Autophosphorylation activity of the Arabidopsis ethylene receptor multigene family

Non-Canonical Inter-Protein Interactions of Key Proteins Belonging to Cytokinin Signaling Pathways

The multistep phosphorelay (MSP) is a conserved signaling system that allows plants to sense and respond to a variety of cues under rapidly changing environmental conditions. The MSP system comprises three main protein types: sensor histidine kinases, phosphotransmitters, and response regulators. There are numerous signaling pathways that use, in whole or in part, this set of proteins to transduce diverse signals. Among them, the cytokinin signal transduction system is the best-studied pathway, which utilizes the entire MSP cascade. Focusing on this system, we review here protein-protein interaction of MSP components that are not directly related to cytokinin signaling. These interactions are likely to play an essential role in hormonal crosstalk and may be promising targets for fine-tuning plant development. In addition, in light of recent advances in the study of cytokinin signaling, we discuss new insights into the putative molecular mechanisms that mediate the pleiotropic action of cytokinins and provide specificity for distinct MSP signals. A detailed network of known non-canonical protein-protein interactions related to cytokinin signaling was demonstrated.

Subcellular dynamics of ethylene signaling drive plant plasticity to growth and stress: Spatiotemporal control of ethylene signaling in Arabidopsis

Volatile compounds, such as nitric oxide and ethylene gas, play a vital role as signaling molecules in organisms. Ethylene is a plant hormone that regulates a wide range of plant growth, development, and responses to stress and is perceived by a family of ethylene receptors that localize in the endoplasmic reticulum. Constitutive Triple Response 1 (CTR1), a Raf-like protein kinase and a key negative regulator for ethylene responses, tethers to the ethylene receptors, but undergoes nuclear translocation upon activation of ethylene signaling. This ER-to-nucleus trafficking transforms CTR1 into a positive regulator for ethylene responses, significantly enhancing stress resilience to drought and salinity. The nuclear trafficking of CTR1 demonstrates that the spatiotemporal control of ethylene signaling is essential for stress adaptation. Understanding the mechanisms governing the spatiotemporal control of ethylene signaling elements is crucial for unraveling the system-level regulatory mechanisms that collectively fine-tune ethylene responses to optimize plant growth, development, and stress adaptation.

Phased grapevine genome sequence of an Rpv12 carrier for biotechnological exploration of resistance to Plasmopara viticola

The downy mildew disease caused by the oomycete Plasmopara viticola is a serious threat for grapevine and can cause enormous yield losses in viticulture. The quantitative trait locus Rpv12, mediating resistance against P. viticola, was originally found in Asian Vitis amurensis. This locus and its genes were analyzed here in detail. A haplotype-separated genome sequence of the diploid Rpv12-carrier Gf.99-03 was created and annotated. The defense response against P. viticola was investigated in an infection time-course RNA-seq experiment, revealing approximately 600 upregulated Vitis genes during host-pathogen interaction. The Rpv12 regions of the resistance and the sensitivity encoding Gf.99-03 haplotype were structurally and functionally compared with each other. Two different clusters of resistance-related genes were identified within the Rpv12 locus. One cluster carries a set of four differentially expressed genes with three ACCELERATED CELL DEATH 6-like genes. The other cluster carries a set of six resistance gene analogs related to qualitative pathogen resistance. The Rpv12 locus and its candidate genes for P. viticola resistance provide a precious genetic resource for P. viticola resistance breeding. Newly developed co-segregating simple sequence repeat markers in close proximity to the R-genes enable its improved applicability in marker-assisted grapevine breeding.

Impacts of DNA methylases and demethylases on the methylation and expression of Arabidopsis ethylene signal pathway genes

Arabidopsis ethylene (ET) signal pathway plays important roles in various aspects. Cytosine DNA methylation is significant in controlling gene expression in plants. Here, we analyzed the bisulfite sequencing and mRNA sequencing data from Arabidopsis (de)methylase mutants met1, cmt3, drm1/2, ddm1, ros1-4, and rdd to investigate how DNA (de)methylases influence the DNA methylation and expression of Arabidopsis ET pathway genes. At least 32 genes are found to involved in Arabidopsis ET pathway by text mining. Among them, 14 genes are unmethylated or methylated with very low levels. ACS6 and ACS9 are conspicuously methylated within their upstream regions. The other 16 genes are predominantly methylated at the CG sites within gene body regions in wild-type plants, and mutation of MET1 resulted in almost entire elimination of the CG methylations. In addition, CG methylations within some genes are jointly maintained by MET1 and other (de)methylases. Analyses of mRNA-seq data indicated that some ET pathway genes were differentially expressed between wild-type and diverse mutants. PDF1.2, the marker gene of ET signal pathway, was found being regulated indirectly by the methylases. Eighty-two transposable elements (TEs) were identified to be associated to 15 ET pathway genes. ACS11 is found located in a heterochromatin region that contains 57 TEs, indicating its specific expression and regulation. Together, our results suggest that DNA (de)methylases are implicated in the regulation of CG methylation within gene body regions and transcriptional activity of some ET pathway genes and that maintenance of normal CG methylation is essential for ET pathway in Arabidopsis.

To have or not to have: expression of amino acid transporters during pathogen infection

The interaction between plants and plant pathogens can have significant effects on ecosystem performance. For their growth and development, both bionts rely on amino acids. While amino acids are key transport forms of nitrogen and can be directly absorbed from the soil through specific root amino acid transporters, various pathogenic microbes can invade plant tissues to feed on different plant amino acid pools. In parallel, plants may initiate an immune response program to restrict this invasion, employing various amino acid transporters to modify the amino acid pool at the site of pathogen attack. The interaction between pathogens and plants is sophisticated and responses are dynamic. Both avail themselves of multiple tools to increase their chance of survival. In this review, we highlight the role of amino acid transporters during pathogen infection. Having control over the expression of those transporters can be decisive for the fate of both bionts but the underlying mechanism that regulates the expression of amino acid transporters is not understood to date. We provide an overview of the regulation of a variety of amino acid transporters, depending on interaction with biotrophic, hemibiotrophic or necrotrophic pathogens. In addition, we aim to highlight the interplay of different physiological processes on amino acid transporter regulation during pathogen attack and chose the LYSINE HISTIDINE TRANSPORTER1 (LHT1) as an example.

Phosphoproteome analysis reveals the involvement of protein dephosphorylation in ethylene-induced corolla senescence in petunia

BACKGROUND

Senescence represents the last stage of flower development. Phosphorylation is the key posttranslational modification that regulates protein functions, and kinases may be more required than phosphatases during plant growth and development. However, little is known about global phosphorylation changes during flower senescence.

