.org/10.7554/eLife.00675.001.

Temporal transcriptional response to ethylene gas drives growth hormone cross-regulation in Arabidopsis

The gaseous plant hormone ethylene regulates a multitude of growth and developmental processes. How the numerous growth control pathways are coordinated by the ethylene transcriptional response remains elusive. We characterized the dynamic ethylene transcriptional response by identifying targets of the master regulator of the ethylene signaling pathway, ETHYLENE INSENSITIVE3 (EIN3), using chromatin immunoprecipitation sequencing and transcript sequencing during a timecourse of ethylene treatment. Ethylene-induced transcription occurs in temporal waves regulated by EIN3, suggesting distinct layers of transcriptional control. EIN3 binding was found to modulate a multitude of downstream transcriptional cascades, including a major feedback regulatory circuitry of the ethylene signaling pathway, as well as integrating numerous connections between most of the hormone mediated growth response pathways. These findings provide direct evidence linking each of the major plant growth and development networks in novel ways.

DOI:http://dx.doi.org/10.7554/eLife.00675.001

All multicellular organisms, including plants, produce hormones—chemical messengers that are released in one part of an organism but act in another. The binding of hormones to receptor proteins on the surface of target cells activates signal transduction cascades, leading ultimately to changes in the transcription and translation of genes.

Ethylene is a gaseous plant hormone that acts at trace levels to stimulate or regulate a variety of processes, including the regulation of plant growth, the ripening of fruit and the shedding of leaves. Plants also produce ethylene in response to wounding, pathogen attack or exposure to environmental stresses, such as extreme temperatures or drought. Although the effects of ethylene on plants are well documented, much less is known about how its functions are controlled and coordinated at the molecular level.

Here, Chang et al. reveal how ethylene alters the transcription of DNA into messenger DNA (mRNA) in the plant model organism, Arabidopsis thaliana. Ethylene is known to exert some of its effects via a protein called EIN3, which is a transcription factor that acts as the master regulator of the ethylene signaling pathway. To identify the targets of EIN3, Chang et al. exposed plants to ethylene and then used a technique called ChIP-Seq to identify those regions of the DNA that EIN3 binds to. At the same time, they used genome-wide mRNA sequencing to determine which genes showed altered transcription.

Over the course of 24 hr, ethylene induced four distinct waves of transcription, suggesting that discrete layers of transcriptional control are present. EIN3 binding also controlled a multitude of downstream transcriptional cascades, including a major negative feedback loop. Surprisingly, many of the genes that showed altered expression in response to EIN3 binding were also influenced by hormones other than ethylene.

In addition to extending our knowledge of the role of EIN3 in coordinating the effects of ethylene, the work of Chang et al. reveals the extensive connectivity between pathways regulated by distinct hormones in plants. The results may also make it easier to identify key players involved in hormone signaling pathways in other plant species.

DOI:http://dx.doi.org/10.7554/eLife.00675.002

Emerging topics in the cell biology of mitogen-activated protein kinases

Signaling through mitogen-activated protein kinase (MAPK) cascades is organized in complex interconnected subcellular networks. Upon MAPK activation, signals are transferred to targets in different subcellular compartments able to regulate various cellular processes. Therefore, subcellular dissection of individual MAPK modules is vital to understand how a single MAPK can simultaneously mediate many tasks and how a single stimulus can direct different MAPK modules to separated tasks. In this opinion article, we present a subcellular localization prediction of all members of Arabidopsis thaliana MAPK modules validated wherever possible with experimental data. Furthermore, we propose, that at least in part, the complexity of plant MAPK signaling can be explained by unique strategies of subcellular targeting, which will be worth investigating in the near future.

Functional phosphoproteomic analysis reveals that a serine-62-phosphorylated isoform of ethylene response factor110 is involved in Arabidopsis bolting

Ethylene is a major plant hormone that plays an important role in regulating bolting, although the underlying molecular mechanism is not well understood. In this study, we report the novel finding that the serine-62 (Ser-62) phosphorylation of Ethylene Response Factor110 (ERF110) is involved in the regulation of bolting time. The gene expression and posttranslational modification (phosphorylation) of ERF110 were analyzed among ethylene-response mutants and ERF110 RNA-interfering knockout lines of Arabidopsis (Arabidopsis thaliana). Physiological and biochemical studies revealed that the Ser-62 phosphorylation of ERF110 was closely related to bolting time, that is, the ethylene-enhanced gene expression of ERF110 and the decreased Ser-62 phosphorylation of the ERF110 protein in Arabidopsis. The expression of a flowering homeotic APETALA1 gene was up-regulated by the Ser-62-phosphorylated isoform of the ERF110 transcription factor, which was necessary but not sufficient for normal bolting. The gene expression and phosphorylation of ERF110 were regulated by ethylene via both Ethylene-Insensitive2-dependent and -independent pathways, which constitute a dual-and-opposing mechanism of action for ethylene in the regulation of Arabidopsis bolting.

