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Here we show that in tobacco (Nicotiana tabacum cultivar Samsun NN Rx1) the development of Rx1 gene-mediated, symptomless, extreme resistance to Potato virus X (PVX) is preceded by an early, intensive accumulation of the reactive oxygen species (ROS) superoxide (O2·−), evident between 1 and 6 h after inoculation and associated with increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activities. This suggests a direct contribution of this ROS to virus restriction during symptomless, extreme resistance. Superoxide inhibition in PVX-inoculated leaves by infiltration of antioxidants (superoxide dismutase [SOD] and catalase [CAT]) partially suppresses extreme resistance in parallel with the appearance of localized leaf necrosis resembling a hypersensitive resistance (HR) response. F1 progeny from crosses of Rx1 and ferritin overproducer (deficient in production of the ROS OH·) tobaccos also display a suppressed extreme resistance to PVX, because significantly increased virus levels are coupled to HR, suggesting a role of the hydroxyl radical (OH·) in this symptomless antiviral defense. In addition, treatment of PVX-susceptible tobacco with a superoxide-generating agent (riboflavin/methionine) results in HR-like symptoms and reduced PVX titers. Finally, by comparing defense responses during PVX-elicited symptomless, extreme resistance and HR-type resistance elicited by Tobacco mosaic virus, we conclude that defense reactions typical of an HR (e.g., induction of cell death/ROS-regulator genes and antioxidants) are early and transient in the course of extreme resistance. Our results demonstrate the contribution of early accumulation of ROS (superoxide, OH·) in limiting PVX replication during symptomless extreme resistance and support earlier findings that virus-elicited HR represents a delayed, slower resistance response than symptomless, extreme resistance.

Azelaic acid (AzA, 1,9-nonadienoic acid) is a nine-carbon chain (C

Respiratory burst oxidase homologs (Rbohs) play crucial and diverse roles in plant tissue-mediated production of reactive oxygen species during the development, growth, and response of plants to abiotic and biotic stress. Many studies have demonstrated the contribution of RbohD and RbohF in stress signaling in pathogen response differentially modulating the immune response, but the potential role of the Rbohs-mediated response in plant–virus interactions remains unknown. The present study analyzed, for the first time, the metabolism of glutathione in rbohD-, rbohF-, and rbohD/F-transposon-knockout mutants in response to Turnip mosaic virus (TuMV) infection. rbohD–TuMV and Col-0–TuMV interactions were characterized by susceptible reaction to TuMV, associated with significant activity of GPXLs (glutathione peroxidase-like enzymes) and induction of lipid peroxidation in comparison to mock-inoculated plants, with reduced total cellular and apoplastic glutathione content observed at 7–14 dpi and dynamic induction of apoplast GSSG (oxidized glutathione) at 1–14 dpi. Systemic virus infection resulted in the induction of AtGSTU1 and AtGSTU24, which was highly correlated with significant downregulation of GSTs (glutathione transferases) and cellular and apoplastic GGT (γ-glutamyl transferase) with GR (glutathione reductase) activities. On the contrary, resistant rbohF–TuMV reactions, and especially enhanced rbohD/F–TuMV reactions, were characterized by a highly dynamic increase in total cellular and apoplastic glutathione content, with induction of relative expression of AtGGT1, AtGSTU13, and AtGSTU19 genes. Moreover, virus limitation was highly correlated with the upregulation of GSTs, as well as cellular and apoplastic GGT with GR activities. These findings clearly indicate that glutathione can act as a key signaling factor in not only susceptible rbohD reaction but also the resistance reaction presented by rbohF and rbohD/F mutants during TuMV interaction. Furthermore, by actively reducing the pool of glutathione in the apoplast, GGT and GR enzymes acted as a cell first line in the Arabidopsis–TuMV pathosystem response, protecting the cell from oxidative stress in resistant interactions. These dynamically changed signal transductions involved symplast and apoplast in mediated response to TuMV.