RESULTS

In this work, we quantitatively investigated the petunia phosphoproteome following ethylene or air treatment. In total, 2170 phosphosites in 1184 protein groups were identified, among which 2059 sites in 1124 proteins were quantified. To our surprise, treatment with ethylene resulted in 697 downregulated and only 117 upregulated phosphosites using a 1.5-fold threshold (FDR < 0.05), which showed that ethylene negatively regulates global phosphorylation levels and that phosphorylation of many proteins was not necessary during flower senescence. Phosphoproteome analysis showed that ethylene regulates ethylene and ABA signalling transduction pathways via phosphorylation levels. One of the major targets of ethylene-induced dephosphorylation is the plant mRNA splicing machinery, and ethylene treatment increases the number of alternative splicing events of precursor RNAs in petunia corollas.

CONCLUSIONS

Protein dephosphorylation could play an important role in ethylene-induced senescence, and ethylene treatment increased the number of AS precursor RNAs in petunia corollas.

Histidine Kinases: Diverse Functions in Plant Development and Responses to Environmental Conditions

The two-component system (TCS), which is one of the most evolutionarily conserved signaling pathway systems, has been known to regulate multiple biological activities and environmental responses in plants. Significant progress has been made in characterizing the biological functions of the TCS components, including signal receptor histidine kinase (HK) proteins, signal transducer histidine-containing phosphotransfer proteins, and effector response regulator proteins. In this review, our scope is focused on the diverse structure, subcellular localization, and interactions of the HK proteins, as well as their signaling functions during development and environmental responses across different plant species. Based on data collected from scientific studies, knowledge about acting mechanisms and regulatory roles of HK proteins is presented. This comprehensive summary ofthe HK-related network provides a panorama of sophisticated modulating activities of HK members and gaps in understanding these activities, as well as the basis for developing biotechnological strategies to enhance the quality of crop plants.

Ethylene signaling in rice and Arabidopsis: New regulators and mechanisms

Ethylene is a gaseous hormone which plays important roles in both plant growth and development and stress responses. Based on studies in the dicot model plant species Arabidopsis, a linear ethylene signaling pathway has been established, according to which ethylene is perceived by ethylene receptors and transduced through CONSTITUTIVE TRIPLE RESPONSE 1 (CTR1) and ETHYLENE-INSENSITIVE 2 (EIN2) to activate transcriptional reprogramming. In addition to this canonical signaling pathway, an alternative ethylene receptor-mediated phosphor-relay pathway has also been proposed to participate in ethylene signaling. In contrast to Arabidopsis, rice, a monocot, grows in semiaquatic environments and has a distinct plant structure. Several novel regulators and/or mechanisms of the rice ethylene signaling pathway have recently been identified, indicating that the ethylene signaling pathway in rice has its own unique features. In this review, we summarize the latest progress and compare the conserved and divergent aspects of the ethylene signaling pathway between Arabidopsis and rice. The crosstalk between ethylene and other plant hormones is also reviewed. Finally, we discuss how ethylene regulates plant growth, stress responses and agronomic traits. These analyses should help expand our knowledge of the ethylene signaling mechanism and could further be applied for agricultural purposes.

The Interaction Network and Signaling Specificity of Two-Component System in Arabidopsis

Two-component systems (TCS) in plants have evolved into a more complicated multi-step phosphorelay (MSP) pathway, which employs histidine kinases (HKs), histidine-containing phosphotransfer proteins (HPts), and response regulators (RRs) to regulate various aspects of plant growth and development. How plants perceive the external signals, then integrate and transduce the secondary signals specifically to the desired destination, is a fundamental characteristic of the MSP signaling network. The TCS elements involved in the MSP pathway and molecular mechanisms of signal transduction have been best understood in the model plant Arabidopsis thaliana. In this review, we focus on updated knowledge on TCS signal transduction in Arabidopsis. We first present a brief description of the TCS elements; then, the protein–protein interaction network is established. Finally, we discuss the possible molecular mechanisms involved in the specificity of the MSP signaling at the mRNA and protein levels.

Ethylene signaling in plants

Ethylene is a gaseous phytohormone and the first of this hormone class to be discovered. It is the simplest olefin gas and is biosynthesized by plants to regulate plant development, growth, and stress responses via a well-studied signaling pathway. One of the earliest reported responses to ethylene is the triple response. This response is common in eudicot seedlings grown in the dark and is characterized by reduced growth of the root and hypocotyl, an exaggerated apical hook, and a thickening of the hypocotyl. This proved a useful assay for genetic screens and enabled the identification of many components of the ethylene-signaling pathway. These components include a family of ethylene receptors in the membrane of the endoplasmic reticulum (ER); a protein kinase, called constitutive triple response 1 (CTR1); an ER-localized transmembrane protein of unknown biochemical activity, called ethylene-insensitive 2 (EIN2); and transcription factors such as EIN3, EIN3-like (EIL), and ethylene response factors (ERFs). These studies led to a linear model, according to which in the absence of ethylene, its cognate receptors signal to CTR1, which inhibits EIN2 and prevents downstream signaling. Ethylene acts as an inverse agonist by inhibiting its receptors, resulting in lower CTR1 activity, which releases EIN2 inhibition. EIN2 alters transcription and translation, leading to most ethylene responses. Although this canonical pathway is the predominant signaling cascade, alternative pathways also affect ethylene responses. This review summarizes our current understanding of ethylene signaling, including these alternative pathways, and discusses how ethylene signaling has been manipulated for agricultural and horticultural applications.

Histidine kinase MHZ1/OsHK1 interacts with ethylene receptors to regulate root growth in rice

Ethylene plays essential roles during adaptive responses to water-saturating environments in rice, but knowledge of its signaling mechanism remains limited. Here, through an analysis of a rice ethylene-response mutant mhz1, we show that MHZ1 positively modulates root ethylene responses. MHZ1 encodes the rice histidine kinase OsHK1. MHZ1/OsHK1 is autophosphorylated at a conserved histidine residue and can transfer the phosphoryl signal to the response regulator OsRR21 via the phosphotransfer proteins OsAHP1/2. This phosphorelay pathway is required for root ethylene responses. Ethylene receptor OsERS2, via its GAF domain, physically interacts with MHZ1/OsHK1 and inhibits its kinase activity. Genetic analyses suggest that MHZ1/OsHK1 acts at the level of ethylene perception and works together with the OsEIN2-mediated pathway to regulate root growth. Our results suggest that MHZ1/OsHK1 mediates the ethylene response partially independently of OsEIN2, and is directly inhibited by ethylene receptors, thus revealing mechanistic details of ethylene signaling for root growth regulation.

An evolutionarily conserved iron-sulfur cluster underlies redox sensory function of the Chloroplast Sensor Kinase

Photosynthetic efficiency depends on equal light energy conversion by two spectrally distinct, serially-connected photosystems. The redox state of the plastoquinone pool, located between the two photosystems, is a key regulatory signal that initiates acclimatory changes in the relative abundance of photosystems. The Chloroplast Sensor Kinase (CSK) links the plastoquinone redox signal with photosystem gene expression but the mechanism by which it monitors the plastoquinone redox state is unclear. Here we show that the purified Arabidopsis and Phaeodactylum CSK and the cyanobacterial CSK homologue, Histidine kinase 2 (Hik2), are iron-sulfur proteins. The Fe-S cluster of CSK is further revealed to be a high potential redox-responsive [3Fe-4S] center. CSK responds to redox agents with reduced plastoquinone suppressing its autokinase activity. Redox changes within the CSK iron-sulfur cluster translate into conformational changes in the protein fold. These results provide key insights into redox signal perception and propagation by the CSK-based chloroplast two-component system.

