Activation of ethylene signaling is mediated by nuclear translocation of the cleaved EIN2 carboxyl terminus

EIN2-dependent regulation of acetylation of histone H3K14 and non-canonical histone H3K23 in ethylene signalling

Ethylene gas is essential for many developmental processes and stress responses in plants. EIN2 plays a key role in ethylene signalling but its function remains enigmatic. Here, we show that ethylene specifically elevates acetylation of histone H3K14 and the non-canonical acetylation of H3K23 in etiolated seedlings. The up-regulation of these two histone marks positively correlates with ethylene-regulated transcription activation, and the elevation requires EIN2. Both EIN2 and EIN3 interact with a SANT domain protein named EIN2 nuclear associated protein 1 (ENAP1), overexpression of which results in elevation of histone acetylation and enhanced ethylene-inducible gene expression in an EIN2-dependent manner. On the basis of these findings we propose a model where, in the presence of ethylene, the EIN2 C terminus contributes to downstream signalling via the elevation of acetylation at H3K14 and H3K23. ENAP1 may potentially mediate ethylene-induced histone acetylation via its interactions with EIN2 C terminus.

The translocation of the C-terminal domain of EIN2 to the nucleus is essential for induction of gene expression in response to the plant hormone ethylene. Here, Zhang et al. show that EIN2 is required for ethylene-inducible elevation of histone acetylation marks associated with transcriptional activation.

The regulatory roles of ethylene and reactive oxygen species (ROS) in plant salt stress responses

Soil salinity is one of the most commonly encountered environmental stresses affecting plant growth and crop productivity. Accordingly, plants have evolved a variety of morphological, physiological and biochemical strategies that enable them to adapt to saline growth conditions. For example, it has long been known that salinity-stress increases both the production of the gaseous stress hormone ethylene and the in planta accumulation of reactive oxygen species (ROS). Recently, there has been significant progress in understanding how the fine-tuning of ethylene biosynthesis and signaling transduction can promote salinity tolerance, and how salinity-induced ROS accumulation also acts as a signal in the mediation of salinity tolerance. Furthermore, recent advances have indicated that ethylene signaling modulates salinity responses largely via regulation of ROS-generating and ROS-scavenging mechanisms. This review focuses on these recent advances in understanding the linked roles of ethylene and ROS in salt tolerance.

Peptides interfering with protein-protein interactions in the ethylene signaling pathway delay tomato fruit ripening

The plant hormone ethylene is involved in the regulation of several processes with high importance for agricultural applications, e.g. ripening, aging and senescence. Previous work in our group has identified a small peptide (NOP-1) derived from the nuclear localization signal of the Arabidopsis ethylene regulator ETHYLENE INSENSITIVE-2 (EIN2) C-terminal part as efficient inhibitor of ethylene responses. Here, we show that NOP-1 is also able to efficiently disrupt EIN2-ETR1 complex formation in tomato, indicating that the NOP-1 inhibition mode is conserved across plant species. Surface application of NOP-1 on green tomato fruits delays ripening similar to known inhibitors of ethylene perception (MCP) and ethylene biosynthesis (AVG). Fruits treated with NOP-1 showed similar ethylene production as untreated controls underlining that NOP-1 blocks ethylene signaling by targeting an essential interaction in this pathway, while having no effect on ethylene biosynthesis.

Banana Transcription Factor MaERF11 Recruits Histone Deacetylase MaHDA1 and Represses the Expression of MaACO1 and Expansins during Fruit Ripening

Phytohormone ethylene controls diverse developmental and physiological processes such as fruit ripening via modulation of ethylene signaling pathway. Our previous study identified that ETHYLENE RESPONSE FACTOR11 (MaERF11), a transcription factor in the ethylene signaling pathway, negatively regulates the ripening of banana, but the mechanism for the MaERF11-mediated transcriptional regulation remains largely unknown. Here we showed that MaERF11 has intrinsic transcriptional repression activity in planta. Electrophoretic mobility shift assay and chromatin immunoprecipitation analyses demonstrated that MaERF11 binds to promoters of three ripening-related Expansin genes, MaEXP2, MaEXP7 and MaEXP8, as well as an ethylene biosynthetic gene MaACO1, via the GCC-box motif. Furthermore, expression patterns of MaACO1, MaEXP2, MaEXP7, and MaEXP8 genes are correlated with the changes of histone H3 and H4 acetylation level during fruit ripening. Moreover, we found that MaERF11 physically interacts with a histone deacetylase, MaHDA1, which has histone deacetylase activity, and the interaction significantly strengthens the MaERF11-mediated transcriptional repression of MaACO1 and Expansins Taken together, these findings suggest that MaERF11 may recruit MaHDA1 to its target genes and repress their expression via histone deacetylation.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Interactions between ethylene, gibberellins, and brassinosteroids in the development of rhizobial and mycorrhizal symbioses of pea

The regulation of arbuscular mycorrhizal development and nodulation involves complex interactions between the plant and its microbial symbionts. In this study, we use the recently identified ethylene-insensitive ein2 mutant in pea (Pisum sativum L.) to explore the role of ethylene in the development of these symbioses. We show that ethylene acts as a strong negative regulator of nodulation, confirming reports in other legumes. Minor changes in gibberellin1 and indole-3-acetic acid levels in ein2 roots appear insufficient to explain the differences in nodulation. Double mutants produced by crosses between ein2 and the severely gibberellin-deficient na and brassinosteroid-deficient lk mutants showed increased nodule numbers and reduced nodule spacing compared with the na and lk single mutants, but nodule numbers and spacing were typical of ein2 plants, suggesting that the reduced number of nodules innaandlkplants is largely due to the elevated ethylene levels previously reported in these mutants. We show that ethylene can also negatively regulate mycorrhizae development when ethylene levels are elevated above basal levels, consistent with a role for ethylene in reducing symbiotic development under stressful conditions. In contrast to the hormone interactions in nodulation, ein2 does not override the effect of lk or na on the development of arbuscular mycorrhizae, suggesting that brassinosteroids and gibberellins influence this process largely independently of ethylene.

