Salicylic acid is a systemic signal and an inducer of pathogenesis-related proteins in virus-infected tobacco

Effect of Temperature Regime and Growth Stage Interaction on Pattern of Virus Presence in TSWV-Resistant Accessions of Capsicum chinense

We studied the resistance to tomato spotted wilt virus in plant introduction accession (PI)-151225 and PI-159236 under Mediterranean climatic conditions. Two temperature regimes were utilized, corresponding to early and late cultivation cycles. Inoculations were made at 2- and 4-leaf stages to determine the effect of early infection. The existence of interaction between temperature regime and developmental stage was also studied. When plants of both PIs were maintained at 30/18°C (day/night), all plants at both growth stages when inoculated developed systemic infection. At 25/18°C, only those plants inoculated at 2-leaf stage became systematically infected; however, those inoculated at the 4-leaf stage behaved as resistant. Thus, there was an interaction between temperature regime and growth stage. There is potential for using this type of resistance in areas with mild climates, providing seedling infections are avoided.

Intermediates of salicylic acid biosynthesis in tobacco

Salicylic acid (SA) is an important component of systemic-acquired resistance in plants. It is synthesized from benzoic acid (BA) as part of the phenylpropanoid pathway. Benzaldehyde (BD), a potential intermediate of this pathway, was found in healthy and tobacco mosaic virus (TMV)-inoculated tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaf tissue at 100 ng/g fresh weight concentrations as measured by gas chromatography-mass spectrometry. BD was also emitted as a volatile organic compound from tobacco tissues. Application of gaseous BD to plants enclosed in jars caused a 13-fold increase in SA concentration, induced the accumulation of the pathogenesis-related transcript PR-1, and increased the resistance of tobacco to TMV inoculation. [13C6]BD and [2H5]benzyl alcohol were converted to BA and SA. Labeling experiments using [13C1]Phe in temperature-shifted plants inoculated with the TMV showed high enrichment of cinnamic acids (72%), BA (34%), and SA (55%). The endogenous BD, however, contained nondetectable enrichment, suggesting that BD was not the intermediate between cinnamic acid and BA. These results show that BD and benzyl alcohol promote SA accumulation and expression of defense responses in tobacco, and provide insight into the early steps of SA biosynthesis.

Four classes of salicylate-induced tobacco genes

We have identified and characterized fragments of 15 salicylic acid (SA) early response genes. The kinetics of induction and response to cycloheximide (CHX) treatment allowed classification of genes into four groups. Classes I-III are characterized by immediate-early responses, showing increased accumulation of mRNA within 30 min of SA treatment. Moreover, CHX did not block induction of these genes, indicating that latent cellular factors mediate the SA response. Class IV genes were induced more slowly, but still within 2 to 3 h of SA treatment, and required protein synthesis for expression. Although identified in this study as SA-responsive genes, several could also be induced by other compounds. Two genes were characterized in more detail, including isolation of cDNA sequences and additional analysis of gene expression. Sequence analysis revealed that the class I gene, C18-1, is the previously identified ethylene response element binding protein 1 (EREBP1), an ethylene-induced transcription factor for basic pathogenesis-related (PR) genes, whereas the class III gene, G8-1, is a novel sequence. G8-1 was found to be strongly induced only by SA and its active analogs and was exquisitely sensitive to low SA concentrations. These and other genes were found to be activated at early times following tobacco mosaic virus infection of resistant tobacco genotypes.

SA, JA, ethylene, and disease resistance in plants

Exciting advances have been made during the past year: isolating mutants affecting plant disease resistance, cloning genes involved in the regulation of various defense responses, and characterizing novel defense signaling pathways. Recent studies have demonstrated that jasmonic acid and ethylene are important for the induction of nonspecific disease resistance through signaling pathways that are distinct from the classical systemic acquired resistance response pathway regulated by salicylic acid.

Glucosylation of Salicylic Acid in Nicotiana tabacum Cv. Xanthi-nc

ABSTRACT Salicylic acid (SA) is a key regulatory component of disease resistance in plants. In tobacco mosaic virus (TMV)-inoculated tobacco (Nicotiana tabacum cv. Xanthi-nc NN genotype), newly synthesized SA is converted primarily to SA 2-O-beta-D-glucoside (SAG) and glucosyl salicylate (GS), a relatively minor metabolite. Similar patterns in the formation of GS and SAG were observed in tobacco inoculated with Pseudomonas syringae pv. phaseolicola, suggesting the accumulation of two glucosylated metabolites is a general phenomenon in tobacco plants. After SA infiltration, GS was synthesized rapidly, reached a maximal level at 6 h, declined, and remained relatively constant for at least 24 h. In contrast, SAG content increased gradually after SA treatment. Our in vitro and in vivo data suggest that a high concentration of free SA triggers transient formation of GS and continuous accumulation of SAG, which is a more stable metabolite of SA. The two distinct SA glucosyltransferases catalyzed the formation of GS and SAG, respectively. The activities of these enzymes were enhanced by TMV or P. syringae pv. phaseolicola inoculation or SA treatment and were found in different fractions of gel filtration chromatography.

Generation of broad-spectrum disease resistance by overexpression of an essential regulatory gene in systemic acquired resistance

The recently cloned NPR1 gene of Arabidopsis thaliana is a key regulator of acquired resistance responses. Upon induction, NPR1 expression is elevated and the NPR1 protein is activated, in turn inducing expression of a battery of downstream pathogenesis-related genes. In this study, we found that NPR1 confers resistance to the pathogens Pseudomonas syringae and Peronospora parasitica in a dosage-dependent fashion. Overexpression of NPR1 leads to enhanced resistance with no obvious detrimental effect on the plants. Thus, for the first time, a single gene is shown to be a workable target for genetic engineering of nonspecific resistance in plants.

