Ethylene hormone receptor action in Arabidopsis

CmERF1 acts as a positive regulator of fruits and leaves growth in melon (Cucumis melo L.)

Melon (Cucumis melo L.) is an important horticultural and economic crop. ETHYLENE RESPONSE FACTOR1 (ERF1) plays an important role in regulating plant development, and the resistance to multiple biotic and abiotic stresses. In this study, developmental biology, molecular biology and biochemical assays were performed to explore the biological function of CmERF1 in melon. Abundant transcripts of CmERF1 were found in ovary at green-yellow bud (GYB) and rapid enlargement (ORE) stages. In CmERF1 promoter, the cis-regulatory elements for indoleacetic acid (IAA), methyl jasmonate (MeJA), salicylic acid (SA), abscisic acid (ABA), gibberellic acid (GA), light and low temperature responses were found. CmERF1 could be significantly induced by ethylene, IAA, MeJA, SA, ABA, and respond to continuous light and low temperature stresses in melon. Ectopic expression of CmERF1 increased the length of siliqua and carpopodium, and expanded the size of leaves in Arabidopsis. Knockdown of CmERF1 led to smaller ovary at anthesis, mature fruit and leaves in melon. In CmERF1-RNAi #2 plants, 75 genes were differently expressed compared with control, and the promoter regions of 28 differential expression genes (DEGs) contained the GCC-box (AGCCGCC) or DRE (A/GCCGAC) cis-acting elements of CmERF1. A homolog of cell division cycle protein 48 (CmCDC48) was proved to be the direct target of CmERF1 by the yeast one-hybrid assay and dual-luciferase (LUC) reporter (DLR) system. These results indicated that CmERF1 was able to promote the growth of fruits and leaves, and involved in multiple hormones and environmental signaling pathways in melon.

Differential gene expression during floral transition in pineapple

Pineapple (Ananas comosus var. comosus) and ornamental bromeliads are commercially induced to flower by treatment with ethylene or its analogs. The apex is transformed from a vegetative to a floral meristem and shows morphological changes in 8 to 10 days, with flowers developing 8 to 10 weeks later. During eight sampling stages ranging from 6 h to 8 days after treatment, 7961 genes were found to exhibit differential expression (DE) after the application of ethylene. In the first 3 days after treatment, there was little change in ethylene synthesis or in the early stages of the ethylene response. Subsequently, three ethylene response transcription factors (ERTF) were up-regulated and the potential gene targets were predicted to be the positive flowering regulator CONSTANS-like 3 (CO), a WUSCHEL gene, two APETALA1/FRUITFULL (AP1/FUL) genes, an epidermal patterning gene, and a jasmonic acid synthesis gene. We confirm that pineapple has lost the flowering repressor FLOWERING LOCUS C. At the initial stages, the SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was not significantly involved in this transition. Another WUSCHEL gene and a PHD homeobox transcription factor, though not apparent direct targets of ERTF, were up-regulated within a day of treatment, their predicted targets being the up-regulated CO, auxin response factors, SQUAMOSA, and histone H3 genes with suppression of abscisic acid response genes. The FLOWERING LOCUS T (FT), TERMINAL FLOWER (TFL), AGAMOUS-like APETELAR (AP2), and SEPETALA (SEP) increased rapidly within 2 to 3 days after ethylene treatment. Two FT genes were up-regulated at the apex and not at the leaf bases after treatment, suggesting that transport did not occur. These results indicated that the ethylene response in pineapple and possibly most bromeliads act directly to promote the vegetative to flower transition via APETALA1/FRUITFULL (AP1/FUL) and its interaction with SPL, FT, TFL, SEP, and AP2. A model based on AP2/ERTF DE and predicted DE target genes was developed to give focus to future research. The identified candidate genes are potential targets for genetic manipulation to determine their molecular role in flower transition.

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.

Cytokinin regulates apical hook development via the coordinated actions of EIN3/EIL1 and PIF transcription factors in Arabidopsis

The apical hook is indispensable for protecting the delicate shoot apical meristem while dicot seedlings emerge from soil after germination in darkness. The development of the apical hook is co-ordinately regulated by multiple phytohormones and environmental factors. Yet, a holistic understanding of the spatial-temporal interactions between different phytohormones and environmental factors remains to be achieved. Using a chemical genetic approach, we identified kinetin riboside, as a proxy of kinetin, which promotes apical hook development of Arabidopsis thaliana in a partially ethylene-signaling-independent pathway. Further genetic and biochemical analysis revealed that cytokinin is able to regulate apical hook development via post-transcriptional regulation of the PHYTOCHROME INTERACTING FACTORs (PIFs), together with its canonical roles in inducing ethylene biosynthesis. Dynamic observations of apical hook development processes showed that ETHYLENE INSENSITVE3 (EIN3) and EIN3-LIKE1 (EIL1) are necessary for the exaggeration of hook curvature in response to cytokinin, while PIFs are crucial for the cytokinin-induced maintenance of hook curvature in darkness. Furthermore, these two families of transcription factors display divergent roles in light-triggered hook opening. Our findings reveal that cytokinin integrates ethylene signaling and light signaling via EIN3/EIL1 and PIFs, respectively, to dynamically regulate apical hook development during early seedling development.

Verticillium dahliae secretory effector PevD1 induces leaf senescence by promoting ORE1-mediated ethylene biosynthesis

Leaf senescence, the final stage of leaf development, is influenced by numerous internal and environmental signals. However, how biotic stresses such as pathogen infection regulate leaf senescence remains largely unclear. In this study, we found that the premature leaf senescence in Arabidopsis caused by the soil-borne vascular fungus Verticillium dahliae was impaired by disruption of a protein elicitor from V. dahliae 1 named PevD1. Constitutive or inducible overexpression of PevD1 accelerated Arabidopsis leaf senescence. Interestingly, a senescence-associated NAC transcription factor, ORE1, was targeted by PevD1. PevD1 could interact with and stabilize ORE1 protein by disrupting its interaction with the RING-type ubiquitin E3 ligase NLA. Mutation of ORE1 suppressed the premature senescence caused by overexpressing PevD1, whereas overexpression of ORE1 or PevD1 led to enhanced ethylene production and thereby leaf senescence. We showed that ORE1 directly binds the promoter of ACS6 and promotes its expression for mediating PevD1-induced ethylene biosynthesis. Loss-of-function of ACSs could suppress V. dahliae-induced leaf senescence in ORE1-overexpressing plants. Furthermore, we found thatPevD1 also interacts with Gossypium hirsutum ORE1 (GhORE1) and that virus-induced gene silencing of GhORE1 delays V. dahliae-triggered leaf senescence in cotton, indicating a possibly conserved mechanism in plants. Taken together, these results suggest that V. dahliae induces leaf senescence by secreting the effector PevD1 to manipulate the ORE1-ACS6 cascade, providing new insights into biotic stress-induced senescence in plants.