Mechanisms of signal transduction by ethylene: overlapping and non-overlapping signalling roles in a receptor family

The plant hormone ethylene regulates growth and development as well as responses to biotic and abiotic stresses. Over the last few decades, key elements involved in ethylene signal transduction have been identified through genetic approaches, these elements defining a pathway that extends from initial ethylene perception at the endoplasmic reticulum to changes in transcriptional regulation within the nucleus. Here, we present our current understanding of ethylene signal transduction, focusing on recent developments that support a model with overlapping and non-overlapping roles for members of the ethylene receptor family. We consider the evidence supporting this model for sub-functionalization within the receptor family, and then discuss mechanisms by which such a sub-functionalization may occur. To this end, we consider the importance of receptor interactions in modulating their signal output and how such interactions vary in the receptor family. In addition, we consider evidence indicating that ethylene signal output by the receptors involves both phosphorylation-dependent and phosphorylation-independent mechanisms. We conclude with a current model for signalling by the ethylene receptors placed within the overall context of ethylene signal transduction.

Differential control of ethylene responses by GREEN-RIPE and GREEN-RIPE LIKE1 provides evidence for distinct ethylene signaling modules in tomato

The factors that mediate specific responses to the plant hormone ethylene are not fully defined. In particular, it is not known how signaling at the receptor complex can control distinct subsets of ethylene responses. Mutations at the Green-ripe (Gr) and reversion to ethylene sensitivity1 (rte1) loci, which encode homologous proteins of unknown function, influence ethylene responses in tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana), respectively. In Arabidopsis, AtRTE1 is required for function of the ETR1 ethylene receptor and acts predominantly through this receptor via direct protein-protein interaction. While most eudicot families including the Brassicaceae possess a single gene that is closely related to AtRTE1, we report that members of the Solanaceae family contain two phylogenetically distinct genes defined by GR and GREEN-RIPE LIKE1 (GRL1), creating the possibility of subfunctionalization. We also show that SlGR and SlGRL1 are differentially expressed in tomato tissues and encode proteins predominantly localized to the Golgi. A combination of overexpression in tomato and complementation of the rte1-3 mutant allele indicates that SlGR and SlGRL1 influence distinct but overlapping ethylene responses. Overexpression of SlGRL1 in the Gr mutant background provides evidence for the existence of different ethylene signaling modules in tomato that are influenced by GR, GRL1, or both. In addition, overexpression of AtRTE1 in tomato leads to reduced ethylene responsiveness in a subset of tissues but does not mimic the Gr mutant phenotype. Together, these data reveal species-specific heterogeneity in the control of ethylene responses mediated by members of the GR/RTE1 family.

Cloning and expression of ethylene receptor ERS1 at various developmental and ripening stages of mango fruit

Ethylene induces characteristic ripening reactions in climacteric fruits through its binding to histidine-kinase (HK) receptors, activating the expression of ripening genes. Ethylene receptors have been found in Arabidopsis thaliana (Brassicaceae) and some fruits; number and expression patterns differ among species. In mango, only ethylene receptor ETR1 was known. We cloned ERS1 cDNA from mango, and evaluated the expression of Mi-ERS1 and Mi-ETR1 by qPCR in developmental and ripening stages of this fruit. The Mi-ERS1 coding sequence is 1890 bp long and encodes 629 amino acids, similar to ERS1 from other fruits. Also, the amino acid sequence of ERS1 C-terminal HK domain shows the cognate fold after molecular modeling. Mi-ERS1 expression levels increased as mangoes ripened, showing the highest levels at the climacteric stage, while Mi-ETR1 levels did not change during development and ripening. We conclude that the patterns of expression of Mi-ERS1 and Mi-ETR1 differ in mango fruit.