Uncoupling PR gene expression from NPR1 and bacterial resistance: characterization of the dominant Arabidopsis cpr6-1 mutant

In Arabidopsis, NPR1 mediates the salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance (SAR). Here, we report the identification of another component, CPR 6, that may function with NPR1 in regulating PR gene expression. The dominant CPR 6-1 mutant expresses the SA/NPR1-regulated PR genes (PR-1, BGL 2, and PR-5) and displays enhanced resistance to Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2 in the absence of SAR induction. cpr 6-1-induced PR gene expression is not suppressed in the cpr 6-1 npr1-1 double mutant but is suppressed when SA is removed by salicylate hydroxylase. Thus, constitutive PR gene expression in cpr 6-1 requires SA but not NPR1. In addition, resistance to P. s. maculicola ES4326 is suppressed in the cpr 6-1 npr1-1 double mutant, despite expression of PR-1, BGL 2, and PR-5. Resistance to P. s. maculicola ES4326 must therefore be accomplished through unidentified antibacterial gene products that are regulated through NPR1. These results show that CPR 6 is an important regulator of multiple signal transduction pathways involved in plant defense.

Induction of Arabidopsis tryptophan pathway enzymes and camalexin by amino acid starvation, oxidative stress, and an abiotic elicitor

The tryptophan (Trp) biosynthetic pathway leads to the production of many secondary metabolites with diverse functions, and its regulation is predicted to respond to the needs for both protein synthesis and secondary metabolism. We have tested the response of the Trp pathway enzymes and three other amino acid biosynthetic enzymes to starvation for aromatic amino acids, branched-chain amino acids, or methionine. The Trp pathway enzymes and cytosolic glutamine synthetase were induced under all of the amino acid starvation test conditions, whereas methionine synthase and acetolactate synthase were not. The mRNAs for two stress-inducible enzymes unrelated to amino acid biosynthesis and accumulation of the indolic phytoalexin camalexin were also induced by amino acid starvation. These results suggest that regulation of the Trp pathway enzymes under amino acid deprivation conditions is largely a stress response to allow for increased biosynthesis of secondary metabolites. Consistent with this hypothesis, treatments with the oxidative stress-inducing herbicide acifluorfen and the abiotic elicitor alpha-amino butyric acid induced responses similar to those induced by the amino acid starvation treatments. The role of salicylic acid in herbicide-mediated Trp and camalexin induction was investigated.

Induction of Arabidopsis tryptophan pathway enzymes and camalexin by amino acid starvation, oxidative stress, and an abiotic elicitor

The tryptophan (Trp) biosynthetic pathway leads to the production of many secondary metabolites with diverse functions, and its regulation is predicted to respond to the needs for both protein synthesis and secondary metabolism. We have tested the response of the Trp pathway enzymes and three other amino acid biosynthetic enzymes to starvation for aromatic amino acids, branched-chain amino acids, or methionine. The Trp pathway enzymes and cytosolic glutamine synthetase were induced under all of the amino acid starvation test conditions, whereas methionine synthase and acetolactate synthase were not. The mRNAs for two stress-inducible enzymes unrelated to amino acid biosynthesis and accumulation of the indolic phytoalexin camalexin were also induced by amino acid starvation. These results suggest that regulation of the Trp pathway enzymes under amino acid deprivation conditions is largely a stress response to allow for increased biosynthesis of secondary metabolites. Consistent with this hypothesis, treatments with the oxidative stress-inducing herbicide acifluorfen and the abiotic elicitor alpha-amino butyric acid induced responses similar to those induced by the amino acid starvation treatments. The role of salicylic acid in herbicide-mediated Trp and camalexin induction was investigated.

Plant resistance to fungal infection induced by nontoxic pokeweed antiviral protein mutants

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytolacca americana inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic ribosomes. Transgenic plants expressing PAP are resistant to a broad spectrum of plant viruses. Nontoxic PAP mutants have been isolated by random mutagenesis and selection in yeast. One of these mutants, PAP-X, had a point mutation at the active-site (E176V) that abolished enzymatic activity, and another mutant, delta C25PAP, had a nonsense mutation near the C-terminus (W237stop) that deleted 25 C-terminal amino acids. Unlike the wild-type PAP, expression of neither mutant was toxic to transgenic plants. We show that both class I (basic) and class II (acidic) isoforms of pathogenesis-related (PR) proteins are overexpressed in transgenic plants expressing PAP and the nontoxic PAP mutants. Although PR-proteins are constitutively expressed, no increase in salicylic acid levels was detected. Homozygous progeny of transgenic plants expressing either PAP or the nontoxic PAP mutants displayed resistance to the fungal pathogen Rhizoctonia solani. These results show that expression of PAP or the nontoxic PAP mutants activates multiple plant defense pathways independently of salicylic acid and confers resistance to fungal infection. The C-terminal 25 amino acids of PAP, which are required for toxicity in vivo, are not critical for resistance to viral or fungal infection, indicating that toxicity of PAP can be separated from pathogen resistance.

The cpr5 mutant of Arabidopsis expresses both NPR1-dependent and NPR1-independent resistance

The cpr5 mutant was identified from a screen for constitutive expression of systemic acquired resistance (SAR). This single recessive mutation also leads to spontaneous expression of chlorotic lesions and reduced trichome development. The cpr5 plants were found to be constitutively resistant to two virulent pathogens, Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2; to have endogenous expression of the pathogenesis-related gene 1 (PR-1); and to have an elevated level of salicylic acid (SA). Lines homozygous for cpr5 and either the SA-degrading bacterial gene nahG or the SA-insensitive mutation npr1 do not express PR-1 or exhibit resistance to P. s. maculicola ES4326. Therefore, we conclude that cpr5 acts upstream of SA in inducing SAR. However, the cpr5 npr1 plants retained heightened resistance to P. parasitica Noco2 and elevated expression of the defensin gene PDF1.2, implying that NPR1-independent resistance signaling also occurs. We conclude that the cpr5 mutation leads to constitutive expression of both an NPR1-dependent and an NPR1-independent SAR pathway. Identification of this mutation indicates that these pathways are connected in early signal transduction steps and that they have overlapping functions in providing resistance.

