Ethylene signaling: from mutants to molecules

Exploring the influence of a single-nucleotide mutation in EIN4 on tomato fruit firmness diversity through fruit pericarp microstructure

Tomato (Solanum lycopersicum) stands as one of the most valuable vegetable crops globally, and fruit firmness significantly impacts storage and transportation. To identify genes governing tomato firmness, we scrutinized the firmness of 266 accessions from core collections. Our study pinpointed an ethylene receptor gene, SlEIN4, located on chromosome 4 through a genome-wide association study (GWAS) of fruit firmness in the 266 tomato core accessions. A single-nucleotide polymorphism (SNP) (A → G) of SlEIN4 distinguished lower (AA) and higher (GG) fruit firmness genotypes. Through experiments, we observed that overexpression of SlEIN4AA significantly delayed tomato fruit ripening and dramatically reduced fruit firmness at the red ripe stage compared with the control. Conversely, gene editing of SlEIN4AA with CRISPR/Cas9 notably accelerated fruit ripening and significantly increased fruit firmness at the red ripe stage compared with the control. Further investigations revealed that fruit firmness is associated with alterations in the microstructure of the fruit pericarp. Additionally, SlEIN4AA positively regulates pectinase activity. The transient transformation assay verified that the SNP (A → G) on SlEIN4 caused different genetic effects, as overexpression of SlEIN4GG increased fruit firmness. Moreover, SlEIN4 exerts a negative regulatory role in tomato ripening by impacting ethylene evolution through the abundant expression of ethylene pathway regulatory genes. This study presents the first evidence of the role of ethylene receptor genes in regulating fruit firmness. These significant findings will facilitate the effective utilization of firmness and ripening traits in tomato improvement, offering promising opportunities for enhancing tomato storage and transportation capabilities.

Recent advances in understanding the regulation of plant secondary metabolite biosynthesis by ethylene-mediated pathways

Plants produce a large repertoire of secondary metabolites. The pathways that lead to the biosynthesis of these metabolites are majorly conserved in the plant kingdom. However, a significant portion of these metabolites are specific to certain groups or species due to variations in the downstream pathways and evolution of the enzymes. These metabolites show spatiotemporal variation in their accumulation and are of great importance to plants due to their role in development, stress response and survival. A large number of these metabolites are in huge industrial demand due to their potential use as therapeutics, aromatics and more. Ethylene, as a plant hormone is long known, and its biosynthetic process, signaling mechanism and effects on development and response pathways have been characterized in many plants. Through exogenous treatments, ethylene and its inhibitors have been used to manipulate the production of various secondary metabolites. However, the research done on a limited number of plants in the last few years has only started to uncover the mechanisms through which ethylene regulates the accumulation of these metabolites. Often in association with other hormones, ethylene participates in fine-tuning the biosynthesis of the secondary metabolites, and brings specificity in the regulation depending on the plant, organ, tissue type and the prevailing conditions. This review summarizes the related studies, interprets the outcomes, and identifies the gaps that will help to breed better varieties of the related crops and produce high-value secondary metabolites for human benefits.

Postharvest fruit quality of tomatoes influenced by an ethylene signaling component during long-term cold storage

Ethylene production is essential for improving cold resistance of postharvest tomatoes. However, the role of ethylene signaling pathway in maintaining fruit quality during long-term cold storage remains poorly understood. Here, we demonstrated that a partial loss of function in ethylene signaling by mutation of Ethylene Response Factor 2 (SlERF2), worsened fruit quality during cold storage, as determined by visual characterization, and physiological analyses of membrane damage and reactive oxygen species metabolism. In addition, the transcriptions of genes related to abscisic acid (ABA) biosynthesis and signaling were also altered by SlERF2 gene in response to cold storage. Furthermore, mutation of SlERF2 gene compromised cold-induced expression of genes in the C-repeat/dehydration-responsive binding factor (CBF) signaling pathway. Therefore, it's concluded that an ethylene signaling component, SlERF2 contributed to the regulations of ABA biosynthesis and signaling, as well as CBF cold signaling pathway, ultimately affecting the fruit quality during long-term cold storage of tomatoes.

Histone deacetylase gene SlHDT1 regulates tomato fruit ripening by affecting carotenoid accumulation and ethylene biosynthesis

Fruit development and ripening is a complicated biological process, that is not only regulated by plant hormones and transcription factors, but also affected by epigenetic modifications. Histone deacetylation is an important way of epigenetic modification, and little information about it is available. In this study, an RNAi vector was constructed and transferred successfully into wild-type tomato for further research on the detailed functions of the histone deacetylation gene SlHDT1. The expression level of PSY1 was upregulated, and the transcription levels of LCY-B, LCY-E and CYC-B were downregulated, which was consistent with the increased accumulation of carotenoids. In addition, the expression levels of ethylene biosynthetic genes (ACS2, ACS4 and ACO1, ACO3), ripening-associated genes (RIN, E4, E8, PG, Pti4 and LOXB) and fruit cell wall metabolism genes (HEX, MAN, TBG4, XTH5 and XYL) were significantly upregulated further strengthening the results, including an increased ethylene content, advanced fruit ripening time and a shortened shelf life of tomato fruits. In addition, the increased total histone H3 acetylation level also provides evidence of a connection between epigenetic regulation by histone deacetylation and fruit development and ripening. Hence, SlHDT1 is a negative regulator and plays an essential role in regulating ethylene and carotenoid biosynthesis during fruit ripening through influences on the acetylation level.

Mapping the helix arrangement of the reconstituted ETR1 ethylene receptor transmembrane domain by EPR spectroscopy

The plant ethylene receptor ETR1 is a key player in the perception of the phytohormone and subsequent downstream ethylene signal transmission, crucial for processes such as ripening, senescence and abscission. However, to date, there is sparse structural knowledge about the transmembrane sensor domain (TMD) of ETR1 that is responsible for the binding of the plant hormone and initiates the downstream signal transmission. Sequence information and ab initio modelling suggest that the TMD consists of three transmembrane helices. Here, we combined site-directed spin labelling with electron paramagnetic resonance spectroscopy and obtained distance restraints for liposome-reconstituted ETR1_TMD on the orientation and arrangement of the transmembrane helices. We used these data to scrutinize different computational structure predictions of the TMD.

