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

Modulation of plant growth and development through altered ethylene binding affinity of the ethylene receptor ETR1

Arabidopsis senses ethylene through a five-member family of ethylene receptors, of which the ethylene receptor ETR1 plays the major role. We examined how changes in ethylene binding affinity of ETR1 can regulate physiological and molecular responses to ethylene, taking advantage of an Asp25Asn mutation that still produces a functional ETR1 receptor (ETR1D25N) but one with 100-fold reduced ethylene binding affinity compared to wild-type ETR1 (ETR1wt). Analysis was performed in a genetic background that lacks the five native members of the receptor family so that the specific role of ETR1 in plant growth and development could be assessed. From this analysis, we determined that changes in ethylene binding affinity of ETR1 are reflected in plant growth and responses to ethylene. Differences in plant growth and ethylene responses for the ETR1wt and ETR1D25N lines were uncovered in seedlings grown under light or dark conditions, and when assayed for short- or long-term responses to ethylene. Dose response analysis revealed that differences in the ethylene responses for ETR1wt and ETR1D25N lines are proportional to the binding affinity of the corresponding receptor variants. Results from the characterization of the ETR1wt line and an etr1 etr2 ein4 triple mutant demonstrate that plants have greater sensitivity to ethylene than previously recognized.

Phosphorylation at serine-260 of Toc33 is essential for chloroplast biogenesis

Chloroplast biogenesis, essential for photosynthesis, depends on the import of nuclear-encoded proteins through the translocon at the outer envelope of chloroplasts (TOC) complexes. Despite its importance, the mechanisms regulating this process remain largely elusive. We identify serine-260 (S260) as a critical phosphorylation site in Toc33, a core TOC component. This phosphorylation stabilizes Toc33 by preventing its ubiquitination and degradation. Constitutive triple response 1 (CTR1), a negative regulator of ethylene signaling, and its paralog RAF-like kinase are involved in phosphorylating Toc33. Disruption of Toc33 phosphorylation impairs its stability and photosynthetic protein import, consequently affecting chloroplast structural integrity and biogenesis. Our findings underscore the essential role of TOC phosphorylation in chloroplast biogenesis and reveal an unexpected regulatory network involving RAF-like kinases in organelle development.

Regulatory Mechanisms of Bud Dormancy: Environmental, Hormonal, and Genetic Perspectives

Dormancy is a vital adaptive strategy in temperate and boreal plants, particularly fruit trees, enabling them to withstand harsh winter conditions and ensure survival and synchronized growth resumption in spring. This review comprehensively examines dormancy, focusing on its physiological, environmental, and molecular mechanisms. Dormancy is characterized by two distinct phases: endodormancy, which is regulated by internal plant signals and requires cold temperatures for release, and ecodormancy, which is influenced by external environmental factors. These stages are intricately linked to seasonal temperature fluctuations and the plant's ability to synchronize growth cycles, ensuring survival through harsh winters and optimal growth in warmer seasons. The review delves into the role of chilling requirements, temperature thresholds, and hormonal regulation in the dormancy process, highlighting how these factors influence critical growth events such as budbreak, flowering, and fruiting. Plant hormones, including abscisic acid, gibberellins, and cytokinins, regulate dormancy by modulating gene expression and growth activity. Additionally, we explore the historical development of dormancy research, from early observations of chilling requirements to the formulation of the chilling hours model. Considering ongoing climate change, the review examines how rising winter temperatures may disrupt dormancy cycles, potentially affecting the timing of flowering, fruiting, and overall crop productivity. This shift necessitates new strategies for managing dormancy, particularly in regions experiencing inconsistent or insufficient chilling. The review concludes by discussing practical approaches to enhance dormancy release and mitigate the impact of environmental stress on deciduous fruit tree growth, offering insights into improving agricultural practices amidst a changing climate.

Study on the expression regulation of the CTR1 gene in the ethylene signaling pathway

The CONSTITUTIVE TRIPLERESPONSE1 (CTR1) is a crucial component in the ethylene signaling pathway. CTR1 transmits signals perceived by ethylene receptors to downstream EIN2 proteins through phosphorylation/dephosphorylation. Although some studies have explored the functions and mechanisms of CTR1, research on its expression and regulation remains relatively limited. This study investigates the tissue-specific expression of the Arabidopsis CTR1 gene and its expression and regulatory mechanisms under ethylene induction. Arabidopsis was treated with ethylene, and changes in CTR1 gene expression were detected using real-time quantitative PCR. The experimental results show that in rosette leaves of 28-day-old Arabidopsis, CTR1 expression is induced by ethylene. To investigate its molecular mechanism, the promoter sequence of the CTR1 was cloned and vectors were constructed by linking the promoter sequence with luciferase and GUS genes. Stable transgenic Arabidopsis lines were obtained, and promoter activity in these materials was analyzed. Promoter activity analysis confirmed that CTR1 promoter activity is ethylene-inducible and that this induction is dependent on the functions of proteins such as EIN2, EIN3, and EILs. Additionally, the study found that CTR1 expression is higher during seed germination and maintained at lower levels in mature leaves and plants. This study provides a detailed observation of CTR1 gene expression and, for the first time, identifies that the CTR1 promoter is regulated by ethylene induction, offering new options for designing ethylene signaling pathway reporter systems.

Revisit and explore the ethylene-independent mechanism of sex expression in cucumber (Cucumis sativus)

KEY MESSAGE

This review provides a thorough and comprehensive perspective on the topic of cucumber sexual expression. Specifically, insights into sex expression mediated by pathways other than ethylene are highlighted. Cucumber (Cucumis sativus L.) is a common and important commercial crop that is cultivated and consumed worldwide. Additionally, this species is commonly used as a model for investigating plant sex expression. Cucumbers exhibit a variety of floral arrangements, comprising male, female, and hermaphroditic (bisexual) flowers. Generally, cucumber plants that produce female flowers are typically preferred due to their significant impact on the overall output. Various environmental conditions, such as temperature, light quality, and photoperiod, have been also shown to influence the sex expression in this species. Multiple lines of evidence indicate that ethylene and its biosynthesis genes are crucial in regulating cucumber sex expression. Gibberellins, another well-known phytohormone, can similarly influence cucumber sex expression via an ethylene-independent route. Further studies employing the next-generation sequencing technology also visualized a deeper slice of the molecular mechanism such as the role of the cell cycle program in the cucumber sex expression. This review aims to provide an overview of the sex expression of cucumber including its underlying molecular mechanism and regulatory aspects based on recent investigations.

