Analysis of stress-induced or salicylic acid-induced expression of the pathogenesis-related 1a protein gene in transgenic tobacco

Functional Characterization of Verticillium dahliae Race 3-Specific Gene VdR3e in Virulence and Elicitation of Plant Immune Responses

Verticillium dahliae is a soilborne fungal pathogen that causes disease on many economically important crops. Based on the resistance or susceptibility of differential cultivars in tomato, isolates of V. dahliae are divided into three races. Avirulence (avr) genes within the genomes of the three races have also been identified. However, the functional role of the avr gene in race 3 isolates of V. dahliae has not been characterized. In this study, bioinformatics analysis showed that VdR3e, a cysteine-rich secreted protein encoded by the gene characterizing race 3 in V. dahliae, was likely obtained by horizontal gene transfer from the fungal genus Bipolaris. We demonstrate that VdR3e causes cell death by triggering multiple defense responses. In addition, VdR3e localized at the periphery of the plant cell and triggered immunity depending on its subcellular localization and the cell membrane receptor BAK1. Furthermore, VdR3e is a virulence factor and shows differential pathogenicity in race 3-resistant and -susceptible hosts. These results suggest that VdR3e is a virulence factor that can also interact with BAK1 as a pathogen-associated molecular pattern (PAMP) to trigger immune responses. IMPORTANCE Based on the gene-for-gene model, research on the function of avirulence genes and resistance genes has had an unparalleled impact on breeding for resistance in most crops against individual pathogens. The soilborne fungal pathogen, Verticillium dahliae, is a major pathogen on many economically important crops. Currently, avr genes of the three races in V. dahliae have been identified, but the function of avr gene representing race 3 has not been described. We investigated the characteristics of VdR3e-mediated immunity and demonstrated that VdR3e acts as a PAMP to activate a variety of plant defense responses and induce plant cell death. We also demonstrated that the role of VdR3e in pathogenicity was host dependent. This is the first study to describe the immune and virulence functions of the avr gene from race 3 in V. dahliae, and we provide support for the identification of genes mediating resistance against race 3.

In vivo Imaging Enables Understanding of Seamless Plant Defense Responses to Wounding and Pathogen Attack

Plants are exposed to varied biotic stresses, including sequential or simultaneous attack by insects and pathogens. To overcome these complex stresses, plants must perceive each of the stresses, then integrate and relay the information throughout the plant body and eventually activate local and systemic resistance responses. Previous molecular genetic studies identified jasmonic acid and salicylic acid as key plant hormones of wound and immune responses. These hormones, combined with their antagonistic interaction, play critical roles in the initiation and regulation of defense responses against insects and pathogens. Aside from molecular and genetic information, the latest in vivo imaging technology has revealed that plant defense responses are regulated spatially and temporally. In this review, we summarize the current knowledge of local and systemic defense responses against wounding and diseases with a focus on past and recent advances in imaging technologies. We discuss how imaging-based multiparametric analysis has improved our understanding of the spatiotemporal regulation of dynamic plant stress responses. We also emphasize the importance of compiling the knowledge generated from individual studies on plant wounding and immune responses for a more seamless understanding of plant defense responses in the natural environment.

Polymer-Conjugated Carbon Nanotubes for Biomolecule Loading

Nanomaterials have emerged as an invaluable tool for the delivery of biomolecules such as DNA and RNA, with various applications in genetic engineering and post-transcriptional genetic manipulation. Alongside this development, there has been an increasing use of polymer-based techniques, such as polyethylenimine (PEI), to electrostatically load polynucleotide cargoes onto nanomaterial carriers. However, there remains a need to assess nanomaterial properties, conjugation conditions, and biocompatibility of these nanomaterial-polymer constructs, particularly for use in plant systems. In this work, we develop mechanisms to optimize DNA loading on single-walled carbon nanotubes (SWNTs) with a library of polymer-SWNT constructs and assess DNA loading ability, polydispersity, and both chemical and colloidal stability. Counterintuitively, we demonstrate that polymer hydrolysis from nanomaterial surfaces can occur depending on polymer properties and attachment chemistries, and we describe mitigation strategies against construct degradation. Given the growing interest in delivery applications in plant systems, we also assess the stress response of plants to polymer-based nanomaterials and provide recommendations for future design of nanomaterial-based polynucleotide delivery strategies.

Polymer-Conjugated Carbon Nanotubes for Biomolecule Loading

Nanomaterials have emerged as an invaluable tool for the delivery of biomolecules such as DNA and RNA, with various applications in genetic engineering and post-transcriptional genetic manipulation. Alongside this development, there has been an increasing use of polymer-based techniques, such as polyethylenimine (PEI), to electrostatically load polynucleotide cargoes onto nanomaterial carriers. However, there remains a need to assess nanomaterial properties, conjugation conditions, and biocompatibility of these nanomaterial-polymer constructs, particularly for use in plant systems. In this work, we develop mechanisms to optimize DNA loading on single-walled carbon nanotubes (SWNTs) with a library of polymer-SWNT constructs and assess DNA loading ability, polydispersity, and both chemical and colloidal stability. Counterintuitively, we demonstrate that polymer hydrolysis from nanomaterial surfaces can occur depending on polymer properties and attachment chemistries, and we describe mitigation strategies against construct degradation. Given the growing interest in delivery applications in plant systems, we also assess the stress response of plants to polymer-based nanomaterials and provide recommendations for future design of nanomaterial-based polynucleotide delivery strategies.

Sulfur Deprivation Modulates Salicylic Acid Responses via Nonexpressor of Pathogenesis-Related Gene 1 in Arabidopsis thaliana

Mineral nutrients are essential for plant growth and reproduction, yet only a few studies connect the nutritional status to plant innate immunity. The backbone of plant defense response is mainly controlled by two major hormones: salicylic acid (SA) and jasmonic acid (JA). This study investigated changes in the macronutrient concentration (deficiency/excess of nitrogen, phosphorus, potassium, magnesium, and sulfur) on the expression of PR1, a well-characterized marker in the SA-pathway, and PDF1.2 and LOX2 for the JA-pathway, analyzing plants carrying the promoter of each gene fused to GUS as a reporter. After histochemical GUS assays, we determined that PR1 gene was strongly activated in response to sulfur (S) deficiency. Using RT-PCR, we observed that the induction of PR1 depended on the function of Non-expressor of Pathogenesis-Related gene 1 (NPR1) and SA accumulation, as PR1 was not expressed in npr1-1 mutant and NahG plants under S-deprived conditions. Plants treated with different S-concentrations showed that total S-deprivation was required to induce SA-mediated defense responses. Additionally, bioassays revealed that S-deprived plants, induced resistance to the hemibiotrophic pathogen Pseudomonas syringae pv. DC3000 and increase susceptibility to the necrotrophic Botrytis cinerea. In conclusion, we observed a relationship between S and SA/JA-dependent defense mechanisms in Arabidopsis.

