Three ethylene-responsive transcription factors in tobacco with distinct transactivation functions

Studies on transcriptional regulation of endogenous genes by ERF2 transcription factor in tobacco cells

In this study, we showed that overexpression of ethylene-responsive transcription factor (ERF) 2 activated the expression of endogenous genes that have the GCC box in their promoter region, in tobacco plants. These include not only a defense-related gene, CHN50, encoding class I basic chitinase, but also a transcriptional repressor gene, ERF3. In tobacco plants constitutively expressing ERF2:glucocorticoid receptor fusion protein, treatment with dexamethazone induced a rapid increase of ERF3 mRNA and a slow increase of CHN50 mRNA. These results suggest that an antagonistic interplay of ERF2 and ERF3 is involved in the transcriptional regulation of the class I basic chitinase genes in tobacco.

Promoter activation of pepper class II basic chitinase gene, CAChi2, and enhanced bacterial disease resistance and osmotic stress tolerance in the CAChi2-overexpressing Arabidopsis

The activation of the CAChi2 promoter as the result of bacterial infection and osmotic stresses was examined using the Agrobacterium-mediated transient expression assay. Several stress-related cis-acting elements were revealed within the upstream genomic sequence of the CAChi2 gene. In tobacco leaf tissues transiently transformed with the CAChi2 promoter-beta-glucuronidase (GUS) gene, the CAChi2 promoter was up-regulated by Pseudomonas syringae pv. tabaci infection. The CAChi2-GUS activation was closely related to osmotic stresses, including treatment with mannitol and NaCl. The -378 CAChi2 promoter was sufficient for the CAChi2 gene induction by salicylic acid treatment. CAChi2 overexpression in the transgenic Arabidopsis plants enhanced bacterial disease resistance against Pseudomonas syringae pv. tomato infection. CAChi2-overexpressing Arabidopsis plants also exhibited increased tolerance to NaCl-induced osmotic stresses during seed germination and seedling growth. CAChi2 overexpression induced the expression of the NaCl stress-responsive gene RD29A in the absence of NaCl stress. The CAChi2-overexpressing transgenic plants exhibited increased sensitivity to abscisic acid during seed germination.

Genome-wide analysis of the ERF gene family in Arabidopsis and rice

Genes in the ERF family encode transcriptional regulators with a variety of functions involved in the developmental and physiological processes in plants. In this study, a comprehensive computational analysis identified 122 and 139 ERF family genes in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa L. subsp. japonica), respectively. A complete overview of this gene family in Arabidopsis is presented, including the gene structures, phylogeny, chromosome locations, and conserved motifs. In addition, a comparative analysis between these genes in Arabidopsis and rice was performed. As a result of these analyses, the ERF families in Arabidopsis and rice were divided into 12 and 15 groups, respectively, and several of these groups were further divided into subgroups. Based on the observation that 11 of these groups were present in both Arabidopsis and rice, it was concluded that the major functional diversification within the ERF family predated the monocot/dicot divergence. In contrast, some groups/subgroups are species specific. We discuss the relationship between the structure and function of the ERF family proteins based on these results and published information. It was further concluded that the expansion of the ERF family in plants might have been due to chromosomal/segmental duplication and tandem duplication, as well as more ancient transposition and homing. These results will be useful for future functional analyses of the ERF family genes.

AtSAP18, an orthologue of human SAP18, is involved in the regulation of salt stress and mediates transcriptional repression in Arabidopsis

In yeast and mammalian systems, it is well established that transcriptional down-regulation by DNA-binding repressors involves core histone deacetylation, mediated by their interaction within a complex containing histone deacetylase (e.g. HDA1), as well as various other proteins (e.g. SIN3, SAP18, SAP30, and RbAp46). Here we identify that a Arabidopsis thaliana gene related in sequence to SAP18, designated AtSAP18, functions in transcription regulation in plants subjected to salt stress. The AtSAP18 loss- of-function mutant is more sensitive to NaCl, and is impaired in chlorophyll synthesis as compared to the wild-type. Using GST pull-down, two-hybrid, and transient transcription assays, we have characterized SAP18 and HDA1 orthologues and provide evidence that SAP18 and HDA1 function as transcriptional repressors. We further demonstrate that they associate with Ethylene-Responsive Element binding Factors (ERFs) to create a hormone-sensitive multimeric repressor complex under conditions of environmental stress. Our results indicate that AtSAP18 functions to link the HDA complex to transcriptional repressors that are bound to chromatin in a sequence-specific manner, thereby providing the specificity of signal transduction accompanying transcriptional repression under stress conditions.

The NAC transcription factors NST1 and NST2 of Arabidopsis regulate secondary wall thickenings and are required for anther dehiscence

In plants, secondary wall thickenings play important roles in various biological processes, although the factors regulating these processes remain to be characterized. We show that expression of chimeric repressors derived from NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2 in Arabidopsis thaliana resulted in an anther dehiscence defect due to loss of secondary wall thickening in anther endothecium. Plants with double, but not single, T-DNA-tagged lines for NST1 and NST2 had the same anther-indehiscent phenotype as transgenic plants that expressed the individual chimeric repressors, indicating that NST1 and NST2 are redundant in regulating secondary wall thickening in anther walls. The activity of the NST2 promoter was particularly strong in anther tissue, while that of the NST1 promoter was detected in various tissues in which lignified secondary walls develop. Ectopic expression of NST1 or NST2 induced ectopic thickening of secondary walls in various aboveground tissues. Epidermal cells with ectopic thickening of secondary walls had structural features similar to those of tracheary elements. However, among genes involved in the differentiation of tracheary elements, only those related to secondary wall synthesis were clearly upregulated. None of the genes involved in programmed cell death were similarly affected. Our results suggest NAC transcription factors as possible regulators of secondary wall thickening in various tissues.

