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

An alternate route of ethylene receptor signaling

The gaseous plant hormone ethylene is perceived by a family of ethylene receptors and mediates an array of ethylene responses. In the absence of ethylene, receptor signaling is conveyed via the C-terminal histidine kinase domain to the N-terminus of the CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) protein kinase, which represses ethylene signaling mediated by ETHYLENE INSENSITIVE2 (EIN2) followed by EIN3. In the presence of ethylene, the receptors are inactivated when ethylene binds to their N-terminal domain, and consequently CTR1 is inactive, allowing EIN2 and EIN3 to activate ethylene signaling. Recent findings have shown that the ethylene receptor N-terminal portion can conditionally mediate the receptor signal output in mutants lacking CTR1, thus providing evidence of an alternative pathway from the ethylene receptors not involving CTR1. Here we highlight the evidence for receptor signaling to an alternative pathway and suggest that receptor signaling is coordinated via the N- and C-termini, as we address the biological significance of the negative regulation of ethylene signaling by the two pathways.

Transcriptome of the inflorescence meristems of the biofuel plant Jatropha curcas treated with cytokinin

Background

Jatropha curcas, whose seed content is approximately 30–40% oil, is an ideal feedstock for producing biodiesel and bio-jet fuels. However, Jatropha plants have a low number of female flowers, which results in low seed yield that cannot meet the needs of the biofuel industry. Thus, increasing the number of female flowers is critical for the improvement of Jatropha seed yield. Our previous findings showed that cytokinin treatment can increase the flower number and female to male ratio and also induce bisexual flowers in Jatropha. The mechanisms underlying the influence of cytokinin on Jatropha flower development and sex determination, however, have not been clarified.

Results

This study examined the transcriptional levels of genes involved in the response to cytokinin in Jatropha inflorescence meristems at different time points after cytokinin treatment by 454 sequencing, which gave rise to a total of 294.6 Mb of transcript sequences. Up-regulated and down-regulated annotated and novel genes were identified, and the expression levels of the genes of interest were confirmed by qRT-PCR. The identified transcripts include those encoding genes involved in the biosynthesis, metabolism, and signaling of cytokinin and other plant hormones, flower development and cell division, which may be related to phenotypic changes of Jatropha in response to cytokinin treatment. Our analysis indicated that Jatropha orthologs of the floral organ identity genes known as ABCE model genes, JcAP1,2, JcPI, JcAG, and JcSEP1,2,3, were all significantly repressed, with an exception of one B-function gene JcAP3 that was shown to be up-regulated by BA treatment, indicating different mechanisms to be involved in the floral organ development of unisexual flowers of Jatropha and bisexual flowers of Arabidopsis. Several cell division-related genes, including JcCycA3;2, JcCycD3;1, JcCycD3;2 and JcTSO1, were up-regulated, which may contribute to the increased flower number after cytokinin treatment.

Conclusions

This study presents the first report of global expression patterns of cytokinin-regulated transcripts in Jatropha inflorescence meristems. This report laid the foundation for further mechanistic studies on Jatropha and other non-model plants responding to cytokinin. Moreover, the identification of functional candidate genes will be useful for generating superior varieties of high-yielding transgenic Jatropha.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-974) contains supplementary material, which is available to authorized users.

The receptor kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 attenuates abscisic acid responses in Arabidopsis

In plants, membrane-bound receptor kinases are essential for developmental processes, immune responses to pathogens and the establishment of symbiosis. We previously identified the Arabidopsis (Arabidopsis thaliana) receptor kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as required for successful infection with the downy mildew pathogen Hyaloperonospora arabidopsidis. We report here that IOS1 is also required for full susceptibility of Arabidopsis to unrelated (hemi)biotrophic filamentous oomycete and fungal pathogens. Impaired susceptibility in the absence of IOS1 appeared to be independent of plant defense mechanism. Instead, we found that ios1-1 plants were hypersensitive to the plant hormone abscisic acid (ABA), displaying enhanced ABA-mediated inhibition of seed germination, root elongation, and stomatal opening. These findings suggest that IOS1 negatively regulates ABA signaling in Arabidopsis. The expression of ABA-sensitive COLD REGULATED and RESISTANCE TO DESICCATION genes was diminished in Arabidopsis during infection. This effect on ABA signaling was alleviated in the ios1-1 mutant background. Accordingly, ABA-insensitive and ABA-hypersensitive mutants were more susceptible and resistant to oomycete infection, respectively, showing that the intensity of ABA signaling affects the outcome of downy mildew disease. Taken together, our findings suggest that filamentous (hemi)biotrophs attenuate ABA signaling in Arabidopsis during the infection process and that IOS1 participates in this pathogen-mediated reprogramming of the host.

Salt-induced stabilization of EIN3/EIL1 confers salinity tolerance by deterring ROS accumulation in Arabidopsis

Ethylene has been regarded as a stress hormone to regulate myriad stress responses. Salinity stress is one of the most serious abiotic stresses limiting plant growth and development. But how ethylene signaling is involved in plant response to salt stress is poorly understood. Here we showed that Arabidopsis plants pretreated with ethylene exhibited enhanced tolerance to salt stress. Gain- and loss-of-function studies demonstrated that EIN3 (ETHYLENE INSENSITIVE 3) and EIL1 (EIN3-LIKE 1), two ethylene-activated transcription factors, are necessary and sufficient for the enhanced salt tolerance. High salinity induced the accumulation of EIN3/EIL1 proteins by promoting the proteasomal degradation of two EIN3/EIL1-targeting F-box proteins, EBF1 and EBF2, in an EIN2-independent manner. Whole-genome transcriptome analysis identified a list of SIED (Salt-Induced and EIN3/EIL1-Dependent) genes that participate in salt stress responses, including several genes encoding reactive oxygen species (ROS) scavengers. We performed a genetic screen for ein3 eil1-like salt-hypersensitive mutants and identified 5 EIN3 direct target genes including a previously unknown gene, SIED1 (At5g22270), which encodes a 93-amino acid polypeptide involved in ROS dismissal. We also found that activation of EIN3 increased peroxidase (POD) activity through the direct transcriptional regulation of PODs expression. Accordingly, ethylene pretreatment or EIN3 activation was able to preclude excess ROS accumulation and increased tolerance to salt stress. Taken together, our study provides new insights into the molecular action of ethylene signaling to enhance plant salt tolerance, and elucidates the transcriptional network of EIN3 in salt stress response.