Nicotiana benthamiana RanBP1-1 Is Involved in the Induction of Disease Resistance via Regulation of Nuclear-Cytoplasmic Transport of Small GTPase Ran

Plant cells enhance the tolerances to abiotic and biotic stresses via recognition of the stress, activation and nuclear import of signaling factors, up-regulation of defense genes, nuclear export of mRNA and translation of defense proteins. Nuclear pore-mediated transports should play critical roles in these processes, however, the regulatory mechanisms of nuclear-cytoplasmic transport during stress responses are largely unknown. In this study, a regulator of nuclear export of RNA and proteins, NbRanBP1-1 (Ran-binding protein1-1), was identified as an essential gene for the resistance of Nicotiana benthamiana to potato blight pathogen Phytophthora infestans. NbRanBP1-1-silenced plants showed delayed accumulation of capsidiol, a sesquiterpenoid phytoalexin, in response to elicitor treatment, and reduced resistance to P. infestans. Abnormal accumulation of mRNA was observed in NbRanBP1-1-silenced plants, indicating that NbRanBP1-1 is involved in the nuclear export of mRNA. In NbRanBP1-1-silenced plants, elicitor-induced expression of defense genes, NbEAS and NbWIPK, was not affected in the early stage of defense induction, but the accumulation of NbWIPK protein was reduced. Nuclear export of the small G-protein NbRan1a was activated during the induction of plant defense, whereas this process was compromised in NbRanBP1-1-silenced plants. Silencing of genes encoding the nuclear pore proteins, Nup75 and Nup160, also caused abnormal nuclear accumulation of mRNA, defects in the nuclear export of NbRan1a, and reduced production of capsidiol, resulting in decreased resistance to P. infestans. These results suggest that nuclear export of NbRan is a key event for defense induction in N. benthamiana, and both RanBP1-1 and nucleoporins play important roles in the process.

The Wheat Bax Inhibitor-1 Protein Interacts with an Aquaporin TaPIP1 and Enhances Disease Resistance in Arabidopsis

Bax inhibitor-1 (BI-1) is an endoplasmic reticulum (ER)-resident cell death suppressor evolutionarily conserved in eukaryotes. The ability of BI-1 to inhibit the biotic and abiotic stresses have been well-studied in Arabidopsis, while the functions of wheat BI-1 are largely unknown. In this study, the wheat BI-1 gene TaBI-1.1 was isolated by an RNA-seq analysis of Fusarium graminearum (Fg)-treated wheat. TaBI-1.1 expression was induced by a salicylic acid (SA) treatment and down-regulated by an abscisic acid (ABA) treatment. Based on β-glucuronidase (GUS) staining, TaBI-1.1 was expressed in mature leaves and roots but not in the hypocotyl or young leaves. Constitutive expression of TaBI-1.1 in Arabidopsis enhanced its resistance to Pseudomonas syringae pv. Tomato (Pst) DC3000 infection and induced SA-related gene expression. Additionally, TaBI-1.1 transgenic Arabidopsis exhibited an alleviation of damage caused by high concentrations of SA and decreased the sensitivity to ABA. Consistent with the phenotype, the RNA-seq analysis of 35S::TaBI-1.1 and Col-0 plants showed that TaBI-1.1 was involved in biotic stresses. These results suggested that TaBI-1.1 positively regulates SA signals and plays important roles in the response to biotic stresses. In addition, TaBI-1.1 interacted with the aquaporin TaPIP1, and both them were localized to ER membrane. Furthermore, we demonstrated that TaPIP1 was up-regulated by SA treatment and TaPIP1 transgenic Arabidopsis enhanced the resistance to Pst DC3000 infection. Thus, the interaction between TaBI-1.1 and TaPIP1 on the ER membrane probably occurs in response to SA signals and defense response.

Salicylic Acid and Jasmonic Acid Pathways are Activated in Spatially Different Domains Around the Infection Site During Effector-Triggered Immunity in Arabidopsis thaliana

The innate immune response is, in the first place, elicited at the site of infection. Thus, the host response can be different among the infected cells and the cells surrounding them. Effector-triggered immunity (ETI), a form of innate immunity in plants, is triggered by specific recognition between pathogen effectors and their corresponding plant cytosolic immune receptors, resulting in rapid localized cell death known as hypersensitive response (HR). HR cell death is usually limited to a few cells at the infection site, and is surrounded by a few layers of cells massively expressing defense genes such as Pathogenesis-Related Gene 1 (PR1). This virtually concentric pattern of the cellular responses in ETI is proposed to be regulated by a concentration gradient of salicylic acid (SA), a phytohormone accumulated around the infection site. Recent studies demonstrated that jasmonic acid (JA), another phytohormone known to be mutually antagonistic to SA in many cases, is also accumulated in and required for ETI, suggesting that ETI is a unique case. However, the molecular basis for this uniqueness remained largely to be solved. Here, we found that, using intravital time-lapse imaging, the JA signaling pathway is activated in the cells surrounding the central SA-active cells around the infection sites in Arabidopsis thaliana. This distinct spatial organization explains how these two phythormone pathways in a mutually antagonistic relationship can be activated simultaneously during ETI. Our results re-emphasize that the spatial consideration is a key strategy to gain mechanistic insights into the apparently complex signaling cross-talk in immunity.

rgs-CaM Detects and Counteracts Viral RNA Silencing Suppressors in Plant Immune Priming

Primary infection of a plant with a pathogen that causes high accumulation of salicylic acid in the plant typically via a hypersensitive response confers enhanced resistance against secondary infection with a broad spectrum of pathogens, including viruses. This phenomenon is called systemic acquired resistance (SAR), which is a plant priming for adaption to repeated biotic stress. However, the molecular mechanisms of SAR-mediated enhanced inhibition, especially of virus infection, remain unclear. Here, we show that SAR against cucumber mosaic virus (CMV) in tobacco plants (Nicotiana tabacum) involves a calmodulin-like protein, rgs-CaM. We previously reported the antiviral function of rgs-CaM, which binds to and directs degradation of viral RNA silencing suppressors (RSSs), including CMV 2b, via autophagy. We found that rgs-CaM-mediated immunity is ineffective against CMV infection in normally growing tobacco plants but is activated as a result of SAR induction via salicylic acid signaling. We then analyzed the effect of overexpression of rgs-CaM on salicylic acid signaling. Overexpressed and ectopically expressed rgs-CaM induced defense reactions, including cell death, generation of reactive oxygen species, and salicylic acid signaling. Further analysis using a combination of the salicylic acid analogue benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) and the Ca2+ ionophore A23187 revealed that rgs-CaM functions as an immune receptor that induces salicylic acid signaling by simultaneously perceiving both viral RSS and Ca2+ influx as infection cues, implying its autoactivation. Thus, secondary infection of SAR-induced tobacco plants with CMV seems to be effectively inhibited through 2b recognition and degradation by rgs-CaM, leading to reinforcement of antiviral RNA silencing and other salicylic acid-mediated antiviral responses.IMPORTANCE Even without an acquired immune system like that in vertebrates, plants show enhanced whole-plant resistance against secondary infection with pathogens; this so-called systemic acquired resistance (SAR) has been known for more than half a century and continues to be extensively studied. SAR-induced plants strongly and rapidly express a number of antibiotics and pathogenesis-related proteins targeted against secondary infection, which can account for enhanced resistance against bacterial and fungal pathogens but are not thought to control viral infection. This study showed that enhanced resistance against cucumber mosaic virus is caused by a tobacco calmodulin-like protein, rgs-CaM, which detects and counteracts the major viral virulence factor (RNA silencing suppressor) after SAR induction. rgs-CaM-mediated SAR illustrates the growth versus defense trade-off in plants, as it targets the major virulence factor only under specific biotic stress conditions, thus avoiding the cost of constitutive activation while reducing the damage from virus infection.