The yellow-fruited tomato 1 (yft1) mutant has altered fruit carotenoid accumulation and reduced ethylene production as a result of a genetic lesion in ETHYLENE INSENSITIVE2

The isolated yft1 allele controls the formation of fruit color in n3122 via the regulation of response to ethylene, carotenoid accumulation and chromoplast development. Fruit color is one of the most important quality traits of tomato (Solanum lycopersicum) and is closely associated with both nutritional and market value. In this study, we characterized a tomato fruit color mutant n3122, named as yellow-fruited tomato 1 (yft1), which produces yellow colored mature fruit. Fruit color segregation of the progeny from an intra-specific cross (M82 × n3122) and an inter-specific cross (n3122 × LA1585) revealed that a single recessive nuclear gene determined the yellow fruit phenotype. Through map-based cloning, the yft1 locus was assigned to an 88.2 kb region at the top of chromosome 9 that was annotated as containing 12 genes. Sequencing revealed that one gene, Solyc09g007870, which encodes ETHYLENE INSENSITIVE2 (EIN2), contained two mutations in yft1: a 13 bp deletion and a 573 bp insertion at position -318 bp upstream of the translation initiation site. We detected that EIN2 expression was substantially lower in yft1 than in the red-fruited M82 wild type and that, in addition, carotenoid accumulation was decreased, ethylene synthesis and perception were impaired and chromoplast development was delayed. The results implied that the reduced expression of EIN2 in yft1 leads to suppressed ethylene signaling which results in abnormal carotenoid production.

Biochemical and molecular characterisation of salt-induced poor grain filling in a rice cultivar

Despite the prevalence of poor grain filling in rice (Oryza sativa L.) under abiotic stress, the reason for this is largely unexplored. Application of 0.75% NaCl to a salt-sensitive rice cultivar at late booting resulted in a >20% yield loss. Spikelets per panicle and the percentage of filled grain decreased significantly in response to NaCl application. The inhibitory effect of NaCl on grain filling was greater in basal than in apical spikelets. Sucrose synthase (SUS) activity was positively correlated with grain weight. The transcript levels of the SUS isoforms differed greatly: the levels of SUS2 increased significantly in response to salt; those of SUS4 decreased drastically. Gene expression studies of starch synthase and ADP-glucose pyrophosphorylase showed that the decreased transcript levels of one isoform was compensated by an increase in those of the other. Salt application also significantly increased the gene expression of the ethylene receptors and the ethylene signalling proteins. The increase in their transcript levels was comparatively greater in basal than in apical spikelets. Significant enhancement in the transcript levels of the ethylene receptors and the increase in the production of ethylene indicated that the salt-induced inhibition of grain filling might be mediated by ethylene. Additionally, the inhibition of chromosomal endoreduplication mediated by decreased transcript levels of B-type cyclin could explain poor grain filling under salt stress. A significant increase in the transcript levels of the ethylene-responsive factors in the spikelets during grain filling in response to salt indicated their possible protective role in grain filling under stress.

Q&A: How do plants respond to ethylene and what is its importance?

Ethylene gas is a major plant hormone that influences diverse processes in plant growth, development and stress responses throughout the plant life cycle. Responses to ethylene, such as fruit ripening, are significant to agriculture. The core molecular elements of the ethylene-signaling pathway have been uncovered, revealing a unique pathway that is negatively regulated. Practical applications of this knowledge can lead to substantial improvements in agriculture.

Activation of HLS1 by Mechanical Stress via Ethylene-Stabilized EIN3 Is Crucial for Seedling Soil Emergence

The seeds of terrestrial flowering plants often start their life cycle in subterranean darkness. To protect the fragile apical meristematic tissues and cotyledons from mechanical injuries during soil penetration, dicotyledonous seedlings form an elegant apical hook at the top of the hypocotyl. The apical hook has been considered as an adaption structure to the subterranean environment. However, the role of the apical hook in seedling emergence and the molecular mechanism of apical hook formation under real-life conditions remain highly speculative. Here, we find that HOOKLESS 1 (HLS1), a critical gene in apical hook formation in Arabidopsis thaliana, is required for seedling emergence from the soil. When grown under soil, hls1 mutant exhibits severe emergence defects. By contrast, HLS1 overexpression in the hls1 background fully restores emergence defects and displays better emergence capacity than that of WT. Our results indicate that HLS1 transcription is stimulated in response to the mechanical stress of soil cover, which is dependent on the function of the transcription factors ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-LIKE 1 (EIL1). Soil-conferred mechanical stress activates the ethylene signaling pathway to stabilize EIN3 by repressing the activity of the F-box proteins EBF1 and EBF2. These combined results reveal a signaling pathway in which plant seedlings transduce the mechanical pressure of soil cover to correctly modulate apical hook formation during soil emergence.

Analysis of Network Topologies Underlying Ethylene Growth Response Kinetics

Most models for ethylene signaling involve a linear pathway. However, measurements of seedling growth kinetics when ethylene is applied and removed have resulted in more complex network models that include coherent feedforward, negative feedback, and positive feedback motifs. The dynamical responses of the proposed networks have not been explored in a quantitative manner. Here, we explore (i) whether any of the proposed models are capable of producing growth-response behaviors consistent with experimental observations and (ii) what mechanistic roles various parts of the network topologies play in ethylene signaling. To address this, we used computational methods to explore two general network topologies: The first contains a coherent feedforward loop that inhibits growth and a negative feedback from growth onto itself (CFF/NFB). In the second, ethylene promotes the cleavage of EIN2, with the product of the cleavage inhibiting growth and promoting the production of EIN2 through a positive feedback loop (PFB). Since few network parameters for ethylene signaling are known in detail, we used an evolutionary algorithm to explore sets of parameters that produce behaviors similar to experimental growth response kinetics of both wildtype and mutant seedlings. We generated a library of parameter sets by independently running the evolutionary algorithm many times. Both network topologies produce behavior consistent with experimental observations, and analysis of the parameter sets allows us to identify important network interactions and parameter constraints. We additionally screened these parameter sets for growth recovery in the presence of sub-saturating ethylene doses, which is an experimentally-observed property that emerges in some of the evolved parameter sets. Finally, we probed simplified networks maintaining key features of the CFF/NFB and PFB topologies. From this, we verified observations drawn from the larger networks about mechanisms underlying ethylene signaling. Analysis of each network topology results in predictions about changes that occur in network components that can be experimentally tested to give insights into which, if either, network underlies ethylene responses.

Relaying the Ethylene Signal: New Roles for EIN2

ETHYLENE INSENSITIVE 2 (EIN2), an endoplasmic reticulum (ER) localized protein, plays a central role in relaying the ethylene signal from ER perception to the nucleus. Two recent reports reveal the novel role for EIN2 in translational control, providing another layer of regulation for ethylene signal transduction.