Retrograde signalling from the mitochondria to the nucleus translates the positive effect of ethylene on dormancy breaking of Arabidopsis thaliana seeds

Ethylene and reactive oxygen species (ROS) regulate seed dormancy alleviation, but the molecular basis of their action and crosstalk remains largely unknown. Here we studied the mechanism of Arabidopsis seed dormancy release by ethylene using cell imaging, and genetic and transcriptomics approaches, in order to tackle its possible interaction with ROS homeostasis. We found that the effect of ethylene on seed germination required ROS production by the mitochondrial electron transport chain. Seed response to ethylene involved a mitochondrial retrograde response (MRR) through nuclear ROS production and upregulation of the MRR components AOX1a and ANAC013, but also required the activation of the ethylene canonical pathway. Together our data allowed deciphering of the mode of action of ethylene on seed germination and the associated dynamics of ROS production. Our findings highlight the occurrence of retrograde signalling in seed germination.

Target of Rapamycin (TOR) Negatively Regulates Ethylene Signals in Arabidopsis

Target of rapamycin (TOR) acts as a master regulator in coordination of cell growth with energy and nutrient availability. Despite the increased appreciation of the essential role of the TOR complex in interaction with phytohormone signaling, little is known about its function on ethylene signaling. Here, through expression analysis, genetic and biochemical approaches, we reveal that TOR functions in the regulation of ethylene signals. Transcriptional analysis indicates that TOR inhibition by AZD8055 upregulated senescence- and ethylene-related genes expression. Furthermore, ethylene insensitive mutants like etr1-1, ein2-5 and ein3 eil1, showed more hyposensitivity to AZD8055 than that of WT in hypocotyl growth inhibition. Similarly, blocking ethylene signals by ethylene action inhibitor Ag+ or biosynthesis inhibitor aminoethoxyvinylglycine (AVG) largely rescued hypocotyl growth even in presence of AZD8055. In addition, we also demonstrated that Type 2A phosphatase-associated protein of 46 kDa (TAP46), a downstream component of TOR signaling, physically interacts with 1-aminocy-clopropane-1-carboxylate (ACC) synthase ACS2 and ACS6. Arabidopsis overexpressing ACS2 or ACS6 showed more hypersensitivity to AZD8055 than WT in hypocotyl growth inhibition. Moreover, ACS2/ACS6 protein was accumulated under TOR suppression, implying TOR modulates ACC synthase protein levels. Taken together, our results indicate that TOR participates in negatively modulating ethylene signals and the molecular mechanism is likely involved in the regulation of ethylene biosynthesis by affecting ACSs in transcription and protein levels.

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.

Assessment of Local and Systemic Changes in Plant Gene Expression and Aphid Responses during Potato Interactions with Arbuscular Mycorrhizal Fungi and Potato Aphids

This research examined aphid and plant responses to distinct levels (none, low, and high) of arbuscular mycorrhizal (AM) fungal root colonization by studying the association between potato aphids (Macrosiphum euphorbiae), potatoes (Solanum tuberosum), and AM fungi (Rhizophagus intraradices). It extends knowledge on gene expression changes, assessed by RT-qPCR, of ten defense-related genes at two time-points post-herbivory (24 h and 10 days), focusing on aphid-infested local leaves, non-infested systemic leaves, and roots. The results showed that aphid fitness was not altered by AM symbiosis. At 24 h, ETHYLENE RECEPTOR 1 gene expression was repressed in roots of aphid-infested non-mycorrhizal plants and aphid-infested plants with a high level of AM fungal root colonization, but not on aphid-infested plants with a low level of AM fungal root colonization. At 10 days, ALLENE OXIDE CYCLASE and POTATO TYPE I PROTEASE INHIBITOR were upregulated exclusively in local leaves of aphid-infested plants with a low level of AM fungal root colonization. In addition, local and systemic changes in plant gene expression appeared to be regulated exclusively by AM status and aphid herbivory. In summary, the gene expression data provide insights on mycorrhizal potato responses to aphid herbivory and serve as a starting point for future studies using this system.

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.

Study on differentially expressed genes related to defoliation traits in two alfalfa varieties based on RNA-Seq

Background

Alfalfa (Medicago sativa) is a widely cultivated, essential commercial forage crop. The protein content in its leaves is the critical factor in determining the quality of alfalfa. Thus far, the understanding of the molecular mechanism of alfalfa defoliation traits remains unclear. The transcriptome database created by RNA-Seq is used to identify critical genes related to defoliation traits.

Results

In this study, we sequenced the transcriptomes of the Zhungeer variety (with easy leaf abscission) and WL319HQ variety (without easy leaf abscission). Among the identified 66,734 unigenes, 706 differentially expressed genes (DEGs) upregulated, and 392 unigenes downregulated in the Zhungeer vs WL319HQ leaf. KEGG pathway annotations showed that 8,414 unigenes were annotated to 87 pathways and contained 281 DEGs. Six DEGs belonging to the “Carotenoid biosynthesis”, “Plant hormone signal transduction” and “Circadian rhythm-plant” pathways involved in defoliation traits were identified and validated by RT-qPCR analyses.

Conclusions

This study used RNA-Seq to discover genes associated with defoliation traits between two alfalfa varieties. Our transcriptome data dramatically enriches alfalfa functional genomic studies. In addition, these data provide theoretical guidance for field production practice and genetic breeding, as well as references for future study of defoliation traits in alfalfa.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5180-1) contains supplementary material, which is available to authorized users.

