Salicylic acid as a signal molecule in plant-pathogen interactions

MdPRX34L, a class III peroxidase gene, activates the immune response in apple to the fungal pathogen Botryosphaeria dothidea

MAIN CONCLUSION

MdPRX34L enhanced resistance to Botryosphaeria dothidea by increasing salicylic acid (SA) and abscisic acid (ABA) content as well as the expression of related defense genes. The class III peroxidase (PRX) multigene family is involved in complex biological processes. However, the molecular mechanism of PRXs in the pathogen defense of plants against Botryosphaeria dothidea (B. dothidea) remains unclear. Here, we cloned the PRX gene MdPRX34L, which was identified as a positive regulator of the defense response to B. dothidea, from the apple cultivar 'Royal Gala.' Overexpression of MdPRX34L in apple calli decreased sensitivity to salicylic acid (SA) and abscisic acid（ABA). Subsequently, overexpression of MdPRX34L in apple calli increased resistance to B. dothidea infection. In addition, SA contents and the expression levels of genes related to SA synthesis and signaling in apple calli overexpressing MdPRX34L were higher than those in the control after inoculation, suggesting that MdPRX34L enhances resistance to B. dothidea via the SA pathway. Interestingly, infections in apple calli by B. dothidea caused an increase in endogenous levels of ABA followed by induction of ABA-related genes expression. These findings suggest a potential mechanism by which MdPRX34L enhances plant-pathogen defense against B. dothidea by regulating the SA and ABA pathways.

Transcriptome analysis reveals pathogenesis-related gene 1 pathway against salicylic acid treatment in grapevine (Vitis vinifera L)

Salicylic acid (SA) is a well-studied phenolic plant hormone that plays an important role in plant defense against the hemi-biothrophic and biothrophic pathogens and depends on the living cells of host for the successful infection. In this study, a pathogenesis test was performed between Vitis davidii and V. vinifera cultivars against grape white rot disease (Coniella diplodiella). V. davidii was found to be resistant against this disease. SA contents were found to be higher in the resistant grape cultivar after different time points. RNA-seq analysis was conducted on susceptible grapevine cultivars after 12, 24, and 48 h of SA application with the hypothesis that SA may induce defense genes in susceptible cultivars. A total of 511 differentially expressed genes (DEGs) were identified from the RNA-seq data, including some important genes, VvWRKY1/2, VvNPR1, VvTGA2, and VvPR1, for the SA defense pathway. DEGs related to phytohormone signal transduction and flavonoid biosynthetic pathways were also upregulated. The quantitative real-time PCR (qRT-PCR) results of the significantly expressed transcripts were found to be consistent with the transcriptome data, with a high correlation between the two analyses. The pathogenesis-related gene 1 (VvPR1), which is an important marker gene for plant defense, was selected for further promoter analysis. The promoter sequence showed that it contains some important cis-elements (W-box, LS7, as-1, and TCA-element) to recruit the transcription factors VvWRKY, VvNPR1, and VvTGA2 to express the VvPR1 gene in response to SA treatment. Furthermore, the VvPR1 promoter was serially deleted into different fragments (-1,837, -1,443, -1,119, -864, -558, -436, and -192 ) bp and constructed vectors with the GUS reporter gene. Deletion analysis revealed that the VvPR1 promoter between -1837 bp to -558 bp induced significant GUS expression with respect to the control. On the basis of these results, the -558 bp region was assumed to be an important part of the VvPR1 promoter, and this region contained the important cis-elements related to SA, such as TCA-element (-1,472 bp), LS7 (-1,428 bp), and as-1 (-520 bp), that recruit the TFs and induce the expression of the VvPR1 gene. This study expanded the available information regarding SA-induced defense in susceptible grapes and recognized the molecular mechanisms through which this defense might be mediated.

The glycosyltransferase UGT76B1 modulates N-hydroxy-pipecolic acid homeostasis and plant immunity

The tradeoff between growth and defense is a critical aspect of plant immunity. Therefore, the plant immune response needs to be tightly regulated. Salicylic acid (SA) is an important plant hormone regulating defense against biotrophic pathogens. Recently, N-hydroxy-pipecolic acid (NHP) was identified as another regulator for plant innate immunity and systemic acquired resistance (SAR). Although the biosynthetic pathway leading to NHP formation is already been identified, how NHP is further metabolized is unclear. Here, we present UGT76B1 as a uridine diphosphate-dependent glycosyltransferase (UGT) that modifies NHP by catalyzing the formation of 1-O-glucosyl-pipecolic acid in Arabidopsis thaliana. Analysis of T-DNA and clustered regularly interspaced short palindromic repeats (CRISPR) knock-out mutant lines of UGT76B1 by targeted and nontargeted ultra-high performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS) underlined NHP and SA as endogenous substrates of this enzyme in response to Pseudomonas infection and UV treatment. ugt76b1 mutant plants have a dwarf phenotype and constitutive defense response which can be suppressed by loss of function of the NHP biosynthetic enzyme FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1). This suggests that elevated accumulation of NHP contributes to the enhanced disease resistance in ugt76b1. Externally applied NHP can move to distal tissue in ugt76b1 mutant plants. Although glycosylation is not required for the long-distance movement of NHP during SAR, it is crucial to balance growth and defense.