Mitogen-activated protein kinase 6 mediates nuclear translocation of ORE3 to promote ORE9 gene expression in methyl jasmonate-induced leaf senescence

Methyl jasmonate (MeJA) is a potent promoter of plant senescence. ORESARA3 (ORE3)/ETHYLENE INSENSITIVE2 (EIN2), a protein similar to the members of the disease-related Nramp metal transporter family, is involved in cross-talk among several senescence processes related to abscisic acid, ethylene, MeJA, age and darkness. Nevertheless, the mechanism involved in the regulation of ORE3/EIN2 in exogenous MeJA-induced leaf senescence remains unclear. The C-terminal end of ORE3/EIN2 (CEND) was cleaved from ORE3/EIN2 located in the endoplasmic reticulum and then transferred to the nucleus during MeJA-induced senescence. Further analyses showed that mitogen-activated protein kinase 6 (MPK6) promoted CEND cleavage and nuclear translocation. Nuclear CEND accumulated ETHYLENE INSENSITIVE3 (EIN3), a transcription factor that accelerates MeJA-induced leaf senescence wherein ORESARA9 (ORE9) expression was suppressed in ein3, ore3, and mpk6 mutant plants. ChIP experiments revealed that EIN3 bound directly to the ORE9 promoter and this binding was enhanced in MeJA-induced leaf senescence. This study revealed the effect of the signalling pathway involving MPK6-ORE3-EIN3-ORE9 on regulating leaf senescence and provided insights into the mechanism of MeJA in promoting leaf senescence in Arabidopsis thaliana.

Seedlings Transduce the Depth and Mechanical Pressure of Covering Soil Using COP1 and Ethylene to Regulate EBF1/EBF2 for Soil Emergence

The survival of seed plants in natural environments requires the successful emergence from the soil. In this process, the ethylene signaling pathway is utilized by plants to sense and respond to the mechanical resistance of the soil. Here, we report that constitutive photomorphogenesis 1 (COP1), a central repressor of light signaling, is a key component required for seedlings to sense the depth of soil overlay. Mutation in COP1 causes severe defects in penetrating soil, due to decreased level of EIN3, a master transcription factor in ethylene pathway that mediates seedling emergence. We show that COP1 directly targets the F box proteins EBF1 and EBF2 for ubiquitination and degradation, thus stabilizing EIN3. As seedlings grow toward the surface, the depth of soil overlay decreases, resulting in a gradual increase of light fluences. COP1 channels the light signals, while ethylene transduces the information on soil mechanical conditions, which cooperatively control EIN3 protein levels to promote seedling emergence from the soil. The COP1-EBF1/2-EIN3 module reveals a mechanism by which plants sense the depth to surface and uncovers a novel regulatory paradigm of an ubiquitin E3 ligase cascade.

Dominant gain-of-function mutations in transmembrane domain III of ERS1 and ETR1 suggest a novel role for this domain in regulating the magnitude of ethylene response in Arabidopsis

Prior work resulted in identification of an Arabidopsis mutant, eer5-1, with extreme ethylene response in conjunction with failure to induce a subset of ethylene-responsive genes, including AtEBP. EER5, which is a TREX-2 homolog that is part of a nucleoporin complex, functions as part of a cryptic aspect of the ethylene signaling pathway that is required for regulating the magnitude of ethylene response. A suppressor mutagenesis screen was carried out to identify second site mutations that could restore the growth of ethylene-treated eer5-1 to wild-type levels. A dominant gain-of-function mutation in the ethylene receptor ETHYLENE RESPONSE SENSOR 1 (ERS1) was identified, with the ers1-4 mutation being located in transmembrane domain III at a point nearly equivalent to the previously described etr1-2 mutation in the other Arabidopsis subfamily I ethylene receptor, ETHYLENE RESPONSE 1 (ETR1). Although both ers1-4 and etr1-2 partially suppress the ethylene hypersensitivity of eer5-1 and are at least in part REVERSION TO ETHYLENE SENSITIVITY 1 (RTE1)-dependent, ers1-4 was additionally found to restore the expression of AtEBP in ers1-4;eer5-1 etiolated seedlings after ethylene treatment in an EIN3-dependent manner. Our work indicates that ERS1-regulated expression of a subset of ethylene-responsive genes is related to controlling the magnitude of ethylene response, with hyperinduction of these genes correlated with reduced ethylene-dependent growth inhibition.

CHR729 Is a CHD3 Protein That Controls Seedling Development in Rice

CHD3 is one of the chromatin-remodeling factors that contribute to controlling the expression of genes associated with plant development. Loss-of-function mutants display morphological and growth defects. However, the molecular mechanisms underlying CHD3 regulation of plant development remain unclear. In this study, a rice CHD3 protein, CHR729, was identified. The corresponding mutant line (t483) exhibited late seed germination, low germination rate, dwarfism, low tiller number, root growth inhibition, adaxial albino leaves, and short and narrow leaves. CHR729 encoded a nuclear protein and was expressed in almost all organs. RNA-sequencing analysis showed that several plant hormone-related genes were up- or down-regulated in t483 compared to wild type. In particular, expression of the gibberellin synthetase gibberellin 20 oxidase 4 gene was elevated in the mutant. Endogenous gibberellin assays demonstrated that the content of bioactive GA₃ was reduced in t483 compared to wild type. Moreover, the seedling dwarfism, late seed germination, and short root length phenotypes of t483 were partially rescued by treatment with exogenous GA₃. These results suggest that the rice CHD3 protein CHR729 plays an important role in many aspects of seedling development and controls this development via the gibberellin pathway.

Comparison of plant hormone signalling systems

Plant growth and development are controlled by nine structurally distinct small molecules termed phytohormones. Over the last 20 years, the molecular basis of their signal transduction, from receptors to transcription factors, has been dissected using mainly Arabidopsis thaliana and rice as model systems. Phytohormones can be broadly classified into two distinct groups on the basis of whether the subcellular localization of their receptors is in the cytoplasm or nucleus, and hence soluble, or membrane-bound, and hence insoluble. Soluble receptors, which control the responses to auxin, jasmonates, gibberellins, strigolactones and salicylic acid, signal either directly or indirectly via the destruction of regulatory proteins. Responses to abscisic acid are primarily mediated by soluble receptors that indirectly regulate the phosphorylation of targeted proteins. Insoluble receptors, which control the responses to cytokinins, brassinosteroids and ethylene, transduce their signal through protein phosphorylation. This chapter provides a comparison of the different components of these signalling systems, and discusses the similarities and differences between them.