High salinity, as a world-wide abiotic stress, restricts root water uptake, damages cell physiology, and limits the productivity of agricultural crops. Ethylene is a major phytohormone that regulates plant development in response to adverse environments, including high salt stress. However, the molecular mechanisms of how ethylene signal exerts its effect and how ethylene signaling is modulated upon salt stress remain to be explored. Here, we report that high salinity induces EIN3/EIL1 protein accumulation and EBF1/2 protein degradation in an EIN2-independent manner. Moreover, the activated EIN3 deters excess ROS accumulation and increases salt tolerance. Transcriptome analysis and functional studies reveal an EIN3-directed gene network in salt stress response. Functional studies of 114 SIED (Salt-Induced and EIN3/EIL1-Dependent) genes identify a novel regulator of ROS dismissal and salt tolerance. This new understanding of ethylene/salt mutual regulation would allow a better manipulation and engineering of EIN3 and its downstream SIED genes to enhance plant tolerance and adaption to salt stress, particularly in those economically important crops in the future.

Ectopic expression of CsCTR1, a cucumber CTR-like gene, attenuates constitutive ethylene signaling in an Arabidopsis ctr1-1 mutant and expression pattern analysis of CsCTR1 in cucumber (Cucumis sativus)

The gaseous plant hormone ethylene regulates many aspects of plant growth, development and responses to the environment. Constitutive triple response 1 (CTR1) is a central regulator involved in the ethylene signal transduction pathway. To obtain a better understanding of this particular pathway in cucumber, the cDNA-encoding CTR1 (designated CsCTR1) was isolated from cucumber. A sequence alignment and phylogenetic analyses revealed that CsCTR1 has a high degree of homology with other plant CTR1 proteins. The ectopic expression of CsCTR1 in the Arabidopsis ctr1-1 mutant attenuates constitutive ethylene signaling of this mutant, suggesting that CsCTR1 indeed performs its function as negative regulator of the ethylene signaling pathway. CsCTR1 is constitutively expressed in all of the examined cucumber organs, including roots, stems, leaves, shoot apices, mature male and female flowers, as well as young fruits. CsCTR1 expression gradually declined during male flower development and increased during female flower development. Additionally, our results indicate that CsCTR1 can be induced in the roots, leaves and shoot apices by external ethylene. In conclusion, this study provides a basis for further studies on the role of CTR1 in the biological processes of cucumber and on the molecular mechanism of the cucumber ethylene signaling pathway.

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

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

Altered sensitivity to ethylene in 'Tardivo', a late-ripening mutant of Clementine mandarin

'Tardivo' mandarin is a mutant of 'Comune' Clementine with a delay in peel degreening and coloration, allowing late harvesting. In this work, we have explored if the late-harvesting phenotype of 'Tardivo' mandarin is related to altered perception and sensitivity to ethylene. The peel degreening rate was examined after a single ethephon treatment or during a continuous ethylene application in fruits at two maturation stages. In general, ethylene-induced peel degreening was considerably delayed and reduced in fruits of 'Tardivo', as well as the concomitant reduction of chlorophyll (Chl) and chloroplastic carotenoids, and the accumulation of chromoplastic carotenoids. Analysis of the expression of genes involved in Chl degradation, carotenoids, ABA, phenylpropanoids and ethylene biosynthesis revealed an impairment in the stimulation of most genes by ethylene in the peel of 'Tardivo' fruits with respect to 'Comune', especially after 5 days of ethylene application. Moreover, ethylene-induced expression of two ethylene receptor genes, ETR1 and ETR2, was also reduced in mutant fruits. Expression levels of two ethylene-responsive factors, ERF1 and ERF2, which were repressed by ethylene, were also impaired to a different extent, in fruits of both genotypes. Collectively, results suggested an altered sensitivity of the peel of 'Tardivo' to ethylene-induced physiological and molecular responses, including fruit degreening and coloration processes, which may be time-dependent since an early moderated reduction in the responses was followed by the latter inability to sustain ethylene action. These results support the involvement of ethylene in the regulation of at least some aspects of peel maturation in the non-climacteric citrus fruit.

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

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

Regulation of epinasty induced by 2,4-dichlorophenoxyacetic acid in pea and Arabidopsis plants

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) causes uncontrolled cell division and malformed growth in plants, giving rise to leaf epinasty and stem curvature. In this study, mechanisms involved in the regulation of leaf epinasty induced by 2,4-D were studied using different chemicals involved in reactive oxygen species (ROS) accumulation (diphenyleniodonium, butylated hydroxyanisole, EDTA, allopurinol), calcium channels (LaCl3), protein phosphorylation (cantharidin, wortmannin) and ethylene emission/perception (aminoethoxyvinyl glycine, AgNO3). The effect of these compounds on the epinasty induced by 2,4-D was analysed in shoots and leaf strips from pea plants. For further insight into the effect of 2,4-D, studies were also made in Arabidopsis mutants deficient in ROS production (rbohD, rbohF, xdh), ethylene (ein 3-1, ctr 1-1, etr 1-1), abscisic acid (aba 3.1), and jasmonic acid (coi 1.1, jar 1.1, opr 3) pathways. The results suggest that ROS production, mainly ·OH, is essential in the development of epinasty triggered by 2,4-D. Epinasty was also found to be regulated by Ca2+, protein phosphorylation and ethylene, although all these factors act downstream of ROS production. The use of Arabidopsis mutants appears to indicate that abscisic and jasmonic acid are not involved in regulating epinasty, although they could be involved in other symptoms induced by 2,4-D.

An integrative analysis of the transcriptome and proteome of the pulp of a spontaneous late-ripening sweet orange mutant and its wild type improves our understanding of fruit ripening in citrus

Fruit ripening is a complex, genetically programmed process that occurs in conjunction with the differentiation of chloroplasts into chromoplasts and involves changes to the organoleptic properties of the fruit. In this study, an integrative analysis of the transcriptome and proteome was performed to identify important regulators and pathways involved in fruit ripening in a spontaneous late-ripening mutant ('Fengwan' orange, Citrus sinensis L. Osbeck) and its wild type ('Fengjie 72-1'). At the transcript level, 628 genes showed a 2-fold or more expression difference between the mutant and wild type as detected by an RNA sequencing approach. At the protein level, 130 proteins differed by 1.5-fold or more in their relative abundance, as indicated by iTRAQ (isobaric tags for relative and absolute quantitation) analysis. A comparison of the transcriptome and proteome data revealed some aspects of the regulation of metabolism during orange fruit ripening. First, a large number of differential genes were found to belong to the plant hormone pathways and cell-wall-related metabolism. Secondly, we noted a correlation between ripening-associated transcripts and sugar metabolites, which suggests the importance of these metabolic pathways during fruit ripening. Thirdly, a number of genes showed inconsistency between the transcript and protein level, which is indicative of post-transcriptional events. These results reveal multiple ripening-associated events during citrus ripening and provide new insights into the molecular mechanisms underlying citrus ripening regulatory networks.