VOCs-mediated hormonal signaling and crosstalk with plant growth promoting microbes

In the natural environment, plants communicate with various microorganisms (pathogenic or beneficial) and exhibit differential responses. In recent years, research on microbial volatile compounds (MVCs) has revealed them to be simple, effective and efficient groups of compounds that modulate plant growth and developmental processes. They also interfere with the signaling process. Different MVCs have been shown to promote plant growth via improved photosynthesis rates, increased plant resistance to pathogens, activated phytohormone signaling pathways, or, in some cases, inhibit plant growth, leading to death. Regardless of these exhibited roles, the molecules responsible, the underlying mechanisms, and induced specific metabolic/molecular changes are not fully understood. Here, we review current knowledge on the effects of MVCs on plants, with particular emphasis on their modulation of the salicylic acid, jasmonic acid/ethylene, and auxin signaling pathways. Additionally, opportunities for further research and potential practical applications presented.

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.

Genome Wide Identification and Expression Profiling of Ethylene Receptor Genes during Soybean Nodulation

It has long been known that the gaseous plant hormone ethylene plays a key role in nodulation in legumes. The perception of ethylene by a family of five membrane-localized receptors is necessary to trigger the ethylene signaling pathway, which regulates various biological responses in Arabidopsis. However, a systematic analysis of the ethylene receptors in leguminous plants and their roles in nodule development is lacking. In this study, we performed a characterization of ethylene receptor genes based on the latest Glycine max genome sequence and a public microarray database. Eleven ethylene receptor family genes were identified in soybean through homology searches, and they were divided into two subgroups. Exon-intron analysis showed that the gene structures are highly conserved within each group. Further analysis of their expression patterns showed that these ethylene receptor genes are differentially expressed in various soybean tissues and organs, including functional nodules. Notably, the ethylene receptor genes showed different responses to rhizobial infection and Nod factors, suggesting a possible role for ethylene receptors and ethylene signaling in rhizobia-host cell interactions and nodulation in soybean. Together, these data indicate the functional divergence of ethylene receptor genes in soybean, and that some of these receptors mediate nodulation, including rhizobial infection, nodule development, and nodule functionality. These findings provide a foundation for further elucidation of the molecular mechanism by which the ethylene signaling pathway regulates nodulation in soybean, as well as other legumes.

Co-infection of Sweet Orange with Severe and Mild Strains of Citrus tristeza virus Is Overwhelmingly Dominated by the Severe Strain on Both the Transcriptional and Biological Levels

Citrus tristeza is one of the most destructive citrus diseases and is caused by the phloem-restricted Closterovirus, Citrus tristeza virus. Mild strain CTV-B2 does not cause obvious symptoms on indicators whereas severe strain CTV-B6 causes symptoms, including stem pitting, cupping, yellowing, and stiffening of leaves, and vein corking. Our laboratory has previously characterized changes in transcription in sweet orange separately infected with CTV-B2 and CTV-B6. In the present study, transcriptome analysis of Citrus sinensis in response to double infection by CTV-B2 and CTV-B6 was carried out. Four hundred and eleven transcripts were up-regulated and 356 transcripts were down-regulated prior to the onset of symptoms. Repressed genes were overwhelmingly associated with photosynthesis, and carbon and nucleic acid metabolism. Expression of genes related to the glycolytic, oxidative pentose phosphate (OPP), tricarboxylic acid cycle (TCA) pathways, tetrapyrrole synthesis, redox homeostasis, nucleotide metabolism, protein synthesis and post translational protein modification and folding, and cell organization were all reduced. Ribosomal composition was also greatly altered in response to infection by CTV-B2/CTV-B6. Genes that were induced were related to cell wall structure, secondary and hormone metabolism, responses to biotic stress, regulation of transcription, signaling, and secondary metabolism. Transport systems dedicated to metal ions were especially disturbed and ZIPs (Zinc Transporter Precursors) showed different expression patterns in response to co-infection by CTV-B2/CTV-B6 and single infection by CTV-B2. Host plants experienced root decline that may have contributed to Zn, Fe, and other nutrient deficiencies. Though defense responses, such as, strengthening of the cell wall, alteration of hormone metabolism, secondary metabolites, and signaling pathways, were activated, these defense responses did not suppress the spread of the pathogens and the development of symptoms. The mild strain CTV-B2 did not provide a useful level of cross-protection to citrus against the severe strain CTV-B6.

Molecular Cloning and Characterization of Four Genes Encoding Ethylene Receptors Associated with Pineapple (Ananas comosus L.) Flowering

Exogenous ethylene, or ethephon, has been widely used to induce pineapple flowering, but the molecular mechanism behind ethephon induction is still unclear. In this study, we cloned four genes encoding ethylene receptors (designated AcERS1a, AcERS1b, AcETR2a, and AcETR2b). The 5' flanking sequences of these four genes were also cloned by self-formed adaptor PCR and SiteFinding-PCR, and a group of putative cis-acting elements was identified. Phylogenetic tree analysis indicated that AcERS1a, AcERS1b, AcETR2a, and AcETR2b belonged to the plant ERS1s and ETR2/EIN4-like groups. Quantitative real-time PCR showed that AcETR2a and AcETR2b (subfamily 2) were more sensitive to ethylene treatment compared with AcERS1a and AcERS1b (subfamily 1). The relative expression of AcERS1b, AcETR2a, and AcETR2b was significantly increased during the earlier period of pineapple inflorescence formation, especially at 1-9 days after ethylene treatment (DAET), whereas AcERS1a expression changed less than these three genes. In situ hybridization results showed that bract primordia (BP) and flower primordia (FP) appeared at 9 and 21 DAET, respectively, and flowers were formed at 37 DAET. AcERS1a, AcERS1b, AcETR2a, and AcETR2b were mainly expressed in the shoot apex at 1-4 DAET; thereafter, with the appearance of BP and FP, higher expression of these genes was found in these new structures. Finally, at 37 DAET, the expression of these genes was mainly focused in the flower but was also low in other structures. These findings indicate that these four ethylene receptor genes, especially AcERS1b, AcETR2a, and AcETR2b, play important roles during pineapple flowering induced by exogenous ethephon.