The RNA helicase LOS4 regulates pre-mRNA splicing of key genes (EIN2, ERS2, CTR1) in the ethylene signaling pathway

KEY MESSAGE

The Arabidopsis RNA helicase LOS4 plays a key role in regulating pre-mRNA splicing of the genes EIN2, CTR1, and ERS2 in ethylene signaling pathway. The plant hormone ethylene plays diverse roles in plant growth, development, and responses to stress. Ethylene is perceived by the membrane-bound ethylene receptors complex, and then triggers downstream components, such as EIN2, to initiate signal transduction into the nucleus, leading to the activation of ethylene-responsive genes. Over the past decades, substantial information has been accumulated regarding gene cloning, protein-protein interactions, and downstream gene expressions in the ethylene pathway. However, our understanding of mRNA post-transcriptional processing and modification of key genes in the ethylene signaling pathway remains limited. This study aims to provide evidence demonstrating the involvement of the Arabidopsis RNA helicase LOS4 in pre-mRNA splicing of the genes EIN2, CTR1, and ERS2 in ethylene signaling pathway. Various genetic approaches including RNAi gene silencing, CRISPR-Cas9 gene editing, and amino acid mutations were employed in this study. When LOS4 was silenced or knocked down, the ethylene sensitivity of etiolated seedlings was significantly enhanced. Further investigation revealed errors in the EIN2 pre-mRNA splicing when LOS4 was knocked down. In addition, aberrant pre-mRNA splicing was observed in the ERS2 and CTR1 genes in the pathway. Biochemical assays indicated that the los4-2 (E94K) mutant protein exhibited increased ATP binding and enhanced ATP hydrolytic activity. Conversely, the los4-1 (G364R) mutant had reduced substrate RNA binding and lower ATP binding activities. These findings significantly advanced our comprehension of the regulatory functions and molecular mechanisms of RNA helicase in ethylene signaling.

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.

Recent Advances in Dissecting the Function of Ethylene in Interaction between Host and Pathogen

Pathogens influence the growth and development of plants, resulting in detrimental damage to their yields and quality. Ethylene, a gaseous phytohormone, serves a pivotal function in modulating diverse physiological processes in plants, including defense mechanisms against pathogen invasion. Ethylene biosynthesis is involved in both plants and pathogens. Recent empirical research elucidates the intricate interactions and regulatory mechanisms between ethylene and pathogens across various plant species. In this review, we provide a comprehensive overview of the latest findings concerning ethylene's role and its regulatory networks in host-pathogen interactions. Additionally, we explore the crosstalk between ethylene and other phytohormones. Points regarding ethylene emission and its modulation by pathogens are also emphasized. Moreover, we also discuss potential unresolved issues in the field that warrant further investigation.

Transcriptomics combined with physiological analysis provided new insights into the Zn enrichment capacity and tolerance mechanism of Dendrobium denneanum Kerr

In this study, we investigated the tolerance and accumulation capacity of Dendrobium denneanum Kerr (D.denneanum) by analyzing the growth and physiological changes of D.denneanum under different levels of Zn treatments, and further transcriptome sequencing of D.denneanum leaves to screen and analyze the differentially expressed genes. The results showed that Zn400 treatment (400 mg·kg-1) promoted the growth of D.denneanum while both Zn800 (800 mg·kg-1) and Zn1600 treatment (1600 mg·kg-1) caused stress to D.denneanum. Under Zn800 treatment (800 mg·kg-1), the resistance contribution of physiological indexes was the most obvious: antioxidant system, photosynthetic pigment, osmoregulation, phytochelatins, and ASA-GSH cycle (Ascorbic acid-Glutathione cycle). D.denneanum leaves stored the most Zn, followed by stems and roots. The BCF(Bioconcentration Factor) of the D.denneanum for Zn were all more than 1.0 under different Zn treatments, with the largest BCF (1.73) for Zn400. The transcriptome revealed that there were 1500 differentially expressed genes between Zn800 treatment and group CK, of which 842 genes were up-regulated and 658 genes were down-regulated. The genes such as C4H, PAL, JAZ, MYC2, PP2A, GS, and GST were significantly induced under the Zn treatments. The differentially expressed genes were associated with phenylpropane biosynthesis, phytohormone signaling, and glutathione metabolism. There were three main pathways of response to Zn stress in Dendrobium: antioxidant action, compartmentalization, and cellular chelation. This study provides new insights into the response mechanisms of D.denneanum to Zn stress and helps to evaluate the phytoremediation potential of D.denneanum in Zn-contaminated soils.

Uncovering the proximal proteome of CTR1 through TurboID-mediated proximity labeling

Protein-protein interactions play a crucial role in driving cellular processes and enabling appropriate physiological responses in organisms. The plant hormone ethylene signaling pathway is complex and regulated by the spatiotemporal regulation of its signaling molecules. Constitutive Triple Response 1 (CTR1), a key negative regulator of the pathway, regulates the function of Ethylene-Insensitive 2 (EIN2), a positive regulator of ethylene signaling, at the endoplasmic reticulum (ER) through phosphorylation. Our recent study revealed that CTR1 can also translocate from the ER to the nucleus in response to ethylene and positively regulate ethylene responses by stabilizing EIN3. To gain further insights into the role of CTR1 in plants, we used TurboID-based proximity labeling and mass spectrometry to identify the proximal proteomes of CTR1 in Nicotiana benthamiana. The identified proximal proteins include known ethylene signaling components, as well as proteins involved in diverse cellular processes such as mitochondrial respiration, mRNA metabolism, and organelle biogenesis. Our study demonstrates the feasibility of proximity labeling using the N. benthamiana transient expression system and identifies the potential interactors of CTR1 in vivo, uncovering the potential roles of CTR1 in a wide range of cellular processes.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Endoribonuclease DNE1 Promotes Ethylene Response by Modulating EBF1/2 mRNA Processing in Arabidopsis

The gaseous phytohormone ethylene plays a crucial role in plant growth, development, and stress responses. In the ethylene signal transduction cascade, the F-box proteins EIN3-BINDING F-BOX 1 (EBF1) and EBF2 are identified as key negative regulators governing ethylene sensitivity. The translation and processing of EBF1/2 mRNAs are tightly controlled, and their 3' untranslated regions (UTRs) are critical in these regulations. However, despite their significance, the exact mechanisms modulating the processing of EBF1/2 mRNAs remain poorly understood. In this work, we identified the gene DCP1-ASSOCIATED NYN ENDORIBONUCLEASE 1 (DNE1), which encodes an endoribonuclease and is induced by ethylene treatment, as a positive regulator of ethylene response. The loss of function mutant dne1-2 showed mild ethylene insensitivity, highlighting the importance of DNE1 in ethylene signaling. We also found that DNE1 colocalizes with ETHYLENE INSENSITIVE 2 (EIN2), the core factor manipulating the translation of EBF1/2, and targets the P-body in response to ethylene. Further analysis revealed that DNE1 negatively regulates the abundance of EBF1/2 mRNAs by recognizing and cleaving their 3'UTRs, and it also represses their translation. Moreover, the dne1 mutant displays hypersensitivity to 1,4-dithiothreitol (DTT)-induced ER stress and oxidative stress, indicating the function of DNE1 in stress responses. This study sheds light on the essential role of DNE1 as a modulator of ethylene signaling through regulation of EBF1/2 mRNA processing. Our findings contribute to the understanding of the intricate regulatory process of ethylene signaling and provide insights into the significance of ribonuclease in stress responses.