A High-Throughput Strategy for Recombinant Protein Expression and Solubility Screen in Escherichia coli : A Case of Sensor Histidine Kinase

Determining conditions optimal for host growth, maximal protein yield, and lysis buffer composition is of critical importance for the efficient purification of soluble and well-folded recombinant proteins suitable for functional and/or structural studies. Small-scale optimization of conditions for protein production and stability saves time, labor, and costs. Here we describe a protocol for quick protein production and solubility screen using TissueLyser II system from Qiagen enabling simultaneous processing of 96 protein samples, with application to recombinant proteins encompassing two intracellular domains of ethylene-recognizing sensor histidine kinase ETHYLENE RESPONSE1 (ETR1) from Arabidopsis thaliana. We demonstrate that conditions for expression and cell lysis found in our small-scale screen allow successful large-scale production of pure and functional domains of sensor histidine kinase, providing a strategy potentially transferable to other similar catalytic domains.

ETR1 Integrates Response to Ethylene and Cytokinins into a Single Multistep Phosphorelay Pathway to Control Root Growth

Cytokinins and ethylene control plant development via sensors from the histidine kinase (HK) family. However, downstream signaling pathways for the key phytohormones are distinct. Here we report that not only cytokinin but also ethylene is able to control root apical meristem (RAM) size through activation of the multistep phosphorelay (MSP) pathway. We found that both cytokinin and ethylene-dependent RAM shortening requires ethylene binding to ETR1 and the HK activity of ETR1. The receiver domain of ETR1 interacts with MSP signaling intermediates acting downstream of cytokinin receptors, further substantiating the role of ETR1 in MSP signaling. We revealed that both cytokinin and ethylene induce the MSP in similar and distinct cell types with ETR1-mediated ethylene signaling controlling MSP output specifically in the root transition zone. We identified members of the MSP pathway specific and common to both hormones and showed that ETR1-regulated ARR3 controls RAM size. ETR1-mediated MSP spatially differs from canonical CTR1/EIN2/EIN3 ethylene signaling and is independent of EIN2, indicating that both pathways can be spatially and functionally separated. Furthermore, we demonstrated that canonical ethylene signaling controls MSP responsiveness to cytokinin specifically in the root transition zone, presumably via regulation of ARR10, one of the positive regulators of MSP signaling in Arabidopsis.

Genome-wide identification and interactome analysis of members of two-component system in Banana

BACKGROUND

Ethylene signal transduction in plants is conducted by the two-component system (TCS) which consists of histidine kinase (HK), histidine phosphotransferase (HPT) and response regulators (RRs). This system plays an important role in signal transduction during various cellular processes, including fruit ripening and response to multiple environmental cues. Though members of TCS have been identified in a few plants, no detailed analysis has been carried out in banana.

RESULTS

Through genome-wide analysis, we identified a total of 80 (25 HK, 10 HPT and 45 RR) and 72 (25 HK, 5 HPT and 42 RR) TCS genes in Musa acuminata and Musa balbisiana respectively. The analysis of identified genes revealed that most of the genes are highly conserved however; there are subtle divergences among various members. Comparative expression analysis revealed an involvement of a set of TCS members during banana fruit ripening. Co-expression network analysis identified a working TCS module with direct interactions of HK-HPT and RR members. The molecular dynamics analysis of TCS module showed a significant change in structural trajectories of TCS proteins in the presence of ethylene. Analysis suggests possible interactions between the HK-HPTs and RRs as well as other members leading to banana fruit ripening.

CONCLUSIONS

In this study, we identified and compared the members of TCS gene family in two banana species and showed their diversity, within groups on the basis of whole-genome duplication events. Our analysis showed that during banana fruit ripening TCS module plays a crucial role. We also demonstrated a possible interaction mechanism of TCS proteins in the presence and absence of ethylene by molecular dynamics simulations. These findings will help in understanding the functional mechanism of TCS proteins in plants in different conditions.

Diversification of cytokinin phosphotransfer signaling genes in Medicago truncatula and other legume genomes

BACKGROUND

Legumes can establish on nitrogen-deprived soils a symbiotic interaction with Rhizobia bacteria, leading to the formation of nitrogen-fixing root nodules. Cytokinin phytohormones are critical for triggering root cortical cell divisions at the onset of nodule initiation. Cytokinin signaling is based on a Two-Component System (TCS) phosphorelay cascade, involving successively Cytokinin-binding Histidine Kinase receptors, phosphorelay proteins shuttling between the cytoplasm and the nucleus, and Type-B Response Regulator (RRB) transcription factors activating the expression of cytokinin primary response genes. Among those, Type-A Response Regulators (RRA) exert a negative feedback on the TCS signaling. To determine whether the legume plant nodulation capacity is linked to specific features of TCS proteins, a genome-wide identification was performed in six legume genomes (Cajanus cajan, pigeonpea; Cicer arietinum, chickpea; Glycine max, soybean; Phaseolus vulgaris, common bean; Lotus japonicus; Medicago truncatula). The diversity of legume TCS proteins was compared to the one found in two non-nodulating species, Arabidopsis thaliana and Vitis vinifera, which are references for functional analyses of TCS components and phylogenetic analyses, respectively.

RESULTS

A striking expansion of non-canonical RRBs was identified, notably leading to the emergence of proteins where the conserved phosphor-accepting aspartate residue is replaced by a glutamate or an asparagine. M. truncatula genome-wide expression datasets additionally revealed that only a limited subset of cytokinin-related TCS genes is highly expressed in different organs, namely MtCHK1/MtCRE1, MtHPT1, and MtRRB3, suggesting that this "core" module potentially acts in most plant organs including nodules.

CONCLUSIONS

Further functional analyses are required to determine the relevance of these numerous non-canonical TCS RRBs in symbiotic nodulation, as well as of canonical MtHPT1 and MtRRB3 core signaling elements.