Transcriptome analysis insight into ethylene metabolism and pectinase activity of apricot (Prunus armeniaca L.) development and ripening

Ethylene metabolism is very important for climacteric fruit, and apricots are typical climacteric fruit. The activity of pectinase is closely related to fruit firmness, which further affects fruit quality. To better understand ethylene metabolism, pectinase activity and their molecular regulation mechanisms during the development and ripening of apricot fruit, ethylene metabolism, pectinase activity and the "Luntaibaixing" apricot fruit transcriptome were analyzed at different developmental stages. Ethylene metabolic precursors, enzyme activities and ethylene release increased during fruit development and ripening, with significant differences between the ripening stage and other stages (P < 0.05). Fruit firmness decreased significantly from the S1 to S5 stages, and polygalacturonase, pectin methylesterase, and pectin lyase activities were significantly higher in the S5 stage than in other stages. RNA sequencing (RNA-seq) analysis of fruit resulted in the identification of 22,337 unigenes and 6629 differentially expressed genes (DEGs) during development and ripening, of which 20,989 unigenes are annotated in public protein databases. In functional enrichment analysis, DEGs among the three stages were found to be involved in plant hormone signal transduction. Four key genes affecting ethylene metabolism, six key ethylene signal transduction genes and seven genes related to pectinase in apricot fruit were identified by KEGG pathway analysis. By RNA-sequencing, we not only clarified the molecular mechanism of ethylene metabolism during the ripening of "Luntaibaixing" apricot fruit but also provided a theoretical basis for understanding pectin metabolism in apricot fruit.

ERF4 affects fruit firmness through TPL4 by reducing ethylene production

The firmness of fleshy fruit crops has a significant effect on their quality, consumer preference, shelf life and transportability. In a combined quantitative trait locus and genome-wide association studies study of apple fruit texture, we identified a mutation (C-G) in the ethylene response factor-associated amphiphilic repression (EAR) motif in the coding region of the apple ETHYLENE RESPONSE FACTOR4 (ERF4) gene. Chromatin immunoprecipitation sequencing showed that ERF4 binds to the promoter of ERF3, which is involved in regulation of ethylene biosynthesis. The EAR mutation in ERF4 results in reduced repression of ERF3 expression, which is turn promotes ethylene production and loss of fruit firmness. ERF4 acts as a transcriptional repressor whose activity is modulated by a TOPLESS co-repressor 4 (TPL4)-binding EAR repression motif. Biolayer interferometry analysis showed that the mutation in the EAR motif causes a reduction in the interaction with TPL4. Suppression of ERF4 or TPL4 promoted fruit ripening and ethylene production. Taken together, our results provide insights into how ERF4 allelic variation underlies an important fruit quality trait.

Overview of Sustainable Plant Growth and Differentiation and the Role of Hormones in Controlling Growth and Development of Plants Under Various Stresses

Plant development is different from animals by many fundamental aspects; as they have immobilized cells, a rigid cell wall, and the large central vacuole. Plant growth and cell division are restricted to the specific area of the shoot and root called meristems. Plants have the ability to carry out differentiation, dedifferentiation and redifferentiation. In plants, the growth and differentiation processes are controlled by hormonal and genetic factors. Phytohormones can exert independent/ dependent actions on plant growth and development. A pool of stem cells is placed at the niche of the apex meristem, which is the source of self-renewal of the cell system and its maintenance to provide cells to differentiated tissues. A complex interaction network between hormones and other factors maintains a balance between cell division and differentiation. Auxins promote the growth, gibberellins' function in seed germination, cytokinin's influence on cell division and delay leaf senescence; abscisic acid promotes the stomatal closure and bud dormancy, while salicylic acid promotes resistance against different diseases. Plants are often exposed to different abiotic and biotic stresses, for example, heat, cold, drought, salinity etc., whereas biotic stress arises mainly from fungi, bacteria, insect, etc. Phytohormones play a critical role in well-developed mechanisms that help to perceive the stress signal and enable the plant's optimal growth response. In this review, we studied both the intrinsic and extrinsic factors which govern growth and differentiation of plants under normal and stress condition. This review also deals with genetic modifications occurring in the cell and cell signaling during growth and differentiation.

Auxin Acts Downstream of Ethylene and Nitric Oxide to Regulate Magnesium Deficiency-Induced Root Hair Development in Arabidopsis thaliana

This study examines the association of auxin with ethylene and nitric oxide (NO) in regulating the magnesium (Mg) deficiency-induced root hair development in Arabidopsis thaliana. With Mg deficiency, both ethylene and NO promoted the elevation of root auxin levels in roots by inducing the expression of AUXIN-RESISTANT1 (AUX1), PIN-FORMED 1 (PIN1) and PIN2 transporters. In turn, auxin stimulated ethylene and NO production by activating the activities of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO), ACC synthase (ACS), nitrate reductase (NR) and NO synthase-like (NOS-L). These processes constituted an NO/ethylene-auxin feedback loop. Interestingly, however, the roles of ethylene and NO in regulating Mg deficiency-induced root hair development required the action of auxin, but not vice versa. In summary, these results suggest that Mg deficiency induces a positive interaction between the accumulation of auxin and ethylene/NO in roots, with auxin acting downstream of ethylene and NO signals to regulate Mg deficiency-induced root hair morphogenesis.

Recognition motif and mechanism of ripening inhibitory peptides in plant hormone receptor ETR1

Synthetic peptides derived from ethylene-insensitive protein 2 (EIN2), a central regulator of ethylene signalling, were recently shown to delay fruit ripening by interrupting protein-protein interactions in the ethylene signalling pathway. Here, we show that the inhibitory peptide NOP-1 binds to the GAF domain of ETR1 - the prototype of the plant ethylene receptor family. Site-directed mutagenesis and computational studies reveal the peptide interaction site and a plausible molecular mechanism for the ripening inhibition.