The effects of elevated glutathione levels on defence responses to powdery mildew (Euoidium longipes) were investigated in a salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) and wild-type cv. Xanthi plants, where salicylic acid (SA) contents are normal. Aqueous solutions of reduced glutathione (GSH) and its synthetic precursor R-2-oxothiazolidine-4-carboxylic acid (OTC) were injected into leaves of tobacco plants 3 h before powdery mildew inoculation. SA-deficient NahG tobacco was hyper-susceptible to E. longipes, as judged by significantly more severe powdery mildew symptoms and enhanced pathogen accumulation. Strikingly, elevation of GSH levels in SA-deficient NahG tobacco restored susceptibility to E. longipes to the extent seen in wild-type plants (i.e. enhanced basal resistance). However, expression of the SA-mediated pathogenesis-related gene (NtPR-1a) did not increase significantly in GSH or OTC-pretreated and powdery mildew-inoculated NahG tobacco, suggesting that the induction of this PR gene may not be directly involved in the defence responses induced by GSH. Our results demonstrate that artificial elevation of glutathione content can significantly reduce susceptibility to powdery mildew in SA-deficient tobacco.

Simple Summary Among abiotic stress factors, drought is one of the most detrimental factors in arid and semiarid regions, causing a significant decrease in plant growth and yield in most species, including crops. Under drought conditions, morphological, physiological and biochemical characteristics such as plant height and enzymatic activities are negatively affected. This negative effect may be alleviated with the aid of plant growth-promoting bacteria. Application of plant growth-promoting bacteria such as Paenibacillus, Azospirillum, Rhizobium, Bacillus, Azotobacter, Klebsiella, Pseudomonas and Serratia can enhance hormonal balance, maintain nutrient status and improve plant growth characters as well as increase yield. This review discusses the pivotal role of plant growth-promoting bacteria in mitigating drought stress by improving plant growth characters and yield. Abstract Plant growth-promoting bacteria play an essential role in enhancing the physical, chemical and biological characters of soils by facilitating nutrient uptake and water flow, especially under abiotic stress conditions, which are major constrains to agricultural development and production. Drought is one of the most harmful abiotic stress and perhaps the most severe problem facing agricultural sustainability, leading to a severe shortage in crop productivity. Drought affects plant growth by causing hormonal and membrane stability perturbations, nutrient imbalance and physiological disorders. Furthermore, drought causes a remarkable decrease in leaf numbers, relative water content, sugar yield, root yield, chlorophyll a and b and ascorbic acid concentrations. However, the concentrations of total phenolic compounds, electrolyte leakage, lipid peroxidation, amounts of proline, and reactive oxygen species are considerably increased because of drought stress. This negative impact of drought can be eliminated by using plant growth-promoting bacteria (PGPB). Under drought conditions, application of PGPB can improve plant growth by adjusting hormonal balance, maintaining nutrient status and producing plant growth regulators. This role of PGPB positively affects physiological and biochemical characteristics, resulting in increased leaf numbers, sugar yield, relative water content, amounts of photosynthetic pigments and ascorbic acid. Conversely, lipid peroxidation, electrolyte leakage and amounts of proline, total phenolic compounds and reactive oxygen species are decreased under drought in the presence of PGPB. The current review gives an overview on the impact of drought on plants and the pivotal role of PGPB in mitigating the negative effects of drought by enhancing antioxidant defense systems and increasing plant growth and yield to improve sustainable agriculture.

Heat stress negatively affects barley production and under elevated temperatures defense responses to powdery mildew (Blumeria graminis f. sp. hordei, Bgh) are altered. Previous research has analyzed the effects of short-term (30 s to 2 h) heat stress, however, few data are available on the influence of long-term exposure to heat on powdery mildew infections. We simultaneously assessed the effects of short and long term heat pre-exposure on resistance/susceptibility of barley to Bgh, evaluating powdery mildew infection by analyzing symptoms and Bgh biomass with RT-qPCR in barley plants pre-exposed to high temperatures (28 and 35 °C from 30 s to 5 days). Plant defense gene expression after heat stress pre-exposure and inoculation was also monitored. Our results show that prolonged heat stress (24, 48 and 120 h) further enhanced Bgh susceptibility in a susceptible barley line (MvHV118-17), while a resistant line (MvHV07-17) retained its pathogen resistance. Furthermore, prolonged heat stress significantly repressed the expression of several defense-related genes (BAX inhibitor-1, Pathogenesis related-1b and Respiratory burst oxidase homologue F2) in both resistant and susceptible barley lines. Remarkably, heat-suppressed defense gene expression returned to normal levels only in MvHV07-17, a possible reason why this barley line retains Bgh resistance even at high temperatures.