Canonical and noncanonical ethylene signaling pathways that regulate Arabidopsis susceptibility to the cyst nematode Heterodera schachtii

Plant-parasitic cyst nematodes successfully exploit various phytohormone signaling pathways to establish a new hormonal equilibrium that facilitates nematode parasitism. Although it is largely accepted that ethylene regulates plant responses to nematode infection, a mechanistic understanding of how ethylene shapes plant-nematode interactions remains largely unknown. In this study, we examined the involvement of various components regulating ethylene perception and signaling in establishing Arabidopsis susceptibility to the cyst nematode Heterodera schachtii using a large set of well-characterized single and higher order mutants. Our analyses revealed the existence of two pathways that separately engage ethylene with salicylic acid (SA) and cytokinin signaling during plant response to nematode infection. One pathway involves the canonical ethylene signaling pathway in which activation of ethylene signaling results in suppression of SA-based immunity. The second pathway involves the ethylene receptor ETR1, which signals independently of SA acid to affect immunity, instead altering cytokinin-mediated regulation of downstream components. Our results reveal important mechanisms through which cyst nematodes exploit components of ethylene perception and signaling to affect the balance of hormonal signaling through ethylene interaction with SA and cytokinin networks. This hormonal interaction overcomes plant defense and provokes a susceptible response.

Molecular Analysis of Protein-Protein Interactions in the Ethylene Pathway in the Different Ethylene Receptor Subfamilies

Signal perception and transmission of the plant hormone ethylene are mediated by a family of receptor histidine kinases located at the Golgi-ER network. Similar to bacterial and other plant receptor kinases, these receptors work as dimers or higher molecular weight oligomers at the membrane. Sequence analysis and functional studies of different isoforms suggest that the ethylene receptor family is classified into two subfamilies. In Arabidopsis, the type-I subfamily has two members (ETR1 and ERS1) and the type-II subfamily has three members (ETR2, ERS2, and EIN4). Whereas subfamily-I of the Arabidopsis receptors and their interactions with downstream elements in the ethylene pathway has been extensively studied in the past; related information on subfamily-II is sparse. In order to dissect the role of type-II receptors in the ethylene pathway and to decode processes associated with this receptor subfamily on a quantitative molecular level, we have applied biochemical and spectroscopic studies on purified recombinant receptors and downstream elements of the ethylene pathway. To this end, we have expressed purified ETR2 as a prototype of the type-II subfamily, ETR1 for the type-I subfamily and downstream ethylene pathway proteins CTR1 and EIN2. Functional folding of the purified receptors was demonstrated by CD spectroscopy and autokinase assays. Quantitative analysis of protein-protein interactions (PPIs) by microscale thermophoresis (MST) revealed that ETR2 has similar affinities for CTR1 and EIN2 as previously reported for the subfamily-I prototype ETR1 suggesting similar roles in PPI-mediated signal transfer for both subfamilies. We also used in planta fluorescence studies on transiently expressed proteins in Nicotiana benthamiana leaf cells to analyze homo- and heteromer formation of receptors. These studies show that type-II receptors as well as the type-I receptors form homo- and heteromeric complexes at these conditions. Notably, type-II receptor homomers and type-II:type-I heteromers are more stable than type-I homomers as indicated by their lower dissociation constants obtained in microscale thermophoresis studies. The enhanced stability of type-II complexes emphasizes the important role of type-II receptors in the ethylene pathway.

Different Pathogen Defense Strategies in Arabidopsis: More than Pathogen Recognition

Plants constantly suffer from simultaneous infection by multiple pathogens, which can be divided into biotrophic, hemibiotrophic, and necrotrophic pathogens, according to their lifestyles. Many studies have contributed to improving our knowledge of how plants can defend against pathogens, involving different layers of defense mechanisms. In this sense, the review discusses: (1) the functions of PAMP (pathogen-associated molecular pattern)-triggered immunity (PTI) and effector-triggered immunity (ETI), (2) evidence highlighting the functions of salicylic acid (SA) and jasmonic acid (JA)/ethylene (ET)-mediated signaling pathways downstream of PTI and ETI, and (3) other defense aspects, including many novel small molecules that are involved in defense and phenomena, including systemic acquired resistance (SAR) and priming. In particular, we mainly focus on SA and (JA)/ET-mediated signaling pathways. Interactions among them, including synergistic effects and antagonistic effects, are intensively explored. This might be critical to understanding dynamic disease regulation.

A role for two-component signaling elements in the Arabidopsis growth recovery response to ethylene

Previous studies indicate that the ability of Arabidopsis seedlings to recover normal growth following an ethylene treatment involves histidine kinase activity of the ethylene receptors. As histidine kinases can function as inputs for a two-component signaling system, we examined loss-of-function mutants involving two-component signaling elements. We find that mutants of phosphotransfer proteins and type-B response regulators exhibit a defect in their ethylene growth recovery response similar to that found with the loss-of-function ethylene receptor mutant etr1-7. The ability of two-component signaling elements to regulate the growth recovery response to ethylene functions independently from their well-characterized role in cytokinin signaling, based on the analysis of cytokinin receptor mutants as well as following chemical inhibition of cytokinin biosynthesis. Histidine kinase activity of the receptor ETR1 also facilitates growth recovery in the ethylene hypersensitive response, which is characterized by a transient decrease in growth rate when seedlings are treated continuously with a low dose of ethylene; however, this response was found to operate independently of the type-B response regulators. These results indicate that histidine kinase activity of the ethylene receptor ETR1 performs two independent functions: (a) regulating the growth recovery to ethylene through a two-component signaling system involving phosphotransfer proteins and type-B response regulators and (b) regulating the hypersensitive response to ethylene in a type-B response regulator independent manner.

Arabidopsis CPR5 regulates ethylene signaling via molecular association with the ETR1 receptor

The plant hormone ethylene plays various functions in plant growth, development and response to environmental stress. Ethylene is perceived by membrane-bound ethylene receptors, and among the homologous receptors in Arabidopsis, the ETR1 ethylene receptor plays a major role. The present study provides evidence demonstrating that Arabidopsis CPR5 functions as a novel ETR1 receptor-interacting protein in regulating ethylene response and signaling. Yeast split ubiquitin assays and bi-fluorescence complementation studies in plant cells indicated that CPR5 directly interacts with the ETR1 receptor. Genetic analyses indicated that mutant alleles of cpr5 can suppress ethylene insensitivity in both etr1-1 and etr1-2, but not in other dominant ethylene receptor mutants. Overexpression of Arabidopsis CPR5 either in transgenic Arabidopsis plants, or ectopically in tobacco, significantly enhanced ethylene sensitivity. These findings indicate that CPR5 plays a critical role in regulating ethylene signaling. CPR5 is localized to endomembrane structures and the nucleus, and is involved in various regulatory pathways, including pathogenesis, leaf senescence, and spontaneous cell death. This study provides evidence for a novel regulatory function played by CPR5 in the ethylene receptor signaling pathway in Arabidopsis.