The cpr5 mutant of Arabidopsis expresses both NPR1-dependent and NPR1-independent resistance

The cpr5 mutant was identified from a screen for constitutive expression of systemic acquired resistance (SAR). This single recessive mutation also leads to spontaneous expression of chlorotic lesions and reduced trichome development. The cpr5 plants were found to be constitutively resistant to two virulent pathogens, Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2; to have endogenous expression of the pathogenesis-related gene 1 (PR-1); and to have an elevated level of salicylic acid (SA). Lines homozygous for cpr5 and either the SA-degrading bacterial gene nahG or the SA-insensitive mutation npr1 do not express PR-1 or exhibit resistance to P. s. maculicola ES4326. Therefore, we conclude that cpr5 acts upstream of SA in inducing SAR. However, the cpr5 npr1 plants retained heightened resistance to P. parasitica Noco2 and elevated expression of the defensin gene PDF1.2, implying that NPR1-independent resistance signaling also occurs. We conclude that the cpr5 mutation leads to constitutive expression of both an NPR1-dependent and an NPR1-independent SAR pathway. Identification of this mutation indicates that these pathways are connected in early signal transduction steps and that they have overlapping functions in providing resistance.

Expression of Pokeweed Antiviral Protein in Transgenic Plants Induces Virus Resistance in Grafted Wild-Type Plants Independently of Salicylic Acid Accumulation and Pathogenesis-Related Protein Synthesis

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytolacca americana, inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic ribosomes. Transgenic tobacco (Nicotiana tabacum) plants expressing PAP or a variant (PAP-v) were shown to be resistant to a broad spectrum of plant viruses. Expression of PAP-v in transgenic plants induces synthesis of pathogenesis-related proteins and a very weak (<2-fold) increase in salicylic acid levels. Using reciprocal grafting experiments, we demonstrate here that transgenic tobacco rootstocks expressing PAP-v induce resistance to tobacco mosaic virus infection in both N. tabacum NN and nn scions. Increased resistance to potato virus X was also observed in N. tabacum nn scions grafted on transgenic rootstocks. PAP expression was not detected in the wild-type scions or rootstocks that showed virus resistance, nor was there any increase in salicylic acid levels or pathogenesis-related protein synthesis. Grafting experiments with transgenic plants expressing an inactive PAP mutant demonstrated that an intact active site of PAP is necessary for induction of virus resistance in wild-type scions. These results indicate that enzymatic activity of PAP is responsible for generating a signal that renders wild-type scions resistant to virus infection in the absence of increased salicylic acid levels and pathogenesis-related protein synthesis.

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.

PROGRAMMED CELL DEATH IN PLANT-PATHOGEN INTERACTIONS

Plants cope with pathogen attacks by using mechanisms of resistance that rely both on preformed protective defenses and on inducible defenses. The latter are the most well studied, and progress is being made in determining which induced responses are responsible for limiting pathogen growth. Many plant-pathogen interactions are accompanied by plant cell death. Recent evidence suggests that this cell death is often programmed and results from an active process on the part of the host. The review considers the roles and possible mechanisms of plant cell death in response to pathogens.

The Arabidopsis NPR1 gene that controls systemic acquired resistance encodes a novel protein containing ankyrin repeats

The Arabidopsis NPR1 gene controls the onset of systemic acquired resistance (SAR), a plant immunity, to a broad spectrum of pathogens that is normally established after a primary exposure to avirulent pathogens. Mutants with defects in NPR1 fail to respond to various SAR-inducing treatments, displaying little expression of pathogenesis-related (PR) genes and exhibiting increased susceptibility to infections. NPR1 was cloned using a map-based approach and was found to encode a novel protein containing ankyrin repeats. The lesion in one npr1 mutant allele disrupted the ankyrin consensus sequence, suggesting that these repeats are important for NPR1 function. Furthermore, transformation of the cloned wild-type NPR1 gene into npr1 mutants not only complemented the mutations, restoring the responsiveness to SAR induction with respect to PR-gene expression and resistance to infections, but also rendered the transgenic plants more resistant to infection by P. syringae in the absence of SAR induction.

Systemic acquired resistance

This paper examines induced resistance (SAR) in plants against various insect and pathogenic invaders. SAR confers quantitative protection against a broad spectrum of microorganisms in a manner comparable to immunization in mammals, although the underlying mechanisms differ. Discussed here are the molecular events underlying SAR: the mechanisms involved in SAR, including lignification and other structural barriers, pathogenesis-related proteins and their expression, and the signals for SAR including salicylic acid. Recent findings on the biological role of systemin, ethylene, jasmonates, and electrical signals are reviewed. Chemical activators of SAR comprise inorganic compounds, natural compounds, and synthetic compounds. Plants known to exhibit SAR and induced systemic resistance are listed.

Coordinate regulation of the tryptophan biosynthetic pathway and indolic phytoalexin accumulation in Arabidopsis

Little is known about the mechanisms that couple regulation of secondary metabolic pathways to the synthesis of primary metabolic precursors. Camalexin, an indolic secondary metabolite, appears to be the major phytoalexin in Arabidopsis. It was previously shown that camalexin accumulation is caused by infection with plant pathogens, by abiotic elicitors, and in spontaneous lesions in the accelerated cell death mutant acd2. We demonstrate that the accumulation of this phytoalexin is accompanied by the induction of the mRNAs and proteins for all of the tryptophan biosynthetic enzymes tested. A strong correlation was observed between the magnitude of camalexin accumulation and the induction of tryptophan biosynthetic proteins, indicating coordinate regulation of these processes. Production of disease symptoms is not sufficient for the response because systemic infection with cauliflower mosaic virus or cucumber mosaic virus did not induce the tryptophan pathway enzymes or camalexin accumulation. Salicylic acid appears to be required, but unlike other documented pathogenesis-related proteins, it is not sufficient for the coordinate induction. Results with trp mutants suggest that the tryptophan pathway is not rate limiting for camalexin accumulation. Taken together, these results are consistent with the hypothesis that the regulation of the tryptophan pathway in plants responds to needs for biosynthesis of secondary metabolites.