Here we show that in tobacco (

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Twelve bacteriophage isolates of Erwinia amylovora, the causal agent of fire blight, were isolated from blighted apple, pear and quince trees from different sites in Hungary. According to morphological characteristics they were assigned to the order Caudovirales, two isolates belonging to the Podoviridae and ten to the Myoviridae families. Examining plaque morphology, host range and molecular characterization by PCR established that these phages are not identical neither to the three North American strains used as references nor the earlier isolated Hungarian Siphoviridae strains. Studying the efficacy of selected phages in apple blossoms and green pear fruit slices it was found that a combination of three phage isolates (ΦEaH2A, ΦEaH5K and ΦEaH7B) significantly reduced bacterial multiplication and fire blight symptoms as compared to untreated controls. Combined application of these new E. amylovora-specific phages as biocontrol agents may contribute to a better control of E. amylovora under field conditions.

Earlier studies showed that the artificial elevation of endogenous glutathione (GSH) contents can markedly increase the resistance of plants against different viruses. On the other hand, salicylic acid (SA)-deficient NahG plants display enhanced susceptibility to viral infections. In the present study, the biochemical mechanisms underlying GSH-induced resistance were investigated in various tobacco biotypes displaying markedly different GSH and SA levels. The endogenous GSH levels of Nicotiana tabacum cv. Xanthi NN and N. tabacum cv. Xanthi NN NahG tobacco leaves were increased by infiltration of exogenous GSH or its synthetic precursor R-2-oxo-4-thiazolidine-carboxylic acid (OTC). Alternatively, we also used tobacco lines containing high GSH levels due to transgenes encoding critical enzymes for cysteine and GSH biosynthesis. We crossed Xanthi NN and NahG tobaccos with the GSH overproducer transgenic tobacco lines in order to obtain F1 progenies with increased levels of GSH and decreased levels of SA. We demonstrated that in SA-deficient NahG tobacco the elevation of in planta GSH and GSSG levels either by exogenous GSH or by crossing with glutathione overproducing plants confers enhanced resistance to Tobacco mosaic virus (TMV) manifested as both reduced symptoms (i.e. suppression of hypersensitive-type localized necrosis) and lower virus titers. The beneficial effects of elevated GSH on TMV resistance was markedly stronger in NahG than in Xanthi NN leaves. Infiltration of exogenous GSH and OTC or crossing with GSH overproducer tobacco lines resulted in a substantial rise of bound SA and to a lesser extent of free SA levels in tobacco, especially following TMV infection. Significant increases in expression of pathogenesis related (NtPR-1a, and NtPRB-1b), and glutathione S-transferase (NtGSTtau, and NtGSTphi) genes were evident in TMV-inoculated leaves in later stages of pathogenesis. However, the highest levels of defense gene expression were associated with SA-deficiency, rather than enhanced TMV resistance. In summary, elevated levels of glutathione in TMV-infected tobacco can compensate for SA deficiency to maintain virus resistance. Our results suggest that glutathione-induced redox changes are important components of antiviral signaling in tobacco.

Nonhost resistance is the most common form of disease resistance exhibited by plants against most pathogenic microorganisms. Type I nonhost resistance is symptomless (i.e. no macroscopically visible cell/tissue death), implying an early halt of pathogen growth. The timing/speed of defences is much more rapid during type I nonhost resistance than during type II nonhost and host ("gene-for-gene") resistance associated with a hypersensitive response (localized necrosis, HR). However, the mechanism(s) underlying symptomless (type I) nonhost resistance is not entirely understood. Here we assessed accumulation dynamics of the reactive oxygen species superoxide (O

Probably the most known and best studied type of plant resistance to pathogenic infections is the hypersensitive response (HR), a form of localized programmed cell death associated with restriction or killing of pathogens that often leads to macroscopically visible localized tissue necrosis. It is generally assumed that cell death and resistance within the HR are physiologically and genetically linked. However, there has been considerable speculation about whether cell death is an absolute requirement for resistance conditioned by the HR. This review discusses the relation of cell death and resistance in the HR, in particular, the importance of cell death in this process. We intend to focus on the increasing amount of research evidence showing that in several plant-pathogen interactions, the two main components of the HR – resistance and cell death – can be physiologically, genetically and temporally uncoupled. In other words, HR should be considered as a combination of resistance and cell death responses, where cell death may be dispensable for plant disease resistance. The varying contribution of these two components (i.e. cell death and resistance) generates an array of defense strategies differing in efficiency. Thus, a very early and rapid defense response seems to contribute to the development of macroscopically symptomless (extreme) resistance, while a moderately early defense response results in resistance with the concomitant development of controlled and limited cell and tissue death (HR). Accordingly, a delayed and failed attempt by the host to elicit resistance responses would result in massively stressed plant tissues (e.g. “systemic HR”) and a partial or almost complete loss of control over pathogen invasion. The dynamic nature of resistance responses in plants implies that resistance can be effective with or without cell death but its outcome and efficiency may depend primarily on the timing and speed of the host response.