Comparative Analyses of Four Chemicals Used to Control Black Mold Disease in Tomato and Its Effects on Defense Signaling Pathways, Productivity and Quality Traits

The field application of safe chemical inducers plays a vital role in the stimulation of systematic acquired resistance (SAR) of plants. In this study, the efficacy use of three and six field applications with chitosan, lithovit, and K-thiosulfate at 4 gL-1 and salicylic acid at 1.5 gL-1 in improving tomato productivity, quality, and modifying the defense signaling pathways to the Alternaria alternata infection was investigated. Salicylic acid was the most effective in vitro where it completely inhibited the growth of Alternaria alternata. The highest yield quantity was recorded with six applications with Chitosan followed by Salicylic acid; also, they were the most effective treatments in controlling the Alternaria alternata infection in tomato fruits. The maximum increase in chitinase and catalase activity of tomato fruits was observed at five days after inoculation, following treatment with six sprays of salicylic acid followed by chitosan. The transcript levels of seven defense-related genes: ethylene-responsive transcription factor 3 (RAP), xyloglucan endotransglucosylase 2 (XET-2), catalytic hydrolase -2 (ACS-2), proteinase inhibitor II (PINII), phenylalanine ammonia-lyase 5 (PAL5), lipoxygenase D (LOXD), and pathogenesis-related protein 1 (PR1) were upregulated in response to all treatments. The highest expression levels of the seven studied genes were recorded in response to six foliar applications with chitosan. Chitosan followed by salicylic acid was the most effective among the tested elicitors in controlling the black mold rot in tomato fruits. In conclusion, pre-harvest chitosan and salicylic acid in vivo application with six sprays could be recommended as effective safe alternatives to fungicides against black mold disease in tomato fruits.

Endoplasmic reticulum-mediated unfolded protein response is an integral part of singlet oxygen signalling in plants

Singlet oxygen (¹ O₂ ) is a by-product of photosynthesis that triggers a signalling pathway leading to stress acclimation or to cell death. By analyzing gene expressions in a ¹ O₂ -overproducing Arabidopsis mutant (ch1) under different light regimes, we show here that the ¹ O₂ signalling pathway involves the endoplasmic reticulum (ER)-mediated unfolded protein response (UPR). ch1 plants in low light exhibited a moderate activation of UPR genes, in particular bZIP60, and low concentrations of the UPR-inducer tunicamycin enhanced tolerance to photooxidative stress, together suggesting a role for UPR in plant acclimation to low ¹ O₂ levels. Exposure of ch1 to high light stress ultimately leading to cell death resulted in a marked upregulation of the two UPR branches (bZIP60/IRE1 and bZIP28/bZIP17). Accordingly, mutational suppression of bZIP60 and bZIP28 increased plant phototolerance, and a strong UPR activation by high tunicamycin concentrations promoted high light-induced cell death. Conversely, light acclimation of ch1 to ¹ O₂ stress put a limitation in the high light-induced expression of UPR genes, except for the gene encoding the BIP3 chaperone, which was selectively upregulated. BIP3 deletion enhanced Arabidopsis photosensitivity while plants treated with a chemical chaperone exhibited enhanced phototolerance. In conclusion, ¹ O₂ induces the ER-mediated UPR response that fulfils a dual role in high light stress: a moderate UPR, with selective induction of BIP3, is part of the acclimatory response to ¹ O₂ , and a strong activation of the whole UPR is associated with cell death.

A comparison between the homocyclic aromatic metabolic pathways from plant-derived compounds by bacteria and fungi

Aromatic compounds derived from lignin are of great interest for renewable biotechnical applications. They can serve in many industries e.g. as biochemical building blocks for bioplastics or biofuels, or as antioxidants, flavor agents or food preservatives. In nature, lignin is degraded by microorganisms, which results in the release of homocyclic aromatic compounds. Homocyclic aromatic compounds can also be linked to polysaccharides, tannins and even found freely in plant biomass. As these compounds are often toxic to microbes already at low concentrations, they need to be degraded or converted to less toxic forms. Prior to ring cleavage, the plant- and lignin-derived aromatic compounds are converted to seven central ring-fission intermediates, i.e. catechol, protocatechuic acid, hydroxyquinol, hydroquinone, gentisic acid, gallic acid and pyrogallol through complex aromatic metabolic pathways and used as energy source in the tricarboxylic acid cycle. Over the decades, bacterial aromatic metabolism has been described in great detail. However, the studies on fungal aromatic pathways are scattered over different pathways and species, complicating a comprehensive view of fungal aromatic metabolism. In this review, we depicted the similarities and differences of the reported aromatic metabolic pathways in fungi and bacteria. Although both microorganisms share the main conversion routes, many alternative pathways are observed in fungi. Understanding the microbial aromatic metabolic pathways could lead to metabolic engineering for strain improvement and promote valorization of lignin and related aromatic compounds.