Molecular association of Arabidopsis RTH with its homolog RTE1 in regulating ethylene signaling

The plant hormone ethylene affects many biological processes during plant growth and development. Ethylene is perceived by ethylene receptors at the endoplasmic reticulum (ER) membrane. The ETR1 ethylene receptor is positively regulated by the transmembrane protein RTE1, which localizes to the ER and Golgi apparatus. The RTE1 gene family is conserved in animals, plants, and lower eukaryotes. In Arabidopsis, RTE1-HOMOLOG (RTH) is the only homolog of the Arabidopsis RTE1 gene family. The regulatory function of the Arabidopsis RTH in ethylene signaling and plant growth is largely unknown. The present study shows Arabidopsis RTH gene expression patterns, protein co-localization with the ER and Golgi apparatus, and the altered ethylene response phenotype when RTH is knocked out or overexpressed in Arabidopsis. Compared with rte1 mutants, rth mutants exhibit less sensitivity to exogenous ethylene, while RTH overexpression confers ethylene hypersensitivity. Genetic analyses indicate that Arabidopsis RTH might not directly regulate the ethylene receptors. RTH can physically interact with RTE1, and evidence supports that RTH might act via RTE1 in regulating ethylene responses and signaling. The present study advances our understanding of the regulatory function of the Arabidopsis RTE1 gene family members in ethylene signaling.

Analysis of Ethylene Receptor Interactions by Co-immunoprecipitation Assays

Ethylene receptors are predominantly localized to the endoplasmic reticulum (ER) membrane, and coordinate ethylene signal output through protein-protein interactions with each other and additional signaling components. Here, we describe a co-immunoprecipitation (Co-IP) assay based on the use of the Tandem Affinity Purification (TAP) tag to examine the interactions of ethylene receptors in plant extracts. Human IgG-agarose beads are used to pull down TAP-tagged versions of the protein of interest from detergent extracts of Arabidopsis membranes, and the precipitate then is analyzed immunologically for co-purification of the ethylene receptors. This method has been successfully used to examine interactions of the receptors with each other as well as with the Raf-like kinase CTR1.

Analysis of Ethylene Receptors: Assay for Histidine Kinase Activity

The ethylene receptors of plants exist in two subfamilies. Members of subfamily-1 have functional histidine kinase domains, whereas members of subfamily-2 have diverged histidine-kinase-like domains that in some cases have been shown to exhibit Ser/Thr kinase activity. Here, we describe a method to biochemically characterize the enzymatic activity of these kinase domains in vitro. For this purpose, the histidine kinase domain of the receptors is transgenically expressed in yeast as a fusion to glutathione-S-transferase (GST) for subsequent affinity purification. Autophosphorylation activity is assessed by the use of an in vitro kinase assay with the purified protein. Acid/base stability of the incorporated phosphate can then be used as a diagnostic for whether His, Asp, or Ser/Thr/Tyr is phosphorylated.

Characterization of the expression of the stress-responsive PpERS1 gene from peach and analysis of its promoter using transgenic tomato

The PpERS1 gene, which encodes an ethylene receptor and responds to abiotic and biotic stresses, was cloned from peach (Prunus persica L. Batsch cv Okubao). The genomic DNA sequence of PpERS1 comprises seven exons which are separated by six introns, interestingly alternative splicing of the first intron produced three different PpERS1 transcripts. In addition, a 2.8-kb sequence including the promoter of PpERS1 was isolated and analyzed by placing expressing of the GUS reporter gene under its control. Several putative cis-elements were identified in the promoter of PpERS1, including two ethylene-responsive elements (EREs), five W boxes, and four putative binding sites for MYB-type transcription factors. Deletion analysis indicated the presence of an enhancer element in the PpERS1 promoter. Temporal and spatial expression analysis of the PpERS1 promoter using histochemical GUS staining showed GUS activity in all tissues examined throughout the development of transgenic tomato plants. Exposure to various stresses caused similar changes in expression patterns in peach and transgenic tomato plants. Overall, our results suggested that PpERS1 gene might play important roles in response to multiple stresses via signal transduction mediated by ethylene receptors. The characterization of the PpERS1 promoter contributes to our understanding of the transcriptional regulation of this ethylene receptor in peach.

Probing the nucleotide-binding activity of a redox sensor: two-component regulatory control in chloroplasts

Two-component signal transduction systems mediate adaptation to environmental changes in bacteria, plants, fungi, and protists. Each two-component system consists of a sensor histidine kinase and a response regulator. Chloroplast sensor kinase (CSK) is a modified sensor histidine kinase found in chloroplasts-photosynthetic organelles of plants and algae. CSK regulates the transcription of chloroplast genes in response to changes in photosynthetic electron transport. In this study, the full-length and truncated forms of Arabidopsis CSK proteins were overexpressed and purified in order to characterise their kinase and redox sensing activities. Our results show that CSK contains a modified kinase catalytic domain that binds ATP with high affinity and forms a quinone adduct that may confer redox sensing activity.

A natural frameshift mutation in Campanula EIL2 correlates with ethylene insensitivity in flowers

BACKGROUND

The phytohormone ethylene plays a central role in development and senescence of climacteric flowers. In ornamental plant production, ethylene sensitive plants are usually protected against negative effects of ethylene by application of chemical inhibitors. In Campanula, flowers are sensitive to even minute concentrations of ethylene.

RESULTS

Monitoring flower longevity in three Campanula species revealed C. portenschlagiana (Cp) as ethylene sensitive, C. formanekiana (Cf) with intermediate sensitivity and C. medium (Cm) as ethylene insensitive. We identified key elements in ethylene signal transduction, specifically in Ethylene Response Sensor 2 (ERS2), Constitutive Triple Response 1 (CTR1) and Ethylene Insensitive 3- Like 1 and 2 (EIL1 and EIL2) homologous. Transcripts of ERS2, CTR1 and EIL1 were constitutively expressed in all species both throughout flower development and in response to ethylene. In contrast, EIL2 was found only in Cf and Cm. We identified a natural mutation in Cmeil2 causing a frameshift which resulted in difference in expression levels of EIL2, with more than 100-fold change between Cf and Cm in young flowers.

CONCLUSIONS

This study shows that the naturally occurring 7 bp frameshift discovered in Cmeil2, a key gene in the ethylene signaling pathway, correlates with ethylene insensitivity in flowers. We suggest that transfer of the eil2 mutation to other plant species will provide a novel tool to engineer ethylene insensitive flowers.

Structural Aspects of Multistep Phosphorelay-Mediated Signaling in Plants

The multistep phosphorelay (MSP) is a central signaling pathway in plants integrating a wide spectrum of hormonal and environmental inputs and controlling numerous developmental adaptations. For the thorough comprehension of the molecular mechanisms underlying the MSP-mediated signal recognition and transduction, the detailed structural characterization of individual members of the pathway is critical. In this review we describe and discuss the recently known crystal and nuclear magnetic resonance structures of proteins acting in MSP signaling in higher plants, focusing particularly on cytokinin and ethylene signaling in Arabidopsis thaliana. We discuss the range of functional aspects of available structural information including determination of ligand specificity, activation of the receptor via its autophosphorylation, and downstream signal transduction through the phosphorelay. We compare the plant structures with their bacterial counterparts and show that although the overall similarity is high, the differences in structural details are frequent and functionally important. Finally, we discuss emerging knowledge on molecular recognition mechanisms in the MSP, and mention the latest findings regarding structural determinants of signaling specificity in the Arabidopsis MSP that could serve as a general model of this pathway in all higher plants.