A Hungarian cherry pepper (Capsicum annuum var. cerasiformé) cultivar ('Szentesi') displays resistance to pepper powdery mildew caused by Leveillula taurica. Resistance also develops in susceptible sweet pepper (C. annuum) when grafted on resistant cherry pepper cv. Szentesi rootstocks. Powdery mildew (PM) resistance is correlated with high levels of the defense regulator reactive oxygen species superoxide (O2 ') even in healthy plants. In order to further elucidate the mechanisms of preformed defense responses in cherry pepper cv. Szentesi we have monitored levels of salicylic acid (SA), a key molecule of plant defense signaling and expression of so-called pathogenesis/defense related (PR) genes in healthy pepper plants. Assays of free and bound (glycosylated) SA by high performance liquid chromatography (HPLC) revealed that in leaves of PM-resistant pepper levels of free SA are ca. twice as high compared to that of PM-sensitive plants. No difference occurred in levels of bound (glycosylated) SA. Expression of the CaPR-1 gene was several times higher in leaves of PM-resistant pepper than in sensitive plants as assayed by real time reverse transcription quantitative polymerase chain reaction (real time RT-qPCR). On the other hand, high expression levels of the CaPR-2 (glucanase) gene did not entirely correlate with PM-resistance, being detectable only in PM-resistant cv. Szentesi plants but neither in PM-resistant sweet pepper cv. Totál grafted on cv. Szentesi rootstocks nor in susceptible controls (cv. Totál). It seems that graft-transmissible PMresistance of the cherry pepper cv. Szentesi is correlated not only with high levels of superoxide but also with elevated levels of free salicylic acid and enhanced expression of the defense-related CaPR-1 gene.

We have investigated the uptake and delivery of Erwinia amylovora-specific bacteriophages in apple plants. The main aim of this study was to assess the potential of phage application as a means for improving phage persistence and thereby the control of fire blight, the disease caused by E. amylovora. Both phage strains tested (ΦEa104 and H5K) were able to translocate in apple seedlings and were detectable by a modified Adams’ drop test and real-time qPCR in plant parts above ground level following their application to the roots. Conversely, phages were detectable in roots after spraying them onto the stem and leaves. A water suspension of phages effectively decreased symptom severity of E. amylovora infection in apple seedlings following treatment of roots or aerial plant parts and application to the cotyledon, as judged by symptom bonitation. A similar effect was achieved by spraying a phage suspension onto flowering firethorn shoots. Interestingly no significant differences in controlling E. amylovora infection were found among the two phage strains tested. It seems that phages specific to E. amylovora can penetrate plants and exhibit a decrease in severity of symptoms caused by the phytopathogen. Demonstrating in planta translocation of E. amylovora-specific bacteriophages and their effect of reducing fire blight symptoms may significantly contribute to a better control of E. amylovora and promote further investigations on penetration and translocation of phages into plants.

High concentrations of the reactive oxygen species (ROS) superoxide (O2•−) and hydrogen peroxide (H2O2) contribute to the induction of plant cell and tissue death (necrosis). In an effort to create transgenic plants with high antioxidant capacity that could resist necrotic symptoms we produced two transgenic tobacco (Nicotiana tabacum cv. SR1) lines (S1 and S2) overexpressing a tomato chloroplast superoxide dismutase (SlChSOD). SOD genes encode for antioxidant enzymes that dismutate superoxide to hydrogen peroxide. Therefore, SOD-overproducing plants may contain high levels of hydrogen peroxide and are sensitive to stress-related necrosis unless sufficient degradation of hydrogen peroxide is conferred by elevated expression of antioxidants like e.g. catalases and peroxidases. Indeed, line S1 displayed elevated expression of a glutathione peroxidase (NtGPX) and a glutathione S-transferase (NtGSTU1b), as compared to wild type plants. Interestingly, however, expression of a catalase (NtCAT1) was repressed in...