Pseudomonas syringae pv. tomato type III effectors AvrPto and AvrPtoB promote ethylene-dependent cell death in tomato

The type III secretion system (TTSS) of Pseudomonas syringae pv. tomato (Pst) injects into the plant cell effector proteins that play an essential role in the formation of bacterial speck disease. To investigate the molecular roles of TTSS effectors in disease formation, we used a cDNA microarray to analyze the expression of approximately 8600 random tomato genes in response to wild-type Pst strain DC3000 and a mutant lacking a functional TTSS. Many of the differentially expressed genes identified encode proteins associated with hormone response or hormone biosynthesis pathways. Using isogenic mutant strains of DC3000, we monitored host transcriptional changes in response to the TTSS effector proteins AvrPto and AvrPtoB, both of which are important virulence factors on susceptible tomato lines. We found that AvrPto and AvrPtoB induce a set of host genes involved in ethylene biosynthesis and signaling, and in particular they regulate the expression of two genes, LeACO1 and LeACO2, encoding the ethylene-forming enzyme ACC oxidase. Analysis of transgenic tomato lines with diminished ACC oxidase activity revealed that ethylene production by the host is required for the full virulence activity of both AvrPto and AvrPtoB. AvrPto and AvrPtoB therefore appear to promote enhanced disease in tomato leaves, in part, by upregulating genes involved in ethylene production.

Activation of pepper basic PR-1 gene promoter during defense signaling to pathogen, abiotic and environmental stresses

The basic PR-1 gene, CABPR1, accumulates in pepper leaf tissues during pathogen infection as well as after ethylene treatment. We isolated and functionally characterized the CABPR1 promoter region in tobacco leaves to identify the cis-acting regulatory sequences that are involved in CABPR1 gene expression. Constructs harboring the 5'-serially deleted CABPR1 promoter, which was fused to the beta-glucuronidase (GUS) gene, were evaluated for their promoter activity in the tobacco leaves. The CABPR1 promoter of 1670 bp in size was locally or systemically induced during a compatible interaction with Pseudomonas syringae pv. tabaci. The CABPR1 promoter also was differentially activated by treatment with ethylene, salicylic acid, nitric oxide, high salinity, drought and low temperature. The expression of the pepper transcription factors, CAZFP1 and CARAV1, activated the CABPR1 promoter. Analyses of a series of 5'-deletions of the CABPR1 promoter indicated that novel cis-acting elements essential for induction by pathogen and abiotic elicitors are localized in the region between -1670 bp and -1466 bp upstream from the translation start site. These results suggest that CABPR1 promoter is essential for regulating CABPR1 gene expression in response to pathogen, abiotic and environmental stresses, possibly by transactivating the CAZFP1 and CARAV1 transcription factors.

Functional analysis of Arabidopsis ethylene-responsive element binding protein conferring resistance to Bax and abiotic stress-induced plant cell death

Arabidopsis (Arabidopsis thaliana) ethylene-responsive element binding protein (AtEBP) gene was isolated as a suppressor of Bax-induced cell death by functional screening in yeast (Saccharomyces cerevisiae). To further examine the cell death suppressive action of AtEBP in plant cells, we established transgenic tobacco (Nicotiana tabacum) plants overexpressing AtEBP as well as transgenic tobacco plants ectopically expressing mouse Bax protein under a dexamethasone-inducible promoter. We prepared the crosses of the selective lines of each transgenic plant, which were evaluated in terms of cell death suppression activity. Results indicate that AtEBP suppressed Bax-induced cell death in tobacco plants, an action also associated with a lowered level of ion leakage. Furthermore, tobacco Bright Yellow-2 cells overexpressing AtEBP conferred resistance to hydrogen peroxide (H(2)O(2)) and heat treatments. AtEBP protein localized in the nucleus and functioned as an in vivo transcription activator as confirmed in transient assays and experiments using stable transgenic system. Up-regulation of defense genes was observed in transgenic Arabidopsis plants overexpressing AtEBP. Based on the analysis of mRNA accumulation in ethylene-related mutants, the position of AtEBP in signaling pathway is presented.

Plant defence responses: conservation between models and crops

Diseases of plants are a major problem for agriculture world wide. Understanding the mechanisms employed by plants to defend themselves against pathogens may lead to novel strategies to enhance disease resistance in crop plants. Much of the research in this area has been conducted with Arabidopsis as a model system, and this review focuses on how relevant the knowledge generated from this model system will be for increasing resistance in crop plants. In addition, the progress made using other model plant species is discussed. While there appears to be substantial similarity between the defence responses of Arabidopsis and other plants, there are also areas where significant differences are evident. For this reason it is also necessary to increase our understanding of the specific aspects of the defence response that cannot be studied using Arabidopsis as a model.