Illuminating light, cytokinin, and ethylene signalling crosstalk in plant development

Integrating important environmental signals with intrinsic developmental programmes is a crucial adaptive requirement for plant growth, survival, and reproduction. Key environmental cues include changes in several light variables, while important intrinsic (and highly interactive) regulators of many developmental processes include the phytohormones cytokinins (CKs) and ethylene. Here, we discuss the latest discoveries regarding the molecular mechanisms mediating CK/ethylene crosstalk at diverse levels of biosynthetic and metabolic pathways and their complex interactions with light. Furthermore, we summarize evidence indicating that multiple hormonal and light signals are integrated in the multistep phosphorelay (MSP) pathway, a backbone signalling pathway in plants. Inter alia, there are strong overlaps in subcellular localizations and functional similarities in components of these pathways, including receptors and various downstream agents. We highlight recent research demonstrating the importance of CK/ethylene/light crosstalk in selected aspects of plant development, particularly seed germination and early seedling development. The findings clearly demonstrate the crucial integration of plant responses to phytohormones and adaptive responses to environmental cues. Finally, we tentatively identify key future challenges to refine our understanding of the molecular mechanisms mediating crosstalk between light and hormonal signals, and their integration during plant life cycles.

Three SAUR proteins SAUR76, SAUR77 and SAUR78 promote plant growth in Arabidopsis

Ethylene perceived by a family of five receptors regulates many developmental processes in Arabidopsis. Here we conducted the yeast two-hybrid assay to screen for additional unidentified proteins that interact with subfamily II ethylene receptor ETR2. Three SAUR proteins, named SAUR76, 77 and 78, were identified to associate with both ETR2 and EIN4 in different assays. Interaction of SAUR76 and SAUR78 with ETR2 was further verified by co-immunoprecipitation and bimolecular fluorescence complementation (BiFC) assays. Expressions of SAUR76-78 are induced by auxin and ethylene treatments. Compared with wild type, SAUR-overexpressing plants exhibit reduced ethylene sensitivity, while SAUR-RNAi lines exhibit enhanced ethylene sensitivity. Overexpressing the three SAURs partially complements the phenotype of subfamily II ethylene receptor loss-of-function double mutant etr2-3ein4-4, which has increased ethylene response and small cotyledon and rosette. saur76 mutation partially suppresses the reduced ethylene sensitivity of etr2-2. SAUR76/78 proteins are regulated by 26S proteasome system and larger tag increases their protein stability. These findings suggest that SAUR76-78 may affect ethylene receptor signaling and promote plant growth in Arabidopsis.

Ethylene receptors in plants - why so much complexity?

Ethylene is a hormone involved in numerous aspects of growth, development, and responses to biotic and abiotic stresses in plants. Ethylene is perceived through its binding to endoplasmic reticulum-localized receptors that function as negative regulators of ethylene signaling in the absence of the hormone. In Arabidopsis thaliana, five structurally and functionally different ethylene receptors are present. These differ in their primary sequence, in the domains present, and in the type of kinase activity exhibited, which may suggest functional differences among the receptors. Whereas ethylene receptors functionally overlap to suppress ethylene signaling, certain other responses are controlled by specific receptors. In this review, I examine the nature of these receptor differences, how the evolution of the ethylene receptor gene family may provide insight into their differences, and how expression of receptors or their accessory proteins may underlie receptor-specific responses.

Ethylene signaling in rice and Arabidopsis: conserved and diverged aspects

Ethylene as a gas phytohormone plays significant roles in the whole life cycle of plants, ranging from growth and development to stress responses. A linear ethylene signaling pathway has been established in the dicotyledonous model plant Arabidopsis. However, the ethylene signaling mechanism in monocotyledonous plants such as rice is largely unclear. In this review, we compare the ethylene response phenotypes of dark-grown seedlings of Arabidopsis, rice, and other monocotyledonous plants (maize, wheat, sorghum, and Brachypodium distachyon) and pinpoint that rice has a distinct phenotype of root inhibition but coleoptile promotion in etiolated seedlings upon ethylene treatment. We further summarize the homologous genes of Arabidopsis ethylene signaling components in these monocotyledonous plants and discuss recent progress. Although conserved in most aspects, ethylene signaling in rice has evolved new features compared with that in Arabidopsis. These analyses provide novel insights into the understanding of ethylene signaling in the dicotyledonous Arabidopsis and monocotyledonous plants, particularly rice. Further characterization of rice ethylene-responsive mutants and their corresponding genes will help us better understand the whole picture of ethylene signaling mechanisms in plants.

Tobacco ankyrin protein NEIP2 interacts with ethylene receptor NTHK1 and regulates plant growth and stress responses

Ethylene is a gaseous hormone that regulates many processes involved in plant growth, development and stress responses. Previously, we found that the tobacco ethylene receptor NTHK1 (Nicotiana tabacum histidine kinase 1) promotes seedling growth and affects plant salt stress responses. In this study, NTHK1 ethylene receptor-interacting protein 2 (NEIP2) was identified and further characterized in relation to these processes. NEIP2 contains three ankyrin repeats that mediate an interaction with NTHK1 as demonstrated by yeast two-hybrid, glutathione S-transferase (GST) pull-down and co-immunoprecipitation assays. NTHK1 phosphorylates NEIP2 in vitro. Salt stress and ethylene treatment induce NEIP2 accumulation in the first few hours and then the NEIP2 can be phosphorylated in planta. The overexpression of NTHK1 enhances NEIP2 accumulation in the presence of ethylene and salt stress. NEIP2 overexpression promotes plant growth but reduces ethylene responses, which is consistent with the functions of NTHK1. Additionally, NEIP2 improves plant performance under salt and oxidative stress. These results suggest that ethylene-induced NEIP2 probably acts as a brake to reduce ethylene response but resumes growth through interaction with NTHK1. Manipulation of NEIP2 may be beneficial for crop improvement.

Ethylene Regulates Levels of Ethylene Receptor/CTR1 Signaling Complexes in Arabidopsis thaliana

The plant hormone ethylene is perceived by a five-member family of receptors in Arabidopsis thaliana. The receptors function in conjunction with the Raf-like kinase CTR1 to negatively regulate ethylene signal transduction. CTR1 interacts with multiple members of the receptor family based on co-purification analysis, interacting more strongly with receptors containing a receiver domain. Levels of membrane-associated CTR1 vary in response to ethylene, doing so in a post-transcriptional manner that correlates with ethylene-mediated changes in levels of the ethylene receptors ERS1, ERS2, EIN4, and ETR2. Interactions between CTR1 and the receptor ETR1 protect ETR1 from ethylene-induced turnover. Kinetic and dose-response analyses support a model in which two opposing factors control levels of the ethylene receptor/CTR1 complexes. Ethylene stimulates the production of new complexes largely through transcriptional induction of the receptors. However, ethylene also induces turnover of receptors, such that levels of ethylene receptor/CTR1 complexes decrease at higher ethylene concentrations. Implications of this model for ethylene signaling are discussed.

