András Künstler

Sulfate supply influences compartment specific glutathione metabolism and confers enhanced resistance to Tobacco mosaic virus during a hypersensitive response

Sufficient sulfate supply has been linked to the development of sulfur induced resistance or sulfur enhanced defense (SIR/SED) in plants. In this study we investigated the effects of sulfate (S) supply on the response of genetically resistant tobacco (Nicotiana tabacum cv. Samsun NN) to Tobacco mosaic virus (TMV). Plants grown with sufficient sulfate (+S plants) developed significantly less necrotic lesions during a hypersensitive response (HR) when compared to plants grown without sulfate (−S plants). In +S plants reduced TMV accumulation was evident on the level of viral RNA. Enhanced virus resistance correlated with elevated levels of cysteine and glutathione and early induction of a Tau class glutathione S-transferase and a salicylic acid-binding catalase gene. These data indicate that the elevated antioxidant capacity of +S plants was able to reduce the effects of HR, leading to enhanced virus resistance. Expression of pathogenesis-related genes was also markedly up-regulated in +S plants after TMV-inoculation. On the subcellular level, comparison of TMV-inoculated +S and −S plants revealed that +S plants contained 55–132 % higher glutathione levels in mitochondria, chloroplasts, nuclei, peroxisomes and the cytosol than −S plants. Interestingly, mitochondria were the only organelles where TMV-inoculation resulted in a decrease of glutathione levels when compared to mock-inoculated plants. This was particularly obvious in −S plants, where the development of necrotic lesions was more pronounced. In summary, the overall higher antioxidative capacity and elevated activation of defense genes in +S plants indicate that sufficient sulfate supply enhances a preexisting plant defense reaction resulting in reduced symptom development and virus accumulation.

Up-Regulation of Antioxidants in Tobacco by Low Concentrations of H 2 O 2 Suppresses Necrotic Disease Symptoms

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.

Reactive Oxygen Species and Plant Disease Resistance

Plants may successfully limit or even kill pathogens at least in part by eliciting spatial patterns of ROS production in different parts of invaded plant cells, e.g., the cell wall and plasma membrane. Recent research also suggests a significant contribution to plant disease resistance by ROS-mediated processes in the plant cuticle and intracellular organelles. The role of temporal patterns (i.e., proper timing) of ROS accumulation in eliciting an effective plant disease resistance is also discussed. Essentially, defense against pathogens could be very effective if it is a rapid, symptomless process, eliminating the pathogen in due time and not overusing resources of the plant, a process likely mediated by ROS. On the other hand, a delayed and failed attempt by the host to elicit resistance may result in massively stressed plant tissues and a partial or almost complete loss of control over pathogen invasion. Thus, it seems that when plants encounter pathogens they need to defend themselves simultaneously against biotic and abiotic stresses (i.e., pathogen accumulation and excessive cell/tissue death) by turning on two different types of—partially overlapping—signaling pathways that may function in parallel. Very recent interesting data suggest a pivotal role of autopropagating ROS waves in these signaling processes.

Penetration and translocation of Erwinia amylovora-specific bacteriophages in apple - a possibility of enhanced control of fire blight

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.

Characterization of Myoviridae and Podoviridae family bacteriophages of Erwinia amylovora from Hungary - potential of application in biological control of fire blight

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.

Cold hardening protects cereals from oxidative stress and necrotrophic fungal pathogenesis

Abstract The effects of cold hardening of cereals on their cross-tolerance to treatments leading to oxidative stress were investigated. Long-term exposure to low non-freezing temperatures provided partial protection to wheat and barley plants from the damage caused by paraquat and hydrogen peroxide treatments. It also conferred resistance in two barley cultivars to the necrotic symptoms and growth of the fungal phytopathogen Pyrenophora teres f. teres. Pathogen-induced oxidative burst was also reduced in cold hardened plants. The possible roles of host-derived redox factors and other signaling components in the observed forms of cereal cross-tolerance are discussed.

The Signaling Roles of Glutathione in Plant Disease Resistance

Early studies showed that glutathione (GSH) as an antioxidant has a role in modulating plant tolerance to biotic stresses by suppressing localized necrotic symptoms following virus infections. The role of GSH in reducing severity of pathogen-induced symptoms in plants was confirmed by employing pharmacological and transgenic approaches. However, later studies have shown that GSH also has a key role in restricting pathogen levels. In fact, it seems that GSH is a pivotal factor responsible for signaling processes related to different types of plant disease resistance, including systemic acquired resistance. The signaling role of GSH in these processes is interconnected with reactive oxygen species and salicylic acid. GSH also regulates the function of plant defense-associated transcription factors and the transcriptional coregulator NPR1 by modulating their redox state. Another layer of regulation is provided by the nitric oxide donor S-nitrosoglutathione that promotes S-nitrosylation of defense-related transcription factors and transcriptional coregulators. Importantly, the role of GSH in mediating plant disease resistance-related signaling processes is independent of its antioxidant function. Changes in GSH levels and redox state triggered during plant biotic stress are not simply passive responses to oxidative damage, since GSH status regulates important elements of cellular signaling that leads to activation of defense responses.