Arabidopsis ERF1 Mediates Cross-Talk between Ethylene and Auxin Biosynthesis during Primary Root Elongation by Regulating ASA1 Expression

The gaseous phytohormone ethylene participates in the regulation of root growth and development in Arabidopsis. It is known that root growth inhibition by ethylene involves auxin, which is partially mediated by the action of the WEAK ETHYLENE INSENSITIVE2/ANTHRANILATE SYNTHASE α1 (WEI2/ASA1), encoding a rate-limiting enzyme in tryptophan (Trp) biosynthesis, from which auxin is derived. However, the molecular mechanism by which ethylene decreases root growth via ASA1 is not understood. Here we report that the ethylene-responsive AP2 transcription factor, ETHYLENE RESPONSE FACTOR1 (ERF1), plays an important role in primary root elongation of Arabidopsis. Using loss- and gain-of-function transgenic lines as well as biochemical analysis, we demonstrate that ERF1 can directly up-regulate ASA1 by binding to its promoter, leading to auxin accumulation and ethylene-induced inhibition of root growth. This discloses one mechanism linking ethylene signaling and auxin biosynthesis in Arabidopsis roots.

Ethylene is a gaseous phytohormone that plays critical roles in plant development and defense. It is well known that ethylene inhibits primary root elongation through effects on auxin. However, it is not clear how ethylene signal is translated into auxin. In this report, the highly ethylene-responsive transcription factor ETHYLENE RESPONSE FACTOR1 (ERF1) is demonstrated to positively regulate ASA1, encoding ANTHRANILATE SYNTHASE α1, a rate-limiting enzyme in Trp biosynthesis where auxin is derived, by directly binding to its promoter and activating ASA1. Consequently, auxin biosynthesis is promoted, leading to increased auxin accumulation and thus inhibition of primary root elongation. This study unravels a molecular mechanism that bridges ethylene signaling and auxin biosynthesis in primary root elongation.

NO Promotes Seed Germination and Seedling Growth Under High Salt May Depend on EIN3 Protein in Arabidopsis

The gas molecule nitric oxide (NO) can cooperate with ethylene to tightly modulate plant growth and stress responses. One of the mechanism of their crosstalk is that NO is able to activate ethylene biosynthesis, possibly through post-translational modification of key enzymes such as ACC synthase and oxidase by S-nitrosylation. In this paper, we focus on the crosstalk of NO with ethylene signaling transduction transcription factor EIN3 (Ethylene Insensitive 3) and downstream gene expression in alleviating germination inhibition and growth damage induced by high salt. The Arabidopsis lines affected in ethylene signaling (ein3eil1) and NO biosynthesis (nia1nia2) were employed to compare with the wild-type Col-0 and overexpressing line EIN3ox. Firstly, the obviously inhibited germination, greater ratio of bleached leaves and enhanced electrolyte leakage were found in ein3eil1 and nia1nia2 lines than in Col-0 plants upon high salinity. However, the line EIN3ox obtained a notably elevated ability to germinate and improved seedling resistance. The experiment with SNP alone or plus high salt mostly enhanced the expression of EIN3 transcripts, compared with ACO4 and ACS2. The western blot and transcript analysis found that high-salt-induced EIN3 stabilization and EIN3 transcripts were largely attenuated in the NO biogenesis mutant nia1nia2 plants than in Col-0 ones. This observation was confirmed by simulation experiments with NO scavenger cPTIO to block NO emission. Taken together, our study provides insights that NO promotes seed germination and seedlings growth under salinity may depend on EIN3 protein.

Ethylene Regulates Energy-Dependent Non-Photochemical Quenching in Arabidopsis through Repression of the Xanthophyll Cycle

Energy-dependent (qE) non-photochemical quenching (NPQ) thermally dissipates excess absorbed light energy as a protective mechanism to prevent the over reduction of photosystem II and the generation of reactive oxygen species (ROS). The xanthophyll cycle, induced when the level of absorbed light energy exceeds the capacity of photochemistry, contributes to qE. In this work, we show that ethylene regulates the xanthophyll cycle in Arabidopsis. Analysis of eto1-1, exhibiting increased ethylene production, and ctr1-3, exhibiting constitutive ethylene response, revealed defects in NPQ resulting from impaired de-epoxidation of violaxanthin by violaxanthin de-epoxidase (VDE) encoded by NPQ1. Elevated ethylene signaling reduced the level of active VDE through decreased NPQ1 promoter activity and impaired VDE activation resulting from a lower transthylakoid membrane pH gradient. Increasing the concentration of CO₂ partially corrected the ethylene-mediated defects in NPQ and photosynthesis, indicating that changes in ethylene signaling affect stromal CO₂ solubility. Increasing VDE expression in eto1-1 and ctr1-3 restored light-activated de-epoxidation and qE, reduced superoxide production and reduced photoinhibition. Restoring VDE activity significantly reversed the small growth phenotype of eto1-1 and ctr1-3 without altering ethylene production or ethylene responses. Our results demonstrate that ethylene increases ROS production and photosensitivity in response to high light and the associated reduced plant stature is partially reversed by increasing VDE activity.

Lace plant ethylene receptors, AmERS1a and AmERS1c, regulate ethylene-induced programmed cell death during leaf morphogenesis

The lace plant, Aponogeton madagascariensis, is an aquatic monocot that forms perforations in its leaves as part of normal leaf development. Perforation formation occurs through developmentally regulated programmed cell death (PCD). The molecular basis of PCD regulation in the lace plant is unknown, however ethylene has been shown to play a significant role. In this study, we examined the role of ethylene receptors during perforation formation. We isolated three lace plant ethylene receptors AmERS1a, AmERS1b and AmERS1c. Using quantitative PCR, we examined their transcript levels at seven stages of leaf development. Through laser-capture microscopy, transcript levels were also determined in cells undergoing PCD and cells not undergoing PCD (NPCD cells). AmERS1a transcript levels were significantly lower in window stage leaves (in which perforation formation and PCD are occurring) as compared to all other leaf developmental stages. AmERS1a and AmERS1c (the most abundant among the three receptors) had the highest transcript levels in mature stage leaves, where PCD is not occurring. Their transcript levels decreased significantly during senescence-associated PCD. AmERS1c had significantly higher transcript levels in NPCD compared to PCD cells. Despite being significantly low in window stage leaves, AmERS1a transcripts were not differentially expressed between PCD and NPCD cells. The results suggested that ethylene receptors negatively regulate ethylene-controlled PCD in the lace plant. A combination of ethylene and receptor levels determines cell fate during perforation formation and leaf senescence. A new model for ethylene emission and receptor expression during lace plant perforation formation and senescence is proposed.

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.

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.