AtCSLD3 and GhCSLD3 mediate root growth and cell elongation downstream of the ethylene response pathway in Arabidopsis

CSLD3, a gene of the cellulose synthase-like D family, affects root hair elongation, but its interactions with ethylene signaling and phosphate-starvation are poorly understood. Here, we aim to understand the role of CSLD3 in the context of the ethylene signaling and phosphate starvation pathways in Arabidopsis plant growth. Therefore, we performed a comparative analysis of the csld3-1 mutant, CSLD3-overexpressing lines, and ethylene-response mutants, such as the constitutive ethylene-response mutant i-ctr1. We found that CSLD3 overexpression enhanced root and hypocotyl growth by increasing cell elongation, and that the root growth was highly sensitive to ethylene treatment (1 µM ACC), in particular under phosphate starvation. However, the CSLD3-mediated hypocotyl elongation occurred independently of the ethylene signaling pathway. Notably, the typical induction of root hair and root elongation by ethylene and phosphate-starvation was completely abolished in the csld3-1 mutant. Furthermore, i-ctr1 csld3-1 double-mutants were hairless like the csld3-1 parent, confirming that CSLD3 acts downstream of the ethylene signaling pathway during root growth. Moreover, the CSLD3 levels positively correlated with cellulose levels, indicating a role of CSLD3 in cellulose synthesis, which may explain the observed growth effects. Our results establish how CSLD3 works in the context of the ethylene signaling and phosphate-starvation pathways during root hair growth, cell elongation, and cell wall biosynthesis.

Overexpression of tomato SlNAC1 transcription factor alters fruit pigmentation and softening

BACKGROUND

Fruit maturation and ripening are genetically regulated processes that involve a complex interplay of plant hormones, growth regulators and multiple biological and environmental factors. Tomato (Solanum lycopersicum) has been used as a model of biological and genetic studies on the regulation of specific ripening pathways, including ethylene, carotenoid and cell wall metabolism. This model has also been used to investigate the functions of upstream signalling and transcriptional regulators. Thus far, many ripening-associated transcription factors that influence fruit development and ripening have been reported. NAC transcription factors are plant specific and play important roles in many stages of plant growth and development, such as lateral root formation, secondary cell wall synthesis, and embryo, floral organ, vegetative organ and fruit development.

RESULTS

Tissue-specific analysis by quantitative real-time PCR showed that SlNAC1 was highly accumulated in immature green fruits; the expression of SlNAC1 increased with fruit ripening till to the highest level at 7 d after the breaker stage. The overexpression of SlNAC1 resulted in reduced carotenoids by altering carotenoid pathway flux and decreasing ethylene synthesis mediated mainly by the reduced expression of ethylene biosynthetic genes of system-2, thus led to yellow or orange mature fruits. The results of yeast one-hybrid experiment demonstrated that SlNAC1 can interact with the regulatory regions of genes related lycopene and ethylene synthesis. These results also indicated that SlNAC1 inhibited fruit ripening by affecting ethylene synthesis and carotenoid accumulation in SlNAC1 overexpression lines. In addition, the overexpression of SlNAC1 reduced the firmness of the fruits and the thickness of the pericarp and produced more abscisic acid, resulting in the early softening of fruits. Hence, in SlNAC1 overexpression lines, both ethylene-dependent and abscisic acid-dependent pathways are regulated by SlNAC1 in fruit ripening regulatory network.

CONCLUSIONS

SlNAC1 had a broad influence on tomato fruit ripening and regulated SlNAC1 overexpression tomato fruit ripening through both ethylene-dependent and abscisic acid-dependent pathways. Thus, this study provided new insights into the current model of tomato fruit ripening regulatory network.

Shedding light on ethylene metabolism in higher plants

Ethylene metabolism in higher plants is regulated by a wide array of endogenous and environmental factors. During most physiological processes, ethylene levels are mainly determined by a strict control of the rate-limiting biosynthetic steps responsible for the production of 1-aminocyclopropane-1-carboxylic acid (ACC) and its subsequent conversion to ethylene. Responsible for these reactions, the key enzymes ACC synthase and ACC oxidase are encoded by multigene families formed by members that can be differentially regulated at the transcription and post-translational levels by specific developmental and environmental signals. Among the wide variety of environmental cues controlling plant ethylene production, light quality, duration, and intensity have consistently been demonstrated to influence the metabolism of this plant hormone in diverse plant tissues, organs, and species. Although still not completely elucidated, the mechanisms underlying the interaction between light signal transduction and ethylene evolution appears to involve a complex network that includes central transcription factors connecting multiple signaling pathways, which can be reciprocally modulated by ethylene itself, other phytohormones, and specific light wavelengths. Accumulating evidence has indicated particular photoreceptors as essential mediators in light-induced signaling cascades affecting ethylene levels. Therefore, this review specifically focuses on discussing the current knowledge of the potential molecular mechanisms implicated in the light-induced responses affecting ethylene metabolism during the regulation of developmental and metabolic plant responses. Besides presenting the state of the art in this research field, some overlooked mechanisms and future directions to elucidate the exact nature of the light-ethylene interplay in higher plants will also be compiled and discussed.

Cyanide is an adequate agonist of the plant hormone ethylene for studying signalling of sensor kinase ETR1 at the molecular level

The plant hormone ethylene is involved in many developmental processes and responses to environmental stresses in plants. Although the elements of the signalling cascade and the receptors operating the ethylene pathway have been identified, a detailed understanding of the molecular processes related to signal perception and transfer is still lacking. Analysis of these processes using purified proteins in physical, structural and functional studies is complicated by the gaseous character of the plant hormone. In the present study, we show that cyanide, a π-acceptor compound and structural analogue of ethylene, is a suitable substitute for the plant hormone for in vitro studies with purified proteins. Recombinant ethylene receptor protein ETR1 (ethylene-resistant 1) showed high level and selective binding of [14C]cyanide in the presence of copper, a known cofactor in ethylene binding. Replacement of Cys65 in the ethylene-binding domain by serine dramatically reduced binding of radiolabelled cyanide. In contrast with wild-type ETR1, autokinase activity of the receptor is not reduced in the ETR1-C65S mutant upon addition of cyanide. Additionally, protein–protein interaction with the ethylene signalling protein EIN2 (ethylene-insensitive 2) is considerably sustained by cyanide in wild-type ETR1, but is not affected in the mutant. Further evidence for the structural and functional equivalence of ethylene and cyanide is given by the fact that the ethylene-responsive antagonist silver, which is known to allow ligand binding but prevent intrinsic signal transduction, also allows specific binding of cyanide, but shows no effect on autokinase activity and ETR1–EIN2 interaction.

