Ethylene responses are negatively regulated by a receptor gene family in Arabidopsis thaliana

Biochemical characterization of a putative cytokinin-responsive His-kinase, CKI1, from Arabidopsis thaliana

His-Asp phosphorelays are evolutionary-conserved powerful biological tactics for intracellular signal transduction. Such a phosphorelay is generally made up of "sensor histidine (His)-kinases", "response regulators", and "histidine-containing (HPt) phosphotransmitters". Results from recent intensive studies suggested that in the higher plant Arabidopsis thaliana, His-Asp phosphorelays may be widely used for propagating environmental stimuli, such as phytohormones (e.g., ethylene and cytokinin). In this study, we characterized, in vitro, the putative cytokinin-responsive CKI1 His-kinase, in terms of His-Asp phosphorelays. It was demonstrated for the first time that the receiver domain in this sensor exhibits a strong phosphohistidine phosphatase activity toward some Arabidopsis HPt phosphotransmitters (AHP1 and AHP2), suggesting the functional importance of the receiver domain for a resumed interaction of the sensor His-kinase with other His-Asp phosphorelay components.

The relationship between ethylene binding and dominant insensitivity conferred by mutant forms of the ETR1 ethylene receptor

Ethylene responses in Arabidopsis are mediated by a small family of receptors, including the ETR1 gene product. Specific mutations in the N-terminal ethylene-binding domain of any family member lead to dominant ethylene insensitivity. To investigate the mechanism of ethylene insensitivity, we examined the effects of mutations on the ethylene-binding activity of the ETR1 protein expressed in yeast. The etr1-1 and etr1-4 mutations completely eliminated ethylene binding, while the etr1-3 mutation severely reduced binding. Additional site-directed mutations that disrupted ethylene binding in yeast also conferred dominant ethylene insensitivity when the mutated genes were transferred into wild-type Arabidopsis plants. By contrast, the etr1-2 mutation did not disrupt ethylene binding in yeast. These results indicate that dominant ethylene insensitivity may be conferred by mutations that disrupt ethylene binding or that uncouple ethylene binding from signal output by the receptor. Increased dosage of wild-type alleles in triploid lines led to the partial recovery of ethylene sensitivity, indicating that dominant ethylene insensitivity may involve either interactions between wild-type and mutant receptors or competition between mutant and wild-type receptors for downstream effectors.

Regulation of differential growth in the apical hook of Arabidopsis

Arabidopsis seedlings develop a hook-like structure at the apical part of the hypocotyl when grown in darkness. Differential cell growth processes result in the curved hypocotyl hook. Time-dependent analyses of the hypocotyl showed that the apical hook is formed during an early phase of seedling growth and is maintained in a sequential phase by a distinct process. Based on developmental genetic analyses of hook-affected mutants, we show that the hookless mutants (hls1, cop2) are involved in an early aspect of hook development. From time-dependent analyses of ethylene-insensitive mutants, later steps in hook maintenance were found to be ethylene sensitive. Regulation of differential growth was further studied through examination of the spatial pattern of expression of two hormone-regulated genes: an ethylene biosynthetic enzyme and the ethylene receptor ETR1. Accumulation of mRNA for AtACO2, a novel ACC (1-aminocyclopropane-1-carboxylic acid) oxidase gene, occurred within cells predominantly located on the outer-side of the hook and was tightly correlated with ethylene-induced exaggeration in the curvature of the hook. ETR1 expression in the apical hook, however, was reduced by ethylene treatment. Based on the expression pattern of ETR1 and AtACO2 in the hook-affected mutants, a model for hook development and maintenance is proposed.

Ethylene perception and signaling: an evolutionary perspective

Ethylene signal transduction, as revealed by studies in Arabidopsis, provides an interesting example of how information-processing systems have evolved in plants. The ethylene signal is perceived by a family of receptors composed of structural elements that are characteristic of bacterial signaling proteins. In plants, these receptors transmit the signal by interacting with proteins that are eukaryotic in origin. The ethylene sensor domain of the receptors forms a membrane-associated structure that uses a copper cofactor to bind ethylene. This novel protein motif appears to have originated early in the evolution of photosynthetic organisms.