Transport of Salicylic Acid in Tobacco Necrosis Virus-Infected Cucumber Plants

The transport of salicylic acid (SA) was studied in cucumber (Cucumis sativus L.) using 14C-labeled benzoic acid that was injected in the cotyledons at the time of inoculation. Primary inoculation with tobacco necrosis virus (TNV) on the cotyledons led to an induction of systemic resistance of the first primary leaf above the cotyledon against Colletotrichum lagenarium as early as 3 d after inoculation. [14C]SA was detected in the phloem or in the first leaf 2 d after TNV inoculation, whereas [14C]benzoic acid was not detected in the phloem during the first 3 d after TNV inoculation of the cotyledons, indicating phloem transport of [14C]SA from cotyledon. In leaf 1, the specific activity of [14C]SA decreased between 1.7 and 8.6 times compared with the cotyledons, indicating that, in addition to transport, leaf 1 also produced more SA. The amount of SA transported after TNV infection of the cotyledon was 9 to 160 times higher than in uninfected control plants. Thus, SA can be transported to leaf 1 before the development of systemic acquired resistance, and SA accumulation in leaf 1 results both from transport from the cotyledon and from synthesis in leaf 1.

Identification of an immediate-early salicylic acid-inducible tobacco gene and characterization of induction by other compounds

Tobacco genes that are induced in response to salicylic acid (SA) treatment with immediate-early kinetics were identified by differential mRNA display. Detailed analysis of IS10a, one cDNA clone identified by this method, revealed induction within 30 min of treatment, with a peak of expression at 3 h, that decayed rapidly thereafter. Treatment with the protein synthesis inhibitor, cycloheximide (CHX), also caused induction of IS10a mRNA to comparable levels, but the IS10a mRNA continued to accumulate after 3 h of induction. In combination, CHX and SA led to a superinduction of IS10a mRNA levels that was also sustained. Half-maximal induction was evident at ca. 100-150 microM SA. In addition to SA, induction of IS10a occurred to varying degrees upon treatment with acetylsalicylic acid, benzoic acid, 2,4-dichlorophenoxyacetic acid, methyl jasmonate, and hydrogen peroxide, whereas treatment with other compounds had no effect. The proteins encoded by IS10a and a second highly homologous cDNA show sequence similarity to UDP-glucose: flavonoid glucosyltransferases.

Ozone-induced responses in Arabidopsis thaliana: the role of salicylic acid in the accumulation of defense-related transcripts and induced resistance

Exposure of Arabidopsis thaliana to ozone results in the expression of a number of defense-related genes that are also induced during a hypersensitive response. A potential common link between the activation of defense gene expression during a hypersensitive response and by ozone treatment is the production of active oxygen species and the accumulation of hydrogen peroxide. Here we report that salicylic acid accumulation, which can be induced by hydrogen peroxide and is required for the expression of both a hypersensitive response and systemic acquired resistance, is also required for the induction of some, but not all, ozone-induced mRNAs examined. In addition, we show that ozone exposure triggers induced resistance of A. thaliana to infection with virulent phytopathogenic Pseudomonas syringae strains. Infection of transgenic plants expressing salicylate hydroxylase, which prevents the accumulation of salicylic acid, or npr1 mutant plants, which are defective in the expression of systemic acquired resistance at a step downstream of salicylic acid, demonstrated that the signaling pathway activated during ozone-induced resistance overlaps with the systemic acquired resistance activation pathway and is salicylic acid dependent. Interestingly, plants expressing salicylate hydroxylase exhibited increased sensitivity to ozone exposure. These results demonstrate that ozone activates at least two distinct signaling pathways, including a salicylic acid dependent pathway previously shown to be associated with the activation of pathogen defense reactions, and that this latter pathway also induces a protective response to ozone.

Leaf senescence in Brassica napus: expression of genes encoding pathogenesis-related proteins

Genes that are expressed during leaf senescence in Brassica napus were identified by the isolation of representative cDNA clones. DNA sequence and deduced protein sequence from two senescence-related cDNAs, LSC94 and LSC222, representing genes that are expressed early in leaf senescence before any yellowing of the leaves is visible, showed similarities to genes for pathogenesis-related (PR) proteins: a PR-1a-like protein and a class IV chitinase, respectively. The LSC94 and LSC222 genes showed differential regulation with respect to each other; an increase in expression was detected at different times during development of healthy leaves. Expression of both genes was induced by salicylic acid treatment. These findings suggest that some PR genes, as well as being induced by pathogen infection, may have alternative functions during plant development, for example in the process of leaf senescence.

Suppression and Restoration of Lesion Formation in Arabidopsis lsd Mutants

Systemic acquired resistance (SAR) is a broad-spectrum, systemic defense response that is activated in many plant species after pathogen infection. We have previously described Arabidopsis mutants that constitutively express SAR and concomitantly develop lesions simulating disease (lsd). Here, we describe two new mutants, lsd6 and lsd7, that develop spontaneous necrotic lesions and possess elevated levels of salicylic acid (SA) as well as heightened disease resistance, similar to the previously characterized lsd and accelerated cell death (acd2) mutants. Genetic analysis of lsd6 and lsd7 showed that the mutant phenotypes segregated as simple dominant traits. When crossed with transgenic Arabidopsis plants containing the SA-degrading enzyme salicylate hydroxylase, the F1 progeny showed suppression of both SAR gene expression and resistance. In addition, salicylate hydroxylase suppressed lesion formation in the F1 progeny, suggesting that SA or some SA-dependent process may have a role in pathogen-associated cell death. Surprisingly, lesions were restored in the lsd6 F1 progeny after the application of either 2,6-dichloroisonicotinic acid or SA. Lesions were not restored by treatment with either compound in the lsd7 F1 plants. Our findings demonstrate that steps early in the signal transduction pathway leading to SAR and disease resistance are potentiated by later events, suggesting feedback control of lesion formation.