The SALT OVERLY SENSITIVE 2-CONSTITUTIVE TRIPLE RESPONSE1 module coordinates plant growth and salt tolerance in Arabidopsis

High salinity stress promotes plant ethylene biosynthesis and triggers the ethylene signalling response. However, the precise mechanism underlying how plants transduce ethylene signalling in response to salt stress remains largely unknown. In this study, we discovered that SALT OVERLY SENSITIVE 2 (SOS2) inhibits the kinase activity of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) by phosphorylating the 87th serine (S87). This phosphorylation event activates the ethylene signalling response, leading to enhanced plant salt resistance. Furthermore, through genetic analysis, we determined that the loss of CTR1 or the gain of SOS2-mediated CTR1 phosphorylation both contribute to improved plant salt tolerance. Additionally, in the sos2 mutant, we observed compromised proteolytic processing of ETHYLENE INSENSITIVE 2 (EIN2) and reduced nuclear localization of EIN2 C-terminal fragments (EIN2-C), which correlate with decreased accumulation of ETHYLENE INSENSITIVE 3 (EIN3). Collectively, our findings unveil the role of the SOS2-CTR1 regulatory module in promoting the activation of the ethylene signalling pathway and enhancing plant salt tolerance.

Fruit ripening under heat stress: The intriguing role of ethylene-mediated signaling

Crop production is significantly influenced by climate, and even minor climate changes can have a substantial impact on crop yields. Rising temperature due to climate change can lead to heat stress (HS) in plants, which not only hinders plant growth and development but also result in significant losses in crop yields. To cope with the different stresses including HS, plants have evolved a variety of adaptive mechanisms. In response to these stresses, phytohormones play a crucial role by generating endogenous signals that regulate the plant's defensive response. Among these, Ethylene (ET), a key phytohormone, stands out as a major regulator of stress responses in plants and regulates many plant traits, which are critical for crop productivity and nutritional quality. ET is also known as a ripening hormone for decades in climacteric fruit and many studies are available deciphering the function of different ET biosynthesis and signaling components in the ripening process. Recent studies suggest that HS significantly affects fruit quality traits and perturbs fruit ripening by altering the regulation of many ethylene biosynthesis and signaling genes resulting in substantial loss of fruit yield, quality, and postharvest stability. Despite the significant progress in this field in recent years the interplay between ET, ripening, and HS is elusive. In this review, we summarized the recent advances and current understanding of ET in regulating the ripening process under HS and explored their crosstalk at physiological and molecular levels to shed light on intricate relationships.

Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.

The role of ethylene in plant temperature stress response

Temperature influences the seasonal growth and geographical distribution of plants. Heat or cold stress occur when temperatures exceed or fall below the physiological optimum ranges, resulting in detrimental and irreversible damage to plant growth, development, and yield. Ethylene is a gaseous phytohormone with an important role in plant development and multiple stress responses. Recent studies have shown that, in many plant species, both heat and cold stress affect ethylene biosynthesis and signaling pathways. In this review, we summarize recent advances in understanding the role of ethylene in plant temperature stress responses and its crosstalk with other phytohormones. We also discuss potential strategies and knowledge gaps that need to be adopted and filled to develop temperature stress-tolerant crops by optimizing ethylene response.

A role for ethylene signaling and biosynthesis in regulating and accelerating CO2 - and abscisic acid-mediated stomatal movements in Arabidopsis

Little is known about long-distance mesophyll-driven signals that regulate stomatal conductance. Soluble and/or vapor-phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance in Arabidopsis thaliana by CO2 /abscisic acid (ABA) was examined. We present a diffusion model which indicates that gaseous signaling molecule/s with a shorter/direct diffusion pathway to guard cells are more probable for rapid mesophyll-dependent stomatal conductance changes. We, therefore, analyzed different Arabidopsis ethylene-signaling and biosynthesis mutants for their ethylene production and kinetics of stomatal responses to ABA/[CO2 ]-shifts. According to our research, higher [CO2 ] causes Arabidopsis rosettes to produce more ethylene. An ACC-synthase octuple mutant with reduced ethylene biosynthesis exhibits dysfunctional CO2 -induced stomatal movements. Ethylene-insensitive receptor (gain-of-function), etr1-1 and etr2-1, and signaling, ein2-5 and ein2-1, mutants showed intact stomatal responses to [CO2 ]-shifts, whereas loss-of-function ethylene receptor mutants, including etr2-3;ein4-4;ers2-3, etr1-6;etr2-3 and etr1-6, showed markedly accelerated stomatal responses to [CO2 ]-shifts. Further investigation revealed a significantly impaired stomatal closure to ABA in the ACC-synthase octuple mutant and accelerated stomatal responses in the etr1-6;etr2-3, and etr1-6, but not in the etr2-3;ein4-4;ers2-3 mutants. These findings suggest essential functions of ethylene biosynthesis and signaling components in tuning/accelerating stomatal conductance responses to CO2 and ABA.

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.

The potato StMKK5-StSIPK module enhances resistance to Phytophthora pathogens through activating the salicylic acid and ethylene signalling pathways

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in plant responses to both biotic and abiotic stress. A screen of a Nicotiana benthamiana cDNA virus-induced gene silencing (VIGS) library for altered plant responses to inoculation with Phytophthora infestans previously identified an NbMKK gene, encoding a clade D MAPKK that we renamed as NbMKK5, which is involved in immunity to P. infestans. To study the role of the potato orthologous gene, referred to as StMKK5, in the response to P. infestans, we transiently overexpressed StMKK5 in N. benthamiana and observed that cell death occurred at 2 days postinfiltration. Silencing of the highly conserved eukaryotic protein SGT1 delayed the StMKK5-induced cell death, whereas silencing of the MAPK-encoding gene NbSIPK completely abolished the cell death response. Further investigations showed that StMKK5 interacts with, and directly phosphorylates, StSIPK. Furthermore, both StMKK5 and StSIPK trigger salicylic acid (SA)- and ethylene (Eth)-related gene expression, and co-expression of the salicylate hydroxylase NahG with the negative regulator of Eth signalling CTR1 hampers StSIPK-triggered cell death. This observation indicates that the cell death triggered by StMKK5-StSIPK is dependent on the combination of SA- and Eth-signalling. By introducing point mutations, we showed that the kinase activity of both StMKK5 and StSIPK is required for triggering cell death. Genetic analysis showed that StMKK5 depends on StSIPK to trigger plant resistance. Thus, our results define a potato StMKK5-SIPK module that positively regulates immunity to P. infestans via activation of both the SA and Eth signalling pathways.