Emerging Importance of Helicases in Plant Stress Tolerance: Characterization of Oryza sativa Repair Helicase XPB2 Promoter and Its Functional Validation in Tobacco under Multiple Stresses

Genetic material always remains at the risk of spontaneous or induced damage which challenges the normal functioning of DNA molecule, thus, DNA repair is vital to protect the organisms against genetic damage. Helicases, the unique molecular motors, are emerged as prospective molecules to engineer stress tolerance in plants and are involved in nucleic acid metabolism including DNA repair. The repair helicase, XPB is an evolutionary conserved protein present in different organisms, including plants. Availability of few efficient promoters for gene expression in plants provoked us to study the promoter of XPB for better understanding of gene regulation under stress conditions. Here, we report the in silico analysis of novel stress inducible promoter of Oryza sativa XPB2 (OsXPB2). The in vivo validation of functionality/activity of OsXPB2 promoter under abiotic and hormonal stress conditions was performed by Agrobacterium-mediated transient assay in tobacco leaves using OsXPB2::GUS chimeric construct. The present research revealed that OsXPB2 promoter contains cis-elements accounting for various abiotic stresses (salt, dehydration, or cold) and hormone (Auxin, ABA, or MeJA) induced GUS expression/activity in the promoter-reporter assay. The promoter region of OsXPB2 contains CACG, GTAACG, CACGTG, CGTCA CCGCCGCGCT cis acting-elements which are reported to be salt, dehydration, cold, MeJA, or ABA responsive, respectively. Functional analysis was done by Agrobacterium-mediated transient assay using agroinfiltration in tobacco leaves, followed by GUS staining and fluorescence quantitative analyses. The results revealed high induction of GUS activity under multiple abiotic stresses as compared to mock treated control. The present findings suggest that OsXPB2 promoter is a multi-stress inducible promoter and has potential applications in sustainable crop production under abiotic stresses by regulating desirable pattern of gene expression.

Current overview of allergens of plant pathogenesis related protein families

Pathogenesis related (PR) proteins are one of the major sources of plant derived allergens. These proteins are induced by the plants as a defense response system in stress conditions like microbial and insect infections, wounding, exposure to harsh chemicals, and atmospheric conditions. However, some plant tissues that are more exposed to environmental conditions like UV irradiation and insect or fungal attacks express these proteins constitutively. These proteins are mostly resistant to proteases and most of them show considerable stability at low pH. Many of these plant pathogenesis related proteins are found to act as food allergens, latex allergens, and pollen allergens. Proteins having similar amino acid sequences among the members of PR proteins may be responsible for cross-reactivity among allergens from diverse plants. This review analyzes the different pathogenesis related protein families that have been reported as allergens. Proteins of these families have been characterized in regard to their biological functions, amino acid sequence, and cross-reactivity. The three-dimensional structures of some of these allergens have also been evaluated to elucidate the antigenic determinants of these molecules and to explain the cross-reactivity among the various allergens.

Cloning and functional characterization of the promoter of PsSEOF1 gene from Pisum sativum under different stress conditions using Agrobacterium-mediated transient assay

PsSEOF1, a SEO (sieve element occlusion) gene family protein (forisome) is calcium powered motor protein and is located close to plasma membrane of sieve element. In sieve element (SE) it senses the calcium ion levels and undergoes ATP-independent conformational shifts. Forisome, meaning gate-bodies (Latin foris: wing of a gate; Greek soma: body). Recent reports show that SEO gene family protein can prevent the loss of nutrient rich photoassimilate upon wound injury. The regulation of SEO protein forisome under abiotic/ biotic stress is still unknown. The analysis of cis-regulatory element present in the upstream region is not well understood. Tissue specific promoters guarantee correct expression when it perceives particular stimuli. Here we report isolation of tissue specific promoter of PsSEOF1 was isolated by gene walking PCR from P. sativum (pea) genomic DNA library constructed by BD genome walker kit. In silico analysis revealed several putative cis element within this promoter sequence like wound response, cold, dehydration. Putative elements which might be required for its vascular tissue specificity has also been identified. The GUS activities of PsSEOF1 promoter-GUS chimeric construct in the agroinfiltrated leaves under different environmental stress abiotic and biotic like wound, cold, salt and phytohormones has shown high level of GUS activity. To identify the activity of PsSEOF1 promoter under different stress condition an Agrobacterium-mediated transient expression of tobacco plants were subjected to histochemical GUS staining. Stress-inducible nature of PsSEOF1 promoter opens possibility for the study of the PsSEOF1 gene regulation under stress condition. The isolated promoter sequence could serve as an important candidate for tissue specific promoter in genetic engineering of plant under stress conditions.

Expression of FcFT1, a FLOWERING LOCUS T-like gene, is regulated by light and associated with inflorescence differentiation in fig (Ficus carica L.)

BACKGROUND

Because the floral induction occurs in many plants when specific environmental conditions are satisfied, most plants bloom and bear fruit during the same season each year. In fig, by contrast, the time interval during which inflorescence (flower bud, fruit) differentiation occurs corresponds to the shoot elongation period. Fig trees thus differ from many species in their reproductive growth characteristics. To date, however, the molecular mechanisms underlying this unorthodox physiology of floral induction and fruit setting in fig trees have not been elucidated.

RESULTS

We isolated a FLOWERING LOCUS T (FT)-like gene from fig and examined its function, characteristics, and expression patterns. The isolated gene, F. carica FT (FcFT1), is single copy in fig and shows the highest similarity at the amino acid level (93.1%) to apple MdFT2. We sequenced its upstream region (1,644 bp) and identified many light-responsive elements. FcFT1 was mainly expressed in leaves and induced early flowering in transgenic tobacco, suggesting that FcFT1 is a fig FT ortholog. Real-time reverse-transcription PCR analysis revealed that FcFT1 mRNA expression occurred only in leaves at the lower nodes, the early fruit setting positions. mRNA levels remained a constant for approximately 5 months from spring to autumn, corresponding almost exactly to the inflorescence differentiation season. Diurnal variation analysis revealed that FcFT1 mRNA expression increased under relative long-day and short-day conditions, but not under continuous darkness.

CONCLUSION

These results suggest that FcFT1 activation is regulated by light conditions and may contribute to fig's unique fruit-setting characteristics.

Response of tobacco to the Pseudomonas syringae pv. Tomato DC3000 is mainly dependent on salicylic acid signaling pathway

Pseudomonas syringae pv. Tomato DC3000 (Pst DC3000) was the first pathogen to be demonstrated to infect Arabidopsis and to cause disease symptoms in the laboratory setting. However, the defense response to Pst DC3000 was unclear in tobacco. In this report, the expression profiles of twelve defense response-related genes were analyzed after treatment with salicylic acid (SA), jasmonic acid (JA), and pathogen Pst DC3000 by qRT-PCR. According to our results, it could be presented that the genes primarily induced by SA were also induced to higher levels after Pst DC3000 infection. SA accumulation could be induced to a higher level than that of JA after Pst DC3000 infection. In addition, SA could result in hypersensitive response (HR), which did not completely depend on accumulation of reactive oxygen species. These results indicated that tobacco mainly depended on SA signaling pathway rather than on JA signaling pathway in response to Pst DC3000. Further study demonstrated that JA could significantly inhibit the accumulation of SA and the generation of the HR induced by Pst DC3000.

Large-scale characterization of promoters from grapevine (Vitis spp.) using quantitative anthocyanin and GUS assay systems

Successful implementation of cisgenic/intragenic/ingenic technology for crop improvement necessitates a better understanding of the function of native promoters for driving desired gene expression in host plant. Although the genome of grapevine (Vitis vinifera) has been determined, efforts to explore promoter resources for the development of cisgenics are still lacking. Particularly, there is a shortage of constitutive promoters for marker and/or target gene expression in this species. In this work, we utilized an anthocyanin-based color histogram analysis method to evaluate quantitatively a large number of promoters for their ability to activate transgene expression. Promoter fragments corresponding to known genes were amplified from various genotypes and used to drive the VvMybA1 gene of 'Merlot' for anthocyanin production in non-pigmented somatic embryo (SE) explants to infer transcriptional activity. Results revealed that among 15 tested promoters belonging to seven ubiquitin genes, at least three promoters generated constitutive activities reaching up to 100% value of the d35S promoter. In particular, the high activity levels of VvUb6-1 and VvUb7-2 promoters were verified by transient GUS quantitative assay as well as stable anthocyanin expression in sepal and corolla of transgenic tobacco. Variations in promoter activity of different ubiquitin genes in grapevine did not correlate with the presence and sizes of 5' UTR intron, but seemed to be related positively and negatively to the number of positive cis-acting elements and root-specific elements respectively. In addition, several of the 13 promoters derived from a PR1 gene and a PAL gene produced a higher basal activity as compared to previously reported inducible promoters and might be useful for further identification of strong inducible promoters. Our study contributed invaluable information on transcriptional activity of many previously uncharacterized native promoters that could be used for genetic engineering of grapevine.