Benzoic acid 2-hydroxylase, a soluble oxygenase from tobacco, catalyzes salicylic acid biosynthesis

Benzoic acid 2-hydroxylase (BA2H) catalyzes the biosynthesis of salicylic acid from benzoic acid. The enzyme has been partially purified and characterized as a soluble protein of 160 kDa. High-efficiency in vivo labeling of salicylic acid with 18O2 suggested that BA2H is an oxygenase that specifically hydroxylates the ortho position of benzoic acid. The enzyme was strongly induced by either tobacco mosaic virus inoculation or benzoic acid infiltration of tobacco leaves and it was inhibited by CO and other inhibitors of cytochrome P450 hydroxylases. The BA2H activity was immunodepleted by antibodies raised against SU2, a soluble cytochrome P450 from Streptomyces griseolus. The anti-SU2 antibodies immunoprecipitated a radiolabeled polypeptide of around 160 kDa from the soluble protein extracts of L-[35S]-methionine-fed tobacco leaves. Purified BA2H showed CO-difference spectra with a maximum at 457 nm. These data suggest that BA2H belongs to a novel class of soluble, high molecular weight cytochrome P450 enzymes.

Is Salicylic Acid a Translocated Signal of Systemic Acquired Resistance in Tobacco?

Salicylic acid (SA) is a likely endogenous signal in the development of systemic acquired resistance (SAR) in some dicotyledonous plants. In tobacco mosaic virus (TMV)-resistant Xanthi-nc tobacco, SA levels increase systemically following the inoculation of a single leaf with TMV. To determine the extent to which systemic increases in SA result from SA export from the inoculated leaf, SA produced in TMV-inoculated or healthy leaves was noninvasively labeled with 18O2. Spatial and temporal distribution of 18O-SA indicated that most of the SA detected in the healthy tissues was synthesized in the inoculated leaf. No significant increase in the activity of benzoic acid 2-hydroxylase, the last enzyme involved in SA biosynthesis, was detected in upper uninoculated leaves, although the basal level of enzyme activity was relatively high. No increases in SA level, pathogenesis-related PR-1 gene expression, or TMV resistance in the upper uninoculated leaf were observed if the TMV-inoculated leaf was detached up to 60 hr after inoculation. Apart from the inoculated tissues, the highest increase in SA was observed in the leaf located directly above the inoculated leaf. The systemic SA increase observed during SAR may be explained by phloem transport of SA from the inoculation sites.

Hydrogen Peroxide Stimulates Salicylic Acid Biosynthesis in Tobacco

Hydrogen peroxide induced the accumulation of free benzoic acid (BA) and salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaves. Six hours after infiltration with 300 mM H2O2, the levels of BA and SA in leaves increased 5-fold over the levels detected in control leaves. The accumulation of BA and SA was preceded by the rapid activation of benzoic acid 2-hydroxylase (BA2H) in the H2O2-infiltrated tissues. This enzyme catalyzes the formation of SA from BA. Enzyme activation could be reproduced in vitro by addition of H2O2 or cumene hydroperoxide to the assay mixture. H2O2 was most effective in vitro when applied at 6 mM. In vitro activation of BA2H by peroxides was inhibited by the catalase inhibitor 3-amino-1,2,4-triazole. We suggest that H2O2 activates SA biosynthesis via two mechanisms. First, H2O2 stimulates BA2H activity directly or via the formation of its substrate, molecular oxygen, in a catalase-mediated reaction. Second, higher BA levels induce the accumulation of BA2H protein in the cells and provide more substrate for this enzyme.

Systemic Responses in Arabidopsis thaliana Infected and Challenged with Pseudomonas syringae pv syringae

Attack of plants by necrotizing pathogens leads to acquired resistance to the same or other pathogens in tissues adjacent to or remotely located from the site of initial attack. We have used Arabidopsis thaliana inoculated with the incompatible pathogen Pseudomonas syringae pv syringae on the lower leaves to test the induction of systemic reactions. When plants were challenged with Pseudomonas syringae pv syringae in the upper leaves, bacterial titers remained stable in those preinfected on the lower leaves. However, there was a distinct decrease in symptoms that correlated with a local and systemic increase in salicylic acid (SA) and in chitinase activity. Peroxidase activity only increased at the site of infection. No changes in catalase activity were observed, either at the local or at the systemic level. No inhibition of catalase could be detected in tissue in which the endogenous levels of SA were elevated either naturally (after infection) or artificially (after feeding SA to the roots). The activity of catalase in homogenates of A. thaliana leaves could not be inhibited in vitro by SA. SA accumulation was induced by H2O2 in leaves, suggesting a link between H2O2 from the oxidative burst commonly observed during the hypersensitive reaction and the induction of a putative signaling molecule leading to system acquired resistance.

Salicylic Acid Inhibits Synthesis of Proteinase Inhibitors in Tomato Leaves Induced by Systemin and Jasmonic Acid

Salicylic acid (SA) and acetylsalicylic acid (ASA), previously shown to inhibit proteinase inhibitor synthesis induced by wounding, oligouronides (H.M. Doherty, R.R. Selvendran, D.J. Bowles [1988] Physiol Mol Plant Pathol 33: 377-384), and linolenic acid (H. Pena-Cortes, T. Albrecht, S. Prat, E.W. Weiler, L. Willmitzer [1993] Planta 191: 123-128), are shown here to be potent inhibitors of systemin-induced and jasmonic acid (JA)-induced synthesis of proteinase inhibitor mRNAs and proteins. The inhibition by SA and ASA of proteinase inhibitor synthesis induced by systemin and JA, as well as by wounding and oligosaccharide elicitors, provides further evidence that both oligosaccharide and polypeptide inducer molecules utilize the octadecanoid pathway to signal the activation of proteinase inhibitor genes. Tomato (Lycopersicon esculentum) leaves were pulse labeled with [35S]methionine, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the inhibitory effects of SA are shown to be specific for the synthesis of a small number of JA-inducible proteins that includes the proteinase inhibitors. Previous results have shown that SA inhibits the conversion of 13S-hydroperoxy linolenic acid to 12-oxo-phytodienoic acid, thereby inhibiting the signaling pathway by blocking synthesis of JA. Here we report that the inhibition of synthesis of proteinase inhibitor proteins and mRNAs by SA in both light and darkness also occurs at a step in the signal transduction pathway, after JA synthesis but preceding transcription of the inhibitor genes.