Endoplasmic Reticulum Plays a Critical Role in Integrating Signals Generated by Both Biotic and Abiotic Stress in Plants

Most studies of environmental adaptations in plants have focused on either biotic or abiotic stress factors in an attempt to understand the defense mechanisms of plants against individual stresses. However, in the natural ecosystem, plants are simultaneously exposed to multiple stresses. Stress-tolerant crops developed in translational studies based on a single stress often fail to exhibit the expected traits in the field. To adapt to abiotic stress, recent studies have identified the need for interactions of plants with various microorganisms. These findings highlight the need to understand the multifaceted interactions of plants with biotic and abiotic stress factors. The endoplasmic reticulum (ER) is an organelle that links various stress responses. To gain insight into the molecular integration of biotic and abiotic stress responses in the ER, we focused on the interactions of plants with RNA viruses. This interaction points toward the relevance of ER in viral pathogenicity as well as plant responses. In this mini review, we explore the molecular crosstalk between biotic and abiotic stress signaling through the ER by elaborating ER-mediated signaling in response to RNA viruses and abiotic stresses. Additionally, we summarize the results of a recent study on phytohormones that induce ER-mediated stress response. These studies will facilitate the development of multi-stress-tolerant transgenic crops in the future.

Potyviral Gene-Silencing Suppressor HCPro Interacts with Salicylic Acid (SA)-Binding Protein 3 to Weaken SA-Mediated Defense Responses

The viral infection process is a battle between host defense response and pathogen antagonizing action. Several studies have established a tight link between the viral RNA silencing suppressor (RSS) and the repression of salicylic acid (SA)-mediated defense responses, nonetheless host factors directly linking an RSS and the SA pathway remains unidentified. From yeast two-hybrid analysis, we identified an interaction between the potyviral RSS helper-component proteinase (HCPro) and SA-binding protein SABP3. Co-localization and bimolecular fluorescence complementation analyses validated the direct in vivo interaction between Turnip mosaic virus (TuMV) HCPro and the Arabidopsis homologue of SABP3, AtCA1. Additionally, transient expression of TuMV HCPro demonstrated its ability to act as a negative regulator of AtCA1. When the plants of the AtCA1 knockout mutant line were inoculated with TuMV, our results indicated that AtCA1 is essential to restrict viral spreading and accumulation, induce SA accumulation, and trigger the SA pathway. Unexpectedly, the AtCA1 overexpression line also displayed a similar phenotype, suggesting that the constitutive expression of AtCA1 antagonizes the SA pathway. Taken together, our results depict AtCA1 as an essential regulator of SA defense responses. Moreover, the interaction of potyviral HCPro with this regulator compromises the SA pathway to weaken host defense responses and facilitate viral infection.

Synthetic plant defense elicitors

To defend themselves against invading pathogens plants utilize a complex regulatory network that coordinates extensive transcriptional and metabolic reprogramming. Although many of the key players of this immunity-associated network are known, the details of its topology and dynamics are still poorly understood. As an alternative to forward and reverse genetic studies, chemical genetics-related approaches based on bioactive small molecules have gained substantial popularity in the analysis of biological pathways and networks. Use of such molecular probes can allow researchers to access biological space that was previously inaccessible to genetic analyses due to gene redundancy or lethality of mutations. Synthetic elicitors are small drug-like molecules that induce plant defense responses, but are distinct from known natural elicitors of plant immunity. While the discovery of some synthetic elicitors had already been reported in the 1970s, recent breakthroughs in combinatorial chemical synthesis now allow for inexpensive high-throughput screens for bioactive plant defense-inducing compounds. Along with powerful reverse genetics tools and resources available for model plants and crop systems, comprehensive collections of new synthetic elicitors will likely allow plant scientists to study the intricacies of plant defense signaling pathways and networks in an unparalleled fashion. As synthetic elicitors can protect crops from diseases, without the need to be directly toxic for pathogenic organisms, they may also serve as promising alternatives to conventional biocidal pesticides, which often are harmful for the environment, farmers and consumers. Here we are discussing various types of synthetic elicitors that have been used for studies on the plant immune system, their modes-of-action as well as their application in crop protection.

The Arabidopsis thaliana At4g13040 gene, a unique member of the AP2/EREBP family, is a positive regulator for salicylic acid accumulation and basal defense against bacterial pathogens

The Arabidopsis genome contains a large number of putative transcription factors, containing a DNA binding domain similar to APETALA2/ethylene response element binding protein (AP2/EREBP), for most of which a function is not known. Phylogenetic analysis divides the Apetala 2 (AP2) super-family into 5 major groups: AP2, RAV, ethylene response factor (ERF), dehydration response element binding protein (DREB) and At4g13040. Similar to ERF and DREB, the At4g13040 protein contains only one AP2 domain; however, its structural uniqueness places it into a distinct group. In this article, we report that At4g13040 (referred herein as Apetala 2 family protein involved in SA mediated disease defense 1 - APD1) is an important regulator for SA mediated plant defense. The APD1 gene is upregulated upon pathogen inoculation, exogenous SA application and in the mutant that constitutively activates SA signaling. The T-DNA insertion lines (inserted in the APD1 promoter), which fail to induce expression upon pathogen inoculation, are compromised for resistance against virulent bacterial pathogens and show reduced induction of pathogenesis related 1 gene. Our results suggest that APD1 functions downstream of PAD4 in Arabidopsis and promotes pathogen-induced SA accumulation. Exogenous SA application completely restores the loss-of-resistance phenotype of the apd1 mutant. Thus, APD1 is a positive regulator of disease defense that functions upstream of SA accumulation.