EIN2, a bifunctional transducer of ethylene and stress responses in Arabidopsis

Ethylene regulates plant growth, development, and responsiveness to a variety of stresses. Cloning of the Arabidopsis EIN2 gene identifies a central component of the ethylene signaling pathway. The amino-terminal integral membrane domain of EIN2 shows similarity to the disease-related Nramp family of metal-ion transporters. Expression of the EIN2 CEND is sufficient to constitutively activate ethylene responses and restores responsiveness to jasmonic acid and paraquat-induced oxygen radicals to mutant plants. EIN2 is thus recognized as a molecular link between previously distinct hormone response pathways. Plants may use a combinatorial mechanism for assessing various stresses by enlisting a common set of signaling molecules.

Involvement of plasma membrane proteins in plant defense responses. Analysis of the cryptogein signal transduction in tobacco

Cryptogein, a 98 amino acid protein secreted by the fungus Phytophthora cryptogea, induces a hypersensitive response and systemic acquired resistance in tobacco plants (Nicotiana tabacum var Xanthi). The mode of action of cryptogein has been studied using tobacco cell suspensions. The recognition of this elicitor by a plasma membrane receptor leads to a cascade of events including protein phosphorylation, calcium influx, potassium and chloride effluxes, plasma membrane depolarization, activation of a NADPH oxidase responsible for active oxygen species (AOS) production and cytosol acidification, activation of the pentose phosphate pathway, and activation of two mitogen-activated protein kinase (MAPK) homologues. The organization of the cryptogein responses reveals that the earliest steps of the signal transduction pathway involve plasma membrane activities. Their activation generates a complex network of second messengers which triggers the specific physiological responses. This study may contribute to our understanding of plant signaling processes because elicitors and a variety of signals including hormones, Nod factors, light, gravity and stresses share some common transduction elements and pathways.

GENETIC ANALYSIS OF HORMONE SIGNALING

Phytohormones influence many diverse developmental processes ranging from seed germination to root, shoot, and flower formation. Recently, mutational analysis using the model plant Arabidopsis thaliana has been instrumental in determining the individual components of specific hormone signal transduction pathways. Moreover, epistasis and suppressor studies are beginning to explain how these genes and their products relate to one another. While no hormone transduction pathway is completely understood, the genes identified to date suggest that simple molecular rules can be established to explain how plant hormone signals are transduced. This review describes some of the shared characteristics of plant hormone signal transduction pathways and the properties for informational transfer common to many of the genes that specify the transduction of the signal.

Stage- and tissue-specific expression of ethylene receptor homolog genes during fruit development in muskmelon

We isolated two muskmelon (Cucumis melo) cDNA homologs of the Arabidopsis ethylene receptor genes ETR1 and ERS1 and designated them Cm-ETR1 (C. melo ETR1; accession no. AF054806) and Cm-ERS1 (C. melo ERS1; accession no. AF037368), respectively. Northern analysis revealed that the level of Cm-ERS1 mRNA in the pericarp increased in parallel with the increase in fruit size and then markedly decreased at the end of enlargement. In fully enlarged fruit the level of Cm-ERS1 mRNA was low in all tissues, whereas that of Cm-ETR1 mRNA was very high in the seeds and placenta. During ripening Cm-ERS1 mRNA increased slightly in the pericarp of fruit before the marked increase of Cm-ETR1 mRNA paralleled climacteric ethylene production. These results indicate that both Cm-ETR1 and Cm-ERS1 play specific roles not only in ripening but also in the early development of melon fruit and that they have distinct roles in particular fruit tissues at particular developmental stages.