Ethylene-orchestrated circuitry coordinates a seedling's response to soil cover and etiolated growth

The early life of terrestrial seed plants often starts under the soil in subterranean darkness. Over time and through adaptation, plants have evolved an elaborate etiolation process that enables seedlings to emerge from soil and acquire autotrophic ability. This process, however, requires seedlings to be able to sense the soil condition and relay this information accordingly to modulate both the seedlings' growth and the formation of photosynthetic apparatus. The mechanism by which soil overlay drives morphogenetic changes in plants, however, remains poorly understood, particularly with regard to the means by which the cellular processes of different organs are coordinated in response to disparate soil conditions. Here, we illustrate that the soil overlay quantitatively activates seedlings' ethylene production, and an EIN3/EIN3-like 1-dependent ethylene-response cascade is required for seedlings to successfully emerge from the soil. Under soil, an ERF1 pathway is activated in the hypocotyl to slow down cell elongation, whereas a PIF3 pathway is activated in the cotyledon to control the preassembly of photosynthetic machinery. Moreover, this latter PIF3 pathway appears to be coupled to the ERF1-regulated upward-growth rate. The coupling of these two pathways facilitates the synchronized progression of etioplast maturation and hypocotyl growth, which, in turn, ultimately enables seedlings to maintain the amount of protochlorophyllide required for rapid acquisition of photoautotrophic capacity without suffering from photooxidative damage during the dark-to-light transition. Our findings illustrate the existence of a genetic signaling pathway driving soil-induced plant morphogenesis and define the specific role of ethylene in orchestrating organ-specific soil responses in Arabidopsis seedlings.

Medicago truncatula histidine-containing phosphotransfer protein: structural and biochemical insights into the cytokinin transduction pathway in plants

Histidine-containing phosphotransfer proteins (HPts) take part in hormone signal transduction in higher plants. The overall pathway of this process is reminiscent of the two-component system initially identified in prokaryotes. HPts function in histidine-aspartate phosphorelays in which they mediate the signal from sensory kinases (usually membrane proteins) to RRs in the nucleus. Here, we report the crystal structure of an HPt protein from Medicago truncatula (MtHPt1) determined at 1.45 Å resolution and refined to an R-factor of 16.7% using low-temperature synchrotron-radiation X-ray diffraction data. There is one MtHPt1 molecule in the asymmetric unit of the crystal lattice with P2(1)2(1)2(1) symmetry. The protein fold consists of six α helices, four of which form a C-terminal helix bundle. The coiled-coil structure of the bundle is stabilized by a network of S-aromatic interactions involving highly conserved sulfur-containing residues. The structure reveals a solvent-exposed side chain of His79, which is the phosphorylation site, as demonstrated by autoradiography combined with site-directed mutation. It is surrounded by highly conserved residues present in all plant HPts. These residues form a putative docking interface for either the receiver domain of the sensory kinase, or for the RR. The biological activity of MtHPt1 was tested by autoradiography. It demonstrated phosphorylation by the intracellular kinase domain of the cytokinin receptor MtCRE1. Complex formation between MtHPt1 and the intracellular fragment of MtCRE1 was confirmed by thermophoresis, with a dissociation constant K(d) of 14 μM.

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

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

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

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

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

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

Genetics and control of tomato fruit ripening and quality attributes

Tomato ripening is a highly coordinated developmental process that coincides with seed maturation. Regulated expression of thousands of genes controls fruit softening as well as accumulation of pigments, sugars, acids, and volatile compounds that increase attraction to animals. A combination of molecular tools and ripening-affected mutants has permitted researchers to establish a framework for the control of ripening. Tomato is a climacteric fruit, with an absolute requirement for the phytohormone ethylene to ripen. This dependence upon ethylene has established tomato fruit ripening as a model system for study of regulation of its synthesis and perception. In addition, several important ripening mutants, including rin, nor, and Cnr, have provided novel insights into the control of ripening processes. Here, we describe how ethylene and the transcription factors associated with the ripening process fit together into a network controlling ripening.

Information theory and the ethylene genetic network

The original aim of the Information Theory (IT) was to solve a purely technical problem: to increase the performance of communication systems, which are constantly affected by interferences that diminish the quality of the transmitted information. That is, the theory deals only with the problem of transmitting with the maximal precision the symbols constituting a message. In Shannon's theory messages are characterized only by their probabilities, regardless of their value or meaning. As for its present day status, it is generally acknowledged that Information Theory has solid mathematical foundations and has fruitful strong links with Physics in both theoretical and experimental areas. However, many applications of Information Theory to Biology are limited to using it as a technical tool to analyze biopolymers, such as DNA, RNA or protein sequences. The main point of discussion about the applicability of IT to explain the information flow in biological systems is that in a classic communication channel, the symbols that conform the coded message are transmitted one by one in an independent form through a noisy communication channel, and noise can alter each of the symbols, distorting the message; in contrast, in a genetic communication channel the coded messages are not transmitted in the form of symbols but signaling cascades transmit them. Consequently, the information flow from the emitter to the effector is due to a series of coupled physicochemical processes that must ensure the accurate transmission of the message. In this review we discussed a novel proposal to overcome this difficulty, which consists of the modeling of gene expression with a stochastic approach that allows Shannon entropy (H) to be directly used to measure the amount of uncertainty that the genetic machinery has in relation to the correct decoding of a message transmitted into the nucleus by a signaling pathway. From the value of H we can define a function I that measures the amount of information content in the input message that the cell's genetic machinery is processing during a given time interval. Furthermore, combining Information Theory with the frequency response analysis of dynamical systems we can examine the cell's genetic response to input signals with varying frequencies, amplitude and form, in order to determine if the cell can distinguish between different regimes of information flow from the environment. In the particular case of the ethylene signaling pathway, the amount of information managed by the root cell of Arabidopsis can be correlated with the frequency of the input signal. The ethylene signaling pathway cuts off very low and very high frequencies, allowing a window of frequency response in which the nucleus reads the incoming message as a varying input. Outside of this window the nucleus reads the input message as an approximately non-varying one. This frequency response analysis is also useful to estimate the rate of information transfer during the transport of each new ERF1 molecule into the nucleus. Additionally, application of Information Theory to analysis of the flow of information in the ethylene signaling pathway provides a deeper insight in the form in which the transition between auxin and ethylene hormonal activity occurs during a circadian cycle. An ambitious goal for the future would be to use Information Theory as a theoretical foundation for a suitable model of the information flow that runs at each level and through all levels of biological organization.