Evaluation of the use of the tobacco PR-1a promoter to monitor defense gene expression by the luciferase bioluminescence reporter system

Because of their marked responsiveness to induction signals, genes encoding pathogenesis-related proteins are used as markers to monitor defense gene expression in plants. To develop a non-invasive bioluminescence reporter assay system, we tested acidic PR-1 gene promoters from tobacco and Arabidopsis. These two promoters share common regulatory elements and are believed to show similar responsiveness to various stimuli but the results of transient expression assays by microprojectile bombardment of various plant cells and npr1 mutant Arabidopsis suggest that the tobacco PR-1a promoter is superior to its Arabidopsis counterpart in terms of responsiveness to salicylic acid treatment. Transgenic Arabidopsis seedlings harboring the tobacco PR-1a promoter fused to firefly luciferase showed marked induction in response to treatment with chemicals that induce defense gene expression in plants. These results suggest that the tobacco PR-1a promoter is applicable in monitoring defense-gene expression in various plant species.

Glutathione signaling acts through NPR1-dependent SA-mediated pathway to mitigate biotic stress

Glutathione (GSH) has widely been known to be a multifunctional molecule especially as an antioxidant uptill now, but has found a new role in plant defense signaling. Research from the past three decades indicate that GSH is a player in pathogen defense in plants, but the mechanism underlying this has not been elucidated fully. We have recently shown that GSH acts as a signaling molecule and mitigates biotic stress through non-expressor of PR genes 1 (NPR1)-dependent salicylic acid (SA)-mediated pathway. Transgenic tobacco with enhanced level of GSH (NtGB lines) was found to synthesize more SA, was capable of enhanced expression of genes belonging to NPR1-dependent SA-mediated pathway, were resistant to Pseudomonas syringae, the biotrophic pathogen and many SA-related proteins were upregulated. These results gathered experimental evidence on the mechanism through which GSH combats biotic stress. In continuation with our previous investigation we show here that the expression of glutathione S-transferase (GST), the NPR1-independent SA-mediated gene was unchanged in transgenic tobacco with enhanced level of GSH as compared to wild-type plants. Additionally, the transgenic plants were barely resistant to Botrytis cinerea, the necrotrophic pathogen. SA-treatment led to enhanced level of expression of pathogenesis-related protein gene (PR1) and PR4 as against short-chain dehydrogenase/reductase family protein (SDRLP) and allene oxide synthase (AOS). These data provided significant insight into the involvement of GSH in NPR1-dependent SA-mediated pathway in mitigating biotic stress.

Over-expression of osa-MIR396c decreases salt and alkali stress tolerance

Salt and alkali stress are two of the main environmental factors limiting rice production. Thus, understanding the mechanisms of salinity and alkali stress tolerance is necessary to modify rice to increase its resistance to salinity and alkaline stress. MicroRNAs (miRNAs) are approximately 21-nucleotide RNAs that are ubiquitous regulators of gene expression in eukaryotic organisms. In plants, miRNAs constitute one of five classes of small RNAs that function primarily as negative regulators for gene expression at the posttranscriptional level. Several plant miRNAs, such as miR396, play vital roles in plant growth, development and resistance to stresses. In this study, we identified osa-MIR396c, which shows dramatic transcript change under salt and alkali stress conditions in Oryza sativa. We designed an experiment to detect miRNA-target interaction and demonstrated that several transcription factors related to growth, development, and stress tolerance are targeted by osa-MIR396c. Transgenic rice and Arabidopsis thaliana plants constitutively over-expressing osa-MIR396c showed reduced salt and alkali stress tolerance compared to that of wild-type plants. Overall, this study further established a link between salt and alkali stress and osa-MIR396c in rice.

Characteristic expression of twelve rice PR1 family genes in response to pathogen infection, wounding, and defense-related signal compounds (121/180)

Pathogenesis-related (PR) proteins have been used as markers of plant defense responses, and are classified into 17 families. However, precise information on the majority members in specific PR families is still limited. We were interested in the individual characteristics of rice PR1 family genes, and selected 12 putatively active genes using rice genome databases for expressed genes. All were upregulated upon compatible and/or incompatible rice-blast fungus interactions; three were upregulated in the early infection period and four in the late infection period. Upon compatible rice-bacterial blight interaction, four genes were upregulated, six were not affected, and one was downregulated. These results are in striking contrast to those among 22 Arabidopsis PR1 genes where only one gene was pathogen-inducible. The responses of individual genes to salicylic acid, jasmonic acid, and ethylene induced defense signaling pathways in rice are likely to be different from those in dicot plants. Transcript levels in healthy leaves, roots, and flowers varied according to each gene. Analysis of the partially overlapping expression patterns of rice PR1 genes in healthy tissues and in response to pathogens and other stresses would be useful to understand their possible functions and for use as characteristic markers for defense-related studies in rice.

Tobacco NIMIN2 proteins control PR gene induction through transient repression early in systemic acquired resistance

NPR1 (for Nonexpressor of PR genes; also known as NIM1) is a positive regulator of systemic acquired resistance (SAR) in Arabidopsis, which controls the induction of Pathogenesis-Related (PR) genes by salicylic acid (SA). NPR1 interacts with members of two protein families, TGA transcription factors and NIMIN (for NIM1-interacting) proteins. In Arabidopsis, NIMIN1, NIMIN2 and NIMIN3 constitute a small gene family of structurally related, yet distinct members. To unravel the biological significance of NIMIN interaction with NPR1, we searched a tobacco yeast two-hybrid cDNA library for NPR1- and NIMIN2-binding proteins. One NPR1 cDNA clone and three clones encoding NIMIN proteins were isolated. Although clearly similar to At NPR1, Nt NPR1 does not interact with At NIMIN3. Furthermore, all Nt NIMIN proteins identified are structurally related to At NIMIN2, thus forming a small NIMIN2 subfamily in tobacco. cDNA clones encoding At NIMIN1 or At NIMIN3 homologues were not identified. The function of NIMIN2 proteins was studied by expression of Nt NIMIN2a chimeric genes in tobacco. While constitutive NIMIN2a over-expression delayed PR-1 protein induction, suppression of NIMIN2 transcripts enhanced the accumulation of PR-1 proteins. In both cases, the effects of altered NIMIN2 transcript levels became evident foremost early in SAR. Notably, Nt NIMIN2 gene expression is elevated prior to the induction of the PR-1a gene. Together, the data suggest that, in tobacco, NIMIN2 proteins control PR-1 gene expression, and that NIMIN2-mediated control is exerted through transient PR-1 repression before SAR has fully developed. Furthermore, although sharing conserved domains and functions, tobacco and Arabidopsis NPR1 and NIMIN proteins are clearly distinct.