Differential expression of two novel members of the tomato ethylene-receptor family

The phytohormone ethylene regulates many aspects of plant growth, development, and environmental responses. Much of the developmental regulation of ethylene responses in tomato (Lycopersicon esculentum) occurs at the level of hormone sensitivity. In an effort to understand the regulation of ethylene responses, we isolated and characterized tomato genes with sequence similarity to the Arabidopsis ETR1 (ethylene response 1) ethylene receptor. Previously, we isolated three genes that exhibit high similarity to ETR1 and to each other. Here we report the isolation of two additional genes, LeETR4 and LeETR5, that are only 42% and 40% identical to ETR1, respectively. Although the amino acids known to be involved in ethylene binding are conserved, LeETR5 lacks the histidine within the kinase domain that is predicted to be phosphorylated. This suggests that histidine kinase activity is not necessary for an ethylene response, because mutated forms of both LeETR4 and LeETR5 confer dominant ethylene insensitivity in transgenic Arabidopsis plants. Expression analysis indicates that LeETR4 accounts for most of the putative ethylene-receptor mRNA present in reproductive tissues, but, like LeETR5, it is less abundant in vegetative tissues. Taken together, ethylene perception in tomato is potentially quite complex, with at least five structurally divergent, putative receptor family members exhibiting significant variation in expression levels throughout development.

RESPONSIVE-TO-ANTAGONIST1, a Menkes/Wilson disease-related copper transporter, is required for ethylene signaling in Arabidopsis

Ethylene is an important regulator of plant growth. We identified an Arabidopsis mutant, responsive-to-antagonist1 (ran1), that shows ethylene phenotypes in response to treatment with trans-cyclooctene, a potent receptor antagonist. Genetic epistasis studies revealed an early requirement for RAN1 in the ethylene pathway. RAN1 was cloned and found to encode a protein with similarity to copper-transporting P-type ATPases, including the human Menkes/Wilson proteins and yeast Ccc2p. Expression of RAN1 complemented the defects of a ccc2delta mutant, demonstrating its function as a copper transporter. Transgenic CaMV 35S::RAN1 plants showed constitutive expression of ethylene responses, due to cosuppression of RAN1. These results provide an in planta demonstration that ethylene signaling requires copper and reveal that RAN1 acts by delivering copper to create functional hormone receptors.

Regulation of soybean nodulation independent of ethylene signaling

Leguminous plants regulate the number of Bradyrhizobium- or Rhizobium-infected sites that develop into nitrogen-fixing root nodules. Ethylene has been implicated in the regulation of nodule formation in some species, but this role has remained in question for soybean (Glycine max). The present study used soybean mutants with decreased responsiveness to ethylene, soybean mutants with defective regulation of nodule number, and Ag+ inhibition of ethylene perception to examine the role of ethylene in the regulation of nodule number. Nodule numbers on ethylene-insensitive mutants and plants treated with Ag+ were similar to those on wild-type plants and untreated plants, respectively. Hypernodulating mutants displayed wild-type ethylene sensitivity. Suppression of nodule numbers by high nitrate was also similar between ethylene-insensitive plants, wild-type plants, and plants treated with Ag+. Ethylene insensitivity of the roots of etr1-1 mutants was confirmed using assays for sensitivity to 1-aminocyclopropane-1-carboxylic acid and for ethylene-stimulated root-hair formation. Additional phenotypes of etr1-1 roots were also characterized. Ethylene-dependent pathways regulate the number of nodules that form on species such as pea and Medicago truncatula, but our data indicate that ethylene is less significant in regulating the number of nodules that form on soybean.

The ethylene gas signal transduction pathway: a molecular perspective

The gaseous hormone ethylene induces diverse effects in plants throughout their life cycle. Ethylene response is regulated at multiple levels, from hormone synthesis and perception to signal transduction and transcriptional regulation. As more genes in the ethylene response pathway are cloned and characterized, they illustrate the precision with which signaling can be controlled. Wounding, pathogenic attack, flooding, fruit ripening, development, senescence, and ethylene treatment itself induce ethylene production. Ethylene binding to receptors with homology to two-component regulators triggers a kinase cascade that is propagated through the CTR1 Raf-like kinase and other components to the nucleus. Activation of the EIN3 family of nuclear proteins leads to induction of the relevant ethylene-responsive genes via other transcription factors, eliciting a response appropriate to the original stimulus.