Expression of ACO1, ERS1 and ERF1 genes in harvested bananas in relation to heat-induced defense against Colletotrichum musae

The aim of this study was to investigate the connection between heat-induced ethylene signal changes and enhanced disease resistance. Heat enhanced ripening and elevated MaACO1 expression in naturally ripened bananas (NRB), while it delayed ripening and reduced MaACO1expression in the ethephon-treated bananas (ETB). However, in both cases, heat reduced lesion sizes infected by Colletotrichum musae. This indicates that heat-induced disease resistance in bananas was independent of ripening rate. The expression of MaERS1 gene was inhibited by heat treatment in both NRB and ETB, implying that heat as a physical signal could be sensed by banana fruits through the inhibition of ethylene receptor gene expression. The intensity of MaERF1 transcript signals was elevated in heated bananas, suggesting that the enhanced accumulation of MaERF1 transcript following heat treatment could play an important role in activation of the defense system. In ETB, inhibition of JA biosynthesis by application of IBU down-regulated the expression of MaERF and significantly weakened disease resistance, suggesting involvement of endogenous JA in induction of the gene expression, which was reconfirmed by the fact that exposure to exogenous MeJA following the combination of heat plus IBU treatment restored part of the gene expression. On the other hand, in NRB, application of IBU elevated level of MaERF1 expression at 24h and enhanced disease resistance, suggesting that, when banana was not exposed to ethephon, the expression of MaERF1 gene was not JA dependent, which was verified by the fact that MeJA application did not enhance MaERF1 gene expression. In conclusion, heat-induced disease resistance in harvested bananas could involve down-regulation of MaERS1 expression and up-regulation of MaERF1 expression and JA pathway could be involved in heat activation of the defense system in bananas exposed to ethephon.

Two-component systems and their co-option for eukaryotic signal transduction

Two-component signaling pathways involve histidine kinases, response regulators, and sometimes histidine-containing phosphotransfer proteins. Prevalent in prokaryotes, these signaling elements have also been co-opted to meet the needs of signal transduction in eukaryotes such as fungi and plants. Here we consider the evolution of such regulatory systems, with a particular emphasis on the roles they play in signaling by the plant hormones cytokinin and ethylene, in phytochrome-mediated perception of light, and as integral components of the circadian clock.

Phosphorylation alters the interaction of the Arabidopsis phosphotransfer protein AHP1 with its sensor kinase ETR1

The ethylene receptor ethylene response 1 (ETR1) and the Arabidopsis histidine-containing phosphotransfer protein 1 (AHP1) form a tight complex in vitro. According to our current model ETR1 and AHP1 together with a response regulator form a phosphorelay system controlling the gene expression response to the plant hormone ethylene, similar to the two-component signaling in bacteria. The model implies that ETR1 functions as a sensor kinase and is autophosphorylated in the absence of ethylene. The phosphoryl group is then transferred onto a histidine at the canonical phosphorylation site in AHP1. For phosphoryl group transfer both binding partners need to form a tight complex. After ethylene binding the receptor is switched to the non-phosphorylated state. This switch is accompanied by a conformational change that decreases the affinity to the phosphorylated AHP1. To test this model we used fluorescence polarization and examined how the phosphorylation status of the proteins affects formation of the suggested ETR1−AHP1 signaling complex. We have employed various mutants of ETR1 and AHP1 mimicking permanent phosphorylation or preventing phosphorylation, respectively. Our results show that phosphorylation plays an important role in complex formation as affinity is dramatically reduced when the signaling partners are either both in their non-phosphorylated form or both in their phosphorylated form. On the other hand, affinity is greatly enhanced when either protein is in the phosphorylated state and the corresponding partner in its non-phosphorylated form. Our results indicate that interaction of ETR1 and AHP1 requires that ETR1 is a dimer, as in its functional state as receptor in planta.

Expression of genes associated with ethylene-signalling pathway in harvested banana fruit in response to temperature and 1-MCP treatment

BACKGROUND

Little attention has been paid to characterising the ethylene-signalling pathway genes in relation to abnormal ripening of harvested banana fruit during storage at high temperature. The aim of the present study was to investigate banana fruit abnormal ripening and the expression of ten genes associated with the ethylene-signalling pathway, namely MaACS1, MaACO1, MaERS1-4 and MaEIL1-4, at high temperature. Changes in these parameters of banana fruit at high temperature in response to 1-MCP pretreatment were also investigated.

RESULTS

High temperature accelerated the decline in fruit firmness, increased ethylene production and inhibited degreening in banana fruit, resulting in fruit abnormal ripening. In addition, the expression of MaACS1, MaACO1, MaERS2, MaERS3, MaERS4, MaEIL1, MaEIL3 and MaEIL4 was enhanced in banana fruit stored at high temperature. However, application of 1-MCP prior to high temperature storage delayed fruit abnormal ripening and simultaneously suppressed the expression of MaACS1, MaERS2, MaERS3, MaEIL1, MaEIL3 and MaEIL4.

CONCLUSION

The findings of this study suggested that the expression of genes associated with the ethylene-signalling pathway might be involved in banana fruit abnormal ripening at high temperature. Application of 1-MCP suppressed the expression of genes associated with the ethylene-signalling pathway, which may be attributed at least partially to 1-MCP delaying fruit abnormal ripening at high temperature.

Cloning and characterisation of two CTR1-like genes in Cucurbita pepo: regulation of their expression during male and female flower development

Ethylene is an essential regulator of flower development in Cucurbita pepo, controlling the sexual expression, and the differentiation and maturation of floral organs. To study the action mechanism of ethylene during the male and female flower development, we have identified two CTR1 homologues from C. pepo, CpCTR1 and CpCTR2, and analysed their expressions during female and male flower development and in response to external treatments with ethylene. CpCTR1 and CpCTR2 share a high homology with plant CTR1-like kinases, but differ from other related kinases such as the Arabidopsis EDR1 and the tomato LeCTR2. The C-terminal ends of both CpCTR1 and CpCTR2 have all the conserved motifs of Ser/Thr kinase domains, including the ATP-binding signature and the protein kinase active site consensus sequence, which suggests that CpCTR1 and CpCTR2 could have the same function as CTR1 in ethylene signalling. The transcripts of both genes were detected in different organs of the plant, including roots, leaves and shoots, but were mostly accumulated in mature flowers. During the development of male and female flowers, CpCTR1 and CpCTR2 expressions were concomitant with ethylene production, which indicates that both genes could be upregulated by ethylene, at least in flowers. Moreover, external treatments with ethylene, although did not alter the expression of these two genes in seedlings and leaves, were able to upregulate their expression in flowers. In the earlier stages of flower development, when ethylene production is very low, the expression of CpCTR1 and CpCTR2 is higher in male floral organs, which agrees with the role of these genes as negative regulators of ethylene signalling, and explain the lower ethylene sensitivity of male flowers in comparison with female flowers. The function of the upregulation of these two genes in later stages of female flower development, when the production of ethylene is also increased, is discussed.