Polyamines attenuate ethylene-mediated defense responses to abrogate resistance to Botrytis cinerea in tomato

Transgenic tomato (Solanum lycopersicum) lines overexpressing yeast spermidine synthase (ySpdSyn), an enzyme involved in polyamine (PA) biosynthesis, were developed. These transgenic lines accumulate higher levels of spermidine (Spd) than the wild-type plants and were examined for responses to the fungal necrotrophs Botrytis cinerea and Alternaria solani, bacterial pathogen Pseudomonas syringae pv tomato DC3000, and larvae of the chewing insect tobacco hornworm (Manduca sexta). The Spd-accumulating transgenic tomato lines were more susceptible to B. cinerea than the wild-type plants; however, responses to A. solani, P. syringae, or M. sexta were similar to the wild-type plants. Exogenous application of ethylene precursors, S-adenosyl-Met and 1-aminocyclopropane-1-carboxylic acid, or PA biosynthesis inhibitors reversed the response of the transgenic plants to B. cinerea. The increased susceptibility of the ySpdSyn transgenic tomato to B. cinerea was associated with down-regulation of gene transcripts involved in ethylene biosynthesis and signaling. These data suggest that PA-mediated susceptibility to B. cinerea is linked to interference with the functions of ethylene in plant defense.

The tomato MADS-box transcription factor RIPENING INHIBITOR interacts with promoters involved in numerous ripening processes in a COLORLESS NONRIPENING-dependent manner

Fruit ripening is a complex developmental process responsible for the transformation of the seed-containing organ into a tissue attractive to seed dispersers and agricultural consumers. The coordinated regulation of the different biochemical pathways necessary to achieve this change receives considerable research attention. The MADS-box transcription factor RIPENING INHIBITOR (RIN) is an essential regulator of tomato (Solanum lycopersicum) fruit ripening but the exact mechanism by which it influences the expression of ripening-related genes remains unclear. Using a chromatin immunoprecipitation approach, we provide evidence that RIN interacts with the promoters of genes involved in the major pathways associated with observed and well-studied ripening phenotypes and phenomena, including the transcriptional control network involved in overall ripening regulation, ethylene biosynthesis, ethylene perception, downstream ethylene response, cell wall metabolism, and carotenoid biosynthesis. Furthermore, in the cases of ethylene and carotenoid biosynthesis, RIN interacts with the promoters of genes encoding rate-limiting activities. We also show that RIN recruitment to target loci is dependent on a normally functioning allele at the ripening-specific transcription factor COLORLESS NONRIPENING gene locus, further clarifying the relationship between these two ripening regulators.

Elicitor-induced cellular and molecular events are responsible for productivity enhancement in hairy root cultures: an insight study

A wide range of external stress stimuli triggers a plant cell to undergo a complex network of reactions that ultimately lead to the synthesis and accumulation of secondary metabolites. These secondary metabolites help the plant to survive under stress challenge. The potential of biotic and abiotic elicitors for the induction and enhancement of secondary metabolite production in various culture systems including hairy root (HR) cultures is well-known. The elicitor-induced defense responses involves signal perception of elicitor by a cell surface receptor followed by its transduction involving some major cellular and molecular events including activation of major secondary message signaling pathways. This result in induction of gene expressions escorting to the synthesis of various proteins mainly associated with plant defense responses and secondary metabolite synthesis and accumulation. The review discusses the elicitor-induced various cellular and molecular events and correlates them with enhanced secondary metabolite synthesis in HR systems. Further, this review also concludes that combining elicitation with in-silico approaches enhances the usefulness of this practice in better understanding and identifying the rate-limiting steps of biosynthetic pathways existing in HRs which in turn can contribute towards better productivity by utilizing metabolic engineering aspects.

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

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

Transcriptional regulation of two RTE-like genes of carnation during flower senescence and upon ethylene exposure, wounding treatment and sucrose supply

RTE1 (REVERSION-TO-ETHYLENE SENSITIVITY1) was identified as a positive regulator of ETR1 (ethylene resistant1) function in Arabidopsis; RTEs are a small gene family. Ethylene plays a crucial role in the senescence of carnation (Dianthus caryophyllus L.) flowers. Two cDNA clones encoding putative RTE-like protein (DCRTE1 and DCRTH1) were obtained from total RNA isolated from senescing carnation petals using RT-PCR and RACE techniques. The predicted proteins of DCRTE1 and DCRTH1 consist of 228 and 233 amino acids, respectively. Interestingly, the deduced DCRTE1 protein, like most other RTEs, includes two putative transmembrane domains, while the deduced DCRTH1 protein includes five putative transmembrane domains, according to the TMHMM database. Northern blots showed that the level of DCRTE1 mRNA in petals first decreased then increased remarkably after ethylene production started, and DCRTE1 expression showed an increasing trend in ovaries during natural flower senescence. The amount of DCRTH1 transcripts increased gradually in both petals and ovaries during natural senescence. Exogenous ethylene increased transcript abundance of DCRTE1 and DCRTH1 to various degrees in both petals and ovaries. STS treatment decreased the level of DCRTH1 mRNA in petals and ovaries compared with the control. DCRTE1 and DCRTH1 showed a rapid increase and then a decrease in mRNA accumulation in leaves after wounding. These results suggest that both DCRTE1 and DCRTH1 could play important roles in flower senescence-related signalling. Sucrose treatment did not remarkably affect the amount of DCRTE1 and DCRTH1 mRNAs.

The role of the pod in seed development: strategies for manipulating yield

Pods play a key role in encapsulating the developing seeds and protecting them from pests and pathogens. In addition to this protective function, it has been shown that the photosynthetically active pod wall contributes assimilates and nutrients to fuel seed growth. Recent work has revealed that signals originating from the pod may also act to coordinate grain filling and regulate the reallocation of reserves from damaged seeds to those that have retained viability. In this review we consider the evidence that pods can regulate seed growth and maturation, particularly in members of the Brassicaceae family, and explore how the timing and duration of pod development might be manipulated to enhance either the quantity of crop yield or its nutritional properties.