Appearance and elaboration of the ethylene receptor family during land plant evolution

Ethylene is perceived following binding to endoplasmic reticulum-localized receptors, which in Arabidopsis thaliana, include ETR1, ERS1, EIN4, ETR2, and ERS2. These receptors fall into two subfamilies based on conservation of features within their histidine kinase domain. Subfamily 1 contains ETR1 and ERS1 whereas subfamily 2 contains EIN4, ETR2, and ERS2. Because ethylene receptors are found only in plants, this raises questions of when each receptor evolved. Here it is shown that subfamily 1 receptors encoded by a multigene family are present in all charophytes examined, these being most homologous to ETR1 based on their evolutionary relationship as well as containing histidine kinase and receiver domains. In charophytes and Physcomitrella patens, one or more gene family members contain the intron characteristic of subfamily 2 genes, indicating the first step in subfamily 2 receptor evolution. ERS1 homologs appear in basal angiosperm species after Amborella trichopoda and, in some early and basal angiosperm species and monocots in general, it is the only subfamily 1 receptor present. Distinct EIN4 and ETR2 homologs appear only in core eudicots and ERS2 homologs appear only in the Brassicaceae, suggesting it is the most recent receptor to evolve. These findings show that a subfamily 1 receptor had evolved and a subfamily 2 receptor had begun to evolve in plants prior to the colonization of land and only these two existed up to the appearance of the first basal angiosperm. The appearance of ERS2 in the Brassicaceae suggests ongoing evolution of the ethylene receptor family.

Structural model of the cytosolic domain of the plant ethylene receptor 1 (ETR1)

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

Cold signal transduction and its interplay with phytohormones during cold acclimation

Cold stress is a major environmental factor that affects plant growth, development, productivity and distribution. In higher plants, the known major cold signaling pathway is the C-repeat (CRT)-binding factor/dehydration-responsive element (DRE) binding factor (CBF/DREB)-mediated transcriptional regulatory cascade, which is essential for the induction of a set of cold responsive (COR) genes. Recent studies indicate that various plant hormones are also involved in responses to cold stress. This review summarizes recent progress in cold signaling and our understanding of phytohormone signaling in the regulation of plant responses to cold stress.

The relationship between the expression of ethylene-related genes and papaya fruit ripening disorder caused by chilling injury

Papaya (Carica papaya L.) is sensitive to low temperature and easy to be subjected to chilling injury, which causes fruit ripening disorder. This study aimed to investigate the relationship between the expression of genes related to ethylene and fruit ripening disorder caused by chilling injury. Papaya fruits were firstly stored at 7°C and 12°C for 25 and 30 days, respectively, then treated with exogenous ethylene and followed by ripening at 25°C for 5 days. Chilling injury symptoms such as pulp water soaking were observed in fruit stored at 7°C on 20 days, whereas the coloration and softening were completely blocked after 25 days, Large differences in the changes in the expression levels of twenty two genes involved in ethylene were seen during 7°C-storage with chilling injury. Those genes with altered expression could be divided into three groups: the group of genes that were up-regulated, including ACS1/2/3, EIN2, EIN3s/EIL1, CTR1/2/3, and ERF1/3/4; the group of genes that were down-regulated, including ACO3, ETR1, CTR4, EBF2, and ERF2; and the group of genes that were un-regulated, including ACO1/2, ERS, and EBF1. The results also showed that pulp firmness had a significantly positive correlation with the expression of ACS2, ACO1, CTR1/4, EIN3a/b, and EBF1/2 in fruit without chilling injury. This positive correlation was changed to negative one in fruit after storage at 7°C for 25 days with chilling injury. The coloring index displayed significantly negative correlations with the expression levels of ACS2, ACO1/2, CTR4, EIN3a/b, ERF3 in fruit without chilling injury, but these correlations were changed into the positive ones in fruit after storage at 7°C for 25 days with chilling injury. All together, these results indicate that these genes may play important roles in the abnormal softening and coloration with chilling injury in papaya.

Loss of the ETR1 ethylene receptor reduces the inhibitory effect of far-red light and darkness on seed germination of Arabidopsis thaliana

When exposed to far-red light followed by darkness, wild-type Arabidopsis thaliana seeds fail to germinate or germinate very poorly. We have previously shown that the ethylene receptor ETR1 (ETHYLENE RESPONSE1) inhibits and ETR2 stimulates seed germination of Arabidopsis during salt stress. This function of ETR1 requires the full-length receptor. These roles are independent of ethylene levels and sensitivity and are mainly mediated by a change in abscisic acid (ABA) sensitivity. In the current study we find that etr1-6 and etr1-7 loss-of-function mutant seeds germinate better than wild-type seeds after illumination with far-red light or when germinated in the dark indicating an inhibitory role for ETR1. Surprisingly, this function of ETR1 does not require the receiver domain. No differences between these mutants and wild-type are seen when germination proceeds after treatment with white, blue, green, or red light. Loss of any of the other four ethylene receptor isoforms has no measurable effect on germination after far-red light treatment. An analysis of the transcript abundance for genes encoding ABA and gibberellic acid (GA) metabolic enzymes indicates that etr1-6 mutants may produce more GA and less ABA than wild-type seeds after illumination with far-red light which correlates with the better germination of the mutants. Epistasis analysis suggests that ETR1 may genetically interact with the phytochromes (phy), PHYA and PHYB to control germination and growth. This study shows that of the five ethylene receptor isoforms in Arabidopsis, ETR1 has a unique role in modulating the effects of red and far-red light on plant growth and development.

Genome-wide identification, phylogeny, duplication, and expression analyses of two-component system genes in Chinese cabbage (Brassica rapa ssp. pekinensis)

In plants, a two component system (TCS) composed of sensor histidine kinases (HKs), histidine phosphotransfer proteins (HPs), and response regulators (RRs) has been employed in cytokinin signal transduction. A TCS exhibits important functions in diverse biological processes, including plant growth, development, and response to environmental stimuli. Conducting an exhaustive search of the Chinese cabbage genome, a total of 20 HK(L) (11 HKs and 9 HKLs), 8 HP (7 authentic and 1 pseudo), and 57 RR (21 Type-A, 17 Type-B, 4 Type-C, and 15 pseudo) proteins were identified. The structures, conserved domains, and phylogenetic relationships of these protein-coding genes were analysed in detail. The duplications, evolutionary patterns, and divergence of the TCS genes were investigated. The transcription levels of TCS genes in various tissues, organs, and developmental stages were further analysed to obtain information of the functions of these genes. Cytokinin-related binding elements were found in the putative promoter regions of Type-A BrRR genes. Furthermore, gene expression patterns to adverse environmental stresses (drought and high salinity) and exogenous phytohormones (tZ and ABA) were investigated. Numerous stress-responsive candidate genes were obtained. Our systematic analyses provided insights into the characterization of the TCS genes in Chinese cabbage and basis for further functional studies of such genes.