Rapid and transient activation of transcription of the ERF3 gene by wounding in tobacco leaves: possible involvement of NtWRKYs and autorepression

This study investigated the regulatory mechanism of rapid and transient induction of a transcriptional repressor ERF3 gene by wounding in tobacco (Nicotiana tabacum) leaves. Deletion and mutation analysis of the promoter region have suggested that the proximal W boxes (TGAC(C/T)) and a GCC box, respectively, may be involved in the positive and negative regulation of wound-induced expression of the ERF3 gene. Electrophoretic mobility shift assays indicated that wounding enhanced the specific binding activity of nuclear factors to the W boxes. NtWRKY1, -2, and -4, which are tobacco group I WRKYs, interacted specifically with the W boxes and activated transcription via the W boxes. On the other hand, deletion of the GCC box from NsERF3 promoter-GUS reporter gene caused a delay in down-regulation of transcription after wound induction. In addition, ERF3 repressed transcription via the NsERF3 promoter activated by NtWRKYs. These results suggest the possible involvement of NtWRKYs and autorepression in the rapid and transient expression of the ERF3 gene by wounding.

Identification of the minimal repression domain of SUPERMAN shows that the DLELRL hexapeptide is both necessary and sufficient for repression of transcription in Arabidopsis

We reported previously that the carboxy-terminal 30 amino acids of SUPERMAN (SUPRD) function as a repression domain in Arabidopsis. In this study, we identified the peptide sequences in SUPRD that is both necessary and sufficient for repression of transcription. To our surprise, the hexapeptide DLELRL was sufficient, by itself, to confer the ability to repress transcription on a DNA-binding domain. A database search revealed that there are 32 TFIIIA-type zinc finger proteins in the Arabidopsis genome that contain a hexapeptide sequence similar or identical to that of DLELRL. These peptides acted as repression domains, suggesting that these zinc finger proteins might function as active repressors. Further mutational analysis within DLELRL revealed that an amphiphilic motif composed of six amino acids (XLxLXL) with preferences at the first and fifth positions is necessary and sufficient for strong repression. An assay of positional effects suggested that GAL4DB-DLELRL might function as a short-range repressor. A possible mechanism of the DLELRL-mediated repression is discussed.

Overexpression of NtERF5, a new member of the tobacco ethylene response transcription factor family enhances resistance to tobacco mosaic virus

A new member of the tobacco (Nicotiana tabacum) AP2/ERF (ethylene response factor) transcription factor family, designated NtERF5, has been isolated by yeast one-hybrid screening. In vitro, recombinant NtERF5 protein weakly binds GCC box cis-elements, which mediate pathogen-regulated transcription of several PR (pathogenesis related) genes. NtERF5 transcription is transiently activated by wounding, by infection with the bacterial pathogen Pseudomonas syringae, as well as by inoculation with Tobacco mosaic virus (TMV). In contrast, NtERF5 transcription is not enhanced after application of salicylic acid, jasmonic acid, or ethylene. Constitutive overexpression of NtERF5 (ERF5-Oex) under control of the 35S promoter results in no visible alterations in plant growth or enhanced resistance to Pseudomonas infection. Furthermore, no constitutive expression of PR genes has been observed. In contrast, ERF5-Oex plants show enhanced resistance to TMV with reference to reduced size of local hypersensitive-response lesions and impaired systemic spread of the virus. Since, in TMV-infected ERF5-Oex plants, the viral RNA accumulates only up to 10 to 30% of the wild-type level, we suggest that NtERF5-regulated gene expression is controlling resistance to viral propagation. Previous research has demonstrated that overexpression of ERF genes enhances resistance to bacterial and fungal pathogens. Here, we provide further evidence that resistance to viral infection can be engineered by overexpression of ERF transcription factors.

Elicitor-induced activation of transcription via W box-related cis-acting elements from a basic chitinase gene by WRKY transcription factors in tobacco

A putative elicitor responsive element with two W boxes (CTGACC/T) has been identified in the region between -125 and -69 of a tobacco class I basic chitinase gene CHN48. We generated transgenic tobacco calli that contained the -125/-69 region fused to a luciferase reporter gene. The expression of the reporter gene was induced upon treatment with an elicitor, xylanase from Trichoderma viride (TvX). This induction required protein kinase activity. We isolated three cDNA clones encoding DNA-binding proteins, designated as NtWRKY1, NtWRKY2, and NtWRKY4, from tobacco cultured cells. Gel mobility shift assays showed that in vitro translation products of NtWRKY1, NtWRKY2 and NtWRKY4 bound to W box of CHN48 gene. These NtWRKY proteins stimulated W box-mediated transcription of a luciferase reporter gene in the transient assay. In addition, the transactivation of W box-mediated transcription by NtWRKY1 and NtWRKY4 was enhanced in response to elicitor treatment, suggesting elicitor-induced posttranscriptional activation of these NtWRKYs. Northern blot analyses showed that mRNAs for NtWRKY1 and NtWRKY2 increased after treatment with the elicitor, whereas mRNAs for NtWRKY4 were expressed constitutively at a low level. These results suggested possible involvement of NtWRKYs in elicitor-responsive transcription of defense genes in tobacco.