CTR1 phosphorylates the central regulator EIN2 to control ethylene hormone signaling from the ER membrane to the nucleus in Arabidopsis

The gaseous phytohormone ethylene C(2)H(4) mediates numerous aspects of growth and development. Genetic analysis has identified a number of critical elements in ethylene signaling, but how these elements interact biochemically to transduce the signal from the ethylene receptor complex at the endoplasmic reticulum (ER) membrane to transcription factors in the nucleus is unknown. To close this gap in our understanding of the ethylene signaling pathway, the challenge has been to identify the target of the CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) Raf-like protein kinase, as well as the molecular events surrounding ETHYLENE-INSENSITIVE2 (EIN2), an ER membrane-localized Nramp homolog that positively regulates ethylene responses. Here we demonstrate that CTR1 interacts with and directly phosphorylates the cytosolic C-terminal domain of EIN2. Mutations that block the EIN2 phosphorylation sites result in constitutive nuclear localization of the EIN2 C terminus, concomitant with constitutive activation of ethylene responses in Arabidopsis. Our results suggest that phosphorylation of EIN2 by CTR1 prevents EIN2 from signaling in the absence of ethylene, whereas inhibition of CTR1 upon ethylene perception is a signal for cleavage and nuclear localization of the EIN2 C terminus, allowing the ethylene signal to reach the downstream transcription factors. These findings significantly advance our understanding of the mechanisms underlying ethylene signal transduction.

A comparative study of ethylene growth response kinetics in eudicots and monocots reveals a role for gibberellin in growth inhibition and recovery

Time-lapse imaging of dark-grown Arabidopsis (Arabidopsis thaliana) hypocotyls has revealed new aspects about ethylene signaling. This study expands upon these results by examining ethylene growth response kinetics of seedlings of several plant species. Although the response kinetics varied between the eudicots studied, all had prolonged growth inhibition for as long as ethylene was present. In contrast, with continued application of ethylene, white millet (Panicum miliaceum) seedlings had a rapid and transient growth inhibition response, rice (Oryza sativa 'Nipponbare') seedlings had a slow onset of growth stimulation, and barley (Hordeum vulgare) had a transient growth inhibition response followed, after a delay, by a prolonged inhibition response. Growth stimulation in rice correlated with a decrease in the levels of rice ETHYLENE INSENSTIVE3-LIKE2 (OsEIL2) and an increase in rice F-BOX DOMAIN AND LRR CONTAINING PROTEIN7 transcripts. The gibberellin (GA) biosynthesis inhibitor paclobutrazol caused millet seedlings to have a prolonged growth inhibition response when ethylene was applied. A transient ethylene growth inhibition response has previously been reported for Arabidopsis ethylene insensitive3-1 (ein3-1) eil1-1 double mutants. Paclobutrazol caused these mutants to have a prolonged response to ethylene, whereas constitutive GA signaling in this background eliminated ethylene responses. Sensitivity to paclobutrazol inversely correlated with the levels of EIN3 in Arabidopsis. Wild-type Arabidopsis seedlings treated with paclobutrazol and mutants deficient in GA levels or signaling had a delayed growth recovery after ethylene removal. It is interesting to note that ethylene caused alterations in gene expression that are predicted to increase GA levels in the ein3-1 eil1-1 seedlings. These results indicate that ethylene affects GA levels leading to modulation of ethylene growth inhibition kinetics.

A large-scale protein phosphorylation analysis reveals novel phosphorylation motifs and phosphoregulatory networks in Arabidopsis

Large-scale protein phosphorylation analysis by MS is emerging as a powerful tool in plant signal transduction research. However, our current understanding of the phosphorylation regulatory network in plants is still very limited. Here, we report on a proteome-wide profiling of phosphopeptides in nine-day-old Arabidopsis (Arabidopsis thaliana) seedlings by using an enrichment method combining the titanium (Ti(4+))-based IMAC and the RP-strong cation exchange (RP-SCX) biphasic trap column-based online RPLC. Through the duplicated RPLC-MS/MS analyses, we identified 5348 unique phosphopeptides for 2552 unique proteins. Among the phosphoproteins identified, 41% of them were first-time identified. Further evolutionary conservation and phosphorylation motif analyses of the phosphorylation sites discovered 100 highly conserved phosphorylation residues and identified 17 known and 14 novel motifs specific for Ser/Thr protein kinases. Gene ontology and pathway analyses revealed that many of the new identified phosphoproteins are important regulatory proteins that are involved in diverse biological processes, particularly in central metabolisms and cell signaling. Taken together, our results provided not only new insights into the complex phosphoregulatory network in plants but also important resources for future functional studies of protein phosphorylation in plant growth and development.