Similarities and differences in the immune systems of plants and animals are discussed in relation to non-specific and specific immunity (resistance), systemic acquired resistance (immune memory), transgenerational immune memory and gene silencing. Furthermore, we attempt to answer the question “what is inhibiting or killing pathogens during the immune (resistance) process”? Therefore, the possible roles of reactive oxygen species and antioxidants in pathogen inhibition are evaluated in different types of plant disease resistance.

We found that resistance to pepper powdery mildew (PM) (Leveillula taurica) develops in a sweet pepper (Capsicum annuum) cultivar (‘Total’) when grafted on a resistant cherry pepper (C. annuum var. cerasiforme) rootstock (cv. Szentesi). Resistance is manifested both towards PM symptoms and pathogen accumulation. In healthy, uninfected plants PM-resistance can be predicted by enhanced accumulation of the reactive oxygen species (ROS) superoxide (O 2 ) and activity of NADPH oxidase, the enzyme mainly responsible for pathogenesis-related superoxide generation. In L. taurica-inoculated PM-resistant ‘Szentesi’ high levels of superoxide and NADPH oxidase activity are sustained even 45 days after inoculation, as opposed to PM-susceptible ‘Total’. This is also true for ‘Total’ grafted on resistant ‘Szentesi’ rootstocks, where PM resistance, enhanced superoxide production and NADPH oxidase activity is likely due to an unknown, graft-transmitted signal. To further elucidate the mechanisms of graft-transmissible PM-resistance we monitored expression of pathogenesis-related (PR) genes in healthy and infected plants. In healthy plants, expression of CaPR-1 is several times higher in leaves of PM-resistant pepper than in sensitive plants, while high expression of CaPR-2 (glucanase) does not entirely correlate with PM-resistance, being detectable only in PM-resistant ‘Szentesi’. However, during advanced stages of PM-pathogenesis (45 DAI) expression of CaPR-1 and CaPR-2 is by far the highest in PM-susceptible ‘Total’. Our results suggest that the direct biochemical cause of graft-transmissible PM-resistance in pepper is the enhanced accumulation of NADPH oxidase-generated superoxide. To our knowledge, this is the first report on the role of ROS (superoxide) in graft-transmissible, pathogen-specific disease resistance.

Local and systemic effects of azelaic acid (AzA) pretreatments on local necrotic lesion formation induced by Tobacco mosaic virus (TMV) were studied in Nicotiana tabacum cv. Xanthi nc plants as measured by a semi-automated, computer-assisted method. Local application of AzA (0.2–1.0 mM) showed no or limited influence (either increase or decrease) on lesion size of TMV-inoculated leaves. In addition, AzA pretreatment did not modify the multiplication of TMV detected by semiquantitative RT-PCR. No significant systemic effect of AzA on lesion size of TMV was detectable in distant leaves. Moreover, AzA treatment had no considerable local and systemic effect on symptom expression and multiplication of incompatible (P. syringae pv. tomato DC3000) and compatible (P. syringae pv. tabaci) bacteria. The response of the viral and bacterial pathogens to AzA was not different if tobacco plants were exposed to light or darkness after AzA treatment. However, the amount of AzA, measured by HPLC–MS, was doubled in phloem exudates of TMV infected leaves as compared to the control leaves, but concentrated exudates containing AzA did not induce local resistance response. In addition, the amounts of two other short-chain dicarboxylic acids (suberic acid and sebacic acid) increased in phloem exudates of virus-infected leaves. All these results suggest that, at least in tobacco, AzA does not induce considerable local or systemic effects on viral and bacterial infections. Therefore, its formerly reported role in signal transduction and/or induction of systemic acquired resistance (SAR) in Arabidopsis cannot be confirmed in tobacco.