Methyl jasmonate-induced ethylene production is responsible for conifer phloem defense responses and reprogramming of stem cambial zone for traumatic resin duct formation

Conifer stem pest resistance includes constitutive defenses that discourage invasion and inducible defenses, including phenolic and terpenoid resin synthesis. Recently, methyl jasmonate (MJ) was shown to induce conifer resin and phenolic defenses; however, it is not known if MJ is the direct effector or if there is a downstream signal. Exogenous applications of MJ, methyl salicylate, and ethylene were used to assess inducible defense signaling mechanisms in conifer stems. MJ and ethylene but not methyl salicylate caused enhanced phenolic synthesis in polyphenolic parenchyma cells, early sclereid lignification, and reprogramming of the cambial zone to form traumatic resin ducts in Pseudotsuga menziesii and Sequoiadendron giganteum. Similar responses in internodes above and below treated internodes indicate transport of a signal giving a systemic response. Studies focusing on P. menziesii showed MJ induced ethylene production earlier and 77-fold higher than wounding. Ethylene production was also induced in internodes above the MJ-treated internode. Pretreatment of P. menziesii stems with the ethylene response inhibitor 1-methylcyclopropene inhibited MJ and wound responses. Wounding increased 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase protein, but MJ treatment produced a higher and more rapid ACC oxidase increase. ACC oxidase was most abundant in ray parenchyma cells, followed by cambial zone cells and resin duct epithelia. The data show these MJ-induced defense responses are mediated by ethylene. The cambial zone xylem mother cells are reprogrammed to differentiate into resin-secreting epithelial cells by an MJ-induced ethylene burst, whereas polyphenolic parenchyma cells are activated to increase polyphenol production. The results also indicate a central role of ray parenchyma in ethylene-induced defense.

Tomato stress-responsive factor TSRF1 interacts with ethylene responsive element GCC box and regulates pathogen resistance to Ralstonia solanacearum

Ethylene responsive factors (ERFs) are important in regulating plant pathogen resistance, abiotic stress tolerance and plant development. Recent studies have greatly enlarged the ERF protein family and revealed more important roles of ERFs in plants. Here, we report our finding of a tomato ERF protein TSRF1, which is transcriptionally up-regulated by ethylene, salicylic acid, or Ralstonia solanacearum strain BJ1057 infection. Biochemical analysis indicates that TSRF1 specifically interacts in vitro with the GCC box, an element present in the promoters of many pathogenesis-related (PR) genes. Further investigation evidences that TSRF1 activates in vivo the expression of reporter beta-glucuronidase gene controlled by GCC box. More importantly, overexpressing TSRF1 in tobacco and tomato constitutively activates the expression of PR genes, and subsequently enhancing transgenic plant resistance to the bacterial wilt caused by Ralstonia solanacearum strain BJ1057. Therefore our investigation not only extends the functions of ERF proteins in plant resistance to R. solanacearum, but also provides further clues to understanding the mechanism of host regulatory proteins in response to the infection of pathogens.

Ectopic overexpression of tomato JERF3 in tobacco activates downstream gene expression and enhances salt tolerance

The ethylene, jasmonic acid and osmotic signaling pathways respond to environmental stimuli and in order to understand how plants adapt to biotic and abiotic stresses it is important to understand how these pathways interact each other. In this paper, we report a novel ERF protein--jasmonate and ethylene-responsive factor 3 (JERF3)--that unites these pathways. JERF3, which functions as an in vivo transcription activator in yeast, binds to the GCC box, an element responsive to ethylene/JA signaling, as well as to DRE, a dehydration-responsive element that responds to dehydration, high salt and low-temperature. Expression of JERF3 in tomato is mainly induced by ethylene, JA, cold, salt or ABA. Constitutive expression of JERF3 in transgenic tobacco significantly activated expression of pathogenesis-related genes that contained the GCC box, resulting in enhanced tolerance to salt. These results indicate that JERF3 functions as a linker in ethylene- and osmotic stress-signaling pathways.

The ethylene-responsive factor like protein 1 (CaERFLP1) of hot pepper (Capsicum annuum L.) interacts in vitro with both GCC and DRE/CRT sequences with different binding affinities: possible biological roles of CaERFLP1 in response to pathogen infection and high salinity conditions in transgenic tobacco plants