Auxin and ethylene induce flavonol accumulation through distinct transcriptional networks

Auxin and ethylene are key regulators of plant growth and development, and thus the transcriptional networks that mediate responses to these hormones have been the subject of intense research. This study dissected the hormonal cross talk regulating the synthesis of flavonols and examined their impact on root growth and development. We analyzed the effects of auxin and an ethylene precursor on roots of wild-type and hormone-insensitive Arabidopsis (Arabidopsis thaliana) mutants at the transcript, protein, and metabolite levels at high spatial and temporal resolution. Indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) differentially increased flavonol pathway transcripts and flavonol accumulation, altering the relative abundance of quercetin and kaempferol. The IAA, but not ACC, response is lost in the transport inhibitor response1 (tir1) auxin receptor mutant, while ACC responses, but not IAA responses, are lost in ethylene insensitive2 (ein2) and ethylene resistant1 (etr1) ethylene signaling mutants. A kinetic analysis identified increases in transcripts encoding the transcriptional regulators MYB12, Transparent Testa Glabra1, and Production of Anthocyanin Pigment after hormone treatments, which preceded increases in transcripts encoding flavonoid biosynthetic enzymes. In addition, myb12 mutants were insensitive to the effects of auxin and ethylene on flavonol metabolism. The equivalent phenotypes for transparent testa4 (tt4), which makes no flavonols, and tt7, which makes kaempferol but not quercetin, showed that quercetin derivatives are the inhibitors of basipetal root auxin transport, gravitropism, and elongation growth. Collectively, these experiments demonstrate that auxin and ethylene regulate flavonol biosynthesis through distinct signaling networks involving TIR1 and EIN2/ETR1, respectively, both of which converge on MYB12. This study also provides new evidence that quercetin is the flavonol that modulates basipetal auxin transport.

New paradigm in ethylene signaling: EIN2, the central regulator of the signaling pathway, interacts directly with the upstream receptors

The membrane protein ETHYLENE INSENSITIVE2 (EIN2), which is supposed to act between the soluble serine/threonine kinase CTR1 and the EIN3/EIL family of transcription factors, is a central and most critical element of the ethylene signaling pathway in Arabidopsis. In a recent study, we have identified that EIN2 interacts tightly with all members of the Arabidopsis ethylene receptor family - proteins that mark the starting point of the signaling pathway. Our studies show consistently that the kinase domain of the receptors is essential for the formation of the EIN2-receptor complex. Furthermore, mutational analysis demonstrates that phosphorylation is a key mechanism in controlling the interaction of EIN2 and the ethylene receptors. Interaction studies in the presence of the ethylene agonist cyanide revealed a causal link between hormone binding and complex formation. In the presence of the plant hormone agonist the auto-kinase activity of the receptors is inhibited and the non-phosphorylated kinase domain of the receptors binds tightly to the carboxyl-terminal domain of EIN2. In the absence of cyanide inhibition of the auto-kinase activity is relieved and complex formation with the phosphorylated kinase domain of the receptors is reduced. Our data suggest a novel model on the integration of EIN2 in the ethylene signaling pathway.

New insight in ethylene signaling: autokinase activity of ETR1 modulates the interaction of receptors and EIN2

Ethylene insensitive 2 (EIN2), an integral membrane protein of the ER network, has been identified as the central regulator of the ethylene signaling pathway. Still, the mechanism by which the ethylene signal is transferred from the receptors to EIN2 has not been solved yet. Here, we show that protein phosphorylation is a key mechanism to control the interaction of EIN2 and the receptors. In vivo and in vitro fluorescence studies reveal that the kinase domain of the receptors is essential for the interaction. Cyanide, an ethylene agonist, which is known to reduce auto-phosphorylation of the ethylene receptor ethylene resistant 1 (ETR1) or a mutation in the kinase domain of ETR1 that prevents auto-phosphorylation (H353A), increases the affinity of the receptors for EIN2. On the other hand, mimicking permanent auto-phosphorylation of ETR1 as in the mutant H353E releases the EIN2-ETR1 interaction from the control by the plant hormone. Based on our data, we propose a novel model on the integration of EIN2 in the ethylene signaling cascade.

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

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

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

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

Listen to genes: dealing with microarray data in the frequency domain

BACKGROUND

We present a novel and systematic approach to analyze temporal microarray data. The approach includes normalization, clustering and network analysis of genes.

METHODOLOGY

Genes are normalized using an error model based uniform normalization method aimed at identifying and estimating the sources of variations. The model minimizes the correlation among error terms across replicates. The normalized gene expressions are then clustered in terms of their power spectrum density. The method of complex Granger causality is introduced to reveal interactions between sets of genes. Complex Granger causality along with partial Granger causality is applied in both time and frequency domains to selected as well as all the genes to reveal the interesting networks of interactions. The approach is successfully applied to Arabidopsis leaf microarray data generated from 31,000 genes observed over 22 time points over 22 days. Three circuits: a circadian gene circuit, an ethylene circuit and a new global circuit showing a hierarchical structure to determine the initiators of leaf senescence are analyzed in detail.

CONCLUSIONS

We use a totally data-driven approach to form biological hypothesis. Clustering using the power-spectrum analysis helps us identify genes of potential interest. Their dynamics can be captured accurately in the time and frequency domain using the methods of complex and partial Granger causality. With the rise in availability of temporal microarray data, such methods can be useful tools in uncovering the hidden biological interactions. We show our method in a step by step manner with help of toy models as well as a real biological dataset. We also analyse three distinct gene circuits of potential interest to Arabidopsis researchers.

Interplay between ethylene, ETP1/ETP2 F-box proteins, and degradation of EIN2 triggers ethylene responses in Arabidopsis

The gaseous plant hormone ethylene can trigger myriad physiological and morphological responses in plants. While many ethylene signaling pathway components have been identified and characterized, little is known about the function of the integral membrane protein ETHYLENE-INSENSITIVE2 (EIN2), a central regulator of all ethylene responses. Here, we demonstrate that Arabidopsis thaliana EIN2 is a protein with a short half-life that undergoes rapid proteasome-mediated protein turnover. Moreover, EIN2 protein accumulation is positively regulated by ethylene. We identified two F-box proteins, EIN2 TARGETING PROTEIN1 (ETP1) and EIN2 TARGETING PROTEIN2 (ETP2), that interact with the EIN2 C-terminal domain (EIN2-CEND), which is highly conserved and sufficient to activate most ethylene responses. Overexpression of ETP1 or ETP2 disrupts EIN2 protein accumulation, and these plants manifest a strong ethylene-insensitive phenotype. Furthermore, knocking down the levels of both ETP1 and ETP2 mRNAs using an artificial microRNA (amiRNA) leads to accumulation of EIN2 protein, resulting in plants that display constitutive ethylene response phenotypes. Finally, ethylene down-regulates ETP1 and ETP2 proteins, impairing their ability to interact with EIN2. Thus, these studies reveal that a complex interplay between ethylene, the regulation of ETP1/ETP2 F-box proteins, and subsequent targeting and degradation of EIN2 is essential for triggering ethylene responses in plants.