Arabidopsis thaliana is susceptible to various different pathogens (fungi, bacteria, viruses). However, the effect of Tobacco necrosis virus (TNV) infection on Arabidopsis is not well known. TNV causes local necrotic and chlorotic lesions in the Arabidopsis ecotype Wassilewskija (Vannini et al., Physiol Mol Plant Pathol 69:26–42, 2006) but virus replication and systemic spread was not determined. Our goal was to monitor reactions to TNV infection in the most widely used A. thaliana ecotype, ‘Columbia’ (Col-0) by assessing in planta virus replication and systemic spread. We have shown that although TNV does not cause visible symptoms in A. thaliana ecotype Col-0 it is capable of replicating in planta as determined by real-time RT-qPCR. TNV levels progressively increased in inoculated leaves up to 4 days after inoculation (DAI) with a further severalfold increase between 4 and 8 DAI. Back inoculation of Nicotiana benthamiana resulted in symptoms typical of TNV infection (i.e., local necrotic lesions) within a few days. Interestingly, only a very low TNV titer (i.e., less than 20 % of that found in inoculated leaves) was detectable in non-inoculated upper leaves of Arabidopsis Col-0 even in an advanced stage of pathogenesis (21 DAI). The almost complete absence of systemic spread suggested that the virus is not seed-transmissible which was confirmed by real-time RT-qPCR analysis of plants grown from seeds produced by TNV-infected individuals. The present study is the first to show that TNV can infect the most widely used A. thaliana ecotype, Col-0, providing a novel system for future research on plant defense mechanisms during symptomless virus infections.

During plant–virus interactions, defence responses are linked to the accumulation of reactive oxygen species (ROS). Importantly, ROS play a dual role by (1) eliciting pathogen restriction and often localized death of host plant cells at infection sites and (2) as a diffusible signal that induces antioxidant and pathogenesis-related defence responses in adjacent plant cells. The outcome of these defences largely depends on the speed of host responses including early ROS accumulation at virus infection sites. Rapid host reactions may result in early virus elimination without any oxidative stress (i.e. a symptomless, extreme resistance). A slower host response allows a certain degree of virus replication and movement resulting in oxidative stress and programmed death of affected plant cells before conferring pathogen arrest (hypersensitive response, HR). On the other hand, delayed host attempts to elicit virus resistance result in an imbalance of antioxidative metabolism and massively stressed systemic plant tissues (e.g. systemic chlorotic or necrotic symptoms). The final consequence of these processes is a partial or almost complete loss of control over virus invasion (compatible infections)., Research in the author's laboratories is supported by grants of the Hungarian Scientific Research Fund (K111995 and PD108455) and the Spanish Ministry of Economy and Competitiveness (Project INIA, RTA2013-00026-C03-00). PDV acknowledges the CSIC and the Spanish Ministry of Economy and Competitiveness for his ‘Ramon y Cajal’ research contract, cofinanced by FEDER funds. MJCM acknowledges the Spanish Ministry of Economy and Competitivenes for her ‘Juan de la Cierva’ research contract.

Pretreatment of tobacco leaves with low concentrations (5 to 10 mM) of H2O2 suppressed hypersensitive-type necrosis associated with resistance to Tobacco mosaic virus (TMV) or Pseudomonas syringae pv. phaseolicola. The same pretreatment resulted in suppression of normosensitive necrosis associated with susceptibility to Botrytis cinerea. This type of H2O2-mediated, induced disease symptom resistance correlated with enhanced host antioxidant capacity, i.e., elevated enzymatic activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POX) after viral and bacterial infections. Induction of genes that encode the antioxidants superoxide dismutase (SOD), CAT, and APX was also enhanced early after TMV infection. Artificial application of SOD and CAT suppressed necroses caused by viral, bacterial, or fungal pathogens similarly as H2O2 pretreatment, implying that H2O2-mediated symptom resistance operates through enhancement of plant antioxidant capacity. Pathogen multiplication was not significantly affected in H2O2-pretreated plants. Salicylic acid (SA), a central component of plant defense, does not seem to function in this type of H2O2-mediated symptom resistance, indicated by unchanged levels of free and bound SA and a lack of early up-regulation of an SA glucosyltransferase gene in TMV-infected H2O2-pretreated tobacco. Taken together, H2O2-mediated, induced resistance to necrotic symptoms in tobacco seems to depend on enhanced antioxidant capacity.