From a pathogen-inoculated hot pepper (Capsicum annuum L. cv. Pukang) leaf EST, we identified a cDNA clone, pCaERFLP1, encoding a putative transcription factor that contains a single ERF/AP2 DNA binding domain. CaERFLP1 was most closely related to tomato LeERF2 (73%), both of which belong to the novel ERF class IV typified by the N-terminal MCGGAIL signature sequence, while it had a limited sequence identity (25-30%) with Arabidopsis AtERFs and tobacco NtERFs. Quantitative gel retardation assays revealed that bacterially expressed full-length CaERFLP1 was able to form a specific complex with both the GCC box and DRE/CRT motif, with its binding affinity for GCC being stronger than for DRE/CRT. When fused to the GAL4 DNA binding domain, the N-terminal CaERFLP1(1-37) and C-terminal CaERFLP1(198-264) mutant polypeptides could function individually as transactivators in yeast. This suggests that two separate domains of CaERFLP1 may play distinct roles in transcription activation. In particle co-bombardment experiments, CaERFLP1 activated the transcription of reporter genes containing the 4X[GCC] element in tobacco cells. In hot pepper plants, the steady-state level of CaERFLP1 mRNA was markedly induced by multiple environmental factors, such as pathogen infection, ethylene, mechanical wounding and high salinity. Furthermore, ectopic expression of CaERFLP1 in transgenic tobacco plants resulted in partially improved tolerance against the bacterial pathogen Pseudomonas syringae and salt stress (100 mM NaCl). Consistently, various defense-related genes, including GCC box-containing PR genes and the DRE/CRT-containing LTI45 (ERD10) gene, were constitutively expressed in 35S::CaERFLP1 tobacco plants. Thus, it appears that CaERFLP1 is functional in tobacco cells, where it induces the transactivation of some GCC- and DRE/CRT-genes to trigger a subset of stress response. Here, the possible biological role(s) of CaERFLP1 is discussed, especially with regard to the possibility that CaERFLP1 has multiple functions in the regulation of GCC- and DRE/CRT-mediated gene expression in hot pepper plants.

The tomato transcription factor Pti4 regulates defense-related gene expression via GCC box and non-GCC box cis elements

The tomato transcription factor Pti4, an ethylene-responsive factor (ERF), interacts physically with the disease resistance protein Pto and binds the GCC box cis element that is present in the promoters of many pathogenesis-related (PR) genes. We reported previously that Arabidopsis plants expressing Pti4 constitutively express several GCC box-containing PR genes and show reduced disease symptoms compared with wild-type plants after inoculation with Pseudomonas syringae pv tomato or Erysiphe orontii. To gain insight into how genome-wide gene expression is affected by Pti4, we used serial analysis of gene expression (SAGE) to compare transcripts in wild-type and Pti4-expressing Arabidopsis plants. SAGE provided quantitative measurements of >20,000 transcripts and identified the 50 most highly expressed genes in Arabidopsis vegetative tissues. Comparison of the profiles from wild-type and Pti4-expressing Arabidopsis plants revealed 78 differentially abundant transcripts encoding defense-related proteins, protein kinases, ribosomal proteins, transporters, and two transcription factors (TFs). Many of the genes identified were expressed differentially in wild-type Arabidopsis during infection by Pseudomonas syringae pv tomato, supporting a role for them in defense-related processes. Unexpectedly, the promoters of most Pti4-regulated genes did not have a GCC box. Chromatin immunoprecipitation experiments confirmed that Pti4 binds in vivo to promoters lacking this cis element. Potential binding sites for ERF, MYB, and GBF TFs were present in statistically significantly increased numbers in promoters regulated by Pti4. Thus, Pti4 appears to regulate gene expression directly by binding the GCC box and possibly a non-GCC box element and indirectly by either activating the expression of TF genes or interacting physically with other TFs.

FRIZZY PANICLE is required to prevent the formation of axillary meristems and to establish floral meristem identity in rice spikelets

Inflorescences of grass species have a distinct morphology in which florets are grouped in compact branches called spikelets. Although many studies have shown that the molecular and genetic mechanisms that control floret organ formation are conserved between monocots and dicots, little is known about the genetic pathway leading to spikelet formation. In the frizzy panicle (fzp) mutant of rice, the formation of florets is replaced by sequential rounds of branching. Detailed analyses revealed that several rudimentary glumes are formed in each ectopic branch, indicating that meristems acquire spikelet identity. However, instead of proceeding to floret formation, axillary meristems are formed in the axils of rudimentary glumes and they either arrest or develop into branches of higher order. The fzp mutant phenotype suggests that FZP is required to prevent the formation of axillary meristems within the spikelet meristem and permit the subsequent establishment of floral meristem identity. The FZP gene was isolated by transposon tagging. FZP encodes an ERF transcription factor and is the rice ortholog of the maize BD1 gene. Consistent with observations from phenotypic analyses, FZP expression was found to be restricted to the time of rudimentary glumes differentiation in a half-ring domain at the base of which the rudimentary glume primordium emerged.

Molecular and biochemical characterization of VR-EILs encoding mung bean ETHYLENE INSENSITIVE3-LIKE proteins