Pyocyanin, a virulence factor produced by Pseudomonas aeruginosa, alters root development through reactive oxygen species and ethylene signaling in Arabidopsis

Pyocyanin acts as a virulence factor in Pseudomonas aeruginosa, a plant and animal pathogen. In this study, we evaluated the effect of pyocyanin on growth and development of Arabidopsis seedlings. Root inoculation with P. aeruginosa PAO1 strain inhibited primary root growth in wild-type (WT) Arabidopsis seedlings. In contrast, single lasI- and double rhlI-/lasI- mutants of P. aeruginosa defective in pyocyanin production showed decreased root growth inhibition concomitant with an increased phytostimulation. Treatment with pyocyanin modulates root system architecture, inhibiting primary root growth and promoting lateral root and root hair formation without affecting meristem viability or causing cell death. These effects correlated with altered proportions of hydrogen peroxide and superoxide in root tips and with an inhibition of cell division and elongation. Mutant analyses showed that pyocyanin modulation of root growth was likely independent of auxin, cytokinin, and abscisic acid but required ethylene signaling because the Arabidopsis etr1-1, ein2-1, and ein3-1 ethylene-related mutants were less sensitive to pyocyanin-induced root stoppage and reactive oxygen species (ROS) distribution. Our findings suggest that pyocyanin is an important factor modulating the interplay between ROS production and root system architecture by an ethylene-dependent signaling.

cGMP and ethylene are involved in maintaining ion homeostasis under salt stress in Arabidopsis roots

KEY MESSAGE

cGMP promotes ethylene production and enhances the perception of ethylene. Endogenous ethylene or cGMP accumulation maintains ion homeostasis to enhancing salt resistance. etr1 - 3 is insensitive to cGMP under salt stress.

ABSTRACT

In the present study, we presented a signaling network involving ethylene and cGMP in salt resistance pathway of Arabidopsis roots. Results showed that the ethylene-insensitive mutant etr1-3 was more sensitive to salt stress than the wild type (WT). etr1-3 displayed a greater electrolyte leakage, thiobarbituric acid reactive substances and Na(+)/K(+) ratio, but a lower plasma membrane (PM) H(+)-ATPase activity compared to WT under the different NaCl contents. Application of 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) or 8-Br-cGMP (the cGMP analog) alleviated NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing PM H(+)-ATPase activity in WT, but not in etr1-3. Roots treated with 8-Br-cGMP could promote ethylene production and enhance the expression of ACC synthase gene in WT. In addition, the 8-Br-cGMP action in NaCl stress was inhibited by aminooxyacetic acid (an inhibitor of ethylene biosynthesis), but 6-Anilino-5,8-quinolinedione (Ly83583, a guanylate cyclase inhibitor) could not affect ACC action in WT. These results suggest that ethylene functions as a downstream signal of cGMP that stimulates the PM H(+)-ATPase activity, which finally results in regulating ion homeostasis in Arabidopsis tolerance to salt. Moreover, cGMP enhanced the perception of ethylene in Arabidopsis under salt stress, which reversed the salt-induced increase of ETR1 and increased ERF1 at the transcript levels in WT. In a word, cGMP modulates salt resistance pathway of ethylene through regulating biosynthesis and perception of ethylene in Arabidopsis roots.

ENHANCING ctr1-10 ETHYLENE RESPONSE2 is a novel allele involved in CONSTITUTIVE TRIPLE-RESPONSE1-mediated ethylene receptor signaling in Arabidopsis

BACKGROUND

The signal output of ethylene receptor family members is mediated by unknown mechanisms to activate the Raf-like protein CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) in negatively regulating ethylene signaling. The physical interaction between the ethylene receptor histidine kinase (HK) domain and CTR1 N terminus is essential to the CTR1-mediated receptor signal output. To advance our knowledge of the involvement of CTR1-mediated ethylene receptor signaling, we performed a genetic screen for mutations that enhanced the constitutive ethylene response in the weak ctr1-10 allele.

RESULTS

We isolated a loss-of-function allele of ENHANCING ctr1-10 ETHYLENE RESPONSE2 (ECR2) and found that ecr2-1 ctr1-10 and the strong allele ctr1-1 conferred a similar, typical constitutive ethylene response phenotype. Genetic analyses and transformation studies suggested that ECR2 acts downstream of the ethylene receptors and upstream of the transcription factors ETHYLENE INSENSITIVE3 (EIN3) and EIN3-LIKE1 (EIL1), which direct the expression of ethylene response genes. Signal output by the N terminus of the ethylene receptor ETHYLENE RESPONSE1 (ETR1) can be mediated by a pathway independent of CTR1. Expression of the N terminus of the ethylene-insensitive etr1-1 but not the full-length isoform rescued the ecr2-1 ctr1-10 phenotype, which indicates the involvement of ECR2 in CTR1-mediated but not -independent, ethylene receptor signaling. ECR2 was mapped to the centromere region on chromosome 2. With incomplete sequence and annotation information and rare chromosome recombination events in this region, the cloning of ECR2 is challenging and still in progress.

CONCLUSIONS

ECR2 is a novel allele involved in the ethylene receptor signaling that is mediated by CTR1. CTR1 activation by ethylene receptors may require ECR2 for suppressing the ethylene response.