MAPKKKs in Plants: Multidimensional Regulators of Plant Growth and Stress Responses

Mitogen-activated protein kinase kinase kinase (MAPKKK, MAP3K) is located upstream of the mitogen-activated protein kinase (MAPK) cascade pathway and is responsible for receiving and transmitting external signals to the downstream MAPKKs. Although a large number of MAP3K genes play important roles in plant growth and development, and response to abiotic and biotic stresses, only a few members' functions and cascade signaling pathways have been clarified, and the downstream MAPKKs and MAPKs of most MAP3Ks are still unknown. As more and more signaling pathways are discovered, the function and regulatory mechanism of MAP3K genes will become clearer. In this paper, the MAP3K genes in plants were classified and the members and basic characteristics of each subfamily of MAP3K were briefly described. Moreover, the roles of plant MAP3Ks in regulating plant growth and development and stress (abiotic and biotic) responses are described in detail. In addition, the roles of MAP3Ks involved in plant hormones signal transduction pathway were briefly introduced, and the future research focus was prospected.

Cellobiose elicits immunity in lettuce conferring resistance to Botrytis cinerea

Cellobiose is the primary product of cellulose hydrolysis and is expected to function as a type of pathogen/damage-associated molecular pattern in evoking plant innate immunity. In this study, cellobiose was demonstrated to be a positive regulator in the immune response of lettuce, but halted autoimmunity when lettuce was exposed to concentrations of cellobiose >60 mg l-1. When lettuce plants were infected by Botrytis cinerea, cellobiose endowed plants with enhanced pre-invasion resistance by activating high β-1,3-glucanase and antioxidative enzyme activities at the initial stage of pathogen infection. Cellobiose-activated core regulatory factors such as EDS1, PTI6, and WRKY70, as well as salicylic acid signaling, played an indispensable role in modulating plant growth-defense trade-offs. Transcriptomics data further suggested that the cellobiose-activated plant-pathogen pathways are involved in microbe/pathogen-associated molecular pattern-triggered immune responses. Genes encoding receptor-like kinases, transcription factors, and redox homeostasis, phytohormone signal transduction, and pathogenesis-related proteins were also up- or down-regulated by cellobiose. Taken together, the findings of this study demonstrated that cellobiose serves as an elicitor to directly activate disease-resistance-related cellular functions. In addition, multiple genes have been identified as potential modulators of the cellobiose-induced immune response, which could aid understanding of underlying molecular events.

Ethylene-triggered subcellular trafficking of CTR1 enhances the response to ethylene gas

The phytohormone ethylene controls plant growth and stress responses. Ethylene-exposed dark-grown Arabidopsis seedlings exhibit dramatic growth reduction, yet the seedlings rapidly return to the basal growth rate when ethylene gas is removed. However, the underlying mechanism governing this acclimation of dark-grown seedlings to ethylene remains enigmatic. Here, we report that ethylene triggers the translocation of the Raf-like protein kinase CONSTITUTIVE TRIPLE RESPONSE1 (CTR1), a negative regulator of ethylene signaling, from the endoplasmic reticulum to the nucleus. Nuclear-localized CTR1 stabilizes the ETHYLENE-INSENSITIVE3 (EIN3) transcription factor by interacting with and inhibiting EIN3-BINDING F-box (EBF) proteins, thus enhancing the ethylene response and delaying growth recovery. Furthermore, Arabidopsis plants with enhanced nuclear-localized CTR1 exhibited improved tolerance to drought and salinity stress. These findings uncover a mechanism of the ethylene signaling pathway that links the spatiotemporal dynamics of cellular signaling components to physiological responses.

Plant Disease Resistance-Related Signaling Pathways: Recent Progress and Future Prospects

Plant-pathogen interactions induce a signal transmission series that stimulates the plant's host defense system against pathogens and this, in turn, leads to disease resistance responses. Plant innate immunity mainly includes two lines of the defense system, called pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is extensive signal exchange and recognition in the process of triggering the plant immune signaling network. Plant messenger signaling molecules, such as calcium ions, reactive oxygen species, and nitric oxide, and plant hormone signaling molecules, such as salicylic acid, jasmonic acid, and ethylene, play key roles in inducing plant defense responses. In addition, heterotrimeric G proteins, the mitogen-activated protein kinase cascade, and non-coding RNAs (ncRNAs) play important roles in regulating disease resistance and the defense signal transduction network. This paper summarizes the status and progress in plant disease resistance and disease resistance signal transduction pathway research in recent years; discusses the complexities of, and interactions among, defense signal pathways; and forecasts future research prospects to provide new ideas for the prevention and control of plant diseases.

Ethylene Response Factor LlERF110 Mediates Heat Stress Response via Regulation of LlHsfA3A Expression and Interaction with LlHsfA2 in Lilies (Lilium longiflorum)

Heat stress seriously affects the quality of cut lily flowers. The ethylene response factors (ERFs) participate in heat stress response in many plants. In this study, heat treatment increased the production of ethylene in lily leaves, and exogenous ethylene treatment enhanced the heat resistance of lilies. LlERF110, an important transcription factor in the ethylene signaling pathway, was found in the high-temperature transcriptome. The coding region of LlERF110 (969 bp) encodes 322 amino acids and LlERF110 contains an AP2/ERF typical domain belonging to the ERF subfamily group X. LlERF110 was induced by ethylene and was expressed constitutively in all tissues. LlERF110 is localized in the nucleus and has transactivation activity. Virus-induced gene silencing of LlERF110 in lilies reduced the basal thermotolerance phenotypes and significantly decreased the expression of genes involved in the HSF-HSP pathway, such as LlHsfA2, LlHsfA3A, and LlHsfA5, which may activate other heat stress response genes; and LlHsp17.6 and LlHsp22, which may protect proteins under heat stress. LlERF110 could directly bind to the promoter of LlHsfA3A and activate its expression according to the yeast one hybrid and dual-luciferase reporter assays. LlERF110 interacts with LlHsfA2 in the nucleus according to BiFC and the yeast two-hybrid assays. In conclusion, these results indicate that LlERF110 plays an important role in the basal thermotolerance of lilies via regulation of the HSF-HSP pathway, which could be the junction of the heat stress response pathway and the ethylene signaling pathway.

Phosphorylation status of Bβ subunit acts as a switch to regulate the function of phosphatase PP2A in ethylene-mediated root growth inhibition

The various combinations and regulations of different subunits of phosphatase PP2A holoenzymes underlie their functional complexity and importance. However, molecular mechanisms governing the assembly of PP2A complex in response to external or internal signals remain largely unknown, especially in Arabidopsis thaliana. We found that the phosphorylation status of Bβ of PP2A acts as a switch to regulate the activity of PP2A. In the absence of ethylene, phosphorylated Bβ leads to an inactivation of PP2A; the substrate EIR1 remains to be phosphorylated, preventing the EIR1-mediated auxin transport in epidermis, leading to normal root growth. Upon ethylene treatment, the dephosphorylated Bβ mediates the formation of the A2-C4-Bβ protein complex to activate PP2A, resulting in the dephosphorylation of EIR1 to promote auxin transport in epidermis of elongation zone, leading to root growth inhibition. Altogether, our research revealed a novel molecular mechanism by which the dephosphorylation of Bβ subunit switches on PP2A activity to dephosphorylate EIR1 to establish EIR1-mediated auxin transport in the epidermis in elongation zone for root growth inhibition in response to ethylene.