MAP kinases function downstream of HSP90 and upstream of mitochondria in TMV resistance gene N-mediated hypersensitive cell death

Although the involvement of heat shock protein 90 (HSP90), mitogen-activated protein kinase (MAPK) cascades and organelle dysfunction in plant hypersensitive cell death has been suggested, the mutual relationship among them has not been elucidated. Here, we show the molecular network of HSP90, the wound-induced protein kinase (WIPK)/salicylic acid-induced protein kinase (SIPK)-mediated MAPK cascade and mitochondrial dysfunction in tobacco mosaic virus (TMV) resistance gene N-dependent cell death. p50, the Avr component for N, NtMEK2(DD), a constitutively active form of a MAPK kinase of WIPK/SIPK, and a mammalian pro-apoptotic factor Bax were used for cell death induction. Suppression of HSP90 and treatment with geldanamycin, a specific inhibitor of HSP90, compromised p50- but not NtMEK2(DD)- or Bax-mediated cell death accompanying the reduction of NtMEK2, WIPK and SIPK activation. In WIPK/SIPK-double knockdown plants, p50- and NtMEK2(DD)- but not Bax-mediated cell death was suppressed. All three types of cell death induced mitochondrial dysfunction, but they were similarly suppressed by Bcl-xL, which is a mammalian anti-apoptotic factor, and prevents mitochondrial dysfunction in plants as it does in animals in the cell death signal pathway. Taken together with the expression profile of hypersensitive reaction marker genes, it was indicated that the MAPK cascade functions downstream of HSP90 and transduces the cell death signal to mitochondria for N gene-dependent cell death. Furthermore, we found that WIPK and SIPK are functionally redundant in cell death signaling using WIPK/SIPK single or double knockdown plants.

A novel wound-responsive cis-element, VWRE, of the vascular system-specific expression of a tobacco peroxidase gene, tpoxN1

The wound-induced expression of tpoxN1, encoding a tobacco peroxidase, is unique because of its vascular system-specific expression and insensitivity to known wound-signal compounds such as jasmonic acid, ethylene, and plant hormones [Sasaki et al. (2002) Plant Cell Physiol 43:108-117]. To study the mechanism of expression, the 2-kbp tpoxN1 promoter region and successive 5'-deletion of the promoter were introduced as GUS fusion genes into tobacco plants. Analysis of GUS activity in transgenic plants indicated that a vascular system-specific and wound-responsive cis-element (VWRE) is present at the -239/-200 region of the promoter. Gel mobility shift assays suggested that a nuclear factor(s) prepared from wounded tobacco stems binds a 14-bp sequence (-229/-215) in the -239/-200 region in a sequence-specific manner. A mutation in this 14-bp region of the -239 promoter fragment resulted in a considerable decrease in wound-responsive GUS activity in transgenic plants. An 11-bp sequence, which completely overlaps with the 14-bp sequence, was found in the 5' distal region (-420/-410) and is thought to contribute to the wound-induced expression together with the 14-bp. The -114-bp core promoter of the tpoxN1 gene was indispensable for wound-induced expression, indicating that the 14-bp region is a novel wound-responsive cis-element VWRE, which may work cooperatively with other factors in the promoter.

Accumulation of the two transcripts of the N gene, conferring resistance to tobacco mosaic virus, is probably important for N gene-dependent hypersensitive cell death

The N gene is a Toll/interleukin-1 receptor (TIR)-nucleotide-binding site (NBS)-leucine-rich repeat (LRR)-type resistance (R) gene that generates two alternative transcripts, N(S) and N(L). N(S) encodes the full-length N protein while N(L) is predicted to encode a truncated form of the protein lacking most of the LRR region. We found that the two transcripts were accumulated at 20 degrees C, a permissive temperature, but not at 30 degrees C, a non-permissive temperature for the N gene, in tobacco mosaic virus (TMV)-inoculated leaves. When N gene-dependent cell death was triggered by transient 20 degrees C treatment for 2-6 h, considerable levels of the transcripts were accumulated just before cell death, although the levels of N(S) were always higher. The accumulation was induced by transient expression of the 50 kDa helicase domain (p50) of TMV replicase which is the Avr component of N, but not by transient expression of NtMEK2 (DD)-mediated cell death or N gene-independent hypersensitive cell death. These results suggest that the accumulation of N(S) and N(L) is associated with the function of N and, above a certain threshold, triggers N-mediated hypersensitive cell death.

The promoter of Milk vetch dwarf virus component 8 confers effective gene expression in both dicot and monocot plants

The activity of a predicted promoter, PMC8, from Milk vetch dwarf virus was evaluated by comparing it with the cauliflower mosaic virus 35S RNA promoter (P35S) and PNCR, a promoter from Soybean chlorotic mottle virus. When the GUS fusion gene was introduced into tobacco, PMC8 showed a similar expression profile to P35S but with a more intense expression in proliferating tissues. The usefulness of PMC8 was confirmed by driving NPTII for selection of kanamycin-resistant tobacco plants with improved transformation efficiency. PMC8 was also effective in transgenic rice plants. Thus, PMC8 is useful as an alternative to P35S in both dicotyledonous and monocotyledonous plants, especially for gene expression in proliferating tissues.

A novel oxidative stress-inducible peroxidase promoter from sweetpotato: molecular cloning and characterization in transgenic tobacco plants and cultured cells

A strong oxidative stress-inducible peroxidase (POD) promoter was cloned from sweetpotato (Ipomoea batatas) and characterized in transgenic tobacco plants and cultured cells in terms of environmental stress. A POD genomic clone (referred to as SWPA2) consisted of 1824 bp of sequence upstream of the translation start site, two introns (743 bp and 97 bp), and a 1073 bp coding region. SWPA2 had previously been found to encode an anionic POD which was highly expressed in response to oxidative stress. The SWPA2 promoter contained several cis-element sequences implicated in oxidative stress such as GCN-4, AP-1, HSTF, SP-1 reported in animal cells and a plant specific G-box. Employing a transient expression assay in tobacco protoplasts, with five different 5'-deletion mutants of the SWPA2 promoter fused to the beta-glucuronidase (GUS) reporter gene, the 1314 bp mutant deletion mutant showed about 30 times higher GUS expression than the CaMV 35S promoter. The expression of GUS activity in transgenic tobacco plants under the control of the -1314 SWPA2 promoter was strongly induced in response to environmental stresses including hydrogen peroxide, wounding and UV treatment. Furthermore, GUS activity in suspension cultures of transgenic cells derived from transgenic tobacco leaves containing the -1314 bp SWPA2 promoter-GUS fusion was strongly expressed after 15 days of subculture compared to other deletion mutants. We anticipate that the -1314 bp SWPA2 promoter will be biotechnologically useful for the development of transgenic plants with enhanced tolerance to environmental stress and particularly transgenic cell lines engineered to produce key pharmaceutical proteins.

Cloning and identification of the promoter of the tobacco Sar8.2b gene, a gene involved in systemic acquired resistance

Expression of the Sar8.2 gene family is induced by salicylic acid (SA) in tobacco during induction of systemic acquired resistance. Expression of Sar8.2b, one member of this 12-member family, was detected as early as 12 h after treatment with SA and was maximal 36 h after SA treatment. In NahG transgenic tobacco plants, benzothiadiazole and dichloroisonicotinic acid induced expression of Sar8.2b but SA did not, suggesting that expression of the Sar8.2b gene is SA-dependent. Several putative cis-acting elements were found in the Sar8.2b gene promoter region, including an as-1 element and GT-1 and Dof binding sequences. We constructed a series of progressive deletion mutations in the Sar8.2b promoter region linked to the beta-glucuronidase (GUS) coding region and analyzed GUS activities by stable expression in transformants of Arabidopsis thaliana. Deletions between -728 and -927 bp or between -351 and -197 bp of the promoter region resulted in a significant reduction in GUS activity induced by SA treatment as shown in stable transformants of A. thaliana. The -197 bp fragment of the promoter region was found to confer a relatively low level of GUS activity induced by SA treatment in stable expression of transformants in A. thaliana. The results suggest that 927 bp of the Sar8.2b gene promoter confers full promoter activity and that cis-acting elements required for high-level SA-inducible expression of the Sar8.2b gene may exist within the regions -728 to -927 bp and -197 to -351 bp.