Possible modulation of Arabidopsis ETR1 N-terminal signaling by CTR1

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

Ligand-induced alterations in the phosphorylation state of ethylene receptors in tomato fruit

Perception of the plant hormone ethylene is essential to initiate and advance ripening of climacteric fruits. Since ethylene receptors negatively regulate signaling, the suppression is canceled upon ethylene binding, permitting responses including fruit ripening. Although receptors have autophosphorylation activity, the mechanism whereby signal transduction occurs has not been fully determined. Here we demonstrate that LeETR4, a critical receptor for tomato (Solanum lycopersicum) fruit ripening, is multiply phosphorylated in vivo and the phosphorylation level is dependent on ripening stage and ethylene action. Treatment of preclimacteric fruits with ethylene resulted in accumulation of LeETR4 with reduced phosphorylation whereas treatments of ripening fruits with ethylene antagonists, 1-methylcyclopropene and 2,5-norbornadiene, induced accumulation of the phosphorylated isotypes. A similar phosphorylation pattern was also observed for Never ripe, another ripening-related receptor. Alteration in the phosphorylation state of receptors is likely to be an initial response upon ethylene binding since treatments with ethylene and 1-methylcyclopropene rapidly influenced the LeETR4 phosphorylation state rather than protein abundance. The LeETR4 phosphorylation state closely paralleled ripening progress, suggesting that the phosphorylation state of receptors is implicated in ethylene signal output in tomato fruits. We provide insights into the nature of receptor on and off states.

Expression profiles of a MhCTR1 gene in relation to banana fruit ripening

The banana (Musa spp.) is a typical climacteric fruit of high economic importance. The development of bananas from maturing to ripening is characterized by increased ethylene production accompanied by a respiration burst. To elucidate the signal transduction pathway involved in the ethylene regulation of banana ripening, a gene homologous to Arabidopsis CTR1 (constitutive triple response 1) was isolated from Musa spp. (Hsien Jin Chiao, AAA group) and designated as MhCTR1. MhCTR1 spans 11.5 kilobases and consists of 15 exons and 14 introns with consensus GT-AG nucleotides situated at their boundaries. MhCTR1 encodes a polypeptide of 805 amino acid residues with a calculated molecular weight of 88.6 kDa. The deduced amino acid sequence of MhCTR1 demonstrates 55%, 56% and 55% homology to AtCTR1, RhCTR1, and LeCTR1, respectively. MhCTR1 is expressed mostly in the mature green pulp and root organs. During fruit development MhCTR1 expression increases just before ethylene production rises. Moreover, MhCTR1 expression was detected mainly in the pulps at ripening stage 3, and correlated with the onset of peel yellowing, while MhCTR1 was constitutively expressed in the peels. MhCTR1 expression could be induced by ethylene treatment (0.01 μL L(-1)), and MhCTR1 expression decreased in both peel and pulp 24 h after treatment. Overall, changes observed in MhCTR1 expression in the pulp closely related to the regulation of the banana ripening process.

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.

Reduced expression of CTR1 gene modulated by mitochondria causes enhanced ethylene response in cytoplasmic male-sterile Brassica juncea

We studied how mitochondria affect ethylene response via modulation of CTR1 expression in cytoplasmic male-sterile (CMS) Brassica juncea. The expression of CTR1 gene was reduced in CMS compared with male-fertile (MF) lines. We observed that hypocotyl and root lengths were shorter than in the MF line during germination in the dark. An enhanced ethylene response was observed in CMS plants as shown by the CMS and maintainer line phenotypes treated with 1-aminocyclopropane-1-carboxylic acid. The phenotype in CMS plants could be recovered to the maintainer line when treated with Ag(+) . One ethylene response gene, plant defensin gene, was detected to be induced in CMS. The behavior of this phenotype could be mimicked by treating the maintainer line with antimycin A that disturbs mitochondrial function, which showed reduced length of hypocotyl and roots, and resulted in similar expression patterns of ethylene-related genes as in CMS. The reduced length of hypocotyl and roots could be recovered to the maintainer line by treatment with gibberellic acid (GA(3) ). In addition, the GA(3) content was reduced in CMS plants and in the MF line treated with antimycin A. Ethylene treatment markedly affects GA(3) content; however, GA(3) did not significantly affect ethylene-related gene expression in regards to regulation of hypocotyl and root length, which suggests that ethylene acts upstream via gibberellin to regulate hypocotyls and root development. Taken together, our results suggest a link between mitochondrial modulation of the ethylene and gibberellin pathway that regulates the development of hypocotyl and roots.