Heteromeric interactions of ripening-related ethylene receptors in tomato fruit

Ripening of climacteric fruits is initiated when the gaseous plant hormone ethylene is perceived by the cell. Ethylene binding to membrane-associated ethylene receptors (ETRs) triggers a series of biochemical events through multiple components, resulting in the induction of numerous ripening-related genes. In tomato (Solanum lycopersicum L.), there are seven members of the ETR family, which each contribute to the regulation of fruit ripening. However, the relative contribution of each individual receptor to ethylene signaling remains unknown. Here, we demonstrated the formation of heteromeric receptor complexes across the two ETR subfamilies in tomato fruit. Immunoprecipitation of subfamily II SlETR4 resulted in co-purification of subfamily I (SlETR1, SlETR2, and SlETR3), but not subfamily II members (SlETR5, SlETR6, and SlETR7). Such biased interactions were verified in yeast two-hybrid assays, and in transgenic Arabidopsis plants, in which heterologous SlETR4 interacts with subfamily I ETRs. Our analysis also revealed that the receptor complexes engage the Raf-like protein kinases SlCTR1 and SlCTR3, which are potential regulators of signaling. Here, we suggest that tomato receptor members form heteromeric complexes to fine-tune signal output to the downstream pathway, which is similar to that of the Arabidopsis system but appears to be partially diverged.

MicroRNAs Mediated Plant Responses to Salt Stress

One of the most damaging issues to cultivatable land is soil salinity. While salt stress influences plant growth and yields at low to moderate levels, severe salt stress is harmful to plant growth. Mineral shortages and toxicities frequently exacerbate the problem of salinity. The growth of many plants is quantitatively reduced by various levels of salt stress depending on the stage of development and duration of stress. Plants have developed various mechanisms to withstand salt stress. One of the key strategies is the utilization of microRNAs (miRNAs) that can influence gene regulation at the post-transcriptional stage under different environmental conditions, including salinity. Here, we have reviewed the miRNA-mediated adaptations of various plant species to salt stress and other abiotic variables. Moreover, salt responsive (SR)-miRNAs, their targets, and corresponding pathways have also been discussed. The review article concludes by suggesting that the utilization of miRNAs may be a vital strategy to generate salt tolerant crops ensuring food security in the future.

A molecular framework of ethylene-mediated fruit growth and ripening processes in tomato

Although the role of ethylene in tomato (Solanum lycopersicum) fruit ripening has been intensively studied, its role in tomato fruit growth remains poorly understood. In addition, the relationship between ethylene and the developmental factors NON-RIPENING (NOR) and RIPENING INHIBITOR (RIN) during ripening is under debate. Here, we carried out comprehensive genetic analyses of genome-edited mutants of tomato ETHYLENE INSENSITIVE 2 (SlEIN2), four EIN3-like genes (SlEIL1-4), and three EIN3 BINDING F-box protein genes (SlEBF1-3). Both slein2-1 and the high-order sleil mutant (sleil1 sleil2 sleil3/SlEIL3 sleil4) showed reduced fruit size, mainly due to decreased auxin biosynthesis. During fruit maturation, slein2 mutants displayed the complete cessation of ripening, which was partially rescued by slebf1 but not slebf2 or slebf3. We also discovered that ethylene directly activates the expression of the developmental genes NOR, RIN, and FRUITFULL1 (FUL1) via SlEIL proteins. Indeed, overexpressing these genes partially rescued the ripening defects of slein2-1. Finally, the signal intensity of the ethylene burst during fruit maturation was intimately connected with the progression of full ripeness. Collectively, our work uncovers a critical role of ethylene in fruit growth and supports a molecular framework of ripening control in which the developmental factors NOR, RIN, and FUL1 act downstream of ethylene signaling.

MdERF1B-MdMYC2 module integrates ethylene and jasmonic acid to regulate the biosynthesis of anthocyanin in apple

Ethylene and jasmonic acid (JA) are crucial hormones that promote anthocyanin synthesis in apple (Malus × domestica). However, the mechanism by which these hormones cooperate to modulate anthocyanin production in apple is unclear. According to our results, MdERF1B expression was strongly induced by ethylene and JA. Physiological phenotypes and the results of molecular biological analyses indicated that MdERF1B encodes a positive regulator of anthocyanin synthesis. Specifically, MdERF1B was capable of combining directly with the MdMYC2 promoter to promote gene expression. Additionally, MdERF1B interacted with two JA signaling pathway inhibitors, namely MdJAZ5 and MdJAZ10. The MdERF1B-MdJAZ5/10 protein complex decreased the ability of MdERF1B to activate the MdMYC2 promoter. Furthermore, MdEIL1, which is a crucial protein for ethylene signal transduction, was observed to bind directly to the MdERF1B promoter, thereby upregulating gene expression. These results suggest that MdERF1B is a core gene responsive to JA and ethylene signals. The encoded protein, together with MdMYC2, MdJAZ5/10, and MdEIL1, modulates anthocyanin synthesis in apple. This study clarifies the synergistic mechanism by which JA and ethylene regulate anthocyanin production in apple.

Increasing yield on dry fields: molecular pathways with growing potential

Drought stress constitutes one of the major constraints to agriculture all over the world, and its devastating effect is only expected to increase in the following years due to climate change. Concurrently, the increasing food demand in a steadily growing population requires a proportional increase in yield and crop production. In the past, research aimed to increase plant resilience to severe drought stress. However, this often resulted in stunted growth and reduced yield under favorable conditions or moderate drought. Nowadays, drought tolerance research aims to maintain plant growth and yield under drought conditions. Overall, recently deployed strategies to engineer drought tolerance in the lab can be classified into a 'growth-centered' strategy, which focuses on keeping growth unaffected by the drought stress, and a 'drought resilience without growth penalty' strategy, in which the main aim is still to boost drought resilience, while limiting the side effects on plant growth. In this review, we put the scope on these two strategies and some molecular players that were successfully engineered to generate drought-tolerant plants: abscisic acid, brassinosteroids, cytokinins, ethylene, ROS scavenging genes, strigolactones, and aquaporins. We discuss how these pathways participate in growth and stress response regulation under drought. Finally, we present an overview of the current insights and future perspectives in the development of new strategies to improve drought tolerance in the field.