A wound-inducible tobacco peroxidase gene expresses preferentially in the vascular system

A tobacco peroxidase gene tpoxN1 was reported to be expressed within 1 h after wounding in leaves [Hiraga et al. (2000a) Plant Cell Physiol. 41: 165]. We describe here further results on the wound-induced tpoxN1 expression. The quick tpoxN1 induction occurred preferentially in stems and petioles, but was negligible in leaf blades even 8 h after wounding. Induced GUS activity was also detected rapidly after wounding in the stem of transgenic tobacco plants carrying the tpoxN1 promoter::GUS fusion gene, localized mainly in the vascular systems where it was maintained this level for 14 d or more. Strong GUS activity was also found in the petiole and veinlet as well as the epidermal tissue in the stem. Treatment of known inducers for wound-responsive genes such as jasmonate, 1-aminocyclopropane-1-carboxylate, spermine, phytohormones and other stress treatments did not enhance wound-induced tpoxN1 gene expression in stems at all, but rather repressed it in some cases. Studies using metabolic inhibitors suggested that phosphorylation and dephosphorylation of proteins together with de novo protein synthesis are likely to be involved in the wound-induced tpoxN1 expression as well as some other wound-responsive genes. Thus, tpoxN1 is a unique wound-inducible and possible wound-healing gene which is rapidly expressed being maintained for a long time in veins via an unknown wound-signaling pathway(s).

Involvement of the Arabidopsis alpha-DOX1 fatty acid dioxygenase in protection against oxidative stress and cell death

alpha-dioxygenases (alpha-DOXs) catalyze the primary oxygenation of fatty acids into a newly identified group of oxylipins. Here we show that expression of the Arabidopsis alpha-DOX1 gene is induced in response to both incompatible and compatible bacterial infections. However, the level of alpha-DOX1 mRNA and dioxygenase activity appears earlier and reaches higher values when infection promotes a hypersensitive reaction. Furthermore, whereas gene expression is confined to necrotic lesions during the hypersensitive response, it occurs throughout the chlorotic area during a compatible interaction. Accumulation of alpha-DOX1 transcripts is impaired in SA-compromised plants and induced by SA and by chemicals generating nitric oxide (NO), intracellular superoxide or singlet oxygen, three signals mediating host cell death. Transgenic plants with altered levels of alpha-dioxygenase react like wild-type plants to a compatible pathogen. In contrast, plants with reduced activity develop a more rapid and severe necrotic response than wild-type plants to incompatible bacteria and paraquat treatment, respectively, and a milder response when alpha-DOX1 is overproduced. Our results suggest that plant alpha-dioxygenases are used to generate lipid-derived molecules for a process that protects plant tissues from oxidative stress and cell death.

Molecular cloning and characterization of a cDNA for a novel ethylene receptor, NT-ERS1, of tobacco (Nicotiana tabacum L.)

The cDNA encoding a novel member (NT-ERS1) of ethylene receptor family of tobacco (Nicotiana tabacum L.) was obtained by a combination of RT-PCR and 5'-/3'-RACE cloning. The cDNA was 2,092 nucleotides long and had an open reading frame of 1,911 bp encoding 637 amino acids. The deduced polypeptide lacked a response regulator domain, indicating that the ethylene receptor belongs to an ERS-group. The amino acid sequence was similar to respective members of the tobacco ethylene receptor family: 67.8% to NT-ETR1, 39.1% to NTHK1 and 31.1% to NTHK2. Comparison of amino acid sequence suggested that NT-ERS1 is the counterpart of Nr in the ethylene receptor family of tomato, which belongs to Solanaceae as does tobacco. Northern blot analysis showed that mRNA of NT-ERS1 was present in leaf, shoot and root tissues, and accumulated in leaves treated with exogenous ethylene. A mutated NT-ERS1 cDNA transgene, obtained by introducing one nucleotide substitution into NT-ETR1 cDNA, conferred ethylene insensitivity in tobacco plants, indicating that the translation product of the cDNA actually functioned in the plants.

Local and systemic induction of two defense-related subtilisin-like protease promoters in transgenic Arabidopsis plants. Luciferin induction of PR gene expression

Following a pathogenic attack, plants are able to mount a defense response with the coordinated activation of a battery of defense-related genes. In this study we have characterized the mode of expression of the P69B and P69C genes from tomato (Lycopersicon esculentum Mill.), which encodes two closely related subtilisin-like proteases associated with the defense response. We have compared the mode of gene regulation in heterologous transgenic Arabidopsis plants harboring promoter-beta-glucuronidase (GUS) and promoter-luciferase (LUC) gene fusions for these two genes. These studies revealed that the P69B and P69C promoters are induced by salicylic acid as well as during the course of both a compatible and an incompatible interaction with Pseudomonas syringae. Furthermore, P69B and P69C expression takes place in both the local and the distal (noninoculated) leaves upon inoculation with bacteria but following different and unique tissue-specific patterns of expression that are also different to that described for most other classical PR genes. Also, we report that luciferin, the substrate for the reporter luciferase (LUC) gene, is able to activate expression of PR genes, and this may pose a problem when using this gene reporter system in studies related to plant defense.

Induced expression of sarcotoxin IA enhanced host resistance against both bacterial and fungal pathogens in transgenic tobacco

We demonstrate here that induced expression of sarcotoxin IA, a bactericidal peptide from Sarcophaga peregrina, enhanced the resistance of transgenic tobacco plants to both bacterial and fungal pathogens. The peptide was produced with a modified PR1a promoter, which is further activated by salicylic acid treatment and necrotic lesion formation by pathogen infection. Host resistance to infection of bacteria Erwinia carotovora subsp. carotovora and Pseudomonas syringae pv. tabaci was shown to be dependent on the amounts of sarcotoxin IA expressed. Since we found antifungal activity of the peptide in vitro, transgenic seedlings were also inoculated with fungal pathogens Rhizoctonia solani and Pythium aphanidermatum. Transgenic plants expressing higher levels of sarcotoxin were able to withstand fungal infection and remained healthy even after 4 weeks, while control plants were dead by fungal infection after 2 weeks.

Ozone sensitivity in hybrid poplar correlates with insensitivity to both salicylic acid and jasmonic acid. The role of programmed cell death in lesion formation

Our earlier studies demonstrated that the ozone-sensitive hybrid poplar clone NE-388 displays an attenuated level of ozone-, wound-, and phytopathogen-induced defense gene expression. To determine if this reduced gene activation involves signal transduction pathways dependent on salicylic acid (SA) and/or jasmonic acid (JA), we compared the responses of NE-388 and an ozone-tolerant clone, NE-245, to these signal molecules. JA levels increased in both clones in response to ozone, but only minimal increases in SA levels were measured for either clone. Treatment with SA and methyl jasmonate induced defense gene expression only in NE-245, indicating that NE-388 is insensitive to these signal molecules. DNA fragmentation, an indicator of programmed cell death (PCD), was detected in NE-245 treated with either ozone or an avirulent phytopathogen, but was not detected in NE-388. We conclude that these clones undergo two distinct mechanisms of ozone-induced lesion formation. In NE-388, lesions appear to be due to toxic cell death resulting from a limited ability to perceive and subsequently activate SA- and/or JA-mediated antioxidant defense responses. In NE-245, SA-dependent PCD precedes lesion formation via a process related to the PCD pathway activated by phytopathogenic bacteria. These results support the hypothesis that ozone triggers a hypersensitive response.