Salicylic acid determines differential senescence produced by two Turnip mosaic virus strains involving reactive oxygen species and early transcriptomic changes

Losses produced by virus diseases depend mostly on symptom severity. Turnip mosaic virus (TuMV) is one of the most damaging and widespread potyvirus infecting members of the family Brassicaceae, including Arabidopsis thaliana. We used JPN1 and UK1 TuMV strains to characterize viral infections regarding symptom development, senescence progression, antioxidant response, reactive oxygen species (ROS) accumulation, and transcriptional profiling. Both isolates, despite accumulating similar viral titers, induced different symptomatology and strong differences in oxidative status. Early differences in several senescence-associated genes linked to the ORE1 and ORS1 regulatory networks as well as persistent divergence in key ROS production and scavenging systems of the plant were detected. However, at a later stage, both strains induced nutrient competition, indicating that senescence rates are influenced by different mechanisms upon viral infections. Analyses of ORE1 and ORS1 levels in infected Brassica juncea plants showed a similar pattern, suggesting a conserved differential response to both strains in Brassicaceae spp. Transcriptional analysis of the ORE1 and ORS1 regulons showed similarities between salicylic acid (SA) response and the early induction triggered by UK1, the most severe strain. By means of SA-defective NahG transgenic plants, we found that differential senescence progression and ROS accumulation between strains rely on an intact SA pathway.

Profiling of genes related to cross protection and competition for NbTOM1 by HLSV and TMV

Cross protection is the phenomenon through which a mild strain virus suppresses symptoms induced by a closely related severe strain virus in infected plants. Hibiscus latent Singapore virus (HLSV) and Tobacco mosaic virus (TMV) are species within the genus tobamovirus. HLSV can protect Nicotianabenthamiana against TMV-U1 strain, resulting in mild symptoms instead of severe systemic necrosis. The mechanism of cross protection between HLSV and TMV is unknown. In the past, some researchers suggest that the protecting virus strain might occupy virus-specific replication sites within a cell leaving no room for the challenge virus. Quantitative real-time RT-PCR was performed to detect viral RNA levels during cross protection. HLSV accumulation increased in cross protected plants compared with that of single HLSV infected plants, while TMV decreased in cross protected plants. This suggests that there is a competition for host factors between HLSV and TMV for replication. To investigate the mechanism under the cross protection between HLSV and TMV, microarray analysis was conducted to examine the transcriptional levels of global host genes during cross protection, using Tobacco Gene Expression Microarray, 4 x 44 k slides. The transcriptional level of some host genes corresponded to accumulation level of TMV. Some host genes were up-regulated only by HLSV. Tobamovirus multiplication gene 1 (TOM1), essential for tobamovirus multiplication, was involved in competition for replication by HLSV and TMV during cross protection. Both HLSV and TMV accumulation decreased when NbTOM1 was silenced. A large quantity of HLSV resulted in decreased TMV accumulation in HLSV+TMV (100:1) co-infection. These results indicate that host genes involved in the plant defense response and virus multiplication are up-regulated by challenge virus TMV but not by protecting virus HLSV during cross protection.

Synergistic activation of defense responses in Arabidopsis by simultaneous loss of the GSL5 callose synthase and the EDR1 protein kinase

Loss-of-function mutations in the EDR1 gene of Arabidopsis confer enhanced resistance to Golovinomyces cichoracearum (powdery mildew). Disease resistance mediated by the edr1 mutation is dependent on an intact salicylic acid (SA) signaling pathway, but edr1 mutant plants do not constitutively express the SA-inducible gene PR-1 and are not dwarfed. To identify other components of the EDR1 signaling network, we screened for mutations that enhanced the edr1 mutant phenotype. Here, we describe an enhancer of edr1 mutant, eed3, which forms spontaneous lesions in the absence of pathogen infection, constitutively expresses both SA- and methyl jasmonate (JA)-inducible defense genes, and is dwarfed. Positional cloning of eed3 revealed that the mutation causes a premature stop codon in GLUCAN SYNTHASE-LIKE 5 (GSL5, also known as POWDERY MILDEW RESISTANT 4), which encodes a callose synthase required for pathogen-induced callose production. Significantly, gsl5 single mutants do not constitutively express PR-1 or AtERF1 (a JA-inducible gene) and are not dwarfed. Thus, loss of both EDR1 and GSL5 function has a synergistic effect. Our data suggest that EDR1 and GSL5 negatively regulate SA and JA production or signaling by independent mechanisms and that negative regulation of defense signaling by GSL5 may be independent of callose production.