Latest findings about the interplay of auxin, ethylene and nitric oxide in the regulation of Fe deficiency responses by Strategy I plants

Under Fe deficiency, Strategy I (non-graminaceous) plants up-regulate the expression of many Fe acquisition genes and develop morphological changes in their roots. The regulation of these responses is not completely known, but since the 1980's different results suggest a role for auxin, ethylene and, more recently, nitric oxide. The up-regulation of the Fe acquisition genes does not depend solely on these hormones, that would act as activators, but also on some other signals, probably phloem Fe, that would act as an inhibitor. It is not known which of the hormones considered is the last activator of the Fe acquisition genes, but some results suggest that auxin acts upstream of ethylene and NO and that, perhaps, ethylene is the last activator.

Analysis of the functional conservation of ethylene receptors between maize and Arabidopsis

Ethylene, a regulator of plant growth and development, is perceived by specific receptors that act as negative regulators of the ethylene response. Five ethylene receptors, i.e., ETR1, ERS1, EIN4, ETR2, and ERS2, are present in Arabidopsis and dominant negative mutants of each that confer ethylene insensitivity have been reported. In contrast, maize contains just two types of ethylene receptors: ZmERS1, encoded by ZmERS1a and ZmERS1b, and ZmETR2, encoded by ZmETR2a and ZmETR2b. In this study, we introduced a Cys to Tyr mutation in the transmembrane domain of ZmERS1b and ZmETR2b that is present in the etr1-1 dominant negative mutant and expressed each protein in Arabidopsis. Mutant Zmers1b and Zmetr2b receptors conferred ethylene insensitivity and Arabidopsis expressing Zmers1b or Zmetr2b were larger and exhibited a delay in leaf senescence characteristic of ethylene insensitive Arabidopsis mutants. Zmers1b and Zmetr2b were dominant and functioned equally well in a hemizygous or homozygous state. Expression of the Zmers1b N-terminal transmembrane domain was sufficient to exert dominance over endogenous Arabidopsis ethylene receptors whereas the Zmetr2b N-terminal domain failed to do so. Neither Zmers1b nor Zmetr2b functioned in the absence of subfamily 1 ethylene receptors, i.e., ETR1 and ERS1. These results suggest that Cys65 in maize ZmERS1b and ZmETR2b plays the same role that it does in Arabidopsis receptors. Moreover, the results demonstrate that the mutant maize ethylene receptors are functionally dependent on subfamily 1 ethylene receptors in Arabidopsis, indicating substantial functional conservation between maize and Arabidopsis ethylene receptors despite their sequence divergence.

Ethylene signalling and ethylene-targeted transcription factors are required to balance beneficial and nonbeneficial traits in the symbiosis between the endophytic fungus Piriformospora indica and Arabidopsis thaliana

*The endophytic fungus Piriformospora indica colonizes the roots of the model plant Arabidopsis thaliana and promotes its growth and seed production. The fungus can be cultivated in axenic culture without a host, and therefore this is an excellent system to investigate plant-fungus symbiosis. *The growth of etr1, ein2 and ein3/eil1 mutant plants was not promoted or even inhibited by the fungus; the plants produced less seeds and the roots were more colonized compared with the wild-type. This correlates with a mild activation of defence responses. The overexpression of ETHYLENE RESPONSE FACTOR1 constitutively activated defence responses, strongly reduced root colonization and abolished the benefits for the plants. *Piriformospora indica-mediated stimulation of growth and seed yield was not affected by jasmonic acid, and jasmonic acid-responsive promoter beta-glucuronidase gene constructs did not respond to the fungus in Arabidopsis roots. *We propose that ethylene signalling components and ethylene-targeted transcription factors are required to balance beneficial and nonbeneficial traits in the symbiosis. The results show that the restriction of fungal growth by ethylene signalling components is required for the beneficial interaction between the two symbionts.

Biochemical characterization of plant hormone cytokinin-receptor histidine kinases using microorganisms

Results of recent studies on the model higher plant Arabidopsis thaliana have led us to learn about the generality and versatility of two-component systems (TCS) in eukaryotes. In the plant, TCS are crucially involved in certain signal transduction mechanisms underlying the regulation of plant development in response to a subset of plant hormones, namely, cytokinin and ethylene. Results of extensive plant genomics revealed that these hormone-responsive TCS are evolutionarily conserved in many other plants, including mosses, grasses, crops, and trees. In particular, the conserved cytokinin-responsive TCS is typical in the sense that the signaling pathway consists of cytokinin-receptor histidine kinases (HK), histidine-containing phosphotransfer (HPt) factors, and downstream phosphoaccepting response regulators (RR), which together act as His-to-Asp multistep phosphorelay components, and which together modulate the downstream network of cytokinin-responsive gene regulation. The ethylene-responsive TCS is atypical in that ethylene-receptor HKs appear to directly interact with the downstream mitogen-activated protein kinase (MAPK) cascade. The ethylene-responsive HKs have already been introduced in the previous edition of Methods in Enzymology [Schaller, G. E., and Binder, B. M. (2007). Biochemical characterization of plant ethylene receptors following transgenic expression in yeast. Methods Enzymol. 422, 270-287]. Hence, here we focus on the cytokinin-receptor HKs, which are capable of functioning in microorganisms, such as Escherichia coli and Saccharomyces cerevisiae. Some versatile protocols useful for analyzing plant TCS factors by employing these microorganisms will be introduced.