The concept of using bacteriophages (bacterial viruses) as biocontrol agents in pest management emerged shortly after their discovery. Although research on phage-based biopesticides temporarily stopped with the advent of antibiotics, the appearance of antibiotic resistant bacterial strains led to a renewed interest in phage therapy for control of plant diseases. In the past twenty years numerous successful experiments have been reported on bacteriophage-based biocontrol measures, and several comprehensive studies have recently been published discussing detailed results of phage application practices in pest management, mainly from North American authors. The present review focuses on bacteriophage-mediated control of fire blight (caused by Erwinia amylovora (Burill) Winslow et al.), the most devastating bacterial disease of pome fruits. Research results from North America are discussed along with recent data from European laboratories.

In spite of the enormous information from research on genetics of plant disease resistance, the question still remains unresolved: what is directly inhibiting or killing pathogens and suppressing symptoms in resistant plants? This is particularly true for resistance to viral infections. Here we show that externally applied reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) or ROS-producing (O 2·− [superoxide] and H2O2) chemical systems infiltrated into tobacco leaves 2 hours after inoculation suppress replication of Tobacco mosaicvirus (TMV) in the susceptible Samsun (nn) cultivar. This was determined by a biological and a real-time PCR method. Infiltration of leaves of the resistant Xanthi (NN) cultivar with the ROS-producing chemicals and H2O2 significantly suppressed local necrotic lesions (i.e. the hypersensitive response) after inoculation of tobacco leaves with TMV. Accordingly, an early accumulation or external application of ROS, such as O 2·− and H2O2, in tobacco may contribute to the...

Sulfur-induced resistance, also known as sulfur-enhanced defense (SIR/SED) was investigated in Nicotiana tabacum cv. Samsun nn during compatible interaction with Tobacco mosaic virus (TMV) in correlation with glutathione metabolism. To evaluate the influence of sulfur nutritional status on virus infection, tobacco plants were treated with nutrient solutions containing either sufficient sulfate (+S) or no sulfate (−S). Sufficient sulfate supply resulted in a suppressed and delayed symptom development and diminished virus accumulation over a period of 14 days after inoculation as compared with −S conditions. Expression of the defense marker gene PR-1a was markedly upregulated in sulfate-treated plants during the first day after TMV inoculation. The occurrence of SIR/SED correlated with a higher level of activity of sulfate assimilation, cysteine, and glutathione metabolism in plants treated with sulfate. Additionally, two key genes involved in cysteine and glutathione biosynthesis (encoding adenosine 5′-phosphosulfate reductase and γ-glutamylcysteine synthetase, respectively) were upregulated within the first day after TMV inoculation under +S conditions. Sulfate withdrawal from the soil was accelerated at the beginning of the infection, whereas it declined in the long term, leading to an accumulation of sulfur in the soil of plants grown with sulfate. This observation could be correlated with a decrease in sulfur contents in TMV-infected leaves in the long term. In summary, this is the first study that demonstrates a link between the activation of cysteine and glutathione metabolism and the induction of SIR/SED during a compatible plant-virus interaction in tobacco plants, indicating a general mechanism behind SIR/SED.

To evaluate the importance of the oxylipin pathway in plant defense against viruses we studied the induction of this pathway in a compatible and an incompatible virus–pepper interaction. Obuda pepper virus (ObPV) inoculation led to the appearance of hypersensitive necrotic lesions (incompatible interaction), to a marked induction of lipoxygenase (LOX) enzyme activity, and to elevated transcription of three LOX genes in leaves of the pepper cultivar TL 1791 harbouring the L 3 resistance gene. In the same pepper cultivar Pepper mild mottle virus (PMMoV) inoculation caused very mild chlorotic symptoms (compatible interaction), and exerted only negligible effects on LOX activity and gene expression. The transcription of an allene oxide synthase ( CaAOS ) and a hydroperoxide lyase ( CaHPL ) gene was not modified significantly by any virus inoculation. The expression of a novel divinyl ether synthase ( CaDES ) gene was massively induced in ObPV-inoculated leaves, but only very slightly by PMMoV inoculation. Our results showed that LOX and DES genes may be specifically involved in resistance responses against virus infections.