Dissection of salicylic acid-mediated defense signaling networks

The small phenolic molecule salicylic acid (SA) plays a key role in plant defense. Significant progress has been made recently in understanding SA-mediated defense signaling networks. Functional analysis of a large number of genes involved in SA biosynthesis and regulation of SA accumulation and signal transduction has revealed distinct but interconnecting pathways that orchestrate the control of plant defense. Further studies utilizing combinatorial approaches in genetics, molecular biology, biochemistry and genomics will uncover finer details of SA-mediated defense networks as well as further insights into the crosstalk of SA with other defense signaling pathways. The complexity of defense networks illustrates the capacity of plants to integrate multiple developmental and environmental signals into a tight control of the costly defense responses.

Antibiotic effect of exogenously applied salicylic acid on in vitro soilborne pathogen, Fusarium oxysporum f.sp.niveum

Salicylic acid, which is biosynthesized inside plant and is often found and accumulated in soil due to plant debris decaying, is considered as a signaling substance during plant-microbe interactions. It is involved in the cycling of biogeochemistry and related to plant resistance to biotic and abiotic stress. The antibiotic effect of salicylic acid on Fusarium oxysporum f.sp.niveum (FON) was studied to investigate the relationships between the salicylic acid and the fungus in the ecological interaction of plant-microbe. Results showed that the biomass, colony diameter, number of conidium germination and conidium production of FON were decreased by 52.0%, 25.7%, 100% and 100% at concentrations of 800 mg L(-1). However, mycotoxin yield was increased by 233%, pectinase activity raised by 168.0% and cellulase activity increased by 1325% compared to control at higher concentrations. It was concluded that salicylic acid as an allelochemical greatly inhibited FON growth and conidia formation and germination, though stimulated mycotoxin production and activities of hydrolytic enzymes by FON.

Analysis of low molecular weight acids by negative mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Free 9-aminoacridine base is demonstrated to be a suitable matrix for negative mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometric (MALDI-TOFMS) analysis of a wide range of low molecular weight organic acids including aliphatic (from acetic to palmitic acid), aromatic acids, phytohormones (e.g. jasmonic and salicylic acids), and amino acids. Low limits of quantitation in the femtomolar range (jasmonic - 250 fmol; caffeic - 160 fmol and salicylic - 12.5 fmol) and linear detector response over two concentration orders in the pico- and femtomolar range are extremely encouraging for the direct study of such acids in complex biological matrices.

Induction of an anionic peroxidase in cowpea leaves by exogenous salicylic acid

Two isoperoxidases were detected in cowpea (Vigna unguiculata) leaves. Treatment of the primary leaves with 10mM salicylic acid increased the total peroxidase activity contributed by the anionic isoform. To isolate both the anionic and cationic peroxidases the leaf crude extract was loaded on a Superose 12 HR 10/30 column followed by chromatography on Mono-Q HR 5/5. Both enzymes were stable in a pH range from 5 to 7. The optimum-temperatures for the cationic and anionic peroxidase isoforms were, respectively, 20-30 degrees C and 30 degrees C. The dependence of guaiacol oxidation rate varying its concentration at constant H(2)O(2) concentration showed, for both enzymes, Michaelis-Menten-type kinetic. Apparent K(m)(s) were 0.8 and 4.8 microM for the cationic and anionic isoperoxidases, respectively.

Structure-function analysis of the plasma membrane- localized Arabidopsis defense component ACD6

The ACCELERATED CELL DEATH 6 (ACD6) protein, composed of an ankyrin-repeat domain and a predicted transmembrane region, is a necessary positive regulator of Arabidopsis defenses. ACD6 overexpression confers enhanced disease resistance by priming stronger and quicker defense responses during pathogen infection, plant development or treatment with an agonist of the key defense regulator salicylic acid (SA). Modulation of ACD6 affects both SA-dependent and SA-independent defenses. ACD6 localizes to the plasma membrane and is an integral membrane protein with a cytoplasmic ankyrin domain. An activated version of ACD6 with a predicted transmembrane helix mutation called ACD6-1 has the same localization and overall topology as the wild-type protein. A genetic screen for mutants that suppress acd6-1-conferred phenotypes identified 17 intragenic mutations of ACD6. The majority of these mutations reside in the ankyrin domain and in predicted transmembrane helices, suggesting that both ankyrin and transmembrane domains are important for ACD6 function. One mutation (S638F) also identified a key residue in a putative loop between two transmembrane helices. This mutation did not alter the stability or localization of ACD6, suggesting that S635 is a critical residue for ACD6 function. Based on structural modeling, two ankyrin domain mutations are predicted to be in surface-accessible residues. As ankyrin repeats are protein interaction modules, these mutations may disrupt protein-protein interactions. A plausible scenario is that information exchange between the ankyrin and transmembrane domains is involved in activating defense signaling.