DNA sequence analysis of a cyclophilin gene from maize: developmental expression and regulation by salicylic acid

In plants, such as maize, cyclophilin (Cyp) genes are expressed at a basal level in all tissues. Amounts of Cyp mRNA above the basic level are observed in germinating seedlings, in growing tissues/organs such as roots and leaf meristematic tissue of young maize plants, nodes and embryonic female inflorescences of adult plants and also in non-proliferating tissues such as the internodes of adult plants. Salicylic acid (SA) enhances the transcription of maize Cyp genes. The possible involvement of SA in the pathway leading to defense responses induced by abiotic stresses such as mercuric chloride treatment is discussed. A maize Cyp genomic clone isolated using a maize Cyp cDNA probe contains 737 bp of the 5' upstream and the entire coding region. This Cyp gene is not interrupted by intervening sequences. In the 5' upstream region, characteristic transcription signals as well as putative regulatory sequences were identified. Two TATA boxes are found at positions -56 bp and -66 bp with respect to the transcription start site. Two putative heat shock elements were identified in the promoter region; a metal regulatory element and a third heat shock element were localized in the 5' untranslated leader. Several putative polyadenylation signals and (G)T-rich sequence motifs were identified in the 3' untranslated region.

Major allergen Phl p Vb in timothy grass is a novel pollen RNase

A cDNA coding for the major group V allergen Phl p Vb was isolated from a timothy grass pollen cDNA library by immunoscreening with a specific monoclonal antibody. It was discovered for the first time that the recombinant Phl p Vb pollen allergen after expression and purification has ribonuclease activity. High homology of Phl p Vb to other group V allergens in grass pollen indicates similar function. By RNase activity gel of natural pollen extract of timothy grass and consecutive Western blot analysis of the excised proteins, the RNase active bands were shown to be group V allergens. Additionally it was demonstrated that an homologous protein to Phl p Vb in the mother plant could be induced by salicylic acid. This indicates that group Vb allergens may be involved in host-pathogen interactions because in pollen they are quickly exported RNases and in the mother plant they depend on a hormone which is related to expression of plant resistance genes.

Air pollutant stress changes the steady-state transcript levels of three photosynthesis genes

Six-week-old tobacco plants (Nicotiana tabacum L., cv. SR-1) were fumigated with different, but defined and realistic combinations of NO(2), automobile exhaust and ozone for two days. To determine the effect of an additional environmental stress during exposure, plants were either wounded mechanically or treated with salicylic acid to simulate pathogen attack. The steady-state mRNA levels of genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS), chlorophyll a/b-binding protein (cab) and a 10 kDa protein of the water-evolving complex of photosystem II (ST-LS1) were probed in Northern analysis. Our results clearly show a decline of the mRNA levels for all three photosynthesis genes under automobile exhaust and/or ozone fumigation. Wounding and also elicitor treatment enhance this effect. In contrast, exposure to NO(2) either increased the transcription level of all three genes, or counteracted the negative effect of automobile exhaust and ozone on their expression. At this time no major changes in the concentrations of the corresponding proteins could be detected by Western blot analyses. These results are discussed in the present paper.

Transcriptional activation of plant defence genes by short-term air pollutant stress

The expression of defence genes was monitored by RNA blot analyses in tobacco plants (Nicotiana tabacum cv. SR-1) treated with various air pollutants at realistic concentrations that prevail in urban areas. Six-week-old plants responded with an increase in the steady-state mRNA levels of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), chitinase and beta-1,3-glucanase, when exposed to defined and subnecrotic concentrations of automobile exhaust and/or ozone over a period of 48 h. An enhanced expression of genes encoding mitochondrial and cytosolic superoxide dismutases suggested that air pollutants induced considerable oxidative stress. Moreover, wounding or elicitor treatment of plants already exposed to automobile exhaust and/or ozone additionally increased the expression of the above defence genes, but not so in NO(2). Since the main difference between NO(2) and exhaust gas is the absence of the hydrocarbon compounds in the former, we regard hydrocarbons as favourite candidates for the toxic effect of exhaust gas, and they possibly act by generating an enhanced oxidative stress.

An epitope of rice threonine- and hydroxyproline-rich glycoprotein is common to cell wall and hydrophobic plasma-membrane glycoproteins

A monoclonal antibody, LM1, has been derived that has a high affinity for an epitope of hydroxyproline-rich glycoproteins (HRGPs). In suspension-cultured rice (Oryza sativa L.) cells the epitope is carried by three major proteins with different biochemical properties. The most abundant is the 95-kDa extracellular rice extensin, a threonine- and hydroxyproline-rich glycoprotein (THRGP) occurring in the cell wall and secreted into the medium. This THRGP can be selectively oxidatively cross-linked in the presence of hydrogen peroxide and an endogenous peroxidase with the result that it does not enter a protein gel. A second polypeptide with the LM1 epitope (180 kDa), also occurring in the suspension-cultured cells and medium, is not oxidatively cross-linked. Three further polypeptides (52, 65 and 110 kDa) with the characteristics of hydrophobic proteins of the plasma-membrane also carry the LM1 epitope as determined by immuno-blotting of detergent/aqueous partitions of a plasma-membrane preparation and immuno-fluorescence studies with rice protoplasts. At the rice root apex the LM1 epitope is carried by four glycoproteins and is developmentally regulated. The major locations of the epitope are at the surface of cells associated with the developing protoxylem and metaxylem in the stele, the longitudinal radial walls of epidermal cells and a sheath-like structure at the surface of the root apex.