Mechanism of kinetin-induced death of Vicia faba ssp. minor root cortex cells

Cell death (CD) may be induced by endogenous or exogenous factors and contributes to all the steps of plant development. This paper presents results related to the mechanism of CD regulation induced by kinetin (Kin) in the root cortex of Vicia faba ssp. minor. To explain the process, 6-(2-hydroxy-3-methylbenzylamino)purine (PI-55), adenine (Ad), 5'-amine-5'-deoxyadenosine (Ado) and N-(2-chloro-4-piridylo)-N'-phenylurea (CPPU) were applied to (i) block cytokinin receptors (CKs) and inhibit the activities of enzymes of CK metabolism, i.e., (ii) phosphoribosyltransferase, (iii) kinases, and (iv) oxidases, respectively. Moreover, ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), lanthanum chloride (LaCl3), ruthenium red (RRed) and cyclosporine A (CS-A) were applied to (i) chelate extracellular calcium ions (Ca2+) as well as blocks of (ii) plasma-, (iii) endoplasmic reticulum- (ER) membrane Ca2+ ion channels and (iv) mitochondria- (MIT) Ca2+ ions release by permeability transition por (PTP), respectively. The measured physiological effectiveness of these factors was the number of living and dying cortex cells estimated with orange acridine (OA) and ethidium bromide (EB), the amounts of cytosolic Ca2+ ions with chlortetracycline (CTC) staining and the intensity of chromatin and Ca2+-CTC complex fluorescence, respectively. Moreover, the role of sorafenib, an inhibitor of RAF kinase, on the vitality of cortex cells and ethylene levels as well as the activities of RAF-like kinase and MEK2 with Syntide-2 and Mek2 as substrates were studied. The results clarified the previously presented suggestion that Kin is converted to appropriate ribotides (5'-monophosphate ribonucleotides), which cooperate with the ethylene and Ca2+ ion signalling pathways to transduce the signal of kinetin-programmed cell death (Kin-PCD). Based on the present and previously published results related to Kin-PCD, the crosstalk between ethylene and MAP kinase signalling, as well as inhibitors of CK receptors and enzymes of their metabolism, is proposed.

The B'ζ subunit of protein phosphatase 2A negatively regulates ethylene signaling in Arabidopsis

Ethylene is a major plant hormone that regulates plant growth, development, and defense responses to biotic and abiotic stresses. The major pieces of the ethylene signaling pathway have been put together, although several details still need to be elucidated. For instance, the phosphorylation and dephosphorylation processes controlling the ethylene responses are poorly understood and need to be further explored. The type 2A protein phosphatase (PP2A) was suggested to play an important role in the regulation of ethylene biosynthesis, where the A1 subunit of PP2A was shown to be involved in the regulation of the rate-limiting enzyme of the ethylene biosynthetic pathway. However, whether other subunits of PP2A play roles in the ethylene signal transduction pathway is yet to be answered. In this study, we demonstrate that a B subunit, PP2A-B'ζ, positively regulates plant germination and seedling development, as a pp2a-b'ζ mutant is very sensitive to ethylene treatment. Furthermore, PP2A-B'ζ interacts with and stabilizes the kinase CTR1 (Constitutive Triple Response 1), a key enzyme in the ethylene signal transduction pathway, and like CTR1, PP2A-B'ζ negatively regulates ethylene signaling in plants.

Identification of Raf-Like Kinases B Subfamily Genes in Gossypium Species Revealed GhRAF42 Enhanced Salt Tolerance in Cotton

Salinity is a critical abiotic factor that significantly reduces agricultural production. Cotton is an important fiber crop and a pioneer on saline soil, hence genetic architecture that underpins salt tolerance should be thoroughly investigated. The Raf-like kinase B-subfamily (RAF) genes were discovered to regulate the salt stress response in cotton plants. However, understanding the RAFs in cotton, such as Enhanced Disease Resistance 1 and Constitutive Triple Response 1 kinase, remains a mystery. This study obtained 29, 28, 56, and 54 RAF genes from G. arboreum, G. raimondii, G. hirsutum, and G. barbadense, respectively. The RAF gene family described allopolyploidy and hybridization events in allotetraploid cotton evolutionary connections. Ka/Ks analysis advocates that cotton evolution was subjected to an intense purifying selection of the RAF gene family. Interestingly, integrated analysis of synteny and gene collinearity suggested dispersed and segmental duplication events involved in the extension of RAFs in cotton. Transcriptome studies, functional validation, and virus-induced gene silencing on salt treatments revealed that GhRAF42 is engaged in salt tolerance in upland cotton. This research might lead to a better understanding of the role of RAFs in plants and the identification of suitable candidate salt-tolerant genes for cotton breeding.

Recent Advances in Phytohormone Regulation of Apple-Fruit Ripening

Apple (Malus domestica) is, globally, one of the largest fruits in terms of cultivated area and yield. Apple fruit is generally marketed after storage, which is of great significance for regulating the market supply in the off-season of fruit production. Apple-fruit ripening, which culminates in desirable changes in structural and textural properties, is governed by a complex regulatory network. Much is known about ethylene as one of the most important factors promoting apple-fruit ripening. However, the dynamic interplay between phytohormones also plays an important part in apple-fruit ripening. Here, we review and evaluate the complex regulatory network concerning the action of phytohormones during apple-fruit ripening. Interesting future research areas are discussed.

The RING E3 ligase SDIR1 destabilizes EBF1/EBF2 and modulates the ethylene response to ambient temperature fluctuations in Arabidopsis

The gaseous phytohormone ethylene mediates numerous aspects of plant growth and development as well as stress responses. The F-box proteins EIN3-binding F-box protein 1 (EBF1) and EBF2 are key components that ubiquitinate and degrade the master transcription factors ethylene insensitive 3 (EIN3) and EIN3-like 1 (EIL1) in the ethylene response pathway. Notably, EBF1 and EBF2 themselves undergo the 26S proteasome-mediated proteolysis induced by ethylene and other stress signals. However, despite their importance, little is known about the mechanisms regulating the degradation of these proteins. Here, we show that a really interesting new gene (RING)-type E3 ligase, salt- and drought-induced ring finger 1 (SDIR1), positively regulates the ethylene response and promotes the accumulation of EIN3. Further analyses indicate that SDIR1 directly interacts with EBF1/EBF2 and targets them for ubiquitination and proteasome-dependent degradation. We show that SDIR1 is required for the fine tuning of the ethylene response to ambient temperature changes by mediating temperature-induced EBF1/EBF2 degradation and EIN3 accumulation. Thus, our work demonstrates that SDIR1 functions as an important modulator of ethylene signaling in response to ambient temperature changes, thereby enabling plant adaptation under fluctuating environmental conditions.

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.

Genetic mapping and identification of a QTL determining tolerance to freezing stress in Fragaria vesca L

Extreme cold and frost cause significant stress to plants which can potentially be lethal. Low temperature freezing stress can cause significant and irreversible damage to plant cells and can induce physiological and metabolic changes that impact on growth and development. Low temperatures cause physiological responses including winter dormancy and autumn cold hardening in strawberry (Fragaria) species, and some diploid F. vesca accessions have been shown to have adapted to low-temperature stresses. To study the genetics of freezing tolerance, a F. vesca mapping population of 143 seedlings segregating for differential responses to freezing stress was raised. The progeny was mapped using ‘Genotyping-by-Sequencing’ and a linkage map of 2,918 markers at 851 loci was resolved. The mapping population was phenotyped for freezing tolerance response under controlled and replicated laboratory conditions and subsequent quantitative trait loci analysis using interval mapping revealed a single significant quantitative trait locus on Fvb2 in the physical interval 10.6 Mb and 15.73 Mb on the F. vesca v4.0 genome sequence. This physical interval contained 896 predicted genes, several of which had putative roles associated with tolerance to abiotic stresses including freezing. Differential expression analysis of the 896 QTL-associated gene predictions in the leaves and crowns from ‘Alta’ and ‘NCGR1363’ parental genotypes revealed genotype-specific changes in transcript accumulation in response to low temperature treatment as well as expression differences between genotypes prior to treatment for many of the genes. The putative roles, and significant interparental differential expression levels of several of the genes reported here identified them as good candidates for the control of the effects of freezing tolerance at the QTL identified in this investigation and the possible role of these candidate genes in response to freezing stress is discussed.