The Ethylene Receptors ETHYLENE RESPONSE1 and ETHYLENE RESPONSE2 Have Contrasting Roles in Seed Germination of Arabidopsis during Salt Stress

In Arabidopsis (Arabidopsis thaliana), ethylene responses are mediated by a family of five receptors that have both overlapping and nonoverlapping roles. In this study, we used loss-of-function mutants for each receptor isoform to determine the role of individual isoforms in seed germination under salt stress. From this analysis, we found subfunctionalization of the receptors in the control of seed germination during salt stress. Specifically, loss of ETHYLENE RESPONSE1 (ETR1) or ETHYLENE INSENSITIVE4 (EIN4) leads to accelerated germination, loss of ETR2 delays germination, and loss of either ETHYLENE RESPONSE SENSOR1 (ERS1) or ERS2 has no measurable effect on germination. Epistasis analysis indicates that ETR1 and EIN4 function additively with ETR2 to control this trait. Interestingly, regulation of germination by ETR1 requires the full-length receptor. The differences in germination between etr1 and etr2 loss-of-function mutants under salt stress could not be explained by differences in the production of or sensitivity to ethylene, gibberellin, or cytokinin. Instead, etr1 loss-of-function mutants have reduced sensitivity to abscisic acid (ABA) and germinate earlier than the wild type, whereas etr2 loss-of-function mutants have increased sensitivity to ABA and germinate slower than the wild type. Additionally, the differences in seed germination on salt between the two mutants and the wild type are eliminated by the ABA biosynthetic inhibitor norflurazon. These data suggest that ETR1 and ETR2 have roles independent of ethylene signaling that affect ABA signaling and result in altered germination during salt stress.

Antibodies against CKI1RD, a receiver domain of the sensor histidine kinase in Arabidopsis thaliana: from antigen preparation to in planta immunolocalization

Immunodetection is a powerful tool in functional studies of all organisms. In plants, the gene redundancy and presence of gene families composed of highly homologous members often impedes the unambiguous identification of individual gene products. A family of eight sensor histidine kinases (HKs) mediates the transduction of diverse signals into Arabidopsis thaliana cells, thereby ensuring the initiation of appropriate adaptive responses. Antibodies recognizing specific members of the HK family would be valuable for studying their functions in Arabidopsis and other plant species including important crops. We have focused on developing and applying antibodies against CYTOKININ-INDEPENDENT 1 (CKI1), which encodes a constitutively active membrane-bound sensor HK that regulates the development of female gametophytes and vascular tissue in Arabidopsis. A coding sequence delimiting the C-terminal receiver domain of CKI1 (CKI1(RD)) was expressed in Escherichia coli using the IPTG-inducible expression system and purified to give a highly pure target protein. The purified CKI1(RD) protein was then used as an antigen for anti-CKI1(RD) antibody production. The resulting polyclonal antibodies had a detection limit of 10 ng of target protein at 1:20,000 dilution and were able to specifically distinguish CKI1, both in vitro and in situ, even in a direct comparison with highly homologous members of the same HK family AHK4, CKI2 and ETR1. Finally, anti-CKI1(RD) antibodies were able to selectively bind CKI1-GFP fusion protein in a pull-down assay using crude lysate from an Arabidopsis cell suspension culture. Our results suggest that the receiver domain is a useful target for the functional characterization of sensor HKs in immunological and biochemical studies.

How plants sense ethylene gas--the ethylene receptors

Ethylene is a hormone that affects many processes important for plant growth, development, and responses to stresses. The first step in ethylene signal transduction is when ethylene binds to its receptors. Numerous studies have examined how these receptors function. In this review we summarize many of these studies and present our current understanding about how ethylene binds to the receptors. The biochemical output of the receptors is not known but current models predict that when ethylene binds to the receptors, the activity of the associated protein kinase, CTR1 (constitutive triple response1), is reduced. This results in downstream transcriptional changes leading to ethylene responses. We present a model where a copper cofactor is required and the binding of ethylene causes the receptor to pass through a transition state to become non-signaling leading to lower CTR1 activity.

Cloning, overexpression, purification and preliminary X-ray analysis of the catalytic domain of the ethylene receptor ETR1 from Arabidopsis thaliana

Ethylene is a gaseous plant hormone which controls many aspects of plant growth and development. It is perceived by membrane-bound receptors with a similarity to bacterial two-component systems. The catalytic and ATP-binding domain of the histidine kinase domain of ETR1 from Arabidopsis thaliana has been cloned, overexpressed and crystallized. The protein was crystallized together with various nucleotides. Crystals obtained in the presence of ADP belonged to space group I222 or I2(1)2(1)2(1) with one molecule per asymmetric unit. They diffracted X-ray radiation to beyond 1.85 Å resolution.

Identification of rice ethylene-response mutants and characterization of MHZ7/OsEIN2 in distinct ethylene response and yield trait regulation

Ethylene plays essential roles in adaptive growth of rice plants in water-saturating environment; however, ethylene signaling pathway in rice is largely unclear. In this study, we report identification and characterization of ethylene-response mutants based on the specific ethylene-response phenotypes of etiolated rice seedlings, including ethylene-inhibited root growth and ethylene-promoted coleoptile elongation, which is different from the ethylene triple-response phenotype in Arabidopsis. We establish an efficient system for screening and a set of rice mutants have been identified. Genetic analysis reveals that these mutants form eight complementation groups. All the mutants show insensitivity or reduced sensitivity to ethylene in root growth but exhibit differential responses in coleoptile growth. One mutant group mhz7 has insensitivity to ethylene in both root and coleoptile growth. We identified the corresponding gene by a map-based cloning method. MHZ7 encodes a membrane protein homologous to EIN2, a central component of ethylene signaling in Arabidopsis. Upon ethylene treatment, etiolated MHZ7-overexpressing seedlings exhibit enhanced coleoptile elongation, increased mesocotyl growth and extremely twisted short roots, featuring enhanced ethylene-response phenotypes in rice. Grain length was promoted in MHZ7-transgenic plants and 1000-grain weight was reduced in mhz7 mutants. Leaf senescent process was also affected by MHZ7 expression. Manipulation of ethylene signaling may improve adaptive growth and yield-related traits in rice.

Cloning, expression, purification and preliminary X-ray analysis of the dimerization domain of ethylene response sensor 1 (ERS1) from Arabidopsis thaliana

Ethylene signalling is initiated by a group of membrane-bound receptors with similarity to two-component systems. ERS1 belongs, together with ETR1, to subfamily 1, which plays a predominant role in ethylene signalling. The dimerization domain of ERS1 was crystallized in space groups C222(1) and P2(1)2(1)2, with two and four molecules per asymmetric unit, respectively. The crystals diffracted X-ray radiation to 1.9 Å resolution.

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.