Ethylene controls autophosphorylation of the histidine kinase domain in ethylene receptor ETR1

Perception of the phytohormone ethylene is accomplished by a small family of integral membrane receptors. In Arabidopsis, five ethylene receptor proteins are known, including ethylene resistant 1 (ETR1). The hydrophobic amino-terminal domain of these receptors contains the ethylene-binding site while the carboxyl-terminal part consists of a histidine kinase domain and a response regulator domain, which are well known elements found in bacterial two-component signaling. The soluble membrane-extrinsic carboxyl-terminal part of the receptor, which is likely to play an important role in signal transduction, showed intrinsic kinase activity when expressed and purified on its own. However, a correlation between signal input and autokinase activity was not established in these studies, as receptors were missing the transmembrane amino-terminal sensor domain. Thus, it is still unclear whether autophosphorylation occurs in response to perception of the ethylene signal. Here, we report on autophosphorylation studies of purified full-length ETR1. Autokinase activity of the purified receptor is controlled by ethylene or by ethylene agonists like the pi-acceptor compound cyanide. In fact, both signal molecules were able to completely turn off the intrinsic kinase activity. Furthermore, the observed inhibition of autophosphorylation in ETR1 by both molecules could be prevented when the ethylene antagonist 1-methyl-cyclopropene (MCP) was applied.

MYB72 is required in early signaling steps of rhizobacteria-induced systemic resistance in Arabidopsis

Colonization of Arabidopsis thaliana roots by nonpathogenic Pseudomonas fluorescens WCS417r bacteria triggers a jasmonate/ethylene-dependent induced systemic resistance (ISR) that is effective against a broad range of pathogens. Microarray analysis revealed that the R2R3-MYB-like transcription factor gene MYB72 is specifically activated in the roots upon colonization by WCS417r. Here, we show that T-DNA knockout mutants myb72-1 and myb72-2 are incapable of mounting ISR against the pathogens Pseudomonas syringae pv tomato, Hyaloperonospora parasitica, Alternaria brassicicola, and Botrytis cinerea, indicating that MYB72 is essential to establish broad-spectrum ISR. Overexpression of MYB72 did not result in enhanced resistance against any of the pathogens tested, demonstrating that MYB72 is not sufficient for the expression of ISR. Yeast two-hybrid analysis revealed that MYB72 physically interacts in vitro with the ETHYLENE INSENSITIVE3 (EIN3)-LIKE3 transcription factor EIL3, linking MYB72 function to the ethylene response pathway. However, WCS417r activated MYB72 in ISR-deficient, ethylene-insensitive ein2-1 plants. Moreover, exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylate induced wild-type levels of resistance in myb72-1, suggesting that MYB72 acts upstream of ethylene in the ISR pathway. Collectively, this study identified the transcriptional regulator MYB72 as a novel ISR signaling component that is required in the roots during early signaling steps of rhizobacteria-mediated ISR.

Transgenic Campanula carpatica plants with reduced ethylene sensitivity

Fertile transgenic Campanula carpatica Jacq. plants with flowers, which had reduced sensitivity to ethylene were obtained by Agrobacterium tumefaciens that mediated transformation. The construct used for transformation contained the etr1-1 gene from Arabidopsis thaliana under control of the flower specific fbp1-promoter from petunia. More than 100 flowering T0 lines were tested for their ethylene sensitivity using 2 microl l(-1) ethylene. The tolerance level to ethylene varied among the lines. While control plants stopped flowering within 3 days of exposure to ethylene, one of the transformed lines flowered for up to 27 days. The presence and the expression pattern of the transgene in various tissues were studied by polymerase chain reaction (PCR) and reverse transcription (RT)-PCR techniques. The expression of etr1-1 was significant in flowers and buds. Transgenic lines did not differ morphologically from control plants. The selected transgenic T0 lines, which were re-established from in vitro cultures showed the same degree of tolerance to exogenous ethylene, confirming the stability of the transgene in in vitro cultures. The rooting ability of the transgenic plants was not affected by the presence of etr1-1. T1 progeny were produced by crossing the transgenic line, which showed the most significant reduction in ethylene sensitivity with a control plant, and the analysis of the T1 plants showed 1:1 segregation in terms of ethylene sensitivity and the presence of the transgene.

GmEREBP1 is a transcription factor activating defense genes in soybean and Arabidopsis

Ethylene-responsive element-binding proteins (EREBPs) are plant-specific transcription factors, many of which have been linked to plant defense responses. Conserved EREBP domains bind to the GCC box, a promoter element found in pathogenesis-related (PR) genes. We previously identified an EREBP gene from soybean (GmEREBP1) whose transcript abundance decreased in soybean cyst-nematode-infected roots of a susceptible cultivar, whereas it increased in abundance in infected roots of a resistant cultivar. Here, we report further characterization of this gene. Transient expression analyses showed that GmEREBP1 is localized to the plant nucleus and functions as a transcriptional activator in soybean leaves. Transgenic soybean plants expressing GmEREBP1 activated the expression of the ethylene (ET)-responsive gene PR2 and the ET- and jasmonic acid (JA)-responsive gene PR3, and the salicylic acid (SA)-responsive gene PR1 but not the SA-responsive PR5. Similarly, transgenic Arabidopsis plants expressing GmEREBP1 showed elevated mRNA abundance of the ET-regulated gene PR3 and the ET- and JA-regulated defense-related gene PDF1.2 but not the ET-regulated GST2, and the SA-regulated gene PR1 but not the SA-regulated PR2 and PR5. Transgenic soybean and Arabidopsis plants inoculated with cyst nematodes did not display a significantly altered susceptibility to nematode infection. These results collectively show that GmEREBP1 functions as a transacting inducer of defense gene expression in both soybean and Arabidopsis and mediates the expression of both ET- and JA- and SA-regulated defense-related genes in these plant species.

How ethylene works in the reproductive organs of higher plants: a signaling update from the third millennium

Ethylene (ET) is a notable signaling molecule in higher plants. In the year 1993 the ET receptor gene, ETR1, was identified; this ETR1 receptor protein being the first plant hormone receptor to be isolated. It is striking that there are six ET receptors in tomato instead of five in Arabidopsis, the two best-known signaling-model systems. Even though over the last few years great progress has been made in elucidating the genes and proteins involved in ET signaling, the complete pathway remains to be established. The present review examines the most representative successive advances that have taken place in this millennium in terms of the signaling pathway of ET, as well as the implications of the signaling in the reproductive organs of plants (i.e., flowers, fruits, seeds and pollen grains). A detailed comparative study is made on the advances in knowledge in the last decade, showing how the characterization of ET signaling provides clues for understanding how higher plants regulate their ET sensitivity. Also, it is indicated that ET signaling is at present sparking interest within phytohormonal molecular physiology and biology, and it is explained why several socio-economic aspects (flowering and fruit ripening) are undoubtedly involved in ET physiology.