A W-box is required for full expression of the SA-responsive gene SFR2

Transcripts of SFR2, a member of the S family of receptor kinase genes, accumulate rapidly in Brassica oleracea leaves in response to wounding, bacterial infection and following treatment with salicylic acid (SA). Expression of a chimeric gene consisting of the SFR2 5' flanking sequence fused to the gusA reporter gene is also induced in wounded and SA-treated Arabidopsis plants indicating that the observed response is conferred by the SFR2 promoter. We show here that, in Arabidopsis plants carrying the salicylate hydroxylase (NahG) transgene, wound induction of the SFR2 promoter-gusA reporter fusion was abolished, indicating that, as has previously been shown for the response to bacterial infection, SA is required for the response to wounding. Deletion analysis of the SFR2 promoter identified a region necessary for full expression following SA treatment. This region, which includes two putative W-boxes, is conserved in the promoter of the Arabidopsis SFR2 homologue, ARK3. Deletion of a 12 bp region containing the two W-box motifs reduced the response to SA treatment. Tandem repeats of the W-box-containing element fused upstream of a CaMV 35S minimal promoter enhanced reporter gene expression in transgenic Arabidopsis both in the absence and presence of SA. Gel-mobility shift assays showed that Arabidopsis leaf extracts contained factors that bound to a fragment of the promoter spanning the putative W-boxes and that a fragment in which these motifs were mutated was unable to compete for binding. In summary, induction of the SFR2 promoter in response to bacterial infection and wounding requires SA, and full expression of the induced gene requires the presence of a functional element containing W-box motifs in the SFR2 promoter. The involvement of two W-boxes indicates that transcription factors of the WRKY family may play a key role in mediating these responses.

The gene encoding glutathione-dependent formaldehyde dehydrogenase/GSNO reductase is responsive to wounding, jasmonic acid and salicylic acid

It has recently been discovered that glutathione-dependent formaldehyde dehydrogenase (FALDH) exhibits a strong S-nitrosoglutathione reductase activity. Plants use NO and S-nitrosothiols as signaling molecules to activate defense mechanisms. Therefore, it is interesting to investigate the regulation of FALDH by mechanical wounding and plant hormones involved in signal transduction. Our results show that the gene encoding FALDH in Arabidopsis (ADH2) is down-regulated by wounding and activated by salicylic acid (SA). In tobacco, FALDH levels and enzymatic activity decreased after jasmonate treatment, and increased in response to SA. This is the first time that regulation of FALDH in response to signals associated with plant defense has been demonstrated.

Monitoring the switch from housekeeping to pathogen defense metabolism in Arabidopsis thaliana using cDNA arrays

Plants respond to pathogen attack by deploying several defense reactions. Some rely on the activation of preformed components, whereas others depend on changes in transcriptional activity. Using cDNA arrays comprising 13,000 unique expressed sequence tags, changes in the transcriptome of Arabidopsis thaliana were monitored after attempted infection with the bacterial plant pathogen Pseudomonas syringae pv. tomato carrying the avirulence gene avrRpt2. Sampling at four time points during the first 24 h after infiltration revealed significant changes in the steady state transcript levels of approximately 650 genes within 10 min and a massive shift in gene expression patterns by 7 h involving approximately 2,000 genes representing many cellular processes. This shift from housekeeping to defense metabolism results from changes in regulatory and signaling circuits and from an increased demand for energy and biosynthetic capacity in plants fighting off a pathogenic attack. Concentrating our detailed analysis on the genes encoding enzymes in glycolysis, the Krebs cycle, the pentose phosphate pathway, the biosynthesis of aromatic amino acids, phenylpropanoids, and ethylene, we observed interesting differential regulation patterns. Furthermore, our data showed potentially important changes in areas of metabolism, such as the glyoxylate metabolism, hitherto not suspected to be components of plant defense.

The ethylene pathway: a paradigm for plant hormone signaling and interaction

To dissect the web of signals that control plant growth, it is important to understand how the individual components of the pathway are modulated. Ethylene is a plant hormone involved in a large number of developmental processes. Biochemical and genetic approaches have provided a detailed view of the biosynthetic and signal transduction pathways of this hormone in the reference plant Arabidopsis thaliana. The effects of several hormones and of developmental changes on the regulation of the key enzymes of ethylene biosynthesis, ACC synthase and ACC oxidase, serve as a clear example of interaction between signals in the generation of complex responses. We now have a picture of how ethylene is sensed by the ethylene receptors and how the signal is further transduced to the nucleus. Although some of the ethylene receptors show a tissue-specific pattern of expression, little is known about the regulation of the components of the ethylene transduction cascade by other hormones or developmental factors. Once the ethylene signal reaches the nucleus, it activates a transcriptional cascade that results in changes in the expression of a number of genes. We describe some of the results that suggest an interaction at the transcriptional level between ethylene, other hormones, and stress signals.

H(2)O(2) mediates Ca(2+)- and MLC(20) phosphorylation-independent contraction in intact and permeabilized vascular muscle

One purpose of the current study was to establish whether vasoconstriction occurs in all vessel types in response to H(2)O(2). Isometric force was measured in pulmonary venous and arterial rings, and isobaric contractions were measured in mesenteric arteries and veins in response to H(2)O(2). A second purpose was to determine whether H(2)O(2)-induced contraction is calcium independent. The addition of H(2)O(2) to calcium-depleted (using the Ca(2+) ionophore ionomycin in zero calcium EGTA buffer) muscle caused contraction. Furthermore, permeabilized muscle contracted in response to H(2)O(2) even in zero Ca(2+). The final purpose was to determine whether the 20-kDa regulatory myosin light chain (MLC(20)) phosphorylation plays a role in H(2)O(2)-induced contraction. Pulmonary arterial strips were freeze-clamped at various time points during H(2)O(2)-induced contractions, and the relative amounts of phosphorylated MLC(20) were measured. H(2)O(2) caused dose-dependent contractions that were independent of MLC(20) phosphorylation. ML-9, a myosin light chain kinase inhibitor, had no effect on the H(2)O(2) contractile response. In conclusion, H(2)O(2) induces Ca(2+)- and MLC(20) phosphorylation-independent contraction in pulmonary and systemic arterial and venous smooth muscle.