RPW8.1 enhances the ethylene-signaling pathway to feedback-attenuate its mediated cell death and disease resistance in Arabidopsis

The Arabidopsis RESISTANCE TO POWDERY MILDEW 8.1 (RPW8.1) activates confined cell death and defense against different pathogens. However, the underlying regulatory mechanisms still remain elusive. Here, we show that RPW8.1 activates ethylene signaling that, in turn, negatively regulates RPW8.1 expression. RPW8.1 binds and stabilizes 1-aminocyclopropane-1-carboxylate oxidase 4 (ACO4), which may in part explain increased ethylene production and signaling in RPW8.1-expressing plants. In return, ACO4 and other key components of ethylene signaling negatively regulate RPW8.1-mediated cell death and disease resistance via suppressing RPW8.1 expression. Loss of function in ACO4, EIN2, EIN3 EIL1, ERF6, ERF016 or ORA59 increases RPW8.1-mediated cell death and defense response. By contrast, overexpression of EIN3 abolishes or significantly compromises RPW8.1-mediated cell death and disease resistance. Furthermore, ERF6, ERF016 and ORA59 appear to act as trans-repressors of RPW8.1, with OAR59 being able to directly bind to the RPW8.1 promoter. Taken together, our results have revealed a feedback regulatory circuit connecting RPW8.1 and the ethylene-signaling pathway, in which RPW8.1 enhances ethylene signaling, and the latter, in return, negatively regulates RPW8.1-mediated cell death and defense response via suppressing RPW8.1 expression to attenuate its defense activity.

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.

TREE1-EIN3-mediated transcriptional repression inhibits shoot growth in response to ethylene

Ethylene is an important plant hormone that regulates plant growth, in which the master transcriptionactivator EIN3 (Ethylene Insensitive 3)-mediated transcriptional activation plays vital roles. However, the EIN3-mediated transcriptional repression in ethylene response is unknown. We report here that a Transcriptional Repressor of EIN3-dependent Ethylene-response 1 (TREE1) interacts with EIN3 to regulate transcriptional repression that leads to an inhibition of shoot growth in response to ethylene. Tissue-specific transcriptome analysis showed that most of the genes are down-regulated by ethylene in shoots, and a DNA binding motif was identified that is important for this transcriptional repression. TREE1 binds to the DNA motif to repress gene expression in an EIN3-dependent manner. Genetic validation demonstrated that repression of TREE1-targeted genes leads to an inhibition of shoot growth. Overall, this work establishes a mechanism by which transcriptional repressor TREE1 interacts with EIN3 to inhibit shoot growth via transcriptional repression in response to ethylene.

The fungal elicitor AsES requires a functional ethylene pathway to activate the innate immunity in strawberry

Acremonium strictum Elicitor Subtilisin (AsES) is a fungal elicitor that activates innate immunity, conferring disease resistance in strawberry (Fragaria × ananassa Duch.), Arabidopsis and other plant species. The aim of the present work was to evaluate the involvement of the ethylene (ET) signalling pathway in AsES-mediated immune response in strawberry. Ethylene production and expression of the genes responsible for ET synthesis, perception and response were measured after AsES treatment. ROS (H₂ O₂ ) accumulation and immunity induced by AsES were studied after ET perception was blocked by 1-methylcyclopropene (1-MCP). Biochemical and molecular results showed that AsES induced a marked increase in local and systemic biosynthesis of ET, both in a biphasic manner. Blocking of ET perception by 1-MCP prior to AsES induction reduced production of ROS (H₂ O₂ ) and prevented AsES from eliciting defence against fungal pathogens having different lifestyles, such as Botrytis cinerea (necrotrophic) and Colletotrichum acutatum (hemibiotrophic). These findings contribute to elucidate the mode of action of the novel elicitor subtilase, AsES, specifically regarding the role of ET signalling in the activation of plant innate immunity, in addition to the multitude of processes regulated by ET in plants.

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.

Investigation of the EIL/EIN3 Transcription Factor Gene Family Members and Their Expression Levels in the Early Stage of Cotton Fiber Development

The ethylene-insensitive3-like/ethylene-insensitive3 (EIL/EIN3) protein family can serve as a crucial factor for plant growth and development under diverse environmental conditions. EIL/EIN3 protein is a form of a localized nuclear protein with DNA-binding activity that potentially contributes to the intricate network of primary and secondary metabolic pathways of plants. In light of recent research advances, next-generation sequencing (NGS) and novel bioinformatics tools have provided significant breakthroughs in the study of the EIL/EIN3 protein family in cotton. In turn, this paved the way to identifying and characterizing the EIL/EIN3 protein family. Hence, the high-throughput, rapid, and cost-effective meta sequence analyses have led to a remarkable understanding of protein families in addition to the discovery of novel genes, enzymes, metabolites, and other biomolecules of the higher plants. Therefore, this work highlights the recent advance in the genomic-sequencing analysis of higher plants, which has provided a plethora of function profiles of the EIL/EIN3 protein family. The regulatory role and crosstalk of different metabolic pathways, which are apparently affected by these transcription factor proteins in one way or another, are also discussed. The ethylene hormone plays an important role in the regulation of reactive oxygen species in plants under various environmental stress circumstances. EIL/EIN3 proteins are the key ethylene-signaling regulators and play important roles in promoting cotton fiber developmental stages. However, the function of EIL/EIN3 during initiation and early elongation stages of cotton fiber development has not yet been fully understood. The results provided valuable information on cotton EIL/EIN3 proteins, as well as a new vision into the evolutionary relationships of this gene family in cotton species.