Transgene expression systems in plant, a natural bioreactor

Plants are important resources that have been providing us food from the earliest times. The rapid advances that have taken place in plant genetic engineering have made it possible to modify plants to increase food production and contribute to environmental purification. Transgenic plants are gaining increasing attention from the industry as a natural bioreactor for the production of industrial and chemical products. Useful expression systems based on promoters to optimize transgene expression in plant cells, hold the key to maximizing the potential of this concept of molecular-farming or industrial plants. This review, which is devoted to the use of plants for heterologous protein production, is divided into three parts. First, we introduce the nature of plant promoters and strategies for the isolation of novel promoters. In the second part, various promoters showing high-level constitutive, organ-specific, or inducible expression, are summarized as useful tools for realizing the efficient transcription of transgenes. Finally, problems in the expression of foreign gene in plant cells and future prospects in plant biotechnology are discussed.

Requirement of functional ethylene-insensitive 2 gene for efficient resistance of Arabidopsis to infection by Botrytis cinerea

Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola results in systemic induction of genes encoding a plant defensin (PDF1.2), a basic chitinase (PR-3), and an acidic hevein-like protein (PR-4). Pathogen-induced induction of these three genes is almost completely abolished in the ethylene-insensitive Arabidopsis mutant ein2-1. This indicates that a functional ethylene signal transduction component (EIN2) is required in this response. The ein2-1 mutants were found to be markedly more susceptible than wild-type plants to infection by two different strains of the gray mold fungus Botrytis cinerea. In contrast, no increased fungal colonization of ein2-1 mutants was observed after challenge with avirulent strains of either Peronospora parasitica or A. brassicicola. Our data support the conclusion that ethylene-controlled responses play a role in resistance of Arabidopsis to some but not all types of pathogens.

Spermine is a salicylate-independent endogenous inducer for both tobacco acidic pathogenesis-related proteins and resistance against tobacco mosaic virus infection

Intercellular spaces are often the first sites invaded by pathogens. In the spaces of tobacco mosaic virus (TMV)-infected and necrotic lesion-forming tobacco (Nicotiana tabacum L.) leaves, we found that an inducer for acidic pathogenesis-related (PR) proteins was accumulated. The induction activity was recovered in gel-filtrated fractions of low molecular mass with a basic nature, into which authentic spermine (Spm) was eluted. We quantified polyamines in the intercellular spaces of the necrotic lesion-forming leaves and found 20-fold higher levels of free Spm than in healthy leaves. Among several polyamines tested, exogenously supplied Spm induced acidic PR-1 gene expression. Immunoblot analysis showed that Spm treatment increased not only acidic PR-1 but also acidic PR-2, PR-3, and PR-5 protein accumulation. Treatment of healthy tobacco leaves with salicylic acid (SA) caused no significant increase in the level of endogenous Spm, and Spm did not increase the level of endogenous SA, suggesting that induction of acidic PR proteins by Spm is independent of SA. The size of TMV-induced local lesions was reduced by Spm treatment. These results indicate that Spm accumulates outside of cells after lesion formation and induces both acidic PR proteins and resistance against TMV via a SA-independent signaling pathway.

An as-1-like motif controls the level of expression of the gene for the pathogenesis-related protein 1a from tobacco

Pathogenesis-related proteins of group 1 (PR-1) are strongly induced in plants by pathogen attack, exposure of the plants to (acetyl)salicylic acid (ASA, SA), and by developmental cues. Functional analysis of the PR-1a promoter identified a region of 139 bp (from -691 to -553) mediating expression of the GUS reporter gene in response to ASA. Inspection of this region revealed two TGACG elements reminiscent of activation sequence-1 (as-1). Recently, as-1 has been reported to be responsive to SA in the context of the CaMV 35S RNA promoter. To address the question of whether the as-1-like sequence may be of functional significance for the expression of the PR-1a gene, gel shift assays were performed with TGA1a, a protein been shown to interact with as-1 in vitro. TGA1a was found to bind to the PR-1a as-1-like sequence with similar specificity and affinity as to as-1. Furthermore, mutations were introduced in the as-1-like sequence in the context of the inducible 906 bp PR-1a promoter which are impaired in binding TGA1a in vitro. Significantly reduced levels of GUS reporter gene activity were obtained with the mutant promoter regions as compared to the wild-type PR-1a promoter in response to all stimuli in transgenic tobacco plants. Yet, mutation of the as-1-like sequence did not abolish induction of reporter gene expression. Taken together, these results suggest that the level of expression of the tobacco PR-1a gene is controlled by an as-1-like sequence motif in the PR-1a upstream region, possibly interacting with a factor related to TGA1a.

Mapping the elicitor and necrotic sites of Phytophthora elicitins with synthetic peptides and reporter genes controlled by tobacco defense gene promoters

Elicitins are 10-kDa proteins secreted by Phytophthora and Pythium fungi that elicit a hypersensitive-like necrotic reaction, leading to resistance against fungal and bacterial plant pathogens. Induction of necrosis and resistance were previously shown to be borne by different sites of the molecule. Furthermore, sequence comparison indicated several potential residues necessary for necrosis. The role of one of these residues was previously evidenced with site-directed mutagenesis. In order to locate other necrosis-determining sites and reveal the defense-eliciting sites, we synthesized a series of synthetic peptides. Tests were performed on two types of transgenic tobacco plants, both transformed with a construction containing the beta-glucuronidase reporter gene, in one case controlled by the promoter of the multiple stimulus response gene str 246C and in the other by the promoter of the pathogenesis-related gene PR1a. We report that only certain peptides were found to be active. Whereas PR1a induction was consistently correlated with induction of necrosis, four peptides were observed to induce only str 246C expression without necrosis, which led to differentiate the defense-eliciting sites from the necrotic sites. From the structure-function relationship thus obtained, two different defense pathways were inferred to be independently induced by elicitins.

Two PR-1 genes from tomato are differentially regulated and reveal a novel mode of expression for a pathogenesis-related gene during the hypersensitive response and development

Pathogenesis-related (PR) proteins form a heterogeneous family of plant proteins that are likely to be involved in defense and are inducible by pathogen attacks. One group of PRs, represented by the subfamily PR-1, are low-molecular-weight proteins of unknown biochemical function. Here we describe the cloning and characterization of two closely related genes encoding a basic and an acidic PR-1 protein (PR1b1 and PR1a2) from tomato (Lycopersicon esculentum). We present a comparative study of the mode of transcriptional regulation of these two genes in transgenic tobacco plants using a series of promoter-GUS fusions. Unexpectedly, the chimeric PR1a2/GUS gene is not induced by pathogenic signals but instead shows constitutive expression with a reproducible developmental expression pattern. It is expressed in shoot meristems, trichomes, and cortical cells as well as in vascular and nearby tissues of the mature stem. This constitutive expression pattern may represent preemption of plant defenses against potential pathogens. Conversely, the chimeric PR1b1/GUS gene does not show any constitutive expression in the plant, but it is transcriptionally activated following pathogen attack. Upon infection by tobacco mosaic virus, the PR1b1 gene is strongly activated locally in tissues undergoing the hypersensitive response but not systemically in uninoculated tissues. Furthermore, its expression is induced by both salicylic acid and ethylene precursors, two signals that coexist and apparently mediate the activation of local defenses during the hypersensitive response. We speculate that the different mode of expression of the two genes presented here, together with that reported previously for the induction of other PR-1 genes in systemic, uninoculated tissues, may all be complementary and necessary for the plant to acquire an efficient refractory state to resist pathogen attacks.

CHEMICAL CONTROL OF GENE EXPRESSION

Promoters that respond to otherwise inactive chemicals will enhance the tools available for analyzing gene function in vivo and for altering defined traits of plants at will. Approaches to provide such tools have yielded plant promoters that respond to compounds activating defense genes. In addition, the transfer of regulatory elements from prokaryotes, insects, and mammals has opened new avenues to construct chemically inducible promoters that respond to signals normally not recognized by plants. This review describes results and applications of these two approaches.