MAPK and PKC activity are not required for H(2)O(2)-induced arterial muscle contraction

H(2)O(2)-induced pulmonary arterial smooth muscle (PASM) contractions are independent of Ca(2+) and myosin light chain phosphorylation. The purpose of this study was to determine whether mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK) 1 and ERK2, or protein kinase C (PKC) activation is required for H(2)O(2)-induced contraction. Porcine PASM strips were stimulated with 1 mM H(2)O(2), 120 mM KCl, or 10 microM phorbol myristic acetate and freeze clamped at various times during the contractions. Changes in relative amounts of tyrosine/threonine phosphorylated MAPK compared with total MAPK were measured. MAPK tyrosine phosphorylation levels increased in correlation with tension development. However, 50 microM PD-98059, a MAPK/ERK kinase-MAPK kinase blocker, reduced MAPK phosphorylation below resting levels, even though the magnitude of the isometric tension development was unaltered. Freeze-clamped PASM strips were placed in a PKC activity assay buffer containing (32)P and CaCl(2) to measure the total myelin basic protein phosphorylation. The data show that: 1) the time courses of PKC activity and force produced in response to H(2)O(2) do not correlate, and 2) MAPK activation may be a concurrent event with, or a consequence of, tension development in response to a variety of agonists but is not responsible for contractions to H(2)O(2), high K(+), or phorbol esters.

Riboflavin induces disease resistance in plants by activating a novel signal transduction pathway

ABSTRACT The role of riboflavin as an elicitor of systemic resistance and an activator of a novel signaling process in plants was demonstrated. Following treatment with riboflavin, Arabidopsis thaliana developed systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. Tomato, and tobacco developed systemic resistance to Tobacco mosaic virus (TMV) and Alternaria alternata. Riboflavin, at concentrations necessary for resistance induction, did not cause cell death in plants or directly affect growth of the culturable pathogens. Riboflavin induced expression of pathogenesis-related (PR) genes in the plants, suggesting its ability to trigger a signal transduction pathway that leads to systemic resistance. Both the protein kinase inhibitor K252a and mutation in the NIM1/NPR1 gene which controls transcription of defense genes, impaired responsiveness to riboflavin. In contrast, riboflavin induced resistance and PR gene expression in NahG plants, which fail to accumulate salicylic acid (SA). Thus, riboflavin-induced resistance requires protein kinase signaling mechanisms and a functional NIM1/NPR1 gene, but not accumulation of SA. Riboflavin is an elicitor of systemic resistance, and it triggers resistance signal transduction in a distinct manner.

Inhibition of wound-induced accumulation of allene oxide synthase transcripts in flax leaves by aspirin and salicylic acid

Allene oxide synthase (AOS) mediates the conversion of lipoxygenase-derived fatty acid hydroperoxides to unstable allene epoxides, which supply the precursors for the synthesis of the phytohormone jasmonic acid (JA). In this study the characterization of AOS gene expression in flax (Linum usitatissimum) is reported. AOS was constitutively expressed in different organs of flax plants. Additionally, AOS gene expression was enhanced after mechanical wounding in both the directly damaged leaves and in the systemic tissue located distal to the treated leaves. This wound-induced accumulation of AOS required the de novo biosynthesis of other unknown proteins involved in the signaling pathway modulating wound-induced AOS gene expression. Furthermore, the wound-induced AOS mRNA accumulation was correlated with the increase in the levels of JA. Both JA and its precursor, 12-oxo-phytodienoic acid, activated AOS gene expression in a dose-dependent manner. Thus, JA could activate its own biosynthetic pathway in flax leaves. Moreover, neither salicylic acid (SA) nor aspirin influenced AOS enzymatic activity. It is interesting that pretreatment with SA or aspirin inhibited wound-induced accumulation of AOS transcripts. These results suggest that a potent inhibition of JA biosynthetic capacity in leaves can be affected by SA or aspirin at the level of AOS gene expression.

The biosynthesis of salicylic acid in potato plants

Spraying potato (Solanum tuberosum L.) leaves with arachidonic acid (AA) at 1500 &mgr;g mL-1 led to a rapid local synthesis of salicylic acid (SA) and accumulation of a SA conjugate, which was shown to be 2-O-beta-glucopyranosylsalicylic acid. Radiolabeling studies with untreated leaves showed that SA was synthesized from phenylalanine and that both cinnamic and benzoic acid were intermediates in the biosynthesis pathway. Using radiolabeled phenylalanine as a precursor, the specific activity of SA was found to be lower when leaves were treated with AA than in control leaves. Similar results were obtained when leaves were fed with the labeled putative intermediates cinnamic acid and benzoic acid. Application of 2-aminoindan-2-phosphonic acid at 40 &mgr;M, an inhibitor of phenylalanine ammonia-lyase, prior to treatment with AA inhibited the local accumulation of SA. When the putative intermediates were applied to leaves in the presence of 2-aminoindan-2-phosphonic acid, about 40% of the expected accumulation of free SA was recovered, but the amount of the conjugate remained constant.