Association of cytochrome b5 with ETR1 ethylene receptor signaling through RTE1 in Arabidopsis

Ethylene plays important roles in plant growth, development and stress responses, and is perceived by a family of receptors that repress ethylene responses when ethylene is absent. Repression by the ethylene receptor ETR1 depends on an integral membrane protein, REVERSION TO ETHYLENE SENSITIVITY1 (RTE1), which acts upstream of ETR1 in the endoplasmic reticulum (ER) membrane and Golgi apparatus. To investigate RTE1 function, we screened for RTE1-interacting proteins using the yeast split-ubiquitin assay, which yielded the ER-localized cytochrome b(5) (Cb5) isoform D. Cb5s are small hemoproteins that perform electron transfer reactions in all eukaryotes, but their roles in plants are relatively uncharacterized. Using bimolecular fluorescence complementation (BiFC), we found that all four ER-localized Arabidopsis Cb5 isoforms (AtCb5–B, -C, -D and -E) interact with RTE1 in plant cells. In support of this interaction, atcb5 mutants exhibited phenotypic parallels with rte1 mutants in Arabidopsis. Phenotypes included partial suppression of etr1–2 ethylene insensitivity, and no suppression of RTE1-independent ethylene receptor isoforms. The single loss-of-function mutants atcb5–b, -c and -d appeared similar to the wild-type, but double mutant combinations displayed slight ethylene hypersensitivity. Over-expression of AtCb5–D conferred reduced ethylene sensitivity similar to that conferred by RTE1 over-expression, and genetic analyses suggested that AtCb5–D acts upstream of RTE1 in the ethylene response. These findings suggest an unexpected role for Cb5, in which Cb5 and RTE1 are functional partners in promoting ETR1-mediated repression of ethylene signaling.

How plants sense ethylene gas--the ethylene receptors

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

Interaction between MYC2 and ETHYLENE INSENSITIVE3 modulates antagonism between jasmonate and ethylene signaling in Arabidopsis

Plants have evolved sophisticated mechanisms for integration of endogenous and exogenous signals to adapt to the changing environment. Both the phytohormones jasmonate (JA) and ethylene (ET) regulate plant growth, development, and defense. In addition to synergistic regulation of root hair development and resistance to necrotrophic fungi, JA and ET act antagonistically to regulate gene expression, apical hook curvature, and plant defense against insect attack. However, the molecular mechanism for such antagonism between JA and ET signaling remains unclear. Here, we demonstrate that interaction between the JA-activated transcription factor MYC2 and the ET-stabilized transcription factor ETHYLENE-INSENSITIVE3 (EIN3) modulates JA and ET signaling antagonism in Arabidopsis thaliana. MYC2 interacts with EIN3 to attenuate the transcriptional activity of EIN3 and repress ET-enhanced apical hook curvature. Conversely, EIN3 interacts with and represses MYC2 to inhibit JA-induced expression of wound-responsive genes and herbivory-inducible genes and to attenuate JA-regulated plant defense against generalist herbivores. Coordinated regulation of plant responses in both antagonistic and synergistic manners would help plants adapt to fluctuating environments.

Novel connections and gaps in ethylene signaling from the ER membrane to the nucleus

The signaling of the plant hormone ethylene has been studied genetically, resulting in the identification of signaling components from membrane receptors to nuclear effectors. Among constituents of the hormone signaling pathway, functional links involving a putative mitogen-activated protein kinase kinase CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) and a membrane transporter-like protein ETHYLENE INSENSITIVE2 (EIN2) have been missing for a long time. We now learn that EIN2 is cleaved and its C-terminal end moves to the nucleus upon ethylene perception at the membrane receptors, and then the C-terminal end of EIN2 in the nucleus supports EIN3-dependent ethylene-response gene expression. CTR1 kinase activity negatively controls the EIN2 cleavage process through direct phosphorylation. Despite the novel connection of CTR1 with EIN2 that explains a large portion of the missing links in ethylene signaling, our understanding still remains far from its completion. This focused review will summarize recent advances in the EIN3-dependent ethylene signaling mechanisms including CTR1-EIN2 functions with respect to EIN3 regulation and ethylene responses. This will also present several emerging issues that need to be addressed for the comprehensive understanding of signaling pathways of the invaluable plant hormone ethylene.

The roles of ethylene and transcription factors in the regulation of onset of leaf senescence

Leaf senescence is the last stage of leaf development and is accompanied by cell death. In contrast to senescence in individual organisms that leads to death, leaf senescence is associated with dynamic processes that include the translocation of nutrients from old leaves to newly developing or storage tissues within the same plant. The onset of leaf senescence is largely regulated by age and internal and external stimuli, which include the plant hormone ethylene. Earlier studies have documented the important role of ethylene in the regulation of leaf senescence. The production of ethylene coincides with the onset of leaf senescence, whereas the application of ethylene to plants induces precocious leaf senescence. Recently, many studies have described the components of ethylene signaling and biosynthetic pathways that are involved in modulating the onset of leaf senescence. Particularly, transcription factors (TFs) integrate ethylene signals with those from environmental and developmental factors to accelerate or delay leaf senescence. This review aims to discuss the regulatory cascade involving ethylene and TFs in the regulation of onset of leaf senescence.

Hormonal networks involved in apical hook development in darkness and their response to light

In darkness, the dicot seedlings produce an apical hook as result of differential cell division and extension at opposite sides of the hypocotyl. This hook protects the apical meristem from mechanical damage during seedling emergence from the soil. In darkness, gibberellins act via the DELLA-PIF (PHYTOCHROME INTERACTING FACTORs) pathway, and ethylene acts via the EIN3/EIL1 (ETHYLENE INSENSITIVE 3/EIN3 like 1)-HLS1 (HOOKLESS 1) pathway to control the asymmetric accumulation of auxin required for apical hook formation and maintenance. These core pathways form a network with multiple points of connection. Light perception by phytochromes and cryptochromes reduces the activity of PIFs and (COP1) CONSTITUTIVE PHOTOMORPHOGENIC 1-both required for hook formation in darkness-, lowers the levels of gibberellins, and triggers hook opening as a component of the switch between heterotrophic and photoautotrophic development. Apical hook opening is thus a suitable model to study the convergence of endogenous and exogenous signals on the control of cell division and cell growth.