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

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

Ethylene in vegetative development: a tale with a riddle

The vegetative development of plants is strongly dependent on the action of phytohormones. For over a century, the effects of ethylene on plants have been studied, illustrating the profound impact of this gaseous hormone on plant growth, development and stress responses. Ethylene signaling is under tight self-control at various levels. Feedback regulation occurs on both biosynthesis and signaling. For its role in developmental processes, ethylene has a close and reciprocal relation with auxin, another major determinant of plant architecture. Here, we discuss, in view of novel findings mainly in the reference plant Arabidopsis, how ethylene is distributed and perceived throughout the plant at the organ, tissue and cellular levels, and reflect on how plants benefit from the complex interaction of ethylene and auxin, determining their shape. Furthermore, we elaborate on the implications of recent discoveries on the control of ethylene signaling.

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.

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.

Improvement of stress tolerance in plants by genetic manipulation of mitogen-activated protein kinases

Plant stress tolerance depends on many factors among which signaling by mitogen-activated protein-kinase (MAPK) modules plays a crucial role. Reversible phosphorylation of MAPKs, their upstream activators and downstream targets such as transcription factors can trigger a myriad of transcriptomic, cellular and physiological responses. Genetic manipulation of abundance and/or activity of some of these modular MAPK components can lead to better stress tolerance in Arabidopsis and crop plant species such as tobacco and cereals. The main focus of this review is devoted to the MAPK-related signaling components which show the most promising biotechnological potential. Additionally, recent studies identified MAPK components to be involved both in plant development as well as in stress responses, suggesting that these processes are tightly linked in plants.

Dissecting the role of CHITINASE-LIKE1 in nitrate-dependent changes in root architecture

The root phenotype of an Arabidopsis (Arabidopsis thaliana) mutant of CHITINASE-LIKE1 (CTL1), called arm (for anion-related root morphology), was previously shown to be conditional on growth on high nitrate, chloride, or sucrose. Mutants grown under restrictive conditions displayed inhibition of primary root growth, radial swelling, proliferation of lateral roots, and increased root hair density. We found here that the spatial pattern of CTL1 expression was mainly in the root and root tips during seedling development and that the protein localized to the cell wall. Fourier-transform infrared microspectroscopy of mutant root tissues indicated differences in spectra assigned to linkages in cellulose and pectin. Indeed, root cell wall polymer composition analysis revealed that the arm mutant contained less crystalline cellulose and reduced methylesterification of pectins. We also explored the implication of growth regulators on the phenotype of the mutant response to the nitrate supply. Exogenous abscisic acid application inhibited more drastically primary root growth in the arm mutant but failed to repress lateral branching compared with the wild type. Cytokinin levels were higher in the arm root, but there were no changes in mitotic activity, suggesting that cytokinin is not directly involved in the mutant phenotype. Ethylene production was higher in arm but inversely proportional to the nitrate concentration in the medium. Interestingly, eto2 and eto3 ethylene overproduction mutants mimicked some of the conditional root characteristics of the arm mutant on high nitrate. Our data suggest that ethylene may be involved in the arm mutant phenotype, albeit indirectly, rather than functioning as a primary signal.

Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum

Cytokinins are hormones that are involved in various processes of plant growth and development. The model of cytokinin signalling starts with hormone perception through membrane-localized histidine kinase receptors. Although the biochemical properties and functions of these receptors have been extensively studied, there is no solid proof of their subcellular localization. Here, cell biological and biochemical evidence for the localization of functional fluorophor-tagged fusions of Arabidopsis histidine kinase 3 (AHK3) and 4 (AHK4), members of the cytokinin receptor family, in the endoplasmic reticulum (ER) is provided. Furthermore, membrane-bound AHK3 interacts with AHK4 in vivo. The ER localization and putative function of cytokinin receptors from the ER have major impacts on the concept of cytokinin perception and signalling, and hormonal cross-talk in plants.