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

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

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

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

Fruit load and elevation affect ethylene biosynthesis and action in apple fruit (Malus domestica L. Borkh) during development, maturation and ripening

The influence of internal and external factors such as tree fruit load and elevation on ethylene biosynthesis and action was assessed during apple fruit development and ripening. Ethylene biosynthesis, as well as transcript accumulation of the hormone biosynthetic enzymes (MdACS1 and MdACO1), receptors (MdETR1 and MdERS1) and an element of the transduction pathway (MdCTR1), were evaluated in apples borne by trees with high (HL) and low (LL) fruit load. Orchards were located in two localities differing in elevation and season day degree sum. These parameters significantly affected the date of bloom and commercial harvest, and the length of the fruit developmental cycle. Trees from the low elevation (LE) bloomed and the fruit ripened earlier than those from the high elevation (HE), displaying also a shortened fruit developmental cycle. Dynamics of ethylene evolution was apparently not affected by elevation. The onset of ethylene evolution started 130 days after bloom (DAB) at both elevations. During early ripening, fruits from LL trees produced significantly more ethylene than those from HL trees. Expression analysis of MdACS1, MdACO1 and MdERS1 indicated that the transcript accumulation well correlated with ethylene evolution. MdCTR1 was expressed at constant level throughout fruit growth and development up to 130 DAB, thereafter, the transcript accumulation decreased up to commercial harvest, concurrently with the onset of ethylene evolution.

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

Protein degradation plays an important role in modulating ethylene signal transduction in plants. Here we show that the ethylene receptor ETR2 is one such target for degradation and that its degradation is dependent upon perception of the signaling ligand ethylene. The ETR2 protein is initially induced by ethylene treatment, consistent with an increase in transcript levels. At ethylene concentrations above 1 mul/liter, however, ETR2 protein levels subsequently decrease in a post-transcriptional fashion. Genetic and chemical approaches indicate that ethylene perception by the receptors initiates the reduction in ETR2 protein levels. The ethylene-induced decrease in ETR2 levels is not affected by cycloheximide, an inhibitor of protein biosynthesis, but is affected by proteasome inhibitors, indicating a role for the proteasome in ETR2 degradation. Ethylene-induced degradation still occurs in seedlings treated with brefeldin A, indicating that degradation of ETR2 does not require exit from its subcellular location at the endoplasmic reticulum. These data support a model in which ETR2 is degraded by a proteasome-dependent pathway in response to ethylene binding. Implications of this model for ethylene signaling are discussed.

Transcriptional analysis of petal organogenesis in Gerbera hybrida

Understanding of the molecular interplay, which determines early steps of flower formation has grown considerably during last years. In contrast, genetic actions responsible for how flower organs acquire their size and shape at later phases of organogenesis are still poorly understood. We have exploited the large and anatomically simple Gerbera (Gerbera hybrida var. Terra regina) ray flower petals to describe transcriptional changes during organogenesis. Gerbera 9 K cDNA microarray was utilized to profile gene expression at six different developmental stages of petal organogenesis, at the earliest stage expansion of petals is starting and at the latest stage petals have reached their final size and shape. Genes potentially participating in petal opening were identified based on the similarity in expression with a known marker gene. Our results showed characteristic sets of genes expressed during the cell division and cell expansion phases of petal development. Interestingly, there was a transition stage during which neither cell division nor cell expansion marker genes were abundantly expressed. Moreover, constitutive expression of late petal specific genes indicates that they participate in petal organogenesis throughout the development and they are not involved in stage specific switch points.

Stage- and tissue-expression of genes involved in the biosynthesis and signalling of ethylene in reproductive organs of damson plum (Prunus domestica L. subsp. insititia)

In this work, four cDNA clones (Pd-ACS1,AJ890088; Pd-ETR1 and Pd-ERS1, AJ890092, AJ890091; and Pd-CTR1, AJ890089) encoding an ACC-synthase, two putative ethylene (ET) receptors, and a putative MAPKKK, respectively, were isolated and phylogenetically characterized in Prunus domestica L. subsp. insititia. Their expression was studied by real-time PCR during flower (closed, open and senescent) and fruit (early green, late green, maturation and ripening) development of damson plum, which is climateric. While two peaks of ET production were quantified at early green and ripening stages in whole fruits, the seed was not able to produce it during maturation and ripening stages. All studied genes were differentially expressed during flower and fruit development. In general, the level of transcripts of Pd-ACS1 was higher in fruits than in flowers. However, it was noteworthy that: (1) Pd-ACS1 expression was hardly detected in closed flowers and at low levels during early green stage; and fruit development provoked a notable differential expression in seeds, and pericarp; (2) the results of Pd-ACS1 expression during fruit development suggest a preponderant role of this gene from late green stage onward. The stamen was the only floral organ in which expression of both Pd-ETR1 and Pd-ERS1 receptor genes was not significantly altered during development; however, their expression decreased concomitantly with development of pistil (only floral organ to register a net ET production when fertilized) and during first days of ovary development (the highest ET production during all fruit development). Contrary to Pd-ERS1, the level of Pd-ETR1 mRNA was temporally quite similar in the seed. With regard Pd-ETR1, even its expression was very scarce during maturation of mesocarp, was stimulated during ripening. In the epicarp, Pd-ERS1 and Pd-ETR1 were low expressed during pit hardening increasing onward and decreasing during ripening. Pd-CTR1 expression was in the seed>mesocarp>>epicarp. Spatial and temporal levels of Pd-ACS1, Pd-ETR1, Pd-ERS1 and Pd-CTR1 mRNAs described in this work demonstrate that the expression of these genes is not always constitutive and that control of its transcription may play an important role in regulating the development of reproductive organs of damson plum.

AHK5 histidine kinase regulates root elongation through an ETR1-dependent abscisic acid and ethylene signaling pathway in Arabidopsis thaliana

The Arabidopsis thaliana genome encodes a small family of histidine (His) protein kinases, some of which have redundant functions as ethylene receptors, whereas others serve as cytokinin receptors. The most poorly characterized of these is authentic histidine kinase 5 (AHK5; also known as cytokinin-independent 2, CKI2). Here we characterize three independent ahk5 mutants, and show that they have a common phenotype. Our results suggest that AHK5 His-kinase acts as a negative regulator in the signaling pathway in which ethylene and ABA inhibit the root elongation through ETR1 (an ethylene receptor).