Ethylene promotes hypocotyl growth and HY5 degradation by enhancing the movement of COP1 to the nucleus in the light

In the dark, etiolated seedlings display a long hypocotyl, the growth of which is rapidly inhibited when the seedlings are exposed to light. In contrast, the phytohormone ethylene prevents hypocotyl elongation in the dark but enhances its growth in the light. However, the mechanism by which light and ethylene signalling oppositely affect this process at the protein level is unclear. Here, we report that ethylene enhances the movement of CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) to the nucleus where it mediates the degradation of LONG HYPOCOTYL 5 (HY5), contributing to hypocotyl growth in the light. Our results indicate that HY5 is required for ethylene-promoted hypocotyl growth in the light, but not in the dark. Using genetic and biochemical analyses, we found that HY5 functions downstream of ETHYLENE INSENSITIVE 3 (EIN3) for ethylene-promoted hypocotyl growth. Furthermore, the upstream regulation of HY5 stability by ethylene is COP1-dependent, and COP1 is genetically located downstream of EIN3, indicating that the COP1-HY5 complex integrates light and ethylene signalling downstream of EIN3. Importantly, the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) enriched the nuclear localisation of COP1; however, this effect was dependent on EIN3 only in the presence of light, strongly suggesting that ethylene promotes the effects of light on the movement of COP1 from the cytoplasm to the nucleus. Thus, our investigation demonstrates that the COP1-HY5 complex is a novel integrator that plays an essential role in ethylene-promoted hypocotyl growth in the light.

It is well known that light suppresses hypocotyl growth in seedlings, while the phytohormone ethylene and its precursor 1-aminocyclopropane-1-carboxylate (ACC) enhance hypocotyl growth in the light. However, the mechanism by which light and ethylene oppositely affect this process at the protein level is unclear. Here, we demonstrate that ethylene enhances the movement of CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) to the nucleus where it promotes the degradation of LONG HYPOCOTYL 5 (HY5) in the light, contributing to hypocotyl growth. Our data indicate that HY5 is required for ethylene-promoted hypocotyl growth in the light, but not in the dark. Using genetic and biochemical analyses, we found that HY5 functions downstream of ETHYLENE INSENSITIVE 3 (EIN3) during ethylene-promoted hypocotyl growth. Further, the regulation of HY5 stability by ethylene is COP1-dependent, and COP1 is genetically located downstream of EIN3, indicating that the COP1-HY5 complex integrates light and ethylene signalling downstream of EIN3. Importantly, ACC enriched the nuclear localisation of COP1 in an EIN3-dependent manner in the presence of light, suggesting that ethylene rescued the effects of light on the movement of COP1 from the cytoplasm to the nucleus. Thus, our investigation shows that the COP1-HY5 complex is a novel integrator that plays an essential role in ethylene-promoted hypocotyl growth in the light.

Current understanding on ethylene signaling in plants: the influence of nutrient availability

The plant hormone ethylene is involved in many physiological processes, including plant growth, development and senescence. Ethylene also plays a pivotal role in plant response or adaptation under biotic and abiotic stress conditions. In plants, ethylene production often enhances the tolerance to sub-optimal environmental conditions. This role is particularly important from both ecological and agricultural point of views. Among the abiotic stresses, the role of ethylene in plants under nutrient stress conditions has not been completely investigated. In literature few reports are available on the interaction among ethylene and macro- or micro-nutrients. However, the published works clearly demonstrated that several mineral nutrients largely affect ethylene biosynthesis and perception with a strong influence on plant physiology. The aim of this review is to revisit the old findings and recent advances of knowledge regarding the sub-optimal nutrient conditions on the effect of ethylene biosynthesis and perception in plants. The effect of deficiency or excess of the single macronutrient or micronutrient on the ethylene pathway and plant responses are reviewed and discussed. The synergistic and antagonist effect of the different mineral nutrients on ethylene plant responses is critically analyzed. Moreover, this review highlights the status of information between nutritional stresses and plant response, emphasizing the topics that should be further investigated.

Stable isotope metabolic labeling-based quantitative phosphoproteomic analysis of Arabidopsis mutants reveals ethylene-regulated time-dependent phosphoproteins and putative substrates of constitutive triple response 1 kinase

Ethylene is an important plant hormone that regulates numerous cellular processes and stress responses. The mode of action of ethylene is both dose- and time-dependent. Protein phosphorylation plays a key role in ethylene signaling, which is mediated by the activities of ethylene receptors, constitutive triple response 1 (CTR1) kinase, and phosphatase. To address how ethylene alters the cellular protein phosphorylation profile in a time-dependent manner, differential and quantitative phosphoproteomics based on (15)N stable isotope labeling in Arabidopsis was performed on both one-minute ethylene-treated Arabidopsis ethylene-overly-sensitive loss-of-function mutant rcn1-1, deficient in PP2A phosphatase activity, and a pair of long-term ethylene-treated wild-type and loss-of-function ethylene signaling ctr1-1 mutants, deficient in mitogen-activated kinase kinase kinase activity. In total, 1079 phosphopeptides were identified, among which 44 were novel. Several one-minute ethylene-regulated phosphoproteins were found from the rcn1-1. Bioinformatic analysis of the rcn1-1 phosphoproteome predicted nine phosphoproteins as the putative substrates for PP2A phosphatase. In addition, from CTR1 kinase-enhanced phosphosites, we also found putative CTR1 kinase substrates including plastid transcriptionally active protein and calcium-sensing receptor. These regulatory proteins are phosphorylated in the presence of ethylene. Analysis of ethylene-regulated phosphosites using the group-based prediction system with a protein-protein interaction filter revealed a total of 14 kinase-substrate relationships that may function in both CTR1 kinase- and PP2A phosphatase-mediated phosphor-relay pathways. Finally, several ethylene-regulated post-translational modification network models have been built using molecular systems biology tools. It is proposed that ethylene regulates the phosphorylation of arginine/serine-rich splicing factor 41, plasma membrane intrinsic protein 2A, light harvesting chlorophyll A/B binding protein 1.1, and flowering bHLH 3 proteins in a dual-and-opposing fashion.