Mechanisms of ethylene biosynthesis and response in plants

Ethylene is the simplest unsaturated hydrocarbon, yet it has profound effects on plant growth and development, including many agriculturally important phenomena. Analysis of the mechanisms underlying ethylene biosynthesis and signalling have resulted in the elucidation of multistep mechanisms which at first glance appear simple, but in fact represent several levels of control to tightly regulate the level of production and response. Ethylene biosynthesis represents a two-step process that is regulated at both the transcriptional and post-translational levels, thus enabling plants to control the amount of ethylene produced with regard to promotion of responses such as climacteric flower senescence and fruit ripening. Ethylene production subsequently results in activation of the ethylene response, as ethylene accumulation will trigger the ethylene signalling pathway to activate ethylene-dependent transcription for promotion of the response and for resetting the pathway. A more detailed knowledge of the mechanisms underlying biosynthesis and the ethylene response will ultimately enable new approaches to be developed for control of the initiation and progression of ethylene-dependent developmental processes, many of which are of horticultural significance.

Regulatory role of BOTRYTIS INDUCED KINASE1 in ETHYLENE INSENSITIVE3-dependent gene expression in Arabidopsis

Arabidopsis BIK1 negatively regulates EIN3-depedent gene expression as an immediate cellular response. BIK1 localizes to the plasma membrane and its autophosphorylation and kinase activity involves in EIN3 repression. BOTRYTIS INDUCED KINASE1 (BIK1) is a multifunctional receptor-like kinase that involves in ethylene-mediated plant defense signaling. The loss of function BIK1 becomes insensitive to ethylene, but it still accumulates a higher level of ETHYLENE INSENSITIVE3 (EIN3) that serves as the key transcription activator in ethylene signaling. To unequivocally elucidate BIK1 function on EIN3 regulation in ethylene signaling, we took a combined approach of transient expression assay and stable expression analysis of BIK1. In our cell-based functional assay BIK1 destabilized EIN3 and down-regulated EIN3-dependent transcription. Membrane localization and autophosphorylation of BIK1 were required for full repression of EIN3 function, but its kinase activity potential compromised such regulatory action. Consistently, the analysis of transgenic plants verified BIK1 function on EIN3 repression. Our findings have clarified that autophosphorylated BIK1 in the plasma membrane negatively regulates EIN3-dependent gene expression. Thus, ethylene insensitivity in bik1 appears to be an indirect or a feedback long-term response.

Mechanistic Insights in Ethylene Perception and Signal Transduction

The gaseous hormone ethylene profoundly affects plant growth, development, and stress responses. Ethylene perception occurs at the endoplasmic reticulum membrane, and signal transduction leads to a transcriptional cascade that initiates diverse responses, often in conjunction with other signals. Recent findings provide a more complete picture of the components and mechanisms in ethylene signaling, now rendering a more dynamic view of this conserved pathway. This includes newly identified protein-protein interactions at the endoplasmic reticulum membrane, as well as the major discoveries that the central regulator ETHYLENE INSENSITIVE2 (EIN2) is the long-sought phosphorylation substrate for the CONSTITUTIVE RESPONSE1 protein kinase, and that cleavage of EIN2 transmits the signal to the nucleus. In the nucleus, hundreds of potential gene targets of the EIN3 master transcription factor have been identified and found to be induced in transcriptional waves, and transcriptional coregulation has been shown to be a mechanism of ethylene cross talk.

Role of Ethylene and Its Cross Talk with Other Signaling Molecules in Plant Responses to Heavy Metal Stress

Excessive heavy metals (HMs) in agricultural lands cause toxicities to plants, resulting in declines in crop productivity. Recent advances in ethylene biology research have established that ethylene is not only responsible for many important physiological activities in plants but also plays a pivotal role in HM stress tolerance. The manipulation of ethylene in plants to cope with HM stress through various approaches targeting either ethylene biosynthesis or the ethylene signaling pathway has brought promising outcomes. This review covers ethylene production and signal transduction in plant responses to HM stress, cross talk between ethylene and other signaling molecules under adverse HM stress conditions, and approaches to modify ethylene action to improve HM tolerance. From our current understanding about ethylene and its regulatory activities, it is believed that the optimization of endogenous ethylene levels in plants under HM stress would pave the way for developing transgenic crops with improved HM tolerance.

MAOHUZI6/ETHYLENE INSENSITIVE3-LIKE1 and ETHYLENE INSENSITIVE3-LIKE2 Regulate Ethylene Response of Roots and Coleoptiles and Negatively Affect Salt Tolerance in Rice

Ethylene plays important roles in plant growth, development, and stress responses. The ethylene signaling pathway has been studied extensively, mainly in Arabidopsis (Arabidopsis thaliana). However, the molecular mechanism of ethylene signaling is largely unknown in rice (Oryza sativa). Previously, we have isolated a set of rice ethylene-response mutants. Here, we characterized the mutant maohuzi6 (mhz6). Through map-based cloning, we found that MHZ6 encodes ETHYLENE INSENSITIVE3-LIKE1 (OsEIL1), a rice homolog of ETHYLENE INSENSITIVE3 (EIN3), which is the master transcriptional regulator of ethylene signaling in Arabidopsis. Disruption of MHZ6/OsEIL1 caused ethylene insensitivity mainly in roots, whereas silencing of the closely related OsEIL2 led to ethylene insensitivity mainly in coleoptiles of etiolated seedlings. This organ-specific functional divergence is different from the functional features of EIN3 and EIL1, both of which mediate the incomplete ethylene responses of Arabidopsis etiolated seedlings. In Arabidopsis, EIN3 and EIL1 play positive roles in plant salt tolerance. In rice, however, lack of MHZ6/OsEIL1 or OsEIL2 functions improves salt tolerance, whereas the overexpressing lines exhibit salt hypersensitivity at the seedling stage, indicating that MHZ6/OsEIL1 and OsEIL2 negatively regulate salt tolerance in rice. Furthermore, this negative regulation by MHZ6/OsEIL1 and OsEIL2 in salt tolerance is likely attributable in part to the direct regulation of HIGH-AFFINITY K(+) TRANSPORTER2;1 expression and Na(+) uptake in roots. Additionally, MHZ6/OsEIL1 overexpression promotes grain size and thousand-grain weight. Together, our study provides insights for the functional diversification of MHZ6/OsEIL1 and OsEIL2 in ethylene response and finds a novel mode of ethylene-regulated salt stress response that could be helpful for engineering salt-tolerant crops.