A Salicylic Acid-Binding Activity and a Salicylic Acid-Inhibitable Catalase Activity Are Present in a Variety of Plant Species

Recently, it has been demonstrated that the salicylic acid (SA)-binding protein (SABP) from tobacco (Nicotiana tabacum) is a SA-inhibitable catalase (Z. Chen, H. Silva, D.F. Klessig [1993] Science 262: 1883-1886). Here we report the presence of SABP and SA-inhibitable catalase activity in Arabidopsis, tomato, and cucumber. The cucumber SABP has properties similar to the tobacco SABP, including binding affinity and specificity for SA.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

Expression of the gene for a small GTP binding protein in transgenic tobacco elevates endogenous cytokinin levels, abnormally induces salicylic acid in response to wounding, and increases resistance to tobacco mosaic virus infection

Tobacco plants transformed with rgp1, a gene encoding a Ras-related small GTP binding protein, were previously shown to exhibit a distinct reduction in apical dominance with increased tillering. These abnormal pheno-types were later found to be associated with elevated levels of endogenous cytokinins (zeatin and zeatin riboside). Analysis of the expression of several genes known to be affected by cytokinins identified a clear increase in the mRNA levels of genes encoding acidic pathogenesis-related proteins in both transgenic plants and their progenies. This increase was directly attributable to elevated levels of the acidic pathogenesis-related protein inducers, salicylic acid (SA) and salicylic acid beta-glucoside, due to an abnormal and sensitive response of the transgenic plants to wounding. In contrast, mRNA levels of the gene for proteinase inhibitor II, which is normally induced by wounding, were generally suppressed in the same wounded plants, probably due to SA overproduction. The changes in SA and pathogenesis-related protein levels in the transgenic plants resulted in a distinct increase in their resistance to tobacco mosaic virus infection. In normal plants, the wound and pathogen-induced signal transduction pathways are considered to function independently. However, the wound induction of SA in the transgenic plants suggests that overexpression of this small GTP binding protein somehow interferes with the normal signal pathways, possibly by affecting cytokinin biosynthesis, and results in cross-signaling between these two transduction systems.

Salicylic acid as a signal molecule in plant-pathogen interactions

Significant insight has been gained in the past year into the roles of salicylic acid (SA) in plant-pathogen interactions. The ability to accumulate SA has been shown to be essential for systemic acquired resistance in tobacco plants. Further experiments have shown that SA is apparently not a systemic, vascular-mobile signal, but rather is required for signal transduction at the local level. Its mode of action may include inhibition of catalase activity, leading to increased levels of hydrogen peroxide.

Lipid-derived signals that discriminate wound- and pathogen-responsive isoprenoid pathways in plants: methyl jasmonate and the fungal elicitor arachidonic acid induce different 3-hydroxy-3-methylglutaryl-coenzyme A reductase genes and antimicrobial isoprenoids in Solanum tuberosum L

Induction of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR; EC 1.1.1.34) is essential for the synthesis of steroid derivatives and sesquiterpenoid phytoalexins in solanaceous plants following mechanical injury or pathogen infection. Gene-specific probes corresponding to different HMGR genes (hmg1 and hmg2) were used to study HMGR expression in potato tissue following treatment with methyl jasmonate, a lipoxygenase product of linolenic acid, or arachidonic acid, an elicitor present in the lipids of the potato late blight fungus Phytophthora infestans. Treatment of potato discs (2.2 cm in diameter) with low concentrations (0.45-45 nmol per disc surface) of methyl jasmonate nearly doubled the wound-induced accumulation of hmg1 transcripts and steroid-glycoalkaloid (SGA) accumulation, reduced the abundance of hmg2 transcripts, and did not induce phytoalexins. High concentrations of methyl jasmonate (2-4.5 mol per disc surface) suppressed hmg1 mRNA and SGA accumulation but did not affect hmg2 mRNA abundance or induce phytoalexins. In contrast, arachidonate treatment strongly suppressed hmg1 and strongly induced hmg2 mRNA in a concentration-dependent manner. There was a corresponding suppression of SGA accumulation and an induction of sesquiterpene phytoalexin accumulation by this elicitor. Lipoxygenase inhibitors reduced the wound-induced accumulation of hmg1 transcripts and suppressed SGA levels, effects that were overcome by exogenous methyl jasmonate (45 nmol per disc surface). The results (i) suggest that methyl jasmonate can function as a signal for hmg1 expression and SGA induction following wounding and (ii) indicate that the arachidonate- and jasmonate-response pathways are distinct in relation to HMGR gene expression and isoprenoid product accumulation. The results also are consistent with placement of the HMGR activities encoded by hmg1 and hmg2 within discrete steroid and sesquiterpenoid biosynthetic channels.

Cytochrome and Alternative Pathway Respiration in Tobacco (Effects of Salicylic Acid)

In suspension cultures of NT1 tobacco (Nicotiana tabacum L. cv Bright Yellow) cells the cytochrome pathway capacity increased between d 3 and d 4 following subculturing and reached the highest level observed on d 7. The capacity decreased significantly by d 10 and was at the same level on d 14. Both alternative pathway capacity and the amount of the 35-kD alternative oxidase protein increased significantly between d 5 and d 6, reached the highest point observed on d 7, remained constant until d 10, and decreased by d 14. The highest capacities of the alternative and cytochrome pathways and the highest amount of the 35-kD protein were attained on the day that cell cultures reached a stationary phase of growth. Addition of salicylic acid to cell cultures on d 4 caused a significant increase in alternative pathway capacity and a dramatic accumulation of the 35-kD protein by 12 h. The alternative pathway capacity and the protein level reached the highest level observed by 16 h after salicylic acid addition, and the cytochrome pathway capacity was at about the same level at each time point. The accumulation of the 35-kD alternative oxidase protein was significantly decreased by addition of actinomycin D 1 h before salicylic acid and was blocked by addition of cycloheximide. These results indicate that de novo transcription and translation were necessary for salicylic acid to cause the maximum accumulation of the 35-kD protein.