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

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

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

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

A dominant repressor version of the tomato Sl-ERF.B3 gene confers ethylene hypersensitivity via feedback regulation of ethylene signaling and response components

Ethylene Response Factors (ERFs) are downstream components of the ethylene signal transduction pathway, although their role in ethylene-dependent developmental processes remains poorly understood. As the ethylene-inducible tomato Sl-ERF.B3 has been shown previously to display a strong binding affinity to GCC-box-containing promoters, its physiological significance was addressed here by a reverse genetics approach. However, classical up- and down-regulation strategies failed to give clear clues to its roles in planta, probably due to functional redundancy among ERF family members. Expression of a dominant repressor ERF.B3-SRDX version of Sl-ERF.B3 in the tomato resulted in pleiotropic ethylene responses and vegetative and reproductive growth phenotypes. The dominant repressor etiolated seedlings displayed partial constitutive ethylene response in the absence of ethylene and adult plants exhibited typical ethylene-related alterations such as leaf epinasty, premature flower senescence and accelerated fruit abscission. The multiple symptoms related to enhanced ethylene sensitivity correlated with the altered expression of ethylene biosynthesis and signaling genes and suggested the involvement of Sl-ERF.B3 in a feedback mechanism that regulates components of ethylene production and response. Moreover, Sl-ERF.B3 was shown to modulate the transcription of a set of ERFs and revealed the existence of a complex network interconnecting different ERF genes. Overall, the study indicated that Sl-ERF.B3 had a critical role in the regulation of multiple genes and identified a number of ERFs among its primary targets, consistent with the pleiotropic phenotypes displayed by the dominant repression lines.

Analysis of gene expression during the transition to climacteric phase in carnation flowers (Dianthus caryophyllus L.)

It has been generally thought that in ethylene-sensitive plants such as carnations, senescence proceeds irreversibly once the tissues have entered the climacteric phase. While pre-climacteric petal tissues have a lower sensitivity to ethylene, these tissues are converted to the climacteric phase at a critical point during flower development. In this study, it is demonstrated that the senescence process initiated by exogenous ethylene is reversible in carnation petals. Petals treated with ethylene for 12h showed sustained inrolling and senescence, while petals treated with ethylene for 10h showed inrolling followed by recovery from inrolling. Reverse transcription-PCR analysis revealed differential expression of genes involved in ethylene biosynthesis and ethylene signalling between 10h and 12h ethylene treatment. Ethylene treatment at or beyond 12h (threshold time) decreased the mRNA levels of the receptor genes (DcETR1, DcERS1, and DcERS2) and DcCTR genes, and increased the ethylene biosynthesis genes DcACS1 and DcACO1. In contrast, ethylene treatment under the threshold time caused a transient decrease in the receptor genes and DcCTR genes, and a transient increase in DcACS1 and DcACO1. Sustained DcACS1 accumulation is correlated with decreases in DcCTR genes and increase in DcEIL3 and indicates that tissues have entered the climacteric phase and that senescence proceeds irreversibly. Inhibition of ACS (1-aminocyclopropane-1-carboxylic acid synthase) prior to 12h ethylene exposure was not able to prevent reduction in transcripts of DcCTR genes, yet suppressed transcript of DcACS1 and DcACO1. This leads to the recovery from inrolling of the petals, indicating that DcACS1 may act as a signalling molecule in senescence of flowers.

Rice CONSTITUTIVE TRIPLE-RESPONSE2 is involved in the ethylene-receptor signalling and regulation of various aspects of rice growth and development

In Arabidopsis, the ethylene-receptor signal output occurs at the endoplasmic reticulum and is mediated by the Raf-like protein CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) but is prevented by overexpression of the CTR1 N terminus. A phylogenic analysis suggested that rice OsCTR2 is closely related to CTR1, and ectopic expression of CTR1p:OsCTR2 complemented Arabidopsis ctr1-1. Arabidopsis ethylene receptors ETHYLENE RESPONSE1 and ETHYLENE RESPONSE SENSOR1 physically interacted with OsCTR2 on yeast two-hybrid assay, and green fluorescence protein-tagged OsCTR2 was localized at the endoplasmic reticulum. The osctr2 loss-of-function mutation and expression of the 35S:OsCTR2 (1-513) transgene that encodes the OsCTR2 N terminus (residues 1-513) revealed several and many aspects, respectively, of ethylene-induced growth alteration in rice. Because the osctr2 allele did not produce all aspects of ethylene-induced growth alteration, the ethylene-receptor signal output might be mediated in part by OsCTR2 and by other components in rice. Yield-related agronomic traits, including flowering time and effective tiller number, were altered in osctr2 and 35S:OsCTR2 (1-513) transgenic lines. Applying prolonged ethylene treatment to evaluate ethylene effects on rice without compromising rice growth is technically challenging. Our understanding of roles of ethylene in various aspects of growth and development in japonica rice varieties could be advanced with the use of the osctr2 and 35S:OsCTR2 (1-513) transgenic lines.

Ethylene signaling: simple ligand, complex regulation

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

Involvement of rose aquaporin RhPIP1;1 in ethylene-regulated petal expansion through interaction with RhPIP2;1

Aquaporins (AQPs) are multifunctional membrane channels and facilitate the transport of water across plant cell membranes. Among the plant AQPs, plasma membrane intrinsic proteins (PIPs), which cluster in two phylogenetic groups (PIP1 and PIP2), play a key role in plant growth. Our previous work has indicated that RhPIP2;1, a member of PIP2, is involved in ethylene-regulated cell expansion of rose petals. However, whether PIP1s also play a role in petal expansion is still unclear. Here, we identified RhPIP1;1, a PIP1 subfamily member, from 18 PIPs assemble transcripts in rose microarray database responsive to ethylene. RhPIP1;1 was rapidly and significantly down-regulated by ethylene treatment. RhETRs-silencing also clearly decreased the expression of RhPIP1;1 in rose petals. The activity of the RhPIP1;1 promoter was repressed by ethylene in rosettes and roots of Arabidopsis. RhPIP1;1 is mainly localized on endoplasmic reticulum and plasma membrane. We demonstrated that RhPIP1;1-silencing significantly inhibited the expansion of petals with decreased petal size and cell area, as well as reduced fresh weight when compared to controls. Expression of RhPIP1;1 in Xenopus oocytes indicated that RhPIP1;1 was inactive in terms of water transport, while coexpression of RhPIP1;1 with the functional RhPIP2;1 led to a significant increase in plasma membrane permeability. Yeast growth, β-Galactosidase activity, bimolecular fluorescence complementation, and colocalization assay proved existence of the interaction between RhPIP1;1 and RhPIP2;1. We argue that RhPIP1;1 plays an important role in ethylene-regulated petal cell expansion, at least partially through the interaction with RhPIP2;1.