Negative regulation of defence signalling pathways by the EDR1 protein kinase

The enhanced disease resistance 1 (edr1) mutant of Arabidopsis confers enhanced resistance to bacterial and fungal pathogens. To better understand how edr1-mediated resistance occurs, we performed transcriptome analyses on wild-type and edr1 plants inoculated with the fungal pathogen Golovinomyces cichoracearum (powdery mildew). The expression of many known and putative defence-associated genes was more rapidly induced, and to higher levels, in edr1 plants relative to the wild-type. Many of the genes with elevated expression encoded WRKY transcription factors and there was enrichment for their binding sites in promoters of the genes upregulated in edr1. Confocal microscopy of transiently expressed EDR1 protein showed that a significant fraction of EDR1 was localized to the nucleus, suggesting that EDR1 could potentially interact with transcription factors in the nucleus. Analysis of gene ontology annotations revealed that genes associated with the endomembrane system, defence, reactive oxygen species (ROS) production and protein kinases were induced early in the edr1 mutant, and that elevated expression of the endomembrane system, defence and ROS-related genes was maintained for at least 4 days after infection.

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.

An atypical heterotrimeric G-protein γ-subunit is involved in guard cell K⁺-channel regulation and morphological development in Arabidopsis thaliana

Currently, there are strong inconsistencies in our knowledge of plant heterotrimeric G-proteins that suggest the existence of additional members of the family. We have identified a new Arabidopsis G-protein γ-subunit (AGG3) that modulates morphological development and ABA-regulation of stomatal aperture. AGG3 strongly interacts with the Arabidopsis G-protein β-subunit in vivo and in vitro. Most importantly, AGG3-deficient mutants account for all but one of the 'orphan' phenotypes previously unexplained by the two known γ-subunits in Arabidopsis. AGG3 has unique characteristics never before observed in plant or animal systems, such as its size (more than twice that of canonical γ-subunits) and the presence of a C-terminal Cys-rich domain. AGG3 thus represent a novel class of G-protein γ-subunits, widely spread throughout the plant kingdom but not present in animals. Homologues of AGG3 in rice have been identified as important quantitative trait loci for grain size and yield, but due to the atypical nature of the proteins their identity as G-protein subunits was thus far unknown. Our work demonstrates a similar trend in seeds of Arabidopsis agg3 mutants, and implicates G-proteins in such a crucial agronomic trait. The discovery of this highly atypical subunit reinforces the emerging notion that plant and animal G-proteins have distinct as well as shared evolutionary pathways.

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.

Ethylene signalling is involved in regulation of phosphate starvation-induced gene expression and production of acid phosphatases and anthocyanin in Arabidopsis

• With the exception of root hair development, the role of the phytohormone ethylene is not clear in other aspects of plant responses to inorganic phosphate (Pi) starvation. • The induction of AtPT2 was used as a marker to find novel signalling components involved in plant responses to Pi starvation. Using genetic and chemical approaches, we examined the role of ethylene in the regulation of plant responses to Pi starvation. • hps2, an Arabidopsis mutant with enhanced sensitivity to Pi starvation, was identified and found to be a new allele of CTR1 that is a key negative regulator of ethylene responses. 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene, increases plant sensitivity to Pi starvation, whereas the ethylene perception inhibitor Ag+ suppresses this response. The Pi starvation-induced gene expression and acid phosphatase activity are also enhanced in the hps2 mutant, but suppressed in the ethylene-insensitive mutant ein2-5. By contrast, we found that ethylene signalling plays a negative role in Pi starvation-induced anthocyanin production. • These findings extend the roles of ethylene in the regulation of plant responses to Pi starvation and will help us to gain a better understanding of the molecular mechanism underlying these responses.

Cloning, expression, purification and preliminary X-ray analysis of the protein kinase domain of constitutive triple response 1 (CTR1) from Arabidopsis thaliana

Ethylene, a gaseous plant hormone, is perceived by a group of membrane-bound receptors. Constitutive triple response 1 (CTR1) from Arabidopsis thaliana directly interacts with ethylene receptors and thus links signal reception to the intracellular signalling pathway. The C-terminal protein kinase domain of CTR1 has been crystallized in its wild-type form and as a kinase-dead mutant. The wild-type crystals diffracted X-ray radiation to 3 Å resolution and the crystals of the kinase-dead mutant diffacted to 2.5 Å resolution. The crystals belonged to space groups P4(1)2(1)2 and P4(2)2(1)2, respectively, with two molecules per asymmetric unit in both cases.