Arabidopsis leaf necrosis caused by simulated acid rain is related to the salicylic acid signaling pathway

Arabidopsis leaves treated with simulated acid rain (SiAR) showed phenotypes similar to necrotic lesions caused by biotic stresses like Pseudomonad infiltration. Exposure of Arabidopsis to SiAR resulted in the up-regulation of genes known to be induced by the salicylic acid (SA)-mediated pathogen resistance response. The expression of enhanced disease susceptibility (EDS), nonexpressor of PR (NPR) and pathogen-related 1 (PR1), all of which are involved in the salicylic acid signaling pathway, were increased after SiAR exposure. However, vegetative storage protein (VSP), a member of the jasmonic acid pathway did not show a significant change in transcript level. SiAR treatment of transgenic plants expressing salicylate hydroxylase (Nah-G), which prevents the accumulation of salicylic acid, underwent more extensive necrosis than wild-type plants, indicating that the signaling pathway activated by SiAR may overlap with the SA-dependent, systemic acquired resistance pathway. Both Col-0 and Nah-G plants showed sensitivity to SiAR and sulfuric SiAR (S-SiAR) by developing necrotic lesions. Neither Col-0 plants nor Nah-G plants showed sensitivity to nitric SiAR (N-SiAR). These results suggest that SiAR activates at least the salicylic acid pathway and activation of this pathway is sensitive to sulfuric acid.

The climacteric-like behaviour of young, mature and wounded citrus leaves

Although leaves and other vegetative tissues are generally considered as non-climacteric, citrus leaves show a climacteric system II behaviour after detachment. Upon harvest, young, fully expanded 'Valencia' orange (Citrus sinensis) leaves ( approximately 60-d-old) exhibited two phases of ethylene production. The first phase, up to 6 d after detachment, was characterized by a low and constant ethylene production (system I pathway), associated with a constitutive expression of ACC synthase 2 (CsACS2), CsERS1, and CsETR1. ACC synthase 1 (CsACS1) was not expressed during this phase and autoinhibition of ethylene production was apparent following treatment with exogenous ethylene or propylene. The second phase, 7-12 d after detachment, was characterized by a climacteric rise in ethylene production, preceded by the induction of CsACS1 and ACC oxidase 1 (CsACO1) gene expression in the system II pathway. This induction was accelerated and augmented by exogenous ethylene or propylene, indicating an autocatalytic system II ethylene biosynthesis. Mature leaves (6-8-months-old) behaved similarly, except that the climacteric peak in ethylene production occurred earlier (day 5). Young and mature leaves varied in the timing of the climacteric ethylene rise and CsACS1 and CsACO1 induction. The two phases of ethylene production, system I and system II, were also detected in wounded leaf discs of both young and mature leaves. The first phase peaked 15 min after excision and the second phase peaked after 6 h.

Mutation of a UDP-glucose-4-epimerase alters nematode susceptibility and ethylene responses in Arabidopsis roots

In Arabidopsis, mutation of RHD1, a UDP-glucose-4-epimerase, causes root-specific phenotypes, including hypersusceptibility to the cyst nematode Heterodera schachtii, increased root hair elongation, decreased root length, and root epidermal bulging. Previous experiments suggested that increased ethylene sensitivity or production mediated the rhd1-4 phenotypes. In the present study, double mutant analyses revealed that only rhd1-4 hypersusceptibility to H. schachtii and increased root hair elongation were dependent upon the ethylene signaling genes EIN2 and EIN3 but not upon ethylene signaling mediated by the auxin efflux carrier EIR1. In contrast, the rhd1-4 short root and root epidermal bulging phenotypes did not require EIN2, EIN3, or EIR1. A time-course analysis of RHD1 transcript levels in wild-type plants treated with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid showed a root-specific downregulation of RHD1 expression by ethylene. This observation was corroborated by our finding of increased RHD1 transcript levels in roots of the ethylene-insensitive mutants etr1 and ein2. In addition to ethylene, auxin strongly influences H. schachtii susceptibility and root hair elongation. Therefore, we investigated the sensitivity of rhd1-4 roots to indole-3-acetic acid (IAA). Equivalent IAA concentrations caused a greater reduction in rhd1-4 root elongation compared with wild-type roots. Finally, H. schachtii parasitism was found to strongly downregulate RHD1 expression in the root 3 days after inoculation. We conclude that RHD1 is a likely target of root-specific negative regulation by ethylene and that loss of RHD1 function results in a heightened sensitivity of root tissues to both ethylene and auxin.

Ethylene Synthesis and Auxin Augmentation in Pistil Tissues are Important for Egg Cell Differentiation after Pollination in Maize

The role of ethylene and auxin in stigma-to-ovule signalling was investigated in maize (Zea mays L.). Maturation of the egg cells in an ear was stimulated before actual fertilization by the application of fresh pollen grains or quartz sand to fully receptive stigmas. Ethylene emission by maize ears increased in response to those treatments. Silks and ovaries were involved in ethylene synthesis after pollen or sand was shed over the silks. The content of ethylene precursor [1-aminocyclopropane-1-carboxylic acid (ACC)] increased in both pistil parts soon after pollination. ACC rise was delayed by 4 h in the ovaries, and by 8 h in the silks after mock-pollination with sand. The auxin level increased rapidly in the silks and ovaries after pollination, and it was very high in the pollinated silks due to the high indole-3-acetic acid (IAA) content of pollen grains. IAA rise also appeared in the silks and ovaries after treatment with sand but it was delayed by 8 h. Application of ACC (10 microM) or IAA (6 microM) solutions to non-pollinated silks stimulated maturation of the egg cells. Moreover, the response of the egg cells to pollination was cancelled by l-alpha-(2-aminoethoxyvinyl)-glycine, alpha-aminoisobutyric acid or 2,3,5-triiodobenzoic acid applied to the silks before pollination. Thus ethylene synthesis and polar auxin transport in the silks pollinated with fresh pollen were necessary to evoke accelerated differentiation of the egg cells in maize ovules. Differences in pistil responses found between true- and mock-pollination suggest that signalling pathways are at least partially different for the reception of pollen grains and sand crystals on maize stigma.