Fatty acid hydroperoxide-producing lipoxygenase (LOX) and hydroperoxide-degrading glutathione peroxidase (GPOX) enzyme activities were studied in leaves of virus resistant Xanthi-nc and susceptible Samsun-nn tobacco cultivars after inoculation with Tobacco mosaic virus (TMV). Total LOX activity showed a maximum at pH 5.5 in cell-free extracts of uninfected leaves. LOX activity markedly increased at this pH after TMV inoculation, but a substantial induction was detected also in the basic pH range with an emerging peak around pH = 8.5. TMV-elicited LOX induction was weaker and appeared later in Samsun-nn than in Xanthi-nc leaves. GPOX activity was also substantially induced by TMV infection. However, this induction appeared only 4 days post-inoculation in resistant Xanthi-nc plants in tissues surrounding the localized necrotic lesions. In contrast, GPOX activity did not change in TMV-inoculated, susceptible Samsun-nn leaves. Several glutathione S-transferase (GST) isoenzymes also display GPOX activity. The ...

In plants, recognition of a pathogen as an invader may result in the formation of hypersensitive response (HR) lesions, i.e. localized programmed cell and tissue death associated with restriction of the pathogen to the infection site. A transient suppression of antioxidants is known to occur during relatively early stages of the HR. Here we show that the transient suppression of a catalase and an alternative oxidase gene during virusinduced local lesion formation (HR) has similar kinetics in different hosts regardless of the extent of leaf necrotization. Both Nicotiana edwardsonii var. Columbia and a paraquat tolerant N. tabacum biotype display significantly less and smaller necrotic lesions in response to inoculation by two viruses ( Tobacco mosaic virus and Tobacco necrosis virus ) in comparison to control plants ( N. edwardsonii and N. tabacum cv. Samsun, respectively). We found that all of these plant hosts display a transient suppression of catalase and alternative oxidase transcript levels starting ...

Different forms and mechanisms of plant resistance are compared in this review, differentiating between innate and acquired resistance. Within innate resistance, general non-specific and specific host resistance are treated as regards the phenomena and mechanisms. Various forms of non-specific (e.g. non-host, basal, quantitative etc.) as well as specific resistance (extreme resistance, gene-for-gene resistance with the hypersensitive response, toxin resistance and gene silencing) are discussed. Within acquired resistance, the immune memory vs. stress memory of animal or plant hosts are compared. Furthermore, mechanisms of acquired resistance of plants are treated in relation to salicylate metabolism, systemic signalling, antioxidants and a recently discovered recombination signal inducing somatic mutations. It seems important for future research trends in plant pathology that several new resistance mechanisms have been discovered or at least re-evaluated, in addition to the specific HR-associated resistance, which has been the subject of the majority of previous research.

Analysis of virulence mechanisms of plant pathogens is often limited by the lack of genetic tools that can be used to identify genes that are preferentially expressed during their interactions with plants. In the present study, we used the newly constructed IVET (in vivo expression technique) plasmid pIviGK and the corresponding antibiotic resistance-based selection method to identify genes that encode pathogenicity factors of the soft rot-causing bacterium Pseudomonas viridiflava. These included pel, the gene encoding pectate lyase, which is responsible for the development of soft rot symptoms. We have also isolated and characterized the gene mviNpv encoding a putative novel membrane associated virulence factor of P. viridiflava. A mutation in mviNpv was shown to influence motility as well as virulence of P. viridiflava. The mviNpv gene is expressed to a moderate level in LB media and its expression increases under inducing conditions as was shown by measuring in planta expression dynamics of the fused gfp reporter gene.

A new variety of Nicotiana edwardsonii, designated N. edwardsonii cv. Columbia, expresses pathogenesis-related (PR) proteins in a temporal manner 45 to 49 days postplanting and also exhibits enhanced resistance to Tobacco mosaic virus, Tobacco necrosis virus, and Tomato bushy stunt virus. In contrast, PR proteins were not expressed in the original N. edwardsonii variety at comparable ages but were induced after onset of a hypersensitive response to viral infection. The temporal induction of PR proteins in ‘Columbia’ was correlated with increases in salicylic acid and glycosylated salicylic acid. Earlier studies noted that some Nicotiana hybrids derived from interspecific crosses constitutively express PR proteins, but the genetic basis of this phenomenon had not been investigated, likely because many interspecific Nicotiana crosses are sterile. However, the close genetic relationship between N. edwardsonii and ‘Columbia’ indicated that a hybrid between these two plants might be fertile, and this proved to be true. Genetic crosses between ‘Columbia’ and N. edwardsonii demonstrated that a single, dominant gene conditioned temporal expression of PR proteins and enhanced resistance. This gene was designated TPR1 (for temporal expression of PR proteins).