Functional characterization of PhGR and PhGRL1 during flower senescence in the petunia

Petunia PhGRL1 suppression accelerated flower senescence and increased the expression of the genes downstream of ethylene signaling, whereas PhGR suppression did not. Ethylene plays an important role in flowers senescence. Homologous proteins Green-Ripe and Reversion to Ethylene sensitivity1 are positive regulators of ethylene responses in tomato and Arabidopsis, respectively. The petunia flower has served as a model for the study of ethylene response during senescence. In this study, petunia PhGR and PhGRL1 expression was analyzed in different organs, throughout floral senescence, and after exogenous ethylene treatment; and the roles of PhGR and PhGRL1 during petunia flower senescence were investigated. PhGRL1 suppression mediated by virus-induced gene silencing accelerated flower senescence and increased ethylene production; however, the suppression of PhGR did not. Taken together, these data suggest that PhGRL1 is involved in negative regulation of flower senescence, possibly via ethylene production inhibition and consequently reduced ethylene signaling activation.

Ethylene Inhibits Cell Proliferation of the Arabidopsis Root Meristem

The root system of plants plays a critical role in plant growth and survival, with root growth being dependent on both cell proliferation and cell elongation. Multiple phytohormones interact to control root growth, including ethylene, which is primarily known for its role in controlling root cell elongation. We find that ethylene also negatively regulates cell proliferation at the root meristem of Arabidopsis (Arabidopsis thaliana). Genetic analysis indicates that the inhibition of cell proliferation involves two pathways operating downstream of the ethylene receptors. The major pathway is the canonical ethylene signal transduction pathway that incorporates CONSTITUTIVE TRIPLE RESPONSE1, ETHYLENE INSENSITIVE2, and the ETHYLENE INSENSITIVE3 family of transcription factors. The secondary pathway is a phosphorelay based on genetic analysis of receptor histidine kinase activity and mutants involving the type B response regulators. Analysis of ethylene-dependent gene expression and genetic analysis supports SHORT HYPOCOTYL2, a repressor of auxin signaling, as one mediator of the ethylene response and furthermore, indicates that SHORT HYPOCOTYL2 is a point of convergence for both ethylene and cytokinin in negatively regulating cell proliferation. Additional analysis indicates that ethylene signaling contributes but is not required for cytokinin to inhibit activity of the root meristem. These results identify key elements, along with points of cross talk with cytokinin and auxin, by which ethylene negatively regulates cell proliferation at the root apical meristem.

Ethylene Signaling Influences Light-Regulated Development in Pea

Plant responses to light involve a complex network of interactions among multiple plant hormones. In a screen for mutants showing altered photomorphogenesis under red light, we identified a mutant with dramatically enhanced leaf expansion and delayed petal senescence. We show that this mutant exhibits reduced sensitivity to ethylene and carries a nonsense mutation in the single pea (Pisum sativum) ortholog of the ethylene signaling gene ETHYLENE INSENSITIVE2 (EIN2). Consistent with this observation, the ein2 mutation rescues the previously described effects of ethylene overproduction in mature phytochrome-deficient plants. In seedlings, ein2 confers a marked increase in leaf expansion under monochromatic red, far-red, or blue light, and interaction with phytochromeA, phytochromeB, and long1 mutants confirms that ein2 enhances both phytochrome- and cryptochrome-dependent responses in a LONG1-dependent manner. In contrast, minimal effects of ein2 on seedling development in darkness or high-irradiance white light show that ethylene is not limiting for development under these conditions. These results indicate that ethylene signaling constrains leaf expansion during deetiolation in pea and provide further evidence that down-regulation of ethylene production may be an important component mechanism in the broader control of photomorphogenic development by phytochrome and cryptochrome.

Tobacco Translationally Controlled Tumor Protein Interacts with Ethylene Receptor Tobacco Histidine Kinase1 and Enhances Plant Growth through Promotion of Cell Proliferation

Ethylene is an important phytohormone in the regulation of plant growth, development, and stress response throughout the lifecycle. Previously, we discovered that a subfamily II ethylene receptor tobacco (Nicotiana tabacum) Histidine Kinase1 (NTHK1) promotes seedling growth. Here, we identified an NTHK1-interacting protein translationally controlled tumor protein (NtTCTP) by the yeast (Saccharomyces cerevisiae) two-hybrid assay and further characterized its roles in plant growth. The interaction was further confirmed by in vitro glutathione S-transferase pull down and in vivo coimmunoprecipitation and bimolecular fluorescence complementation assays, and the kinase domain of NTHK1 mediates the interaction with NtTCTP. The NtTCTP protein is induced by ethylene treatment and colocalizes with NTHK1 at the endoplasmic reticulum. Overexpression of NtTCTP or NTHK1 reduces plant response to ethylene and promotes seedling growth, mainly through acceleration of cell proliferation. Genetic analysis suggests that NtTCTP is required for the function of NTHK1. Furthermore, association of NtTCTP prevents NTHK1 from proteasome-mediated protein degradation. Our data suggest that plant growth inhibition triggered by ethylene is regulated by a unique feedback mechanism, in which ethylene-induced NtTCTP associates with and stabilizes ethylene receptor NTHK1 to reduce plant response to ethylene and promote plant growth through acceleration of cell proliferation.

Identification of Regions in the Receiver Domain of the ETHYLENE RESPONSE1 Ethylene Receptor of Arabidopsis Important for Functional Divergence

Ethylene influences the growth and development of Arabidopsis (Arabidopsis thaliana) via five receptor isoforms. However, the ETHYLENE RESPONSE1 (ETR1) ethylene receptor has unique, and sometimes contrasting, roles from the other receptor isoforms. Prior research indicates that the receiver domain of ETR1 is important for some of these noncanonical roles. We determined that the ETR1 receiver domain is not needed for ETR1's predominant role in mediating responses to the ethylene antagonist, silver. To understand the structure-function relationship underlying the unique roles of the ETR1 receiver domain in the control of specific traits, we performed alanine-scanning mutagenesis. We chose amino acids that are poorly conserved and are in regions predicted to have altered tertiary structure compared with the receiver domains of the other two receptors that contain a receiver domain, ETR2 and ETHYLENE INSENSITIVE4. The effects of these mutants on various phenotypes were examined in transgenic, receptor-deficient Arabidopsis plants. Some traits, such as growth in air and growth recovery after the removal of ethylene, were unaffected by these mutations. By contrast, three mutations on one surface of the receiver domain rendered the transgene unable to rescue ethylene-stimulated nutations. Additionally, several mutations on another surface altered germination on salt. Some of these mutations conferred hyperfunctionality to ETR1 in the context of seed germination on salt, but not for other traits, that correlated with increased responsiveness to abscisic acid. Thus, the ETR1 receiver domain has multiple functions where different surfaces are involved in the control of different traits. Models are discussed for these observations.