ETHYLENE INSENSITIVE3 (EIN3) is a transcription factor involved in the ethylene signal transduction pathway in Arabidopsis. Two full-length cDNA clones, pVR-EIL1 and pVR-EIL2, encoding EIN3-LIKE proteins were isolated by reverse transcriptase-polymerase chain reaction and by screening the cDNA library of mung bean (Vigna radiata) hypocotyls. VR-EIL1 and VR-EIL2 share 70% identity and display varying degrees of sequence conservation (39%-65%) with previously isolated EIN3 homologs from Arabidopsis, tobacco (Nicotiana tabacum) and tomato (Lycopersicon esculentum) plants. Gel retardation assay revealed that both VR-EILs were able to interact specifically with optimal binding sequence-1, the recently identified optimal binding sequence for tobacco TEIL, with the binding of VR-EIL2 being more efficient than that of VR-EIL1. Transient expression analysis using a VR-EIL::smGFP fusion gene in onion (Allium cepa) epidermal cells indicated that the VR-EIL proteins were effectively targeted to the nucleus. The fusion protein of VR-EIL2 with GAL4 DNA-binding domain strongly activated transcription of a reporter gene in yeast cells, and an essential domain for transcription-stimulating activity was localized to the amino-terminal acidic region that consists of 50 amino acid residues. In contrast with what has been previously found in EIN3- and TEIL-overexpressing Arabidopsis plants, transgenic tobacco seedlings expressing the VR-EIL genes under the control of cauliflower mosaic virus 35S promoter did not exhibit a constitutive triple response. Instead, they displayed a markedly enhanced proliferation of root hairs, one of the typical ethylene response phenotypes, and increased sensitivity to exogenous ethylene. In addition, the pathogenesis-related (PR) genes encoding beta-1,3-glucanase, osmotin, and PR1 were constitutively expressed in 35S::VR-EIL lines without added ethylene, and were hyperinduced in response to ethylene treatment. These results indicate that VR-EILs are functional in tobacco cells, thereby effectively transactivating the GCC-box-containing PR genes and enhancing sensitivity to ethylene. The possible physiological role of VR-EILs is discussed in the light of the suggestion that they are active components of the ethylene-signaling pathway and their heterologous expressions constitutively turn on a subset of ethylene responses in tobacco plants.

A role for the GCC-box in jasmonate-mediated activation of the PDF1.2 gene of Arabidopsis

The PDF1.2 gene of Arabidopsis encoding a plant defensin is commonly used as a marker for characterization of the jasmonate-dependent defense responses. Here, using PDF1.2 promoter-deletion lines linked to the beta-glucoronidase-reporter gene, we examined putative promoter elements associated with jasmonate-responsive expression of this gene. Using stably transformed plants, we first characterized the extended promoter region that positively regulates basal expression from the PDF1.2 promoter. Second, using promoter deletion constructs including one from which the GCC-box region was deleted, we observed a substantially lower response to jasmonate than lines carrying this motif. In addition, point mutations introduced into the core GCC-box sequence substantially reduced jasmonate responsiveness, whereas addition of a 20-nucleotide-long promoter element carrying the core GCC-box and flanking nucleotides provided jasmonate responsiveness to a 35S minimal promoter. Taken together, these results indicated that the GCC-box plays a key role in conferring jasmonate responsiveness to the PDF1.2 promoter. However, deletion or specific mutations introduced into the core GCC-box did not completely abolish the jasmonate responsiveness of the promoter, suggesting that the other promoter elements lying downstream from the GCC-box region may also contribute to jasmonate responsiveness. In other experiments, we identified a jasmonate- and pathogen-responsive ethylene response factor transcription factor, AtERF2, which when overexpressed in transgenic Arabidopsis plants activated transcription from the PDF1.2, Thi2.1, and PR4 (basic chitinase) genes, all of which contain a GCC-box sequence in their promoters. Our results suggest that in addition to their roles in regulating ethylene-mediated gene expression, ethylene response factors also appear to play important roles in regulating jasmonate-responsive gene expression, possibly via interaction with the GCC-box.

ICE1: a regulator of cold-induced transcriptome and freezing tolerance in Arabidopsis

Cold temperatures trigger the expression of the CBF family of transcription factors, which in turn activate many downstream genes that confer chilling and freezing tolerance to plants. We report here the identification of ICE1 (inducer of CBF expression 1), an upstream transcription factor that regulates the transcription of CBF genes in the cold. An Arabidopsis ice1 mutant was isolated in a screen for mutations that impair cold-induced transcription of a CBF3 promoter-luciferase reporter gene. The ice1 mutation blocks the expression of CBF3 and decreases the expression of many genes downstream of CBFs, which leads to a significant reduction in plant chilling and freezing tolerance. ICE1 encodes a MYC-like bHLH transcriptional activator. ICE1 binds specifically to the MYC recognition sequences in the CBF3 promoter. ICE1 is expressed constitutively, and its overexpression in wild-type plants enhances the expression of the CBF regulon in the cold and improves freezing tolerance of the transgenic plants.

Isolation of tobacco ubiquitin-conjugating enzyme cDNA in a yeast two-hybrid system with tobacco ERF3 as bait and its characterization of specific interaction

Tobacco ETHYLENE-RESPONSIVE FACTOR3 (ERF3) is a member of the ERF-domain transcription factors and has a transcriptional repressor activity, whereas other ERF proteins show activation activity. To understand the regulation of ERF3-repressor activity, protein(s) were screened which interact with ERF3 in a yeast two-hybrid system. A partial sequence (B8) of NtUBC2, a tobacco ubiquitin-conjugating enzyme was isolated. This B8 specifically interacted with ERF3 in the yeast two-hybrid system. Further analyses revealed that the region unique to ERF3 interacted with B8. The physiological functions of NtUBC2 and the stability of ERF3 are discussed in relation to the regulation of the repression activity of ERF3.