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.

UV-B- and oxidative stress-induced increase in nicotinamide and trigonelline and inhibition of defensive metabolism induction by poly(ADP-ribose)polymerase inhibitor in plant tissue

Nicotinamide and trigonelline contents increased in Catharanthus roseus tissue culture after exposure to 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) or vanadylsulfate and in Pisum sativum leaves after exposure to UV-B radiation. Vanadylsulfate increased phenylalanine ammonia-lyase (PAL) activity and the content of reduced and oxidized gluthathione in C. roseus tissue culture. The increases in PAL activity caused by 2 mM AAPH or 0.2mM vanadylsulfate were prevented by 0.1 mM 3-aminobenzamide (3-AB), an inhibitor of poly(ADP-ribose)polymerase. Present results support the hypothesis [Berglund, T., FEBS Lett. (1994) 351, 145-149] that nicotinamide and/or its metabolites may function as signal transmittors in the response to oxidative stress in plants and that poly(ADP-ribose)polymerase has a function in the induction of defensive metabolism.

Inhibition of ascorbate peroxidase by salicylic acid and 2,6-dichloroisonicotinic acid, two inducers of plant defense responses

In recent years, it has become apparent that salicylic acid (SA) plays an important role in plant defense responses to pathogen attack. Previous studies have suggested that one of SA's mechanisms of action is the inhibition of catalase, resulting in elevated levels of H2O2, which activate defense-related genes. Here we demonstrate that SA also inhibits ascorbate peroxoidase (APX), the other key enzyme for scavenging H2O2. The synthetic inducer of defense responses, 2,6-dichloroisonicotinic acid (INA), was also found to be an effective inhibitor of APX. In the presence of 750 microM ascorbic acid (AsA), substrate-dependent IC50 values of 78 microM and 95 microM were obtained for SA and INA, respectively. Furthermore, the ability of SA analogues to block APX activity correlated with their ability to induce defense-related genes in tobacco and enhance resistance to tobacco mosaic virus. Inhibition of APX by SA appears to be reversible, thus differing from the time-dependent, irreversible inactivation by suicide substrates such as p-aminophenol. In contrast to APX, the guaiacol-utilizing peroxidases, which participate in the synthesis and crosslinking of cell wall components as part of the defense response, are not inhibited by SA or INA. The inhibition of both catalase and APX, but not guaiacol peroxidases, supports the hypothesis that SA-induced defense responses are mediated, in part, through elevated H2O2 levels or coupled perturbations of the cellular redox state.

Salicylate triggers heat shock factor differently than heat

Sodium salicylate has the unusual property of partially inducing the human heat shock response (Jurivich, D. A., Sistonen, L., Kroes, R., and Morimoto, R. I. (1992) Science 255, 1243-1245). Salicylate induces the DNA binding state of the human heat shock transcription factor (HSF), but this is insufficient to elevate heat shock gene expression. Because it is not known how HSF enhances heat shock gene expression, further analysis of the transcriptionally inert, salicylate-induced HSF was undertaken to potentially identify components of the heat shock response that are necessary for full transcriptional induction. Like thermal stress, exposure of HeLa cells to salicylate led to the induction of HSF1 into a DNA-bound state. Despite continued exposure of cells to salicylate, HSF1.DNA binding attenuated much more rapidly than a continuous heat shock. Western blot analysis revealed that the salicylate-induced form of HSF1 was not hyperphosphorylated like the heat-induced form. Furthermore, supershifts of the HSF1 bound to an heat shock element (HSE) oligonucleotide by monoclonal antibodies to phosphoamino acids revealed that salicylate induced threonine phosphorylation of HSF1, whereas heat led to a predominance of HSF1 serine phosphorylation. These data suggest that salicylate-independent signals are necessary to convert HSF1 into a transactivator of heat shock gene expression and that brief acquisition of DNA binding by this factor is insufficient to maximally enhance transcription.

Nicotinamide, a missing link in the early stress response in eukaryotic cells: a hypothesis with special reference to oxidative stress in plants

A hypothesis is presented suggesting that nicotinamide (NIC) is an initial signal substance in the response of eukaryotic cells to conditions which cause DNA-strand breakage, especially in connection with oxidative stress. In the stressed cell, NIC is released as a result of the activity of poly(ADP-ribose)polymerase (PADPRP). PADPRP is known to be activated by DNA-strand breakage, caused by e.g. oxidative stress or mutagens. NIC and its metabolite trigonelline (N-methylnicotinic acid) can induce defensive metabolism at the gene level. Connections between NIC and DNA-methylation are also considered. This hypothesis is discussed in the light of own observations and literature reports.