Arabidopsis mutants that suppress the phenotype induced by transgene-mediated expression of cauliflower mosaic virus (CaMV) gene VI are less susceptible to CaMV-infection and show reduced ethylene sensitivity

Protein P6 is the main symptom determinant of cauliflower mosaic virus (CaMV), and transgene-mediated expression in Arabidopsis induces a symptom-like phenotype in the absence of infection. Seeds of a P6-transgenic line, A7, were mutagenized by gamma-irradiation and M2 seedlings were screened for mutants that suppressed the phenotype of chlorosis and stunting. We identified four mutants that were larger and less chlorotic than the A7 parent but which contained an intact and transcriptionally active transgene. The two mutants with the strongest suppression phenotype, were recessive and allelic. The transgene was eliminated by back-crossing with wild-type Arabidopsis. In progeny lines that were homozygous for the putative suppressor mutation the proportion of plants becoming infected following inoculation with CaMV was 40% that of wild-type, although in plants that did become infected, levels of virus DNA in mutants and wild-type did not differ significantly. Symptoms in the mutants were milder and delayed although this was somewhat dependent on the virus isolate. This phenotype was inherited stably. Both mutant alleles showed a partially ethylene-insensitive phenotype in an ethylene triple response assay. P6-transgenic plants were also almost completely insensitive to ethylene in the triple response assay. We suggest that the chlorosis and stunting in P6-transgenic and CaMV-infected plants are dependent on interactions between P6 and components involved in ethylene signalling, and that the suppressor gene product may function to augment these interactions.

A proteomic analysis identifies glutathione S-transferase isoforms whose abundance is differentially regulated by ethylene during the formation of early root epidermis in Arabidopsis seedlings

The plant hormone ethylene has been shown to play an important role in root hair development in Arabidopsis. With the aid of proteomic analysis, we identified three distinct glutathione S-transferase (GST) isoforms, AtGSTF2, AtGSTF8, and AtGSTU19, expressed early in root epidermal establishment in Arabidopsis seedlings. The AtGSTF2 protein was specifically up-regulated by ethylene. A subsequent RNA expression study revealed that the AtGSTF2 gene was highly sensitive to ethylene, whereas the transcripts for AtGSTF8 and AtGSTU19 were constitutively present in new root tissue of 4-day-old seedlings. The steady-state level of AtGSTF2 mRNA was greatly reduced in the roots of ethylene-insensitive mutants, while mutation at the CTR1 locus, which confers an ectopic root hair phenotype, resulted in a markedly elevated level of AtGSTF2 transcript in young root tissue. Although the physiological function of ethylene-induced AtGSTF2 is not yet clear, there are several possibilities for its role during early root development.

OsEIN2 is a Positive Component in Ethylene Signaling in Rice

EIN2 is a central signal transducer in the ethylene-signaling pathway, and a unique membrane-anchored protein. By screening a cDNA library, we have isolated a cDNA clone (OsEIN2) that encodes the rice EIN2 homolog. The full-length ORF clone was obtained by reverse transcriptase-polymerase chain reaction. OsEIN2 shares significant amino acid sequence similarity with Arabidopsis EIN2 (57% similarity and 42% identity). Both the numbers and positions of introns and exons in the OsEIN2 and AtEIN2 coding regions are also conserved. To address whether this structural similarity is indicative of functional conservation of the corresponding proteins, we also generated transgenic lines expressing the antisense construct of OsEIN2. Those plants were stunted and shoot elongation was severely inhibited. Their phenotypes were similar to that found with wild-type rice seedlings that were treated with AgNO3, an ethylene signal inhibitor. In the OsEIN2 antisense plants, the expression levels of two ethylene-responsive genes, SC129 and SC255, were decreased compared with the wild types. These results suggest that OsEIN2 is a positive component of the ethylene-signaling pathway in rice, just as AtEIN2 is in Arabidopsis: Our antisense transgenic plants produced approximately 3.5 times more ethylene than the wild-type plants. Expression analysis of rice ACS and ACO genes showed that the transcript levels of OsACS1 and OsACO1 were elevated in the transgenic plants.

The ethylene signaling pathway: new insights

During the past decade, molecular genetic studies on the reference plant Arabidopsis have established a largely linear signal transduction pathway for the response to ethylene gas. The biochemical modes of action of many of the signaling components are still unresolved. During the past year, however, progress in several areas has been made on several fronts. The different approaches used have included a functional study of the activity of the receptor His kinase, the determination of the ethylene receptor signaling complex at the endoplasmic reticulum and of the regulation of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) activity by these receptors, the identification of a unique MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) cascade, the cloning and characterization of numerous ETHYLENE INSENSITIVE3 (EIN3)/EIN3-like (EIL) transcription factors from many plant species, and the integration of the ethylene and jasmonate response pathways via the ETHYLENE RESPONSE FACTOR (ERF) family of transcription factors. The elucidation of the biochemical mechanisms of ethylene signal transduction and the identification of new components in the ethylene response pathway in Arabidopsis are providing a framework for understanding how all plants sense and respond to ethylene.

NopL, an effector protein of Rhizobium sp. NGR234, thwarts activation of plant defense reactions

Bacterial effector proteins delivered into eukaryotic cells via bacterial type III secretion systems are important virulence factors in plant-pathogen interactions. Type III secretion systems have been found in Rhizobium species that form symbiotic, nitrogen-fixing associations with legumes. One such bacterium, Rhizobium sp. NGR234, secretes a number of type III effectors, including nodulation outer protein L (NopL, formerly y4xL). Here, we show that expression of nopL in tobacco (Nicotiana tabacum) prevents full induction of pathogenesis-related (PR) defense proteins. Transgenic tobacco plants that express nopL and were infected with potato virus Y (necrotic strain 605) exhibited only very low levels of chitinase (class I) and beta-1,3-glucanase (classes I and III) proteins. Northern-blot analysis indicated that expression of nopL in plant cells suppresses transcription of PR genes. Treatment with ethylene counteracted the effect of NopL on chitinase (class I). Transgenic Lotus japonicus plants that expressed nopL exhibited delayed development and low chitinase levels. In vitro experiments showed that NopL is a substrate for plant protein kinases. Together, these data suggest that NopL, when delivered into the plant cell, modulates the activity of signal transduction pathways that culminate in activation of PR proteins.