Identification and characterization of a soybean ethylene-responsive element-binding protein gene whose mRNA expression changes during soybean cyst nematode infection

Ethylene-responsive element-binding proteins (EREBPs) are members of a family of plant transcription factors. Conserved EREBP domains of these proteins bind to the GCC box, an ethylene-responsive promoter element found in many pathogenesis-related (PR) genes. Using degenerate primers to the EREBP domain from diverse plant species, an EREBP homolog was isolated from a soybean cDNA library. Gel mobility-shift assays revealed that the translation product of this cDNA bound specifically to GCC box sequences. We, therefore, named this gene Glycine max ethylene-responsive element-binding protein 1 (GmEREBP1), i.e., a gene coding for the first confirmed GCC box-binding protein of soybean. GmEREBP1 mRNA abundance was analyzed by RNA blot hybridizations in soybean roots and shoots of cultivars Corsoy 79 and Hartwig, which are susceptible and resistant, respectively, to the soybean cyst nematode (Heterodera glycines). These analyses revealed that GmEREBP1 is expressed in a root-preferential manner and that GmEREBP1 mRNA abundance is changed after H. glycines infection. GmEREBP1 mRNA abundance decreased in infected (susceptible) 'Corsoy 79' roots, whereas it increased in abundance in infected (resistant) 'Hartwig' roots. Furthermore, ethephon treatment repressed GmEREBP1 mRNA accumulation in both cultivars, whereas wounding increased expression in both cultivars. These changes in mRNA steady-state levels suggest that GmEREBP1 plays a role in soybean-H. glycines interactions.

Maize DRE-binding proteins DBF1 and DBF2 are involved in rab17 regulation through the drought-responsive element in an ABA-dependent pathway

The abscisic acid-responsive gene rab17 of maize is expressed during late embryogenesis, and is induced by ABA and desiccation in embryo and vegetative tissues. ABRE and DRE cis-elements are involved in regulation of the gene by ABA and drought. Using yeast one-hybrid screening, we isolated two cDNAs encoding two new DRE-binding proteins, designated DBF1 and DBF2, that are members of the AP2/EREBP transcription factor family. Analysis of mRNA accumulation profiles showed that DBF1 is induced during maize embryogenesis and after desiccation, NaCl and ABA treatments in plant seedlings, whereas the DBF2 mRNA is not induced. DNA-binding preferences of DBFs were analysed by electrophoretic mobility shift assays, and showed that both DBF1 and DBF2 bound to the wild-type DRE2 element, but not to the DRE2 mutant or to the DRE1 element which differs only in a single nucleotide. Transactivation activity using particle bombardment showed that DBF1 functioned as activator of DRE2-dependent transcription of rab17 promoter by ABA, whereas DBF2 overexpression had a repression action downregulating not only the basal promoter activity, but also the ABA effect. These results show that ABA plays a role in the regulation of DBF activity, and suggests the existence of an ABA-dependent pathway for the regulation of genes through the C-repeat/DRE element.

Determinants in the sequence specific binding of two plant transcription factors, CBF1 and NtERF2, to the DRE and GCC motifs

Arabidopsis ERF proteins such as DREB1, DREB2, and CBF1 bind to the dehydration-responsive element (DRE), which has the sequence TACCGACAT. Mutation analyses reveal that a central 5 bp CCGAC core of the DRE is the minimal sequence motif (designated as the DRE motif in this paper), to which the ERF domain fragment of CBF1 (CBF1-F) binds specifically with a binding K(d) at the nanomolar level. In contrast, the ERF domain fragment of the tobacco ERF2 (NtERF2-F) does not interact with the DRE motif, but restrictedly recognizes the sequence containing a minimal 6 bp GCCGCC motif (designated as the GCC motif in this paper). However, CBF1-F binds to the GCC motif with a binding activity similar to its binding activity for the DRE motif. These in vitro binding variations were further demonstrated through reporter cotransformation assays, suggesting that the DRE and GCC motifs are two similar sequence motifs sharing a common core region of CCGNC with a discriminating guanine base at the 5'-end of the GCC motif. Binding analyses with the mutated ERF domain show that such a unique binding of NtERF2-F to the GCC motif can be altered by the substitution of A14 with valine in beta-strand 2 of its ERF domain, the mutant NtERF2-F, ERFav, acquiring a binding to the DRE motif with a K(d) comparable to that for CBF1-F binding to the DRE motif. This demonstrates that A14 is an important determinant of the NtERF2-F binding specificity. A possible mechanism of the binding specificity determination is discussed.

Functional analysis of tomato Pti4 in Arabidopsis

Pti4 is a tomato (Lycopersicon esculentum) transcription factor that belongs to the ERF (ethylene-responsive element binding factor) family of proteins. It interacts with the Pto kinase in tomato, which confers resistance to the Pseudomonas syringae pv tomato pathogen that causes bacterial speck disease. To study the function of Pti4, transgenic Arabidopsis plants were generated that expressed tomato Pti4 driven by the strong constitutive promoters, cauliflower mosaic virus 35S and tCUP. Global gene expression analysis by Affimetric GeneChip indicated that expression of Pti4 in transgenic Arabidopsis plants induced the expression of GCC box-containing PR genes. We also demonstrated that Pti4 enhanced GCC box-mediated transcription of a reporter gene. The data suggests that tomato Pti4 could act as a transcriptional activator to regulate expression of GCC box-containing genes. Furthermore, we show that the expression of tomato Pti4 in transgenic Arabidopsis plants produced a phenotype similar to that seen in plants treated with ethylene, thus providing evidence that the Pti4 gene is involved in the regulation of a subset of ethylene-responsive genes containing the GCC box.