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

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

Identification, characterization of an AP2/ERF transcription factor that promotes adventitious, lateral root formation in Populus

Using activation tagging in Populus, we have identified five mutant lines showing changes in their adventitious rooting. Among the affected lines, three showed increased and two decreased adventitious rooting. We have positioned the tag in the mutant lines via recovering genomic sequences flanking the left-hand border of the activation tagging vector and validated the transcriptional activation of the proximal genes. We further characterized one line in which the cause of the observed rooting phenotype was up-regulation of a gene encoding a transcription factor of the AP2/ERF family of unknown function (PtaERF003). We show, through retransformation, that this gene has a positive effect on both adventitious and lateral root proliferation. Comparative expression analyses show that the phenotype does not result from ectopic expression but rather up-regulation of the native expression pattern of the gene. PtaERF003 function is linked to auxin signal transduction pathway, as suggested by the rapid induction and accentuated phenotypes of the transgenic plants in presence of the hormone. Upregulation of PtaERF003 led to most significant metabolic changes in the shoot suggesting of a broader regulatory role of the gene that is not restricted to root growth and development. Our study shows that dominant tagging approaches in poplar can successfully identify novel molecular factors controlling adventitious and lateral root formation in woody plants. Such discoveries can lead to technologies that can increase root proliferation and, thus, have significant economic and environmental benefits.

Genetic identification of a second site modifier of ctr1-1 that controls ethylene-responsive and gravitropic root growth in Arabidopsis thaliana

Ethylene controls myriad aspects of plant growth throughout developmental stages in higher plants. It has been well established that ethylene-responsive growth entails extensive crosstalk with other plant hormones, particularly auxin. Here, we report a genetic mutation, named 1-aminocyclopropane carboxylic acid (ACC) resistant root1-1 (are1-1) in Arabidopsis thaliana (L.) Heynh. The CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) encodes a Raf-related protein, functioning as an upstream negative regulator of ethylene signaling in Arabidopsis thaliana. We found that the ctr1-1, a kinase-inactive allele exhibited slightly, but significantly, longer root length, compared to ACC-treated wild-type or ctr1-3, a null allele. Our genetic studies unveiled the existence of are1-1 mutation in the ctr1-1 mutant, as a second-site modifier which confers root-specific ethylene-resistance. Based on well-characterized crosstalk between ethylene and auxin during ethylene-responsive root growth, we performed various physiological analyses. Whereas are1-1 displayed normal sensitivity to synthetic auxins, it showed modest resistance to an auxin transport inhibitor, 1-Nnaphthylphthalamic acid. In addition, are1-1 mutant exhibited ectopically altered DR5:GUS activity upon ethylenetreatment. The results implicated the involvement of are1-1 in auxin-distribution, but not in auxin-biosynthesis, -uptake, or -sensitivity. In agreement, are1-1 mutant exhibited reduced gravitropic root growth and defective redistribution of DR5:GUS activity upon gravi-stimulation. Taken together with genetic and molecular analysis, our results suggest that ARE1 defines a novel locus to control ethylene-responsive root growth as well as gravitropic root growth presumably through auxin distribution in Arabidopsis thaliana.

Induction of somatic embryos in Arabidopsis requires local YUCCA expression mediated by the down-regulation of ethylene biosynthesis

Somatic embryogenesis is an important experimental model for studying cellular and molecular mechanisms of early embryo development. Although it has long been known that removal of exogenous auxin from medium results in somatic embryogenesis, the mechanisms underlying the initiation of somatic embryos (SEs) are poorly understood. In this study, we showed that YUCCAs (YUCs) encoding key enzymes in auxin biosynthesis are required for SE induction in Arabidopsis. To identify other factors mediating SE initiation, we performed transcriptional profiling and gene expression analysis. The results showed that genes involved in ethylene biosynthesis and its responses were down-regulated during SE initiation. Ethylene level decreased progressively during SE initiation, whereas treatment with the metabolic precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), or mutation of ETHYLENE-OVERPRODUCTION1 (ETO1) disrupted SE induction, suggesting that ethylene plays a role in this process. Suppression of SE induction was also observed in the constitutive triple response 1 (ctr1) mutant, in which ethylene signaling was enhanced. These results indicate that down-regulation of not only ethylene biosynthesis, but also ethylene response is critical for SE induction. We further showed that ethylene disturbed SE initiation through inhibiting YUC expression that might be involved in local auxin biosynthesis and subsequent auxin distribution. Our results provide new information on the mechanisms of hormone-regulated SE initiation.

Ethylene signaling and regulation in plant growth and stress responses

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

Autotoxicity mechanism of Oryza sativa: transcriptome response in rice roots exposed to ferulic acid

Background

Autotoxicity plays an important role in regulating crop yield and quality. To help characterize the autotoxicity mechanism of rice, we performed a large-scale, transcriptomic analysis of the rice root response to ferulic acid, an autotoxin from rice straw.

Results

Root growth rate was decreased and reactive oxygen species, calcium content and lipoxygenase activity were increased with increasing ferulic acid concentration in roots. Transcriptome analysis revealed more transcripts responsive to short ferulic-acid exposure (1- and 3-h treatments, 1,204 genes) than long exposure (24 h, 176 genes). Induced genes were involved in cell wall formation, chemical detoxification, secondary metabolism, signal transduction, and abiotic stress response. Genes associated with signaling and biosynthesis for ethylene and jasmonic acid were upregulated with ferulic acid. Ferulic acid upregulated ATP-binding cassette and amino acid/auxin permease transporters as well as genes encoding signaling components such as leucine-rich repeat VIII and receptor-like cytoplasmic kinases VII protein kinases, APETALA2/ethylene response factor, WRKY, MYB and Zinc-finger protein expressed in inflorescence meristem transcription factors.

Conclusions

The results of a transcriptome analysis suggest the molecular mechanisms of plants in response to FA, including toxicity, detoxicification and signaling machinery. FA may have a significant effect on inhibiting rice root elongation through modulating ET and JA hormone homeostasis. FA-induced gene expression of AAAP transporters may contribute to detoxicification of the autotoxin. Moreover, the WRKY and Myb TFs and LRR-VIII and SD-2b kinases might regulate downstream genes under FA stress but not general allelochemical stress. This comprehensive description of gene expression information could greatly facilitate our understanding of the mechanisms of autotoxicity in plants.