Microarray analysis of the abscission-related transcriptome in the tomato flower abscission zone in response to auxin depletion

The abscission process is initiated by changes in the auxin gradient across the abscission zone (AZ) and is triggered by ethylene. Although changes in gene expression have been correlated with the ethylene-mediated execution of abscission, there is almost no information on the molecular and biochemical basis of the increased AZ sensitivity to ethylene. We examined transcriptome changes in the tomato (Solanum lycopersicum 'Shiran 1335') flower AZ during the rapid acquisition of ethylene sensitivity following flower removal, which depletes the AZ from auxin, with or without preexposure to 1-methylcyclopropene or application of indole-3-acetic acid after flower removal. Microarray analysis using the Affymetrix Tomato GeneChip revealed changes in expression, occurring prior to and during pedicel abscission, of many genes with possible regulatory functions. They included a range of auxin- and ethylene-related transcription factors, other transcription factors and regulatory genes that are transiently induced early, 2 h after flower removal, and a set of novel AZ-specific genes. All gene expressions initiated by flower removal and leading to pedicel abscission were inhibited by indole-3-acetic acid application, while 1-methylcyclopropene pretreatment inhibited only the ethylene-induced expressions, including those induced by wound-associated ethylene signals. These results confirm our hypothesis that acquisition of ethylene sensitivity in the AZ is associated with altered expression of auxin-regulated genes resulting from auxin depletion. Our results shed light on the regulatory control of abscission at the molecular level and further expand our knowledge of auxin-ethylene cross talk during the initial controlling stages of the process.

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.

Identification of novel inhibitors of 1-aminocyclopropane-1-carboxylic acid synthase by chemical screening in Arabidopsis thaliana

Ethylene is a gaseous hormone important for adaptation and survival in plants. To further understand the signaling and regulatory network of ethylene, we used a phenotype-based screening strategy to identify chemical compounds interfering with the ethylene response in Arabidopsis thaliana. By screening a collection of 10,000 structurally diverse small molecules, we identified compounds suppressing the constitutive triple response phenotype in the ethylene overproducer mutant eto1-4. The compounds reduced the expression of a reporter gene responsive to ethylene and the otherwise elevated level of ethylene in eto1-4. Structure and function analysis revealed that the compounds contained a quinazolinone backbone. Further studies with genetic mutants and transgenic plants involved in the ethylene pathway showed that the compounds inhibited ethylene biosynthesis at the step of converting S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Biochemical studies with in vitro activity assay and enzyme kinetics analysis indicated that a representative compound was an uncompetitive inhibitor of ACC synthase. Finally, global gene expression profiling uncovered a significant number of genes that were co-regulated by the compounds and aminoethoxyvinylglycine, a potent inhibitor of ACC synthase. The use of chemical screening is feasible in identifying small molecules modulating the ethylene response in Arabidopsis seedlings. The discovery of such chemical compounds will be useful in ethylene research and can offer potentially useful agrochemicals for quality improvement in post-harvest agriculture.

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.

FERONIA is a key modulator of brassinosteroid and ethylene responsiveness in Arabidopsis hypocotyls

Ethylene signaling is a complex pathway that has been intensively analyzed partly due to its importance to the manifestation of horticultural phenomena, including fruit ripening and tissue senescence. In order to further our understanding of how this pathway is regulated, a screen for Arabidopsis mutants with increased ethylene response was conducted. From this, a mutant was identified as having a dark-grown hypocotyl that is indistinguishable from Col-0 wt in the presence of the ethylene perception inhibitor AgNO₃, yet has extreme responsiveness to even low levels of ethylene. Map-based cloning of the mutation revealed a T-DNA insertion in the coding sequence of the receptor-like kinase FERONIA, which is required for normal pollen tube reception and cell elongation in a currently unknown capacity. In contrast to a previous report, analysis of our feronia knockout mutant shows it also has altered responsiveness to brassinosteroids, with etiolated fer-2 seedlings being partially brassinosteroid insensitive with regard to promotion of hypocotyl elongation. Our results indicate that FERONIA-dependent brassinosteroid response serves to antagonize the effect of ethylene on hypocotyl growth of etiolated seedlings, with loss of proper brassinosteroid signaling disrupting this balance and leading to a greater impact of ethylene on hypocotyl shortening.