Purification and characterization of a soluble salicylic acid-binding protein from tobacco

Previously, we identified a soluble salicylic acid (SA)-binding protein (SABP) in tobacco whose properties suggest that it may play a role in transmitting the SA signal during plant defense responses. This SA-binding activity has been purified 250-fold by conventional chromatography and was found to copurify with a 280-kDa protein. Monoclonal antibodies capable of immunoprecipitating the SA-binding activity also immunoprecipitated the 280-kDa protein, indicating that it was responsible for binding SA. These antibodies also recognized the 280-kDa protein in immunoblots of the partially purified SABP fraction or the crude extract. However, when the crude extract was prepared in the presence of antioxidants, only a 57-kDa protein was recognized. Since the SABP has a native molecular mass of 240 kDa, it appears that the SABP is a complex which contains a 57-kDa subunit and perhaps one or more additional proteins which are covalently crosslinked in the absence of antioxidants. The ability of a variety of phenolic compounds to compete with SA for binding to the SABP was both qualitatively and quantitatively correlated with their biological activity in inducing defense-related genes. Moreover, the inducibility of the pathogenesis-related (PR)-1 genes by SA was proportional to the abundance of the SABP in different organs. These correlations are consistent with a role for the SABP in perceiving and transducing the SA signal in plant defense.

Pathway of Salicylic Acid Biosynthesis in Healthy and Virus-Inoculated Tobacco

Salicylic acid (SA) is a likely endogenous regulator of localized and systemic disease resistance in plants. During the hypersensitive response of Nicotiana tabacum L. cv Xanthi-nc to tobacco mosaic virus (TMV), SA levels rise dramatically. We studied SA biosynthesis in healthy and TMV-inoculated tobacco by monitoring the levels of SA and its likely precursors in extracts of leaves and cell suspensions. In TMV-inoculated leaves, stimulation of SA accumulation is accompanied by a corresponding increase in the levels of benzoic acid. 14C-Tracer studies with cell suspensions and mock-or TMV-inoculated leaves indicate that the label moves from trans-cinnamic acid to SA via benzoic acid. In healthy and TMV-inoculated tobacco leaves, benzoic acid induced SA accumulation. o-Coumaric acid, which was previously reported as a possible precursor of SA in other species, did not increase SA levels in tobacco. In healthy tobacco tissue, the specific activity of newly formed SA was equal to that of the supplied [14C]benzoic acid, whereas in TMV-inoculated leaves some isotope dilution was observed, presumably because of the increase in the pool of endogenous benzoic acid. We observed accumulation of pathogen-esis-related-1 proteins and increased resistance to TMV in benzoic acid- but not in o-coumaric acid-treated tobacco leaves. This is consistent with benzoic acid being the immediate precursor of SA. We conclude that in healthy and virus-inoculated tobacco, SA is formed from cinnamic acid via benzoic acid.

Molecular cloning and expression of a new class of ortho-diphenol-O-methyltransferases induced in tobacco (Nicotiana tabacum L.) leaves by infection or elicitor treatment

In tobacco (Nicotiana tabacum L. cv Samsun NN), three distinct enzymes account for ortho-diphenol-O-methyltransferase (OMT) activity. OMT I is the major enzyme of healthy leaves, whereas enzymes OMT II and III are preferentially induced during the hypersensitive reaction to tobacco mosaic virus (TMV). Using an anti-OMT III antiserum, we isolated a partial OMT III cDNA clone by immunoscreening an expression library made from mRNA of TMV-infected tobacco leaves. Using this OMT III clone as a probe, we isolated a full-length clone with a deduced amino acid sequence encompassing all of the sequences obtained by Edman degradation of both purified proteins II and III. Thus, OMT II and III of tobacco are likely to be encoded by the same genes and to arise from different posttranslational modifications. Sequence analysis showed that this OMT clone represents a new class of OMT enzymes (class II) with a low level of similarity (53-58%) to OMTs cloned previously from other dicotyledonous plants. Southern analysis indicated that a small family of class II OMT genes inherited from ancestors related to Nicotiana sylvestris and Nicotiana tomentosiformis occurs in the tobacco genome. RNA blot analysis demonstrated that class II OMT genes, unlike class I OMT genes, are not expressed at a high constitutive level in lignified tissues of tobacco. Class II OMT transcripts were found to accumulate in tobacco leaves infected with TMV or treated with megaspermin, a proteinaceous elicitor from Phytophthora megasperma, but not in leaves treated with salicylic acid, a molecule known to trigger many defense genes. In TMV-infected or elicitor-treated tissues, a marked increase in catechol-methylating activity accompanied the accumulation of class II OMT gene products.

Identification of methyl jasmonate and salicylic acid response elements from the nopaline synthase (nos) promoter

Transgenic tobacco plants carrying a fusion between the nopaline synthase (nos) promoter and chloramphenicol acetyltransferase (CAT) reporter gene (cat) were studied for their inducibility by salicylic acid (SA) or methyl jasmonate (MJ) treatments. Either chemical significantly increased CAT activity to a level much higher than that achieved by wounding. Northern blot analysis showed a corresponding increase in mRNA levels. After 20 h of induction of flowering plants, the response to MJ treatment was weaker in old leaves compared with young leaves, whereas the SA response was stronger in old leaves. Kinetic experiments showed that the SA response was much faster than the MJ response, suggesting that the induction mechanism of the nos promoter by these chemicals may differ. Deletion analysis showed that both SA and MJ responses require the DNA sequence between -119 and -112 from the transcription initiation site. This region contains the hexamer sequence (TGACGT) that has been found to be an important regulatory element for several promoters. The MJ response was also reduced by deletions of the CAAT box region or the sequence between -112 and -101, whereas the SA response was not significantly affected by these deletions. This suggests that the nos upstream region containing the hexamer motif is essential for the SA or MJ response and that the CAAT box region and the sequence immediately downstream from the hexamer motif are required for maximum induction by MJ.