Expression profiling of CTR1-like and EIN2-like genes in buds and leaves of Populus tremula, and in vitro study of the interaction between their polypeptides

In Arabidopsis, the serine/threonine protein kinase Constitutive Triple Response 1 (CTR1) and Ethylene Insensitive 2 polypeptide (EIN2) functions are key negative and positive components, respectively, in the ethylene signalling route. Here, we report on an in silico study of members of the CTR1-like and EIN2-like polypeptide families from poplars. The expression of CTR1-like and EIN2-like genes such as Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a was studied in Populus tremula buds and leaves in response to dehydration, various light conditions and under senescence. In buds under dehydration, the maximal fold-change of the Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a expression level recorded almost identical values. This suggests that maintenance of a constant ratio between the transcript levels of genes encoding positive and negative ethylene signalling components is required under stress. The expression of the studied genes was 1.4-to 3-fold higher in response to darkness, but 4.5- to 51.2-fold and 21.6- to 51.2-fold higher under the early and moderate leaf senescence, respectively. It is worth noting that the senescence-related Ptre-EIN2a and Ptre-CTR3a expression profiles were very similar. Using in vitro assays, we demonstrated the ability of the catalytic domain of Ptre-CTR1 to phosphorylate the Ptre-EIN2a-like polypeptide, which is similar to that in Arabidopsis. The target substrate, the Ptre-CEND2a polypeptide (C-terminal part of Ptre-EIN2a), was only phosphorylated by the protein kinase Ptre-CTR1 and not by Ptre-CTR3. Moreover, the addition of Ptre-CTR3 polypeptides (-CTR3a or -CTR3b forms) to the reaction mixture had an inhibitory effect on Ptre-CTR1 auto- and trans-phosphorylation. In contrast to Ptre-CTR1, Ptre-CTR3 may act as a positive regulator in ethylene signalling in poplar; however, this hypothesis requires in vivo confirmation. Thus, the ethylene signalling route in poplar seems to be under the control of certain additional mechanisms which have not been reported in Arabidopsis.

The EIL transcription factor family in soybean: Genome-wide identification, expression profiling and genetic diversity analysis

The ETHYLENE INSENSITIVE3-LIKE (EIL) transcription factor family plays a critical role in the ethylene signaling pathway, which regulates a broad spectrum of plant growth and developmental processes, as well as defenses to myriad stresses. Although genome-wide analysis of this family has been carried out for several plant species, no comprehensive analysis of the EIL gene family in soybean has been reported so far. Furthermore, there are few studies on the functions of EIL genes in soybean. In this study, we identified 12 soybean (Gm) EIL genes, which we divided into three groups based on their phylogenetic relationships. We then detected their duplication status and found that most of the GmEIL genes have duplicated copies derived from two whole-genome duplication events. These duplicated genes underwent strong negative selection during evolution. We further analyzed the transcript profiles of GmEIL genes using the transcriptome data and found that their spatio-temporal and stress expression patterns varied considerably. For example, GmEIL1-GmEIL5 were found to be strongly expressed in almost every sample, while GmEIL8-GmEIL12 exhibited low expression, or were not expressed at all. Additionally, these genes showed different responses to dehydration, salinity and phosphate starvation. Finally, we surveyed genetic variations of these genes in 302 resequenced wild soybeans, landraces and improved soybean cultivars. Our data showed that most GmEIL genes are well conserved, and are not modified in domesticated or improved cultivars. Together, these findings provide a potentially valuable resource for characterizing the GmEIL gene family and lay the basis for further elucidation of their molecular mechanisms.

Three distinct regions of cRaf kinase domain interact with membrane

Raf kinases are downstream effectors of small GTPase Ras. Mutations in Ras and Raf are associated with a variety of cancers and genetic disorders. Of the three Raf isoforms, cRaf is most frequently involved in tumor initiation by Ras. Cytosolic Raf is auto-inhibited and becomes active upon recruitment to the plasma membrane. Since the catalytic domain of Raf is its kinase domain, we ask the following: does the kinase domain of Raf has potential to interact with membrane and if yes, what role does the membrane interaction play? We present a model of cRaf kinase domain in complex with a heterogeneous membrane bilayer using atomistic molecular dynamics simulation. We show that the kinase domain of cRaf has three distinct membrane-interacting regions: a polybasic motif (R.RKTR) from the regulatory αC-helix, an aromatic/hydrophobic cluster from the N-terminal acidic region (NtA) and positively charged/aromatic cluster from the activation segment (AS). We show that residues from these regions form an extended membrane-interacting surface that resembles the membrane-interacting residues from known membrane-binding domains. Activating phosphorylatable regions (NtA and AS), make direct contact with the membrane whereas R.RKTR forms specific multivalent salt bridges with PA. PA lipids dwell for longer times around the R.RKTR motif. Our results suggest that membrane interaction of monomeric cRaf kinase domain likely orchestrates the Raf activation process and modulates its function. We show that R.RKTR is a hotspot that interacts with membrane when cRaf is monomeric and becomes part of the interface upon Raf dimerization. We propose that in terms of utilizing a specific hotspot to form membrane interaction and dimer formation, both Raf and its upstream binding partner KRas, are similar.

Something ancient and something neofunctionalized-evolution of land plant hormone signaling pathways

The evolution of land plants from a charophycean algal ancestor was accompanied by an increased diversity of regulatory networks, including signaling pathways mediating cellular communication within plants and between plants and the environment. Canonical land plant hormone signaling pathways were originally identified in angiosperms, and comparative studies in basal taxa show that they have been assembled from both ancient and newly evolved components, both before and during land plant evolution. In this review we present our current understanding, and highlight several uncertainties, of the evolution of hormone signaling pathways, focusing on the biosynthetic pathways generating putative ligands and the downstream perception and signaling pathways often leading to transcriptional responses.

An ETHYLENE INSENSITIVE3-LIKE1 Protein Directly Targets the GEG Promoter and Mediates Ethylene-Induced Ray Petal Elongation in Gerbera hybrida

Petal morphogenesis has a profound influence on the quality of ornamental flowers. Most current research on petal development focuses on the early developmental stage, and little is known about the late developmental stage. Previously, it was reported that the GEG gene [a gerbera homolog of the gibberellin-stimulated transcript 1 (GAST1) from tomato] negatively regulates ray petal growth during the late stage of development by inhibiting longitudinal cell expansion. To explore the molecular mechanisms of the role of GEG in petal growth inhibition, an ethylene insensitive 3-like 1 (EIL1) protein was identified from a Gerbera hybrida cDNA library by yeast one-hybrid screening. Direct binding between GhEIL1 and the GEG promoter was confirmed by electrophoretic mobility shift and dual-luciferase assays. The expression profiles of GhEIL1 and GEG were correlated during petal development, while a transient transformation assay suggested that GhEIL1 regulates GEG expression and may be involved in the inhibition of ray petal elongation and cell elongation. To study the effect of ethylene on ray petal growth, a hormone treatment assay was performed in detached ray petals. The results showed that petal elongation is limited and promoted by ACC and 1-MCP, respectively, and the expression of GhEIL1 and GEG is regulated and coordinated during this process. Taken together, our research suggests that GhEIL1 forms part of the ethylene signaling pathway and activates GEG to regulate ray petal growth during the late developmental stage in G. hybrida.

New Insights in Transcriptional Regulation of the Ethylene Response in Arabidopsis

As any living organisms, plants must respond to a wide variety of environmental stimuli. Plant hormones regulate almost all aspects of plant growth and development. Among all the plant hormones, ethylene is the only gaseous plant hormone that plays pleiotropic roles in plant growth, plant development and stress responses. Transcription regulation is one main mechanism by which a cell orchestrates gene activity. This control allows the cell or organism to respond to a variety of intra- and extracellular signals and thus mount a response. Here we review the progress of transcription regulation in the ethylene response.