Ethylene Inhibits Root Elongation during Alkaline Stress through AUXIN1 and Associated Changes in Auxin Accumulation

Soil alkalinity causes major reductions in yield and quality of crops worldwide. The plant root is the first organ sensing soil alkalinity, which results in shorter primary roots. However, the mechanism underlying alkaline stress-mediated inhibition of root elongation remains to be further elucidated. Here, we report that alkaline conditions inhibit primary root elongation of Arabidopsis (Arabidopsis thaliana) seedlings by reducing cell division potential in the meristem zones and that ethylene signaling affects this process. The ethylene perception antagonist silver (Ag(+)) alleviated the inhibition of root elongation by alkaline stress. Moreover, the ethylene signaling mutants ethylene response1-3 (etr1-3), ethylene insensitive2 (ein2), and ein3-1 showed less reduction in root length under alkaline conditions, indicating a reduced sensitivity to alkalinity. Ethylene biosynthesis also was found to play a role in alkaline stress-mediated root inhibition; the ethylene overproducer1-1 mutant, which overproduces ethylene because of increased stability of 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE5, was hypersensitive to alkaline stress. In addition, the ethylene biosynthesis inhibitor cobalt (Co(2+)) suppressed alkaline stress-mediated inhibition of root elongation. We further found that alkaline stress caused an increase in auxin levels by promoting expression of auxin biosynthesis-related genes, but the increase in auxin levels was reduced in the roots of the etr1-3 and ein3-1 mutants and in Ag(+)/Co(2+)-treated wild-type plants. Additional genetic and physiological data showed that AUXIN1 (AUX1) was involved in alkaline stress-mediated inhibition of root elongation. Taken together, our results reveal that ethylene modulates alkaline stress-mediated inhibition of root growth by increasing auxin accumulation by stimulating the expression of AUX1 and auxin biosynthesis-related genes.

Targeting Plant Ethylene Responses by Controlling Essential Protein-Protein Interactions in the Ethylene Pathway

The gaseous plant hormone ethylene regulates many processes of high agronomic relevance throughout the life span of plants. A central element in ethylene signaling is the endoplasmic reticulum (ER)-localized membrane protein ethylene insensitive2 (EIN2). Recent studies indicate that in response to ethylene, the extra-membranous C-terminal end of EIN2 is proteolytically processed and translocated from the ER to the nucleus. Here, we report that the conserved nuclear localization signal (NLS) mediating nuclear import of the EIN2 C-terminus provides an important domain for complex formation with ethylene receptor ethylene response1 (ETR1). EIN2 lacking the NLS domain shows strongly reduced affinity for the receptor. Interaction of EIN2 and ETR1 is also blocked by a synthetic peptide of the NLS motif. The corresponding peptide substantially reduces ethylene responses in planta. Our results uncover a novel mechanism and type of inhibitor interfering with ethylene signal transduction and ethylene responses in plants. Disruption of essential protein-protein interactions in the ethylene signaling pathway as shown in our study for the EIN2-ETR1 complex has the potential to guide the development of innovative ethylene antagonists for modern agriculture and horticulture.

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

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

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

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

Keeping Control: The Role of Senescence and Development in Plant Pathogenesis and Defense

Many plant pathogens show interactions with host development. Pathogens may modify plant development according to their nutritional demands. Conversely, plant development influences pathogen growth. Biotrophic pathogens often delay senescence to keep host cells alive, and resistance is achieved by senescence-like processes in the host. Necrotrophic pathogens promote senescence in the host, and preventing early senescence is a resistance strategy of plants. For hemibiotrophic pathogens both patterns may apply. Most signaling pathways are involved in both developmental and defense reactions. Increasing knowledge about the molecular components allows to distinguish signaling branches, cross-talk and regulatory nodes that may influence the outcome of an infection. In this review, recent reports on major molecular players and their role in senescence and in pathogen response are reviewed. Examples of pathosystems with strong developmental implications illustrate the molecular basis of selected control strategies. A study of gene expression in the interaction between the hemibiotrophic vascular pathogen Verticillium longisporum and its cruciferous hosts shows processes that are fine-tuned to counteract early senescence and to achieve resistance. The complexity of the processes involved reflects the complex genetic control of quantitative disease resistance, and understanding the relationship between disease, development and resistance will support resistance breeding.

EIN3 and ORE1 Accelerate Degreening during Ethylene-Mediated Leaf Senescence by Directly Activating Chlorophyll Catabolic Genes in Arabidopsis

Degreening, caused by chlorophyll degradation, is the most obvious symptom of senescing leaves. Chlorophyll degradation can be triggered by endogenous and environmental cues, and ethylene is one of the major inducers. ETHYLENE INSENSITIVE3 (EIN3) is a key transcription factor in the ethylene signaling pathway. It was previously reported that EIN3, miR164, and a NAC (NAM, ATAF, and CUC) transcription factor ORE1/NAC2 constitute a regulatory network mediating leaf senescence. However, how this network regulates chlorophyll degradation at molecular level is not yet elucidated. Here we report a feed-forward regulation of chlorophyll degradation that involves EIN3, ORE1, and chlorophyll catabolic genes (CCGs). Gene expression analysis showed that the induction of three major CCGs, NYE1, NYC1 and PAO, by ethylene was largely repressed in ein3 eil1 double mutant. Dual-luciferase assay revealed that EIN3 significantly enhanced the promoter activity of NYE1, NYC1 and PAO in Arabidopsis protoplasts. Furthermore, Electrophoretic mobility shift assay (EMSA) indicated that EIN3 could directly bind to NYE1, NYC1 and PAO promoters. These results reveal that EIN3 functions as a positive regulator of CCG expression during ethylene-mediated chlorophyll degradation. Interestingly, ORE1, a senescence regulator which is a downstream target of EIN3, could also activate the expression of NYE1, NYC1 and PAO by directly binding to their promoters in EMSA and chromatin immunoprecipitation (ChIP) assays. In addition, EIN3 and ORE1 promoted NYE1 and NYC1 transcriptions in an additive manner. These results suggest that ORE1 is also involved in the direct regulation of CCG transcription. Moreover, ORE1 activated the expression of ACS2, a major ethylene biosynthesis gene, and subsequently promoted ethylene production. Collectively, our work reveals that EIN3, ORE1 and CCGs constitute a coherent feed-forward loop involving in the robust regulation of ethylene-mediated chlorophyll degradation during leaf senescence in Arabidopsis.