Taking the plunge. Terminal differentiation in Dictyostelium

During the last stage of Dictyostelium development a motile, cylindrical slug transforms into an immotile, stalked fruiting body and the constituent cells change from amoebae to either refractile spores or vacuolated stalk cells. Analysis of this process using genetics and simple culture techniques is becoming a powerful way of investigating a number of conserved signal transduction processes. A common pathway activating cAMP-dependent protein kinase (PKA) triggers the maturation of spore cells and those stalk cells forming the stalk. It uses a eukaryotic version of the 'bacterial' two-component phospho-relay system to control cAMP breakdown. A second pathway, inhibiting the GSK3 protein kinase, might control the maturation of a distinct set of stalk cells at the base of the fruiting body.

Receptors and signalling components of plant hormones

Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.

Ethylene signalling: redundant receptors all have their say

In Arabidopsis, the hormone ethylene is sensed by five related receptors, all of the 'two-component' variety. The receptors constitutively suppress a downstream signalling pathway, and are inactivated by ethylene, leading to the activation of genes necessary for the various ethylene-regulated biological responses.

Nuclear events in ethylene signaling: a transcriptional cascade mediated by ETHYLENE-INSENSITIVE3 and ETHYLENE-RESPONSE-FACTOR1

Response to the gaseous plant hormone ethylene in Arabidopsis requires the EIN3/EIL family of nuclear proteins. The biochemical function(s) of EIN3/EIL proteins, however, has remained unknown. In this study, we show that EIN3 and EILs comprise a family of novel sequence-specific DNA-binding proteins that regulate gene expression by binding directly to a primary ethylene response element (PERE) related to the tomato E4-element. Moreover, we identified an immediate target of EIN3, ETHYLENE-RESPONSE-FACTOR1 (ERF1), which contains this element in its promoter. EIN3 is necessary and sufficient for ERF1 expression, and, like EIN3-overexpression in transgenic plants, constitutive expression of ERF1 results in the activation of a variety of ethylene response genes and phenotypes. Evidence is also provided that ERF1 acts downstream of EIN3 and all other components of the ethylene signaling pathway. The results demonstrate that the nuclear proteins EIN3 and ERF1 act sequentially in a cascade of transcriptional regulation initiated by ethylene gas.

Ethylene gas: perception, signaling and response

During the last decade a genetic approach based on the Arabidopsis 'triple response' to the hormone ethylene has allowed the identification of numerous components of the signal transduction pathway. Cloning of the genes and biochemical analysis of the proteins that they encode are uncovering the molecular mechanisms that allow a plant cell to perceive and respond to this gaseous regulator of plant growth/stress responses.

The ethylene-receptor family from Arabidopsis: structure and function

The gaseous hormone ethylene regulates many aspects of plant growth and development. Ethylene is perceived by a family of high-affinity receptors typified by the ETR1 protein from Arabidopsis. The ETR1 gene codes for a protein which contains a hydrophobic N-terminal domain that binds ethylene and a C-terminal domain that is related in sequence to histidine kinase-response regulator two-component signal transducers found in bacteria. A structural model for the ethylene-binding domain is presented in which a Cu(I) ion is coordinated within membrane-spanning alpha-helices of the hydrophobic domain. It is proposed that binding of ethylene to the transition metal would induce a conformational change in the sensor domain that would be propagated to the cytoplasmic transmitter domain of the protein. A total of four additional genes that are related in sequence to ETR1 have been identified in Arabidopsis. Specific missense mutations in any one of the five genes leads to ethylene insensitivity in planta. Models for signal transduction that can account for the genetic dominance of these mutations are discussed.