Ethylene induces antifreeze activity in winter rye leaves

Antifreeze activity is induced by cold temperatures in winter rye (Secale cereale) leaves. The activity arises from six antifreeze proteins that accumulate in the apoplast of winter rye leaves during cold acclimation. The individual antifreeze proteins are similar to pathogenesis-related proteins, including glucanases, chitinases, and thaumatin-like proteins. The objective of this study was to study the regulation of antifreeze activity in response to ethylene and salicyclic acid, which are known regulators of pathogenesis-related proteins induced by pathogens. Nonacclimated plants treated with salicylic acid accumulated apoplastic proteins with no antifreeze activity. In contrast, when nonacclimated plants were exposed to ethylene, both antifreeze activity and the concentration of apoplastic protein increased in rye leaves. Immunoblotting revealed that six of the seven accumulated apoplastic proteins consisted of two glucanases, two chitinases, and two thaumatin-like proteins. The ethylene-releasing agent ethephon and the ethylene precursor 1-aminocyclopropane-1-carboxylate also induced high levels of antifreeze activity at 20 degrees C, and this effect could be blocked by the ethylene inhibitor AgNO(3). When intact rye plants were exposed to 5 degrees C, endogenous ethylene production and antifreeze activity were detected within 12 and 48 h of exposure to cold, respectively. Rye plants exposed to drought produced both ethylene and antifreeze activity within 24 h. We conclude that ethylene is involved in regulating antifreeze activity in winter rye in response to cold and drought.

The jasmonate-inducible AP2/ERF-domain transcription factor ORCA3 activates gene expression via interaction with a jasmonate-responsive promoter element

The AP2/ERF-domain transcription factor ORCA3 is a master regulator of primary and secondary metabolism in Catharanthus roseus (periwinkle). Here we demonstrate that ORCA3 specifically binds to and activates gene expression via a previously characterized jasmonate- and elicitor-responsive element (JERE) in the promoter of the terpenoid indole alkaloid biosynthetic gene Strictosidine synthase (Str). Functional characterization of different domains in the ORCA3 protein in yeast and plant cells revealed the presence of an N-terminal acidic activation domain and a serine-rich C-terminal domain with a negative regulatory function. Orca3 mRNA accumulation was rapidly induced by the plant stress hormone methyljasmonate with biphasic kinetics. A precursor and an intermediate of the jasmonate biosynthetic pathway also induced Orca3 gene expression, further substantiating the role for ORCA3 in jasmonate signaling. The protein synthesis inhibitor cycloheximide did not inhibit jasmonate-responsive expression of Orca3, nor of its target genes Str and Tryptophan decarboxylase (Tdc). In conclusion, ORCA3 regulates jasmonate-responsive expression of the Str gene via direct interaction with the JERE. The activating activities of ORCA proteins do not seem to depend on jasmonate-induced de novo protein synthesis, but presumably occur via modification of pre-existing ORCA protein.

A calmodulin binding protein from Arabidopsis is induced by ethylene and contains a DNA-binding motif

Calmodulin (CaM), a key calcium sensor in all eukaryotes, regulates diverse cellular processes by interacting with other proteins. To isolate CaM binding proteins involved in ethylene signal transduction, we screened an expression library prepared from ethylene-treated Arabidopsis seedlings with 35S-labeled CaM. A cDNA clone, EICBP (Ethylene-Induced CaM Binding Protein), encoding a protein that interacts with activated CaM was isolated in this screening. The CaM binding domain in EICBP was mapped to the C-terminus of the protein. These results indicate that calcium, through CaM, could regulate the activity of EICBP. The EICBP is expressed in different tissues and its expression in seedlings is induced by ethylene. The EICBP contains, in addition to a CaM binding domain, several features that are typical of transcription factors. These include a DNA-binding domain at the N terminus, an acidic region at the C terminus, and nuclear localization signals. In database searches a partial cDNA (CG-1) encoding a DNA-binding motif from parsley and an ethylene up-regulated partial cDNA from tomato (ER66) showed significant similarity to EICBP. In addition, five hypothetical proteins in the Arabidopsis genome also showed a very high sequence similarity with EICBP, indicating that there are several EICBP-related proteins in Arabidopsis. The structural features of EICBP are conserved in all EICBP-related proteins in Arabidopsis, suggesting that they may constitute a new family of DNA binding proteins and are likely to be involved in modulating gene expression in the presence of ethylene.

Regulation of ethylene-induced transcription of defense genes

Ethylene-induced gene expression has been studied in systems in which the biosynthesis of ethylene is stimulated during developmental process such as ripening of fruit, senescence of flower petals, or during pathogen infection. Functional analysis of the promoters of these genes revealed that the ethylene-responsive cis-elements of fruit ripening genes and senescence genes differed from that of defense genes whose expression is induced by ethylene in response to pathogen infection. The ethylene-responsive element identified as the GCC box (AGCCGCC) is commonly found in the promoter region of the ethylene-inducible defense genes. The ethylene responsive element binding factors that interact with the GCC box were demonstrated to be the transcription factors, which respond to extracellular signals to modulate GCC box-mediated gene expression positively or negatively.