The ARGOS gene family functions in a negative feedback loop to desensitize plants to ethylene

BACKGROUND

Ethylene plays critical roles in plant growth and development, including the regulation of cell expansion, senescence, and the response to biotic and abiotic stresses. Elements of the initial signal transduction pathway have been determined, but we are still defining regulatory mechanisms by which the sensitivity of plants to ethylene is modulated.

RESULTS

We report here that members of the ARGOS gene family of Arabidopsis, previously implicated in the regulation of plant growth and biomass, function as negative feedback regulators of ethylene signaling. Expression of all four members of the ARGOS family is induced by ethylene, but this induction is blocked in ethylene-insensitive mutants. The dose dependence for ethylene induction varies among the ARGOS family members, suggesting that they could modulate responses across a range of ethylene concentrations. GFP-fusions of ARGOS and ARL localize to the endoplasmic reticulum, the same subcellular location as the ethylene receptors and other initial components of the ethylene signaling pathway. Seedlings with increased expression of ARGOS family members exhibit reduced ethylene sensitivity based on physiological and molecular responses.

CONCLUSIONS

These results support a model in which the ARGOS gene family functions as part of a negative feedback circuit to desensitize the plant to ethylene, thereby expanding the range of ethylene concentrations to which the plant can respond. These results also indicate that the effects of the ARGOS gene family on plant growth and biomass are mediated through effects on ethylene signal transduction.

Transcriptional networks in plant immunity

Next to numerous abiotic stresses, plants are constantly exposed to a variety of pathogens within their environment. Thus, their ability to survive and prosper during the course of evolution was strongly dependent on adapting efficient strategies to perceive and to respond to such potential threats. It is therefore not surprising that modern plants have a highly sophisticated immune repertoire consisting of diverse signal perception and intracellular signaling pathways. This signaling network is intricate and deeply interconnected, probably reflecting the diverse lifestyles and infection strategies used by the multitude of invading phytopathogens. Moreover it allows signal communication between developmental and defense programs thereby ensuring that plant growth and fitness are not significantly retarded. How plants integrate and prioritize the incoming signals and how this information is transduced to enable appropriate immune responses is currently a major research area. An important finding has been that pathogen-triggered cellular responses involve massive transcriptional reprogramming within the host. Additional key observations emerging from such studies are that transcription factors (TFs) are often sites of signal convergence and that signal-regulated TFs act in concert with other context-specific TFs and transcriptional co-regulators to establish sensory transcription regulatory networks required for plant immunity.

Ethylene receptors in plants - why so much complexity?

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

Ethylene signaling in rice and Arabidopsis: conserved and diverged aspects

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

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

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

Friends or foes: new insights in jasmonate and ethylene co-actions

One strategy for sessile plants to adapt to their surrounding environment involves the modulation of their various internal phytohormone signaling and distributions when the plants sense environmental change. There are currently dozens of identified phytohormones in plant cells and they act in concert to regulate plant growth, development, metabolism and defense. It has been determined that phytohormones often act together to achieve certain physiological functions. Thus, the study of hormone-hormone interactions is becoming a competitive research field for deciphering the underlying regulatory mechanisms. Among phytohormones, jasmonate and ethylene present a fascinating case of synergism and antagonism. They are commonly recognized as defense hormones that act synergistically. Plants impaired in jasmonate and/or ethylene signaling are susceptible to infections by necrotrophic fungi, suggesting that these two hormones are both required for defense. Moreover, jasmonate and ethylene also act antagonistically, such as in the regulation of apical hook development and wounding responses. Here, we highlight the recent breakthroughs in the understanding of jasmonate-ethylene co-actions and point out the potential power of studying protein-protein interactions for systematically exploring signal cross-talk.

Unsaturation of very-long-chain ceramides protects plant from hypoxia-induced damages by modulating ethylene signaling in Arabidopsis

Lipid remodeling is crucial for hypoxic tolerance in animals, whilst little is known about the hypoxia-induced lipid dynamics in plants. Here we performed a mass spectrometry-based analysis to survey the lipid profiles of Arabidopsis rosettes under various hypoxic conditions. We observed that hypoxia caused a significant increase in total amounts of phosphatidylserine, phosphatidic acid and oxidized lipids, but a decrease in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Particularly, significant gains in the polyunsaturated species of PC, PE and phosphatidylinositol, and losses in their saturated and mono-unsaturated species were evident during hypoxia. Moreover, hypoxia led to a remarkable elevation of ceramides and hydroxyceramides. Disruption of ceramide synthases LOH1, LOH2 and LOH3 enhanced plant sensitivity to dark submergence, but displayed more resistance to submergence under light than wild type. Consistently, levels of unsaturated very-long-chain (VLC) ceramide species (22:1, 24:1 and 26:1) predominantly declined in the loh1, loh2 and loh3 mutants under dark submergence. In contrast, significant reduction of VLC ceramides in the loh1-1 loh3-1 knockdown double mutant and lacking of VLC unsaturated ceramides in the ads2 mutants impaired plant tolerance to both dark and light submergences. Evidence that C24:1-ceramide interacted with recombinant CTR1 protein and inhibited its kinase activity in vitro, enhanced ER-to-nucleus translocation of EIN2-GFP and stabilization of EIN3-GFP in vivo, suggests a role of ceramides in modulating CTR1-mediated ethylene signaling. The dark submergence-sensitive phenotypes of loh mutants were rescued by a ctr1-1 mutation. Thus, our findings demonstrate that unsaturation of VLC ceramides is a protective strategy for hypoxic tolerance in Arabidopsis.