The PR-1a promoter contains a number of elements that bind GT-1-like nuclear factors with different affinity

The 900 bp promoter region of the tobacco PR-1a gene was divided into eight fragments using PCR. The fragments were tested for their ability to bind to nuclear factors isolated from tobacco leaf. Band shift assays demonstrated that all but one of the fragments specifically interacted with nuclear proteins. From competition experiments it was determined that the same nuclear factors bind various promoter fragments with different affinity. Moreover, efficient competition with a synthetic tetramer of box II of the rbcS promoter indicated that GT-1-like nuclear factors are involved in these interactions. Furthermore, in comparison to extracts from untreated plants, nuclear protein preparations from tobacco mosaic virus-infected tobacco showed a reduced GT-1 binding activity. These results will be discussed in relation to induced PR-1a gene expression.

UV-B-Induced PR-1 Accumulation Is Mediated by Active Oxygen Species

Depletion of the stratospheric ozone layer may result in an increase in the levels of potentially harmful UV-B radiation reaching the surface of the earth. We have found that UV-B is a potent inducer of the plant pathogenesis-related protein PR-1 in tobacco leaves. UV-B fluences required for PR-1 accumulation are similar to those of other UV-B-induced responses. The UV-B-induced PR-1 accumulation was confined precisely to the irradiated area of the leaf but displayed no leaf tissue specificity. A study of some of the possible components of the signal transduction pathway between UV-B and PR-1 induction showed that photosynthetic processes are not essential, and photoreversible DNA damage is not involved. Antioxidants and cycloheximide were able to block the induction of PR-1 by UV-B, and treatment of leaves with a generator of reactive oxygen resulted in the accumulation of PR-1 protein. These results demonstrate an absolute requirement for active oxygen species and protein synthesis in this UV-B signal transduction pathway. In contrast, we also show that other elicitors, notably salicylic acid, are able to elicit PR-1 via nonreactive oxygen species-requiring pathways.

UV-B-Induced PR-1 Accumulation Is Mediated by Active Oxygen Species

Depletion of the stratospheric ozone layer may result in an increase in the levels of potentially harmful UV-B radiation reaching the surface of the earth. We have found that UV-B is a potent inducer of the plant pathogenesis-related protein PR-1 in tobacco leaves. UV-B fluences required for PR-1 accumulation are similar to those of other UV-B-induced responses. The UV-B-induced PR-1 accumulation was confined precisely to the irradiated area of the leaf but displayed no leaf tissue specificity. A study of some of the possible components of the signal transduction pathway between UV-B and PR-1 induction showed that photosynthetic processes are not essential, and photoreversible DNA damage is not involved. Antioxidants and cycloheximide were able to block the induction of PR-1 by UV-B, and treatment of leaves with a generator of reactive oxygen resulted in the accumulation of PR-1 protein. These results demonstrate an absolute requirement for active oxygen species and protein synthesis in this UV-B signal transduction pathway. In contrast, we also show that other elicitors, notably salicylic acid, are able to elicit PR-1 via nonreactive oxygen species-requiring pathways.

Immediate early transcription activation by salicylic acid via the cauliflower mosaic virus as-1 element

Transgenic tobacco plants carrying a number of regulatory sequences derived from the cauliflower mosaic virus 35S promoter were tested for their response to treatment with salicylic acid (SA), an endogenous signal involved in plant defense responses. beta-Glucuronidase (GUS) gene fusions with the full-length (-343 to +8) 35S promoter or the -90 truncation were found to be induced by SA. Time course experiments revealed that, in the continuous presence of SA, the -90 promoter construct (-90 35S-GUS) displayed rapid and transient induction kinetics, with maximum RNA levels at 1 to 4 hr, which declined to low levels by 24 hr. Induction was still apparent in the presence of the protein synthesis inhibitor cycloheximide (CHX). Moreover, mRNA levels continued to accumulate over 24 hr rather than to decline. By contrast, mRNA from the endogenous pathogenesis-related protein-1a (PR-1a) gene began to accumulate at later times during SA treatment and steadily increased through 24 hr; transcription of this gene was almost completely blocked by the presence of CHX. Further dissection of the region from -90 and -46 of the 35S promoter revealed that the SA-responsive element corresponds to the previously characterized activation sequence-1 (as-1). These results represent a definitive analysis of immediate early responses to SA, relative to the late induction of PR genes, and potentially elucidate the early events of SA signal transduction during the plant defense response.

Immediate early transcription activation by salicylic acid via the cauliflower mosaic virus as-1 element

Transgenic tobacco plants carrying a number of regulatory sequences derived from the cauliflower mosaic virus 35S promoter were tested for their response to treatment with salicylic acid (SA), an endogenous signal involved in plant defense responses. beta-Glucuronidase (GUS) gene fusions with the full-length (-343 to +8) 35S promoter or the -90 truncation were found to be induced by SA. Time course experiments revealed that, in the continuous presence of SA, the -90 promoter construct (-90 35S-GUS) displayed rapid and transient induction kinetics, with maximum RNA levels at 1 to 4 hr, which declined to low levels by 24 hr. Induction was still apparent in the presence of the protein synthesis inhibitor cycloheximide (CHX). Moreover, mRNA levels continued to accumulate over 24 hr rather than to decline. By contrast, mRNA from the endogenous pathogenesis-related protein-1a (PR-1a) gene began to accumulate at later times during SA treatment and steadily increased through 24 hr; transcription of this gene was almost completely blocked by the presence of CHX. Further dissection of the region from -90 and -46 of the 35S promoter revealed that the SA-responsive element corresponds to the previously characterized activation sequence-1 (as-1). These results represent a definitive analysis of immediate early responses to SA, relative to the late induction of PR genes, and potentially elucidate the early events of SA signal transduction during the plant defense response.

Developmental, hormonal, and pathogenesis-related regulation of the tobacco class I beta-1,3-glucanase B promoter

The class I beta-1,3-glucanases are antifungal vacuolar proteins implicated in plant defense that show developmental, hormonal, and pathogenesis-related regulation. The tobacco enzymes are encoded by a small gene family with members derived from ancestors related to the present-day species Nicotiana sylvestris and N. tomentosiformis. We studied the expression in transgenic tobacco plants of a chimeric beta-glucuronidase (GUS) reporter gene fused to 1.6 kb of upstream sequence of the tobacco class I beta-1,3-glucanase B (GLB) gene, which is of N. tomentosiformis origin. Expression of the GUS reporter gene and the accumulation of class I beta-1,3-glucanase and its mRNA showed very similar patterns of regulation. In young seedlings the reporter gene was expressed in the roots. In mature tobacco plants it was preferentially expressed in lower leaves and roots and was induced in leaves by ethylene treatment and by infection with tobacco mosaic virus (TMV). Furthermore, it was down-regulated in cultured leaf discs by combinations of the hormones auxin and cytokinin. Histological studies of GUS activity showed that the GLB promoter shows highly localized expression in roots of seedlings. It is also expressed in a ring of cells around necrotic lesions induced by TMV infection, but not in cells immediately adjacent to the lesions or in the lesions themselves. The results of deletion analyses suggest that multiple positive and negative elements in the GLB promoter regulate its activity. The region from -1452 to -1193 containing two copies of the heptanucleotide AGCCGCC, which is highly conserved in plant-stress and defense-related genes, is necessary for high level expression in leaves. Additional regions important for organ-specific and regulated expression were: -568 to -402 for ethylene induction of leaves; -402 to -211 for expression in lower leaves and cultured leaf discs and for TMV induction of leaves; and -211 to -60 for expression in roots.