Ethylene perception is involved in female cucumber flower development

It is well established that ethylene promotes female flower development in cucumber. However, little is known about how the gaseous hormone selectively affects female flowers, and what mechanism it uses. Previously, we found organ-specific DNA damage in the primordial anther of female cucumber flowers. This finding led to a hypothesis that ethylene might promote female flower development via the organ-specific induction of DNA damage in primordial anthers. In this study, we tested this hypothesis first by demonstrating ethylene induction of DNA damage via the ethylene signaling pathway using cucumber protoplasts. Then, using representative component genes of the ethylene signaling pathway as probes, we found that one of the ethylene receptors, CsETR1, was temporally and spatially downregulated in the stamens of stage-6 female cucumber flowers, especially along with the increase of the nodes. Furthermore, by constructing transgenic Arabidopsis plants with organ-specific expression of antisense CsETR1 under the control of an AP3 promoter to downregulate ETR1 expression in the stamens, we generated Arabidopsis 'female flowers', in which the abnormal stamens mimic those of female cucumber flowers. Our data suggest that ethylene perception is involved in the arrest of stamen development in female cucumber flowers through the induction of DNA damage. This opens up a novel perspective and approach to solve the half-century-long puzzle of how gaseous ethylene selectively promotes female flowers in the monoecious cucumber plant.

Evolution and phyletic distribution of two-component signal transduction systems

Two-component signal transduction systems are abundant in prokaryotes. They enable cells to adjust multiple cellular functions in response to changing environmental conditions. These systems are also found, although in much smaller numbers, in lower eukaryotes and plants, where they appear to control a few very specific functions. Two-component systems have evolved in Bacteria from much simpler one-component systems bringing about the benefit of extracellular versus intracellular sensing. We review reports establishing the origins of two-component systems and documenting their occurrence in major lineages of Life.

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.

The role of receptor interactions in regulating ethylene signal transduction

The phytohormone ethylene is perceived in Arabidopsis by a five-member receptor family. Earlier work has demonstrated that the basic functional unit for an ethylene receptor is a disulfide-linked homodimer. We recently reported in The Journal of Biological Chemistry that the ethylene-receptor ETR1 physically associates with other ethylene receptors through higher order interactions, suggesting the existence of receptor clusters. Here we consider the implications of such clusters upon the mechanism of ethylene signal transduction. In particular, we consider how such clustering provides a cooperative mechanism, akin to what has been found for the prokaryotic chemoreceptors, by which plant sensitivity to ethylene may be increased. In addition, we consider how the dominant ethylene insensitivity conferred by some receptor mutations, such as etr1-1, may also be propagated by interactions among members of the ethylene receptor family.

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.

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.

Virus-induced gene silencing reveals the involvement of ethylene-, salicylic acid- and mitogen-activated protein kinase-related defense pathways in the resistance of tomato to bacterial wilt

Bacterial wilt (BW), caused by Ralstonia solanacearum, is a devastating vascular disease of tomato worldwide. However, information on tomato's defense mechanism against infection by this soil-borne bacterium is limited. In this study, virus-induced gene silencing (VIGS) was employed to decipher signaling pathways involved in the resistance of tomato to this pathogen. Defined sequence fragments derived from a group of genes known or predicted to be involved in ethylene (ET) and salicylic acid (SA) signaling transduction pathways and mitogen-activated protein kinase (MAPK) cascades were subjected to VIGS in 'Hawaii 7996', a tomato cultivar with stable resistance to BW, and their effect on resistance was determined. The results indicated that silencing of ACO1/3, EIN2, ERF3, NPR1, TGA2.2, TGA1a, MKK2, MPK1/2 and MPK3 caused significant increase in bacterial proliferation in stembases and/or mid-stems. Partial wilting symptoms appeared on plants in which TGA2.2, TGA2.1a, MKK2 and MPK1/2 were silenced. These results suggested that ET-, SA- and MAPK-related defense signaling pathways are involved in the resistance of tomato to BW. This is the first report elucidating the multiple layers of defense governing the resistance of tomato to BW. The results are discussed to enlighten an important and complex interaction between tomato and a soil-borne vascular pathogen.