Insights into the origin and evolution of the plant hormone signaling machinery

Plant hormones modulate plant growth, development, and defense. However, many aspects of the origin and evolution of plant hormone signaling pathways remain obscure. Here, we use a comparative genomic and phylogenetic approach to investigate the origin and evolution of nine major plant hormone (abscisic acid, auxin, brassinosteroid, cytokinin, ethylene, gibberellin, jasmonate, salicylic acid, and strigolactone) signaling pathways. Our multispecies genome-wide analysis reveals that: (1) auxin, cytokinin, and strigolactone signaling pathways originated in charophyte lineages; (2) abscisic acid, jasmonate, and salicylic acid signaling pathways arose in the last common ancestor of land plants; (3) gibberellin signaling evolved after the divergence of bryophytes from land plants; (4) the canonical brassinosteroid signaling originated before the emergence of angiosperms but likely after the split of gymnosperms and angiosperms; and (5) the origin of the canonical ethylene signaling pathway postdates shortly the emergence of angiosperms. Our findings might have important implications in understanding the molecular mechanisms underlying the emergence of land plants.

Ethylene signaling pathway is not linear, however its lateral part is responsible for sensing and signaling of sulfur status in plants

A secondary, non-linear, lateral part of ethylene signaling pathway has been anticipated and speculated before. Recently, it has been found that part of the proteomic response of Eruca sativa to silver nitrate (which is an inhibitor of ethylene signaling) is related to sulfur metabolism. Using public Arabidopsis thaliana microarray data, I show that silver nitrate mimics the signal of sulfur starvation at the transcriptome level. This, combined with data mined from literature, indicates that ethylene receptors are localized at the beginning of the response to sulfur deficiency in plants. This means that the non-linear, lateral part of ethylene signaling pathway exists and is responsible for transduction of the signal of sulfur deficit. Here, I present a model of such a pathway and anticipate it to be the starting point for more detailed analysis of the lateral part of ethylene signaling pathway and the exact mechanism of sulfur status sensing in plants.

To grow old: regulatory role of ethylene and jasmonic acid in senescence

Senescence, the final stage in the development of an organ or whole plant, is a genetically programmed process controlled by developmental and environmental signals. Age-related signals underlie the onset of senescence in specific organs (leaf, flower, and fruit) as well as the whole plant (monocarpic senescence). Rudimentary to most senescence processes is the plant hormone ethylene, a small gaseous molecule critical to diverse processes throughout the life of the plant. The role of ethylene in senescence was discovered almost 100 years ago, but the molecular mechanisms by which ethylene regulates senescence have been deciphered more recently primarily through genetic and molecular studies in Arabidopsis. Jasmonic acid (JA), another plant hormone, is emerging as a key player in the control of senescence. The regulatory network of ethylene and JA involves the integration of transcription factors, microRNAs, and other hormones. In this review, we summarize the current understanding of ethylene's role in senescence, and discuss the interplay of ethylene with JA in the regulation of senescence.

The RING finger ubiquitin E3 ligase SDIR1 targets SDIR1-INTERACTING PROTEIN1 for degradation to modulate the salt stress response and ABA signaling in Arabidopsis

The plant hormone abscisic acid (ABA) regulates many aspects of plant development and the stress response. The intracellular E3 ligase SDIR1 (SALT- AND DROUGHT-INDUCED REALLY INTERESTING NEW GENE FINGER1) plays a key role in ABA signaling, regulating ABA-related seed germination and the stress response. In this study, we found that SDIR1 is localized on the endoplasmic reticulum membrane in Arabidopsis thaliana. Using cell biology, molecular biology, and biochemistry approaches, we demonstrated that SDIR1 interacts with and ubiquitinates its substrate, SDIRIP1 (SDIR1-INTERACTING PROTEIN1), to modulate SDIRIP1 stability through the 26S proteasome pathway. SDIRIP1 acts genetically downstream of SDIR1 in ABA and salt stress signaling. In detail, SDIRIP1 selectively regulates the expression of the downstream basic region/leucine zipper motif transcription factor gene ABA-INSENSITIVE5, rather than ABA-RESPONSIVE ELEMENTS BINDING FACTOR3 (ABF3) or ABF4, to regulate ABA-mediated seed germination and the plant salt response. Overall, the SDIR1/SDIRIP1 complex plays a vital role in ABA signaling through the ubiquitination pathway.

The Role of Ethylene in Plants Under Salinity Stress

Although the roles of ethylene in plant response to salinity and other stresses have been extensively studied, there are still some obscure points left to be clarified. Generally, in Arabidopsis and many other terrestrial plants, ethylene signaling is indispensable for plant rapid response and tolerance to salinity stress. However, a few studies showed that functional knock-out of some ACSs increased plant salinity-tolerance, while overexpression of them caused more sensitivity. This seems to be contradictory to the known opinion that ethylene plays positive roles in salinity response. Differently, ethylene in rice may play negative roles in regulating seedling tolerance to salinity. The main positive ethylene signaling components MHZ7/OsEIN2, MHZ6/OsEIL1, and OsEIL2 all negatively regulate the salinity-tolerance of rice seedlings. Recently, several different research groups all proposed a negative feedback mechanism of coordinating plant growth and ethylene response, in which several ethylene-inducible proteins (including NtTCTP, NEIP2 in tobacco, AtSAUR76/77/78, and AtARGOS) act as inhibitors of ethylene response but activators of plant growth. Therefore, in addition to a summary of the general roles of ethylene biosynthesis and signaling in salinity response, this review mainly focused on discussing (i) the discrepancies between ethylene biosynthesis and signaling in salinity response, (ii) the divergence between rice and Arabidopsis in regulation of salinity response by ethylene, and (iii) the possible negative feedback mechanism of coordinating plant growth and salinity response by ethylene.

Mitogen-activated protein kinase cascades in signaling plant growth and development

Mitogen-activated protein kinase (MAPK) cascades are ubiquitous signaling modules in eukaryotes. Early research of plant MAPKs has been focused on their functions in immunity and stress responses. Recent studies reveal that they also play essential roles in plant growth and development downstream of receptor-like protein kinases (RLKs). With only a limited number of MAPK components, multiple functional pathways initiated from different receptors often share the same MAPK components or even a complete MAPK cascade. In this review, we discuss how MAPK cascades function as molecular switches in response to spatiotemporal-specific ligand-receptor interactions and the availability of downstream substrates. In addition, we discuss other possible mechanisms governing the functional specificity of plant MAPK cascades, a question central to our understanding of MAPK functions.