An organ-specific role for ethylene in rose petal expansion during dehydration and rehydration

Dehydration is a major factor resulting in huge loss from cut flowers during transportation. In the present study, dehydration inhibited petal cell expansion and resulted in irregular flowers in cut roses, mimicking ethylene-treated flowers. Among the five floral organs, dehydration substantially elevated ethylene production in the sepals, whilst rehydration caused rapid and elevated ethylene levels in the gynoecia and sepals. Among the five ethylene biosynthetic enzyme genes (RhACS1-5), expression of RhACS1 and RhACS2 was induced by dehydration and rehydration in the two floral organs. Silencing both RhACS1 and RhACS2 significantly suppressed dehydration- and rehydration-induced ethylene in the sepals and gynoecia. This weakened the inhibitory effect of dehydration on petal cell expansion. β-glucuronidase activity driven by both the RhACS1 and RhACS2 promoters was dramatically induced in the sepals, pistil, and stamens, but not in the petals of transgenic Arabidopsis. This further supports the organ-specific induction of these two genes. Among the five rose ethylene receptor genes (RhETR1-5), expression of RhETR3 was predominantly induced by dehydration and rehydration in the petals. RhETR3 silencing clearly aggravated the inhibitory effect of dehydration on petal cell expansion. However, no significant difference in the effect between RhETR3-silenced flowers and RhETR-genes-silenced flowers was observed. Furthermore, RhETR-genes silencing extensively altered the expression of 21 cell expansion-related downstream genes in response to ethylene. These results suggest that induction of ethylene biosynthesis by dehydration proceeds in an organ-specific manner, indicating that ethylene can function as a mediator in dehydration-caused inhibition of cell expansion in rose petals.

The TMK subfamily of receptor-like kinases in Arabidopsis display an essential role in growth and a reduced sensitivity to auxin

Mechanisms that govern the size of plant organs are not well understood but believed to involve both sensing and signaling at the cellular level. We have isolated loss-of-function mutations in the four genes comprising the transmembrane kinase TMK subfamily of receptor-like kinases (RLKs) in Arabidopsis. These TMKs have an extracellular leucine-rich-repeat motif, a single transmembrane region, and a cytoplasmic kinase domain. While single mutants do not display discernable phenotypes, unique double and triple mutant combinations result in a severe reduction in organ size and a substantial retardation in growth. The quadruple mutant displays even greater severity of all phenotypes and is infertile. The kinematic studies of root, hypocotyl, and stamen filament growth reveal that the TMKs specifically control cell expansion. In leaves, TMKs control both cell expansion and cell proliferation. In addition, in the tmk double mutants, roots and hypocotyls show reduced sensitivity to applied auxin, lateral root induction and activation of the auxin response reporter DR5: GUS. Thus, taken together with the structural and biochemical evidence, TMKs appear to orchestrate plant growth by regulation of both cell expansion and cell proliferation, and as a component of auxin signaling.

The rhizobacterium Variovorax paradoxus 5C-2, containing ACC deaminase, promotes growth and development of Arabidopsis thaliana via an ethylene-dependent pathway

Many plant-growth-promoting rhizobacteria (PGPR) associated with plant roots contain the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase and can metabolize ACC, the immediate precursor of the plant hormone ethylene, thereby decreasing plant ethylene production and increasing plant growth. However, relatively few studies have explicitly linked ethylene emission and/or action to growth promotion in these plant-microbe interactions. This study examined effects of the PGPR Variovorax paradoxus 5C-2 containing ACC deaminase on the growth and development of Arabidopsis thaliana using wild-type (WT) plants and several ethylene-related mutants (etr1-1, ein2-1, and eto1-1). Soil inoculation with V. paradoxus 5C-2 promoted growth (leaf area and shoot biomass) of WT plants and the ethylene-overproducing mutant eto1-1, and also enhanced floral initiation of WT plants by 2.5 days. However, these effects were not seen in ethylene-insensitive mutants (etr1-1 and ein2-1) even though bacterial colonization of the root system was similar. Furthermore, V. paradoxus 5C-2 decreased ACC concentrations of rosette leaves of WT plants by 59% and foliar ethylene emission of both WT plants and eto1-1 mutants by 42 and 37%, respectively. Taken together, these results demonstrate that a fully functional ethylene signal transduction pathway is required for V. paradoxus 5C-2 to stimulate leaf growth and flowering of A. thaliana.

Proteomics-based investigation of salt-responsive mechanisms in plant roots

Salinity is one of the major abiotic stresses that limits agricultural productivity worldwide. Plant roots function as the primary site of salinity perception. Salt responses in roots are essential for maintaining root functionality, as well as for transmitting the salt signal to shoot for proper salt response and adaptation in the entire plant. Therefore, a thorough understanding of signaling and metabolic mechanisms of salt response in roots is critical for improving plant salt tolerance. Current proteomic studies have provided salt-responsive expression patterns of 905 proteins in 14 plant species. Through integrative analysis of salt-responsive proteins and previous physiological and molecular findings, this review summarizes current understanding of salt responses in roots and highlights proteomic findings on the molecular mechanisms in the fine-tuned salt-responsive networks. At the proteome level, the following processes become dominant in root salt response: (i) salt signal perception and transduction; (ii) detoxification of reactive oxygen species (ROS); (iii) salt uptake/exclusion and compartmentalization; (iv) protein translation and/or turnover dynamics; (v) cytoskeleton/cell wall dynamics; (vi) carbohydrate and energy metabolism; and (vii) other salt-responsive metabolisms. These processes work together to gain cellular homeostasis in roots and determine the overall phenotype of plant growth and development under salt stress.

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

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

Tomato transcriptome and mutant analyses suggest a role for plant stress hormones in the interaction between fruit and Botrytis cinerea

Fruit-pathogen interactions are a valuable biological system to study the role of plant development in the transition from resistance to susceptibility. In general, unripe fruit are resistant to pathogen infection but become increasingly more susceptible as they ripen. During ripening, fruit undergo significant physiological and biochemical changes that are coordinated by complex regulatory and hormonal signaling networks. The interplay between multiple plant stress hormones in the interaction between plant vegetative tissues and microbial pathogens has been documented extensively, but the relevance of these hormones during infections of fruit is unclear. In this work, we analyzed a transcriptome study of tomato fruit infected with Botrytis cinerea in order to profile the expression of genes for the biosynthesis, modification and signal transduction of ethylene (ET), salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA), hormones that may be not only involved in ripening, but also in fruit interactions with pathogens. The changes in relative expression of key genes during infection and assays of susceptibility of fruit with impaired synthesis or perception of these hormones were used to formulate hypotheses regarding the involvement of these regulators in the outcome of the tomato fruit-